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May 13

Other than dogs, what animals mainly perspirate by drooling?

--朝鲜的轮子 (talk) 01:56, 13 May 2013 (UTC)

Dogs don't really perspire by "drooling", they use panting to get rid of excess heat, the drool helps with the efficiency of the panting. Panting is also used for thermoregulation by pigs, cats and many birds. Vespine (talk) 02:56, 13 May 2013 (UTC)

I thought there's a theory about human's running. It says humans are "born to run long distances". People sweat and it helps you get rid of excess heat "in the long run". On the other hand, many other animals are only capable of sprinting. -- Toytoy (talk) 03:15, 13 May 2013 (UTC)

There's something to be said for that; famously in the Man versus Horse Marathon sometimes the man wins. That is, while horses are better, on average, at running a marathon-like distance, it isn't such an overwhelming advantage that some people can't outrace some horses over long distances. Quite notably, as the article Perspiration notes, while many mammals sweat, humans and horses are among the few that do so as an efficient means of thermoregulation; and notably horses are somewhat better long distance runners than humans. --Jayron32 03:31, 13 May 2013 (UTC)

Competitors in the 2006 Man versus Horse Marathon

This is unfair!

That poor horse has a man on his back.

To be fair, the man in front must carry a horse on his back! -- Toytoy (talk) 04:07, 13 May 2013 (UTC)

He already has a horse at his back... :-) Double sharp (talk) 13:41, 13 May 2013 (UTC)

22 miles is rather too short for an advantage for a man against a horse, it should really be over something like twice that distance and wild horses have been caught on foot just by running after them, it can be dangerous for the horse though. However for smaller animals like kudu persistence hunting is used by bushmen in a distance like 20 miles to tire down and kill prey, see [[1]]. Dmcq (talk) 09:18, 13 May 2013 (UTC)

(Speaking of horses...) I know very little about horse perspiration (that's not an article!), but I have rather distinct memories of horses I've been acquainted with being exceptionally drool-y (and that's not a word!) on hot summer days. My best guess at the time was that it was something like dogs' version of thermoregulation, minus the panting. I haven't been able to come up with a better explanation yet. Evanh2008 ^{(talk|contribs)} 06:06, 13 May 2013 (UTC)

Don't horses show froth or foam on their flanks when they've been ridden hard? I know hardly anything about horses, but remember reading it in different places. Alansplodge (talk) 17:24, 13 May 2013 (UTC)

Now that you mention it, that does sound like something I've heard in the past. The odd thing about this is that my most vivid memory of it involves an entire pasture of horses. They all would have been ridden fairly regularly, but I'm certain not all of them had been ridden that day or even that week. Evanh2008 ^{(talk|contribs)} 17:32, 13 May 2013 (UTC)

Frothing on the body depends on the action of friction on the sweat. See discussion. μηδείς (talk) 20:49, 13 May 2013 (UTC)

This was definitely originating from the mouth, rather than the body. Evanh2008 ^{(talk|contribs)} 00:35, 14 May 2013 (UTC)

Is it true that dogs have very few sweat glands except on their heads?--朝鲜的轮子 (talk) 02:57, 14 May 2013 (UTC)

Mammals like cats, dogs and pigs, rely on panting or other means for thermal regulation and have sweat glands only in foot pads and snout. Evanh2008 ^{(talk|contribs)} 03:09, 14 May 2013 (UTC)

Clam meat

Clams are usually sold by weight. Let's say all clams are sold at the same price by weight, and there are large, medium and small clams for sale. What kind of clam would you buy in order to get the maximum amount of edible clam meat? -- Toytoy (talk) 03:11, 13 May 2013 (UTC)

Depends on what you want to use them for. The larger the clam, the tougher the meat. Small clams are good raw or steamed on the half-shell. Larger clams should be used in chowder. What you want to do with them should be of greater concern for you than simply the total amount of meat. Think of it this way: filet mignon costs different than brisket, but you don't use them for the same thing. I'd never make pastrami out of filet mignon, and brisket is far too tough for making grilling steaks out of. Same with the clams. Regardless of which gets you the best "bargain", it isn't a bargain if the quality is not what you want. --Jayron32 03:14, 13 May 2013 (UTC)

To answer the question, I'd go for the larger clams to get most edible meat per weight. Here's my reasoning and assumptions: the shell thickness doesn't vary much, but total weight does. That is, a 4 oz clam may have a thicker shell than a 2 oz clam, but probably not twice as thick. The larger clams should have relatively less of their weight taken up by shell, compared to smaller clams. This is also assuming that we are talking about different size classes of the same species. Note that clam can mean many different species in USA English. SemanticMantis (talk) 14:22, 13 May 2013 (UTC)

A Boy and His Atom

This video is a stop-motion film made by IBM engineers using carbon monoxide molecules (the video describes them as "atoms," which I guess they technically are). Around the molecules, you can see something like a double image, almost a blurry, ripple-like duplicate of the molecules, only it may be many times double. Given the spacing of the atoms and the number of duplicates, it's hard to tell the exact number. My theory so far is that this is an optic effect (for lack of a better term), likely caused by mechanical oscillations in the microscope itself, as the imaging device vibrates, causing something like you may see when someone with an unsteady hand snaps a picture with a digital camera. What I can't figure out is why the molecules themselves aren't distorted in the picture (as you would see with a typical double exposure). But if that is the explanation, I can't figure out why you wouldn't see an identical effect with other pictures of small objects taken with electron microscopes. Any thoughts? Evanh2008 ^{(talk|contribs)} 05:59, 13 May 2013 (UTC)

This question is specifically answered in text and in video form at IBM's website: Made With Atoms. Nimur (talk) 06:18, 13 May 2013 (UTC)

And that's how the ref desk is supposed to work! Thanks, Nimur! :) Evanh2008 ^{(talk|contribs)} 06:23, 13 May 2013 (UTC)

Peroxodisulphate

calculate the volume of 0.01M peroxodisulphate ions required to oxidize 25cm³ of iron(Π) ions to ironΙΙΙ ions.iron(ΙΙ) ions were obtained by dissolving 1.12g of iron in dilute sulphuric acid — Preceding unsigned comment added by Nabwire (talk • contribs) 12:32, 13 May 2013 (UTC)

Please do your own homework.

Welcome to the Wikipedia Reference Desk. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. Red Act (talk) 13:32, 13 May 2013 (UTC)

Zero point energy and the volume of the universe.

Why doesn't the zero-point energy density of the vacuum change with changes in the volume of the universe? And related to that, why doesn't the large constant zero-point energy density of the vacuum cause a large cosmological constant? Is it allowed to postulate / hypothesize on this topic on the reference desk, or is there a separate science forum / talk page for that? Robert van der Hoff (talk) 06:49, 7 May 2013 (UTC)

Robert, the idea is that a header is a **short** (up to about 7 words) pointer to what the question is about, the meat of which then appears below the header. -- Jack of Oz [Talk] 07:32, 7 May 2013 (UTC)

(I made a more concise title and transferred the L-O-N-G title into the message body). SteveBaker (talk) 12:59, 7 May 2013 (UTC)

• We don't encourage using the reference desk simply to initiate discussion - especially when it's to discuss some idea that you had. However, there is a reasonable question here that we can possibly answer. SteveBaker (talk) 13:01, 7 May 2013 (UTC)

You hit the nail right on the head. That is indeed a very deep mystery that is yet to be satisfactorily answer by modern physics. Naive calculations show that the cosmologic is about 120 orders of magnitude off (If memory serves). Supersymmetry improves that to "only" 60 orders of magnitude. Dauto (talk) 22:28, 7 May 2013 (UTC)

And did you read the Zero-point energy article? where energy per particle is ½hν, not necessarily a very high density by cosmological standards. And this article also mentions renormalization to deal with the possibility of the lowest energy level of fields also containing energy. Graeme Bartlett (talk) 08:05, 8 May 2013 (UTC)

It's not ½hν per particle. It's ½hν per vibration mode of each bosonic field which strictly speaking is infinite hence the need for renormalization. Dauto (talk) 17:18, 8 May 2013 (UTC)

I read the article, and also the one about quantum foam, and the relevant sections from the book 'the fabric of the cosmos'. What I don't understand is how it relates to the universe we know. What are the properties or conditions of the quantum foam that triggered the Big Bang?Robert van der Hoff (talk) 13:38, 13 May 2013 (UTC)

Isn't this question listed in List of unsolved problems in physics? RJFJR (talk) 16:15, 13 May 2013 (UTC)

Correct, but there seems to be little movement in trying to solve it. I still hope that somebody will try to answer my question.Robert van der Hoff (talk) 11:29, 14 May 2013 (UTC) It's radio silence out there, so I'll just talk to myself. I postulate that the quantum foam is the real universe, infinite and eternal, and that our Big Bang was just a local outburst triggered by something that happened within the quantum foam. I wonder if that notion could put to rest the paradox of the large cosmological constant?Robert van der Hoff (talk) 07:15, 16 May 2013 (UTC)

Carbohydrate jacket potato overdose?

I was told earlier (during lunch funny enough) that it's possible to overdose on jacket potato...I'm aware you can overdose on vitamins, but there aren't any high vitamin values in jacket potatoes are there? The only decent Google references to this i could find point to someone named "Harvey" managing to "carb overdose" on 3 jacket potatoes. So really my question is how safe would it be to eat a jacket potato on a daily basis? Thanks ツ Jenova20 ^(email) 14:53, 13 May 2013 (UTC)

Think this is a miss use of semantics. Overdose usually in the vernacular means: taking too much of a drug. [2] Too much carbohydrate (at the expense of protein) will only leave you with stunted growth and that is a dietary thing. One just requires a balanced diet. If you do wrestling, then you could have a huge intake of calories and still be healthy. Sumo's eat amassing quantities of rice but burn it of in training.--Aspro (talk) 15:20, 13 May 2013 (UTC)

That's jacket potato, in case anyone else is unfamiliar with the term. Otherwise I agree with Aspro, this is a very informal usage (or just incorrect, e.g. overdose). Compare to "He overdosed on Breaking Bad, watching 50 episodes in two days!" SemanticMantis (talk) 15:36, 13 May 2013 (UTC)

I'm already stumped on this one... ツ Jenova20 ^(email) 15:45, 13 May 2013 (UTC)

A diet of whole milk and potatoes would supply almost all of the food elements necessary for the maintenance of the human body. Count Iblis (talk) 15:52, 13 May 2013 (UTC)

That's certainly interesting. So there's no issues of any overdose, like with a potassium overdose from eating hundreds of bananas? Thanks ツ Jenova20 ^(email) 16:02, 13 May 2013 (UTC)

. An organization whose sole raison d'être is to sell more potatoes is not where I would go for information on whether I should eat more potatoes. --Jayron32 16:17, 13 May 2013 (UTC)

I eat about 1 kg of potatoes a day, I doubt most people could eat this much. While 1 kg of potatoes contains a lot of nutrients, I still need to eat a lot of vegetables, fruits etc. to get all the nutrients I need. So, overdosing on potatoes isn't really feasible as our stomachs are too small. Perhaps big animals that eat foods low in nutrients like e.g. cows or elephants could overdose on potatoes. Count Iblis (talk) 16:31, 13 May 2013 (UTC)

• See hyperglycemia, diabetes, ketoacidosis, and death. μηδείς (talk) 17:48, 13 May 2013 (UTC)

Diabetes? Is there really much sugar in potato? I know it's starchy but i'm not really really technical with this stuff. Thanks ツ Jenova20 ^(email) 19:48, 13 May 2013 (UTC)

Starch is sugar, essentially. All carbohydrates are simply chains of sugar molecules of varying length, from one (Glucose and fructose) to two (sucrose and maltose) to shortish chains starch to long, complicatedly branching chains (cellulose and dietary fiber). The key is Glycemic index which tells how fast the digestion of various carbohydrates releases glucose into the blood stream. Foods with low glycemic index slowly release glucose, so you don't get high glucose concentrations, which is good, diabetically speaking. Foods which are high in glycemic index cause large spikes in blood glucose levels, which is bad diabetically speaking. Baked russet potatoes are among the worst foods in terms of glycemic index: 111, per this table published by Harvard Medical School, which makes it worse than pure glucose. Even boiled or mashed potatoes are pretty high, ranking in the 80s. It's hard to do worse, in terms of picking foods which are bad for blood glucose, than potatoes. --Jayron32 20:47, 13 May 2013 (UTC)

Starch is actually a chain of pure sugar with some water extracted. It converts entirely to sugar, starting even in the mouth with the action of pepsin in the saliva. μηδείς (talk) 20:45, 13 May 2013 (UTC)

Would it be possible to get an excessive amount of solanine or related compounds from eating potato skins? RJFJR (talk) 21:00, 13 May 2013 (UTC)

This article says that most commercial varieties of potato have between 2-13 mg/100 g of solanine in them. The TDLo for humans is 2.8 mg/kg: www.look chem.com/SOLANINE/. So, an 80 kg person would have to eat 224 mg of solanine to be toxic, that would mean eating 1.72 kg of potatoes, assuming the 13 mg/100 g at the high end of solanine concentration. The average Russet potato (one of the larger varieties) is 299 g: www.food facts.com/NutritionFacts/Fruits-Nuts-Seeds-Vegetables/Potatoes-Russet-Raw--Large-Potato-w-Skin-/21020. That would mean you'd need to eat 5.75 whole russet potatoes in a sitting to get toxic effects from solanine, assuming the worst case scenario. Unlikely, but not impossible. (refs hit spam filter. Take out spaces to get them) --Jayron32 21:16, 13 May 2013 (UTC)

You might try our article on solanine, just not the green potatoes. μηδείς (talk) 22:03, 13 May 2013 (UTC)

All potatoes contain some solanine, the data above is for average, standard store-bought potatoes of good eating quality. Green potatoes would, of course, contain even more solanine that what is noted above. --Jayron32 22:09, 13 May 2013 (UTC)

Hmmm, I'm not far below this limit with my 59 kg bodyweight and 1 kg of potatoes per day. Count Iblis (talk) 22:42, 13 May 2013 (UTC)

Some naturopaths claim that potatoes aggravate arthritis - a disease of elderly people. I've never believed it myself, as I am elderly, arthritis free, and eat a lot of potatoes, which contain vitamin C, but how old is Count Iblis? Wickwack 121.221.30.176 (talk) 01:55, 14 May 2013 (UTC)

One issue to bear in mind is that something like half of the solanine is in the outer few millimeters of the surface of the potato. So if you were to preferentially eat potato skins (and I've seen those served as appetizers in several US restaurant chains), you could easily go over the safe dose level. Fortunately, most people who serve them deep-fry them and that gets rid of most of the solanine. However, pan-frying or microwaving doesn't get rid of it...so beware! However, a corollary to that is that if you eat baked potatoes and leave the skin behind - or if you peel your potatoes, then the risk is greatly reduced. SteveBaker (talk) 16:59, 16 May 2013 (UTC)

Hot Jupiters

With Hot Jupiters, how is their mass, density, and composition determined? Bubba73 ^{You talkin' to me?} 16:25, 13 May 2013 (UTC)

AFAIK, you need planets that you can detect both via the Doppler method and the Transit method. In that case, you know that the orbital plane of the planet is (very nearly) aligned with a line from the star to the observer, and hence that all of the radial velocity difference visible in the doppler shift. Then you have the ratio of masses and the orbital period, and you can solve for mass. The transit observation shows you how much of the star is covered by the planet during transit, and hence the diameter of the planet. From that you get the volumen, and with the mass (and math ;-) the density. I don't think you can completely determine the composition, but the density gives you a good initial guess, and in the best cases, you can get spectroscopic data from the atmosphere during transits. --Stephan Schulz (talk) 16:38, 13 May 2013 (UTC)

OK, I see how you get the size, but how do you get the mass? Bubba73 ^{You talkin' to me?} 16:46, 13 May 2013 (UTC)

From the radial velocity changes, you get the relative ratio of the masses (the heavier the planet, the more it pulls the star around). You also know the orbital period (from the timing of transitions or doppler shifts). That gives you two equations for two variables. --Stephan Schulz (talk) 16:50, 13 May 2013 (UTC)

OK, the ratio of the mass of the planet and the star.

Resolved

Bubba73 ^{You talkin' to me?} 17:04, 13 May 2013 (UTC)

galactic plane all the same?

How closely aligned to our solar system's are the orbital planes of other planetary systems on average? μηδείς (talk) 17:45, 13 May 2013 (UTC)

They are not aligned at all. Ruslik_Zero 19:14, 13 May 2013 (UTC)

Ugh? You didn't give any references, so that's really no answer at all. [3] suggests that all stars, planets and star polars start up the same way. But the op is asking for an average or better description, like a bell curve as it applies to OUR solar system in OUR galaxy to other planetary systems in OUR galaxy --Aspro (talk) 19:44, 13 May 2013 (UTC)

I'm sorry, but I cannot find anything about the orbital planes of planetary systems in the linked article (that may be my failure). As far as I can tell, it only talks about the orbits of stars in the galactic plane. --Stephan Schulz (talk) 21:11, 13 May 2013 (UTC)

She didn't specify it was confined to our galaxy. -- Jack of Oz ^[Talk] 20:19, 13 May 2013 (UTC)

Quote the OP : "How closely aligned to our solar system's are the orbital planes of other planetary systems on average?"

What is there in that to suggest other galactical bodies other than our own? Use a modicum of commons sense.--Aspro (talk) 20:33, 13 May 2013 (UTC)

What is there in that to suggest other galaxies are excluded? Keep your remarks civil, please. -- Jack of Oz ^[Talk] 20:41, 13 May 2013 (UTC)

I didn't think it was necessary to mention the galactic plane twice, I usually assume common sense on behalf of the regulars here. Also, how many planetary systems in other galaxies do we have information on? Also, is it not obvious other galaxies all spin at random angles to ours? I am basically curious if there's any information; average, anecdotal, from a study, theoretical, whatever; that suggests there's a better than chance alignment of planetary planes in the galaxy. μηδείς (talk) 20:42, 13 May 2013 (UTC)

At least Kepler data is interpreted with the assumption that orbital planes of other stars are randomly aligned. Kepler is also looking "up", so this assumption has also been build into the mission (if planetary orbits were aligned with ours, Kepler would miss them all). --Stephan Schulz (talk) 21:19, 13 May 2013 (UTC)

Well, that article shows a field that looks pretty close to the galactic plane near Cygnus, so it's a matter of degree - it's certainly not pointed straight out to deep space. Wnt (talk) 16:57, 14 May 2013 (UTC)

Damned reference system confusion! I was talking about "up" from a solar system perspective, where "up" from the ecliptic is the side where Earth North Pole is, not "up" in a galactic reference system. --Stephan Schulz (talk) 19:27, 14 May 2013 (UTC)

Transits of planets close to stars in our galaxy are barely detectable. Yet, if you are insatiable (and I can think of better terms) you can search for them here. [[4]] For other planets in other galaxies far, far, aware -forget it -the orders of magnitudes are not yet within our realms detectability – Jesus (etc.,) -we can’t see planets on the other side of our own universes let alone other galaxies. Quote: ”Also, is it not obvious other galaxies all spin at random angles to ours?” Have you hear off “big bang" and space telescopes show everything flowing – blooming off from the centre?” Ballooning other analogies? Hubble & other images show it. Perhaps not obvious (to you) but it is very evident evident. Its not planetary planes but galactic planes. Quote: “other galaxies all spin at random angles to ours” they did not! Computer modelling has shown that the universe ended up in its present state according to to the physics that we now understand.Aspro (talk) 22:28, 13 May 2013 (UTC)

Your comment is very confusing, Aspro, and you seem to be conflating the words galaxy and universe, perhaps because galaxies used to be called island universes. In any case, I am talking about objects like the Milky Way and Andromeda when I speak of galaxies in general. It is quite obvious from both catalog and deep field images that their spin is random in relation to the spin of our galaxy. In any case, my question is only about the various planetary systems in our galaxy alone. μηδείς (talk) 23:56, 13 May 2013 (UTC)

Sorry, but nothing is "flying off from the centre", unless (maybe) you think in more than the customary dimensions. The metric expansion of space has no center. And the Hubble Deep Field and Galaxy Zoo do not seem to find much bias in galaxy orientation in the universe at large. I'm also confused by your plural use of "universes", and the idea of "the other side" of our universe(s). But yes, as far as I know, we have not yet detected any planets in another galaxy. We can resolve individual stars in nearby galaxies, so in principle it could be possible to use the transit method. --Stephan Schulz (talk) 22:54, 13 May 2013 (UTC)

When I was a child, the classroom dictionary said that "the universe" and "the milky way galaxy" were synonyms. Since that dictionary was printed, astronomers determined that there were other galaxies in the "universe." Edison (talk) 02:01, 14 May 2013 (UTC)

Are you the Edison and have invented a longevity machine, or was your school using really outdated books? The topic was settled late, but it was settled by 1930 or so. --Stephan Schulz (talk) 06:30, 14 May 2013 (UTC)

I was a child a long time ago, the classroom dictionary was an old one, and dictionary writers apparently took a surprisingly long time to stop saying that the Milky way galaxy equalled "the universe." Edison (talk) 16:05, 14 May 2013 (UTC)

Extragalactic planets. Dragons flight (talk) 23:23, 13 May 2013 (UTC)

And specifically: An extra-galactic planet has been detected in Andromeda, lovingly named OGLE-2005-BLG-390Lb. They used gravitational microlensing techniques to find it. SteveBaker (talk) 19:58, 14 May 2013 (UTC)

That page says it's ~20,000 ly away and in the Scorpius constellation, while Andromeda is 2.5 million ly away and in the Andromeda constellation. --Sean 20:58, 14 May 2013 (UTC)

Ack! Sorry - I was thinking of this: http://www.newscientist.com/article/dn17287-first-extragalactic-exoplanet-may-have-been-found.html - but that seems more tentative than I believed it to be. SteveBaker (talk) 20:05, 15 May 2013 (UTC)

It seems doubtful that any planets could be detected by wobble or by transit, the only means of analysis currently available, if they were not in our own galaxy. So the question must apply to planets in the Milky Way galaxy. The transit method only reveals a planet if its orientation is has an axis such that it passes between us and its star. I'm not so sure about the wobble method. Edison (talk) 01:54, 14 May 2013 (UTC)

I actually studied at Cornell as an undergrad because I was a huge fan of Carl Sagan, although I never did meet him. So I can speculate on my own. The extragalactic article mentions an unrepeatable observation, an unconfirmed one, and a rogue planet of assumed extraterrestrial origin, but no information on intragalactic (or intergalactic) systems. I can draw surmises from the Kepler article, but they are surmises. I do believe we have some professional or at least credentialed astronomers here, so I hope there will be more comment. μηδείς (talk) 03:57, 14 May 2013 (UTC)

I want to have new discussion about this , has solar system sidereal rotation?--Akbarmohammadzade (talk) 05:47, 14 May 2013 (UTC)

See your question "Has solar system sidereal rotation?" below. Wickwack 60.230.230.117 (talk) 06:38, 14 May 2013 (UTC)

In the astrophysics literature, it is standard to assume that the orbital planes of other stars are completely random. Every statistic you see in the news relating to exoplanet frequencies (i.e. "there are 2 billion Earth-like planets in the habitable zones of Sun-like stars in the Milky Way") is going to have that assumption built in. That's how we calculate the geometric transit probability of detected planets, and from there, work backwards to deduce how many of that type of planet there must be.

As of now, there's no way to test the assumption of random orientation. If a planet transits its star, we can deduce its inclination with respect to the plane of the sky, but this will always be close to 90 degrees (or else the planet wouldn't transit). To determine its 3D orientation, one more angle is required, and that's the direction of the orbital axis in the sky plane. There's no way to measure this angle because for almost every star, the star itself is a point of light, the planet is invisible, and the star does not move enough due to the planet to be detectable by astrometry.

That said, it would be quite surprising if orbital planes weren't randomly oriented. The molecular gas clouds that stellar systems originate from are expected to have tiny rotation rates determined by chance and internal dynamics, not the galaxy (see [5]). When they collapse, whatever direction the cloud's net angular momentum happens to be pointing in defines the stellar system's orbital plane. Also, we have evidence that the orbital planes of binary stars are randomly oriented, and the formation process of binary stars is roughly analogous to that of planets: [6]. --Bowlhover (talk) 08:13, 14 May 2013 (UTC)

The plane of rotation is defined by the net angular momentum. Physicists believe, rather religiously, actually, that conservation of angular momentum is one of the most fundamental properties of the universe - and is never violated. If many different objects - planetary or stellar systems - have the same plane of rotation, then they have aligned angular momentum - which doesn't cancel out if you look at the ensemble. The whole group therefore has angular momentum, and always had angular momentum. Consider two possible consequences of that hypothetical observation, spanning all cases:

• This is a local observation. Suppose we broaden our search and find that more distant systems do not align with the local systems. Elsewhere in the universe, if we looked on a bigger scale, the angular momentum is oriented differently, and does cancel out. Therefore, the universe is strongly inhomogeneous at very large length scales. Some parts "birthed" into existence carrying different values of built-in momentum.
• This is a global observation. Suppose we hypothetically observe very far into the universe; and everywhere we look, in the entire universe, all the planes of rotation line up. The universe has therefore a net angular momentum; this defines an axis of rotation: and therefore, the universe has a preferred direction. The universe is strongly anisotropic at very large length scales. The universe "birthed" into existence already spinning along a particular axis.

Now, as observationalist physicists, we can't seriously conclude either case to be true, but we can quantitatively bound how probable these cases can be, based on our observations of apparently random orientations of stellar systems. We can say that for case one, if such inhomogeneity exists, it must occur on length scales larger than L... where L can be specified based on our observation. Or, we can say that for case two, if such anisotropy exists, its net "strength" must be weaker than S based on our counterobservations at small length scales.

I have frequently stated that the only actual questions left in cosmology boil down to these two: how anisotropic is the universe? How inhomogeneous is the universe? And we can say to both, "less than some maximum, and more than some minimum, based on what we observe..." Either way, we can't make a very strong claim, and our bounds are so very broad as to be almost useless. This is because our observations are very sparse, and are limited by our technological capability to study distant stellar systems from here at Earth. But, we've quantitfied the limits of our knowledge!

Finally we're thinking and talking like real actual cosmologists, rather than pop-pulp-science-fiction-writers! Nimur (talk) 14:22, 14 May 2013 (UTC)

I have no idea what you're trying to say. The OP asked about planetary systems in the Milky Way. You're talking about cosmology. Cosmological effects are utterly negligible on the scale of a galaxy, especially because the expansion of space does not affect gravitationally bound objects like galaxies. The answer to the OP is still "orbital planes are randomly oriented". --Bowlhover (talk) 16:44, 14 May 2013 (UTC)

Sorry if my post was unclear. I was attempting to describe why, from first principles of physics, we expect random alignment of the orbital planes, irrespective of the various length-scales we choose for our observational sample. If you still find my explanations unclear, you might consider reading my favorite resource on the topic, de Pater and Lissauer, Planetary Science, which spends most of chapter 13 discussing formation and evolution of stellar discs and describes their constraints by applying physical principles to inferred cosmological initial conditions. Perhaps my regurgitation of these concepts was a little more disordered than a textbook explanation. Nimur (talk) 18:02, 14 May 2013 (UTC)

Apparently Nimur's given a theoretical argument against, while these images show that our ecliptic is not aligned to the galactic plane. It appears the answer is that there is no evidence in favor of the hypothesis that planetary systems within our galaxy are aligned. I suspect that makes sense for any system that evolved from a nebula that was not coeval with the original condensation of the galaxy, given the alignments of nova ejecta would not be expected to align with the galactic plane. Does that sound correct? μηδείς (talk) 02:09, 15 May 2013 (UTC)

I seemed to have confused μηδείς with my last post. Said, Quote “our own universe” not “the universe”. Galaxies (as far as we know, start of as a flat plain (think of a pitza). Over time gravity (including dark matter so I'm reliably informed) pulls it together and it thickens out. [7]. Now think of spinning a bicycle wheel axil on your hand. Hold it tight. Move it up down, rotate it left to right. If one holds it firmly (as gravity will do) then it does not keep pointing in the same direction (as a free floating gyro would do ) but the axis of rotation goes off somewhere else. So, therefore, and thus, etc., etcetera; our own Sun's planetary plane is not exactly on our own galactic plain. Because we (you, me, Martha Stewart et. al) are no longer on out galaxy's 'exact galactic plain. So, do other star systems in 'our' and other galaxies when the star bunch up. But now I'm forgetting what useful point of μηδείς question is about. Back to you μηδείς: For what useful purpose do you need to know this percentage?Aspro (talk) 22:50, 15 May 2013 (UTC)

Determination of titratable acidity in apple juice

Hi! So this is our project (sort of) for an analytical chemistry lab. And If get the idea, since the commercial apple juice (which is what we're gonna use) is more or less colorless we can use an indicator (I'm unsure of whether they'll give us pH-meters and I'm not gonna risk) and apparently Malic acid is the dominant acid in apple juice (the natural one at least) and I've read in sources here and there that at least for the apple juice the pH of equivalence point is about 8, which helps me choose an indicator. The problem, however, is that I need a reliable source on the internet which I can use to cite, and Google searching it didn't give me any method which cites reliable sources. so where can I find a source for that? (sorry for potential grammatical/spelling errors)--Irrational number (talk) 18:47, 13 May 2013 (UTC)

The equivalence point will depend on which base you are titrating with, but if you are using a strong base like NaOH, then you need to calculate the pH of sodium malate, which will be the only species present at the equivalence point. There's a description of how to do so here, it's not hard if you know how to do pH calculations in general. Since you're calculating the pH of a solution of a weak base (sodium malate), you need it's Kb=10^-14/Ka of malic acid (obtained from the malic acid article). If you have an estimate of the concentration of the sodium malate (based on what you expect the concentration of malic acid is in apple juice) you can make a decent guestimation of what pH the equivalence point will be. --Jayron32 21:00, 13 May 2013 (UTC)

Thank you, but I was actually looking for a source I could cite for writing my lab report. It's so important for them.--Irrational number (talk) 09:46, 14 May 2013 (UTC)

The source you cite is any source for the Ka of malic acid, as it is the only number you need to look up, rather than calculate. All of the rest of the numbers either come from the lab procedure your teacher gave you, or you calculate them. The Ka values are given in the Wikipedia article, but if your teacher doesn't want you to cite Wikipedia, the values have a footnote so you can follow that to a non-Wikipedia reliable source. --Jayron32 12:39, 14 May 2013 (UTC)

Yeah, but I need a source for the procedure, not the values that I need. The teacher didn't give us any procedure, she asked us to find one for determining the titratable acidity of apple juice. I can think of a procedure (calculating the pH value of the equivalence point by estimating the concentration (and choosing the best indicator for it), making NaOH, standardizing it, using it to titrate the sample in presence the proper indicator), but I don't have any source that says this is the right procedure. I know science must not be based so much on authority and all that, but that's what they want from us... Sorry if I'm annoying...--Irrational number (talk) 13:06, 14 May 2013 (UTC)

The source of the procedure would be the source for any titration of a weak, diprotic acid with a strong base, which isn't functionally different than any titration. Literally every single general chemistry lab manual for first year chemistry students at both high school and college level has a titration lab like this. The procedure itself doesn't have to be modified from any titration, except that malic acid is a diprotic acid, so has two equivalence points. --Jayron32 21:07, 14 May 2013 (UTC)

May 14

has solar system sidereal rotation

As the sun and solar system rotate round center of milky way galaxy per 220milion years , it is clear that the system might rotate sidereal round sun , is there any information about such rotation? "I have inventive idea for this fact".--Akbarmohammadzade (talk) 05:23, 14 May 2013 (UTC)

May it be tidal locked or faster ,i suppose it be faster , but not be sensed by man for his short duration of observation time !!--Akbarmohammadzade (talk) 05:23, 14 May 2013 (UTC)

Sidereal just means "in an inertial frame of reference", that is, with respect to essentially motionless objects outside of the system being measured. Sidereal measurements of motion refer to measurements made against such a background; thus a sidereal day is the earth's rotation as measured against the stars, rather than the sun, which defines a solar day. I'm not how you are using it in this context. --Jayron32 06:11, 14 May 2013 (UTC)

May be the solar system ,or global stars or whole Orion cloud rotate round any center of mass ,when we are moving round center of galaxy . the planet day is its period of self rotating , i want to see , either the solar system or the global stars ,or Orion cloud has self rotating .then the sidereal rotation comes from comparing any inertial frame of reference, may it be very difficult to observe .(for the reason of our short duration of observation time)--Akbarmohammadzade (talk) 06:33, 14 May 2013 (UTC)

Our sun follows an orbital path around our galaxy. The timing of this orbital period will be perturbed by any orbital path through the universe that the galaxy may take, if the timing is with reference to something external to the universe. We have no such reference. Some boffins think that the universe does rotate, but as you surmised, it is difficult to prove. No accepted theory requires it. It has no practical importance. Wickwack 60.230.230.117 (talk) 06:45, 14 May 2013 (UTC)

Why it has no practical importance? A.M IRAN— Preceding unsigned comment added by 81.12.40.3 (talk) 14:51, 14 May 2013 (UTC)

We don't live long enough to notice the difference, and even as an evolving species, are not likely to. There is conjecture that the universe is ~17 billion years old. If our descendents are still around in another similar length of time, we might be in a postion to notice. But without a reference frame outside the universe, we can never be in a position to know anyway. And if we solve that one, we would still have to develop clocks billions of times better in precision that the best atomic clocks we have now, in order to measure the variation. Wickwack 120.145.220.218 (talk) 15:08, 14 May 2013 (UTC)

when the sun and its global system for example alpha-Centures and Orion cloud are rotating round any center of mass , then the branch of galaxy which we are in has special form of equilibrium. in the other hand if the solar system does rotate so ,that has other meaning . how can any branch of galaxy be able to have both movement and conserve its own shape?A.M IRAN

Does it mean that we have special gravitational dynamic in galaxies other than planetary system ?A.mohammad zade IRAN — Preceding unsigned comment added by 81.12.40.3 (talk) 15:06, 14 May 2013 (UTC) Lets I name this Dynamics ""galactic rotation""

I have no idea what you are trying to say. I realise that English is not your first language, but please take more care to write clearly. Wickwack 120.145.220.218 (talk) 15:10, 14 May 2013 (UTC)

To a good approximation, the solar system is orbiting the galactic center without rotating the plane of the ecliptic. See conservation of angular momentum. To visualize this, assume taking a plate, holding in vertically in front of your breast, and walk in a circle around some fixed object, but always facing (e.g.) north. The plate will always be oriented along the east/west axis. Of course, the sidereal period is originally observed with reference to the fixed stars, and for time periods similar to the orbit of the sun around the center of the galaxy (~250 million years), there are basically no fixed stars ;-). --Stephan Schulz (talk) 15:28, 14 May 2013 (UTC)

ok ,with this suppose our system will be tidal locked to center of galaxy . determining of it is very difficult ,and without any further dynamic result. when the center of gravity field effects on system (as I assumed whole Orion cloud or neighborhood stars )we will have special rotation , not because of conservation of angular momentum , but for their own equilibrium against such force.--Akbarmohammadzade (talk) 14:40, 15 May 2013 (UTC)

The solar system is certainly not tidally locked with respect to the center of the galaxy. This doesn't even make sense - tidal locking needs an at least somewhat deformable and minimally rigid body, since rotational energy is lost via deformation of the body. Also, tidal force is the difference in gravitational attraction between a nearer and a farther part of a body with respect to a source of gravity. On the scale of the galaxy, even the solar system is so small that this effect is negligible. --Stephan Schulz (talk) 17:39, 15 May 2013 (UTC)

which factor gives the velocity of our sun rotation round galaxy center ? the galaxy rotation curve is very different than which gives Newton formula . then the vector of such force which causes own rotation of system large amount.--Akbarmohammadzade (talk) 14:40, 15 May 2013 (UTC)

for the planets in our solar system:

The rotational period varies indirectly with the mass and distance of planets and satellites. As for neutron stars, however it is less than one second. We also have synchronous planets and moons with one gravitational sidereal rotation per year. The fact that any primordial birth event causes the rotational spin of sun and planets is clear, but this variation seems to be mulch-factorial, from tidal energy, hypothetical events and mass variations.--Akbarmohammadzade (talk) 14:54, 15 May 2013 (UTC)

Why does Jupiter Rotate So Fast?

Jupiter and Saturn are very fast and that doesn’t obey any regular process or regime based on, for example, the planet's mass or distance from the sun. Satellites effect the volume of a planet (and so on ….) which was not clear until now--Akbarmohammadzade (talk) 14:54, 15 May 2013 (UTC)

For observation of supposed either the solar system or the global neighborhood
bodies angular rotation , we need far away reference ,sun has its own rotation 

,but all its family are moving together ,may whole the galaxy has rotation for

conserving its branches .--Akbarmohammadzade (talk) 03:59, 16 May 2013 (UTC)

• 

rotation.jpg

• File:Http:/upwiki/wikipedia/commons/thumb/b/b9/GalacticRotation2.svg/250px-GalacticRotation2.svg.png

orbital.jpg--Akbarmohammadzade (talk) 08:12, 16 May 2013 (UTC)

r=x+a√1-x2/b2 =r.cos ω.t +a(√1-r2.cos2 ω.t /b2)

                                ṙ= - r. ω .sin ω.t-[a/(√1-r2.cos2 ω.t /b2)] r. ω .sin ω.t
                               V= ṙ= - r. ω .sin ω.t[1+ a/(√1-r2.cos2 ω.t /b2)]
                               a =  ȑ=- r. ω2 (sinω.t.cosω.t+ cosω.t)--Akbarmohammadzade (talk) 08:12, 16 May 2013 (UTC)

• File:Http:/upwiki/wikipedia/commons/b/b9/GalacticRotation2.svg

"Nipple delay"?

Reading the coverage of Angelina Jolie's preventative mastectomy, I saw the term "nipple delay" mentioned. Some Googling suggests that this might be this: http://www.ncbi.nlm.nih.gov/pubmed/22829005 , but this just refers in turn to a "surgical delay procedure" -- as in "Surgical delay is a technique that has been used for more than 400 years to improve survival of skin flaps." Does anyone know what this might involve? Is it the same thing as "delay of flap" (see http://www.springerreference.com/docs/html/chapterdbid/349885.html ) -- The Anome (talk) 08:53, 14 May 2013 (UTC)

As per http://www.ncbi.nlm.nih.gov/pubmed/15871701. It's not a common procedure, even in traditional nipple-sparing surgery. The vascular supply through the underlying glandular breast tissue to the nipple is obstructed (usually coagulation) under LA, and the nipple then left in place for two weeks or so for vascular collaterals in the skin to open up. Nickopotamus (talk) 23:15, 14 May 2013 (UTC)

Animal defense

Shimmering behaviour in Giant honeybees

Where in Wikipedia is an article about patterns of defense tactics of animals (show of force, threatening, pasive/active defense)? Examples: peacoks spread feathers, cats make themselfs big (fur and corpus), some tropical fish form schools, and some beatles make loud noises. --77.4.36.43 (talk) 10:17, 14 May 2013 (UTC)

Beatles do indeed make loud noises (or did, anyway, until Yoko Ono came along...) 24.23.196.85 (talk) 23:51, 14 May 2013 (UTC)

Ethologists usually classify some of those as agonistic behavior. There is also some info at territory_(animal). Then there is herd behavior which covers some of the others. For the "passive defense", see Armour_(anatomy), and maybe animal coloration. Does that cover all your examples? I'll check back later. SemanticMantis (talk) 12:38, 14 May 2013 (UTC)

I would agree, though, that we could legitimately have an overview article on that topic. Unfortunately high-level articles like that are the hardest kind to write, since they require the greatest level of expertise. Looie496 (talk) 14:26, 14 May 2013 (UTC)

I was thinking the same thing. Maybe it would be appropriate to start a stub (rather than limiting and contentious redirects)? For instance, a page titled "Animal defense" that has the above links, and a few more, separated into behavioral and morphological... SemanticMantis (talk) 16:11, 14 May 2013 (UTC)

That'll be great, too bad I'm not a behavioral scientist. --77.4.36.43 (talk) 16:48, 14 May 2013 (UTC)

Big Bang

Why is the expansion of the universe evidence for the Big Bang? That sounds like horseshit to me. Thats the equivalent of seeing a car going eastward on the M25 and concluding "that car started its journey in South London". Aren't physicists similarly jumping to conclusions? Pass a Method talk 10:25, 14 May 2013 (UTC)

That's not a fair comparisson - the M25 just keeps going and going, there is no end of the road where you can't go any further. The whole point of the expansion evidence, is that there is such a limit, where you can't go further back in time. Plasmic Physics (talk) 11:15, 14 May 2013 (UTC)

Technically the M25 doesn't 'just [keep] going and going' - there's a break for the Dartford Crossing. AndrewWTaylor (talk) 13:37, 14 May 2013 (UTC)

Moreover, the M25 has many intersections, the car could have driven onto the M25 by any one of those. The Big Bang expansion has no intersections, it started at a specific point in time. We've got no evidence to the contrary. Plasmic Physics (talk) 11:21, 14 May 2013 (UTC)

Is there any other evidence for the Big Bang besides expansion? From my perspective, the expansion of the universe could be explained by a modified version of the Big Bounce - perhaps an infinite universe without the gravitational singularity. Pass a Method talk 11:40, 14 May 2013 (UTC)

Cosmic Microwave Background radiation is predicted by, and consisted with, the Big Bang theory; an expanding universe which did not originate in a singularity might not be expected to have this. AlexTiefling (talk) 12:14, 14 May 2013 (UTC)

Actually, as far as I understand it (and this is hazy, year or so old knowledge from a physics degree), the CMB originates from the surface of last scattering, rather than the singularity itself, so a "big bounce" type theory which gets close to, but does not reach, a singularity would still show CMB similar to what we see now. Of course, with any such theory you run into the problem of the regions where it is substantially different being near impossible to observe. 91.208.124.126 (talk) 12:33, 14 May 2013 (UTC)

What happened before the Big Bang is a whole different pot of stew. Feel free to adhere to any theory you like, like the Big Bounce. Expansion only indicates the starting point of the Big Bang. Plasmic Physics (talk) 12:45, 14 May 2013 (UTC)

Try reading the article Big Bang (it exists for a purpose). The first two sentences of section Timeline of the Big Bang should tell you that the singularity is the result of an extrapolation of the standard cosmological model beyond the range of validity of known physical theories (general relativity and quantum mechanics). Simply put, the "big bang" (i.e. the singularity) is not actually part of "big bang theory". From observations we can infer that the universe was hot and dense at earlier times. This is directly observable to the last scattering surface/recombination when the CMB originated, temperature about 3000 K; indirectly to much earlier times based on a cosmological model with the observable universe given as initial conditions.--Wrongfilter (talk) 13:13, 14 May 2013 (UTC)

(edit conflict) Also note that the so-called "singularity", that is the point of infinite density with zero dimensions, is not itself actually proven, or even well believed, by anyone. Mathematical infinities show up frequently in physics calculations, but whenever we run into such an infinity, it is generally taken to mean that, at some arbitrarily large or small point, our theories cannot any longer match reality. See Gravitational singularity, for example, which states "Many theories in physics have mathematical singularities of one kind or another. Equations for these physical theories predict that the ball of mass of some quantity becomes infinite or increases without limit. This is generally a sign for a missing piece in the theory, as in the ultraviolet catastrophe, renormalization, and instability of a hydrogen atom predicted by the Larmor formula." (bold mine). That is, what we call the Big Bang is the massive expansion of the universe from its initial state; while most models take that initial state to be a singularity, that singularity just means "our theories break down as we get that far back" not "there really was an infinitely small point". The "Big Bang" itself could be taken to mean cosmological inflation, which was a period from .000000000000000000000000000000000001 seconds after creation to .000000000000000000000000000000001 seconds after creation. We have some grasp on what happened before that, down to perhaps the Planck epoch, which ended .0000000000000000000000000000000000000000001 seconds after creation, though often the term includes all of the events that happened from creation through the inflationary period. There are still many unanswered questions about how certain things happened in the transition from these very early time, such as Baryogenesis and Baryon asymmetry, or why we have more matter than antimatter. --Jayron32 13:06, 14 May 2013 (UTC)

'Bang' is a misnomer, that's been established, it's more like a 'Hum'. I guess, it's a bang in the sense of the rapidness of the event. In any case, something certainly went 'Big', that's been settled. Plasmic Physics (talk) 13:35, 14 May 2013 (UTC)

The evidence that should convince a lay jury to find the Big Bang guilty is:

1) The universe is expanding (galaxies are receding approximately according to Hubble's law).

2) Existence of the cosmic background radiation.

3) The age of the oldest white dwarf stars as inferred from the cooling rate is below the age of the universe as inferred from the expansion of the universe, while a hypothetical white dwarf of hundreds of billions years old would still be easily detectable.

4) The present day ratio of the abundance of hydrogen, helium and deuterium exactly fits the prediction of the Big bang model.

Count Iblis (talk) 13:48, 14 May 2013 (UTC)

My contention is mainly with the gravitational singularity in the Big Bang model. Not the rest. Pass a Method talk 13:53, 14 May 2013 (UTC)

As others have pointed out above, the Big Bang model is only useful to explain the evolution of the universe from some age onward. It has to break down when the universe was very young. We know this because the extremely high temperatures predicted by the Big Bang model would have led to the creation of magnetic monopoles and so much energy would have been in the form of these monopoles that the universe would have closed in on itself and collapsed soon after the Big Bang. This and other problems (e.g. the fact that the cosmic background is extremely uniform) are fixed by the inflation model. Count Iblis (talk) 14:02, 14 May 2013 (UTC)

When you use words such as "young" you are implying that we know whether the universe is infinite or not. The reality is we don't know. Let's be humble here Pass a Method talk 15:32, 14 May 2013 (UTC)

At that level, we don't know anything beyond our own existence, individually speaking (see solipsism). At some level, we need to accept an amount of uncertainty. The level of uncertainty present in modern scientific cosmogeny is not significantly more than the level of uncertainty necessary for you to operate in the world, and certainly not more so than any other branch of science. We have a pretty good handle on stuff down to pretty small units of time from the moment of creation. Science is very humble, but it also requires us to accept, until shown to be incorrect, theories that match observation well. And most of the theory in this case matches observation very well. --Jayron32 15:55, 14 May 2013 (UTC)

Let's not be pedantic either. It's perfectly OK to describe the universe as young without implying that nothing happened before the Big Bang. We don't know if the universe really started at the Big Bang but we can take the Big Bang time as time = zero, and measure the age of the universe from there. Dauto (talk) 15:58, 14 May 2013 (UTC)

The talk of "creation" is fruitless. We admit we have no idea what things would be like in the Planck epoch (and indeed, disputes about what happened later are so severe that the lay person gets the notion that at least some physicists don't know anything about what happened then). So we have no real proof of any kind that there was a "moment of creation", or what that means; it is a point where the math and theory breaks down. Wnt (talk) 16:54, 14 May 2013 (UTC)

Creation is as good of a word as any for that moment we define as the "zero" spot for our calculations of events as they occurred in what we call the "young" or "early" history, with all of the silly caveats we have to throw in about us not knowing if it is a real "zero" or merely an impenetrable barrier beyond which we can't measure, but still exists, and for which all of the other caveats we have to put on words like "early" and "young" and all that. But there's nothing inherently wrong with the word itself, it's a good heuristic word for a concept which, under Occam's razor, is the simplest explanation for what happened before that moment (i.e. "nothing", which is simpler than "something we can't ever know about, as far as we can tell") --Jayron32 19:42, 14 May 2013 (UTC)

Not to mention Last Thursdayism, which pretty much makes the idea of "creation" unfalsifiable. -- The Anome (talk) 18:46, 14 May 2013 (UTC)

That's certainly true - and can be taken to the extreme by saying that the only thing that exists is this precise instant - with time not existing at all - so no past and no future whatever. However, even in such a world-view, it is scientifically reasonable to ask why there are all of these signs and symptoms that make it look an awful lot like the universe was created 14 billion years ago - and to study those signs and symptoms to see why last Thursday's creation process resulted in such a bizarre set of seemingly self-consistent facts. If the universe was created last Thursday, why does it have cosmic background radiation? Certainly we can't falsify last-Thursdayism - but Occam's razor says that: like religion, the existence of ghosts and Russel's teapot - we're better off ignoring the unfalsifiable and pressing on with the study of what we can actually measure and conduct experiments upon. Occam's razor doesn't have the force of law - but its a damned good principle that is right a lot more often than it's wrong. SteveBaker (talk) 19:41, 14 May 2013 (UTC)

The Universe

I've always wondered, say the Universe is finite, it must have shape, right? If so, what is outside the shape? And since the Universe is said to expand, where is it expanding into? Say it were infinite, could the human mind ever comprehend how it looks like? ☯ Bonkers The Clown \(^_^)/ Nonsensical Babble ☯ 12:52, 14 May 2013 (UTC)

Start at Shape of the Universe and read on. --Jayron32 13:10, 14 May 2013 (UTC)

Short answer: nobody knows. Pass a Method talk 13:27, 14 May 2013 (UTC)

That's a terrible short answer! Here is a better short answer: cosmologists use the word "shape" to mean something different from what many people think. If you would like to learn what they mean, you'll need a longer answer, as others have pointed out. Nimur (talk) 15:55, 14 May 2013 (UTC)

It could have a topological configuration (eg a hypertorus) without needing to be embedded in any real higher space, so 'outsideness' does not necessarily enter into consideration. AlexTiefling (talk) 13:28, 14 May 2013 (UTC)

Yes, and hypertorus should probably redirect to Torus#n-dimensional_torus, which has a very nice animation. SemanticMantis (talk) 14:19, 14 May 2013 (UTC)

From our article on infinities, Infinity#Cosmology: "In 1584, the Italian philosopher and astronomer Giordano Bruno proposed an unbounded universe in On the Infinite Universe and Worlds "Innumerable suns exist; innumerable earths revolve around these suns in a manner similar to the way the seven planets revolve around our sun. Living beings inhabit these worlds." Modocc (talk) 14:24, 14 May 2013 (UTC)

It's hard to mentally visualize a finite shape without boundaries - but imagine that if the universe was MUCH smaller than it really is...say no bigger than the solar system. Now imagine you have an impossibly good telescope, then you could look towards the horizon - see the earth as a tiny dot out in space - then zoom in and see the back of your own head - you could look straight up and (if you're standing someplace in North America) see another copy of the earth - and zoom in to see the middle of the pacific ocean. Space itself could be curved back around in some extra dimension and looped back on itself...and it might do that in every direction you could look. What you'd see would be time-delayed, repeating copies of the same volume of space in every direction.

That wouldn't work in practice for a couple of reasons - one being that the speed of light wouldn't allow you to see that far without you seeing something from the impossibly distant past, before stars and planets existed - the other related problem being that no matter what, the "observable universe" is still smaller than the complete universe.

But it's certainly possible (even quite likely) that there are no boundaries and yet the universe can still be finite. That's a weird thing to comprehend - but no weirder than an infinite universe.

In a sense though, it doesn't matter. No matter which way we look, or how good our telescopes may become, the further we look in distance, the further back in time we see - so out beyond 45 billion lightyears, all we can see in any direction is the big bang. SteveBaker (talk) 16:59, 14 May 2013 (UTC)

• Imagine the universe is the size of a room in a normal sized house, with all the galaxies floating in it like motes of dust. That room has four walls, each with a door in it, and trap doors on the floor and ceiling. If you open the door facing east, the door facing west opens and you step in from the west as you leave from the east. Likewise north-south, and floor ceiling. Now imagine the doors are gone and you can see the back of your head by looking through any doorway. Now imagine the doorways expand and the walls disappear until there is no framework to the room any more, just a universe the size of the room in which it is possible to move in any direction infinitely without coming to a boundary. That is an finite unbounded universe. μηδείς (talk) 17:11, 14 May 2013 (UTC)

• There is a natural intuition for people who haven't studied advanced geometry that a curved space is always embedded in a higher dimensional flat space -- in the same way we visualize the surface of a sphere as embedded in 3-dimensional Euclidean space. But that's a false intuition. It is mathematically possible to have a curved space without having it embedded in anything, and even if it is embedded, it may be impossible to infer properties of the embedding space from the properties of the curved subspace. Looie496 (talk) 17:13, 14 May 2013 (UTC)

• For a smooth curved space it is generally true that one can do all the math one ever needs to do simply by considering local changes within the space. It is also true that any smooth curved space with a finite number of holes can always be mathematically embedded in a higher dimensional flat space. Whether or not such an embedding is useful will depend on the details of the problem one is considering, but the existence of such an embedding is really a statement about the nature of mathematics rather than anything physical. If people want to imagine that the universe lives within some empty, flat, higher dimensional space, then they are free to do that. Whether or not it is ever actually useful to do that is a separate issue. Dragons flight (talk) 18:17, 14 May 2013 (UTC)

As Dragons Flight says, it's possible to embed any any sufficiently well-behaved manifold in a higher-dimensional flat space, if only formally, for the purposes of discussing it, so it's not a bad way to visualise these things if you can't rid yourself of imagining everything in terms of curved things embedded in Euclidean space. -- The Anome (talk) 18:20, 14 May 2013 (UTC)

I find this stuff totally counter intuitive and mind bending. The thing to remember is that there isn't ANYTHING outside the universe. It's not like there is a boundary at the perimeter of the universe, like a bubble, with "universe stuff" on one side and some kind of "other stuff" on the other. It's kind of meaningless to talk about 'shapes', our normal undetstanding of things completely breaks down. Vespine (talk) 22:55, 14 May 2013 (UTC)

Do you find the room analogy I gave difficult, Vespine? μηδείς (talk) 02:01, 15 May 2013 (UTC)

I didn't find the analogy difficult, but that will only work if the universe is positively curved, which could very well be wrong. I believe the prevailing theory right now is that the universe is flat, which means you will never reach the same spot. Vespine (talk) 06:20, 15 May 2013 (UTC)

That is not correct. You can very well have closed flat spaces, in exactly the way that μηδείς described. Think of a cylinder, a two-dimensional flat surface (it is indeed geometrically flat, you can roll up a piece of paper without tearing it) that is closed in one dimension. Here, you can mentally identify the open ends of the cylinder to have a flat two-dimensional hypertorus (not possible in practice though, embedded in 3D space). Observations currently show no evidence that the Universe is closed - if it is, the "room" is probably larger than the observable Universe. --Wrongfilter (talk) 16:34, 16 May 2013 (UTC)

The most common example to help with understanding a finite universe with no boundaries is the surface of a balloon. Imagine you were a 2D ant living on the surface of the balloon, and for some reason, can't look up or down. The ant could move around the balloon and observe the effects of curvature, just like how astronomers can deduce our universe's curvature by looking at the CMB, but it will never come to the edge of its universe. --Bowlhover (talk) 06:27, 15 May 2013 (UTC)

It's certainly the most common analogy - but I think it's wrong. A torus (donut) is a better choice than a sphere. If the 3D universe was the surface of a hypersphere then I think we could easily tell. Imagine we're ants in your 2D analogy ants-on-a-balloon universe...if you and a dozen of your ant-friends walk in a straight line (let's suppose you walk around the equator) - and for every inch you walk, one of your friends stops walking and takes a rest. Now, after a prearranged amount of time, everyone turns right through 90 degrees and starts walking in a straight line again. In a "balloon universe", you'll be reunited with your ant-friends at the north or south pole. If you do that same thought experiment on a toroidal universe - you won't ever meet up again - each ant walks from the equator, through the donut hole and up the other side...no two ants ever meet again.

Now imagine the ants all have flashlights tied to their backs, as they converged at the poles, they'd get closer together - but the light rays would arrive at your eyes on the equator as parallel rays. If our universe was a balloon universe, objects in the far distance would be hugely magnified...and as far as we can tell, they aren't.

SteveBaker (talk) 19:57, 15 May 2013 (UTC)

No, a sphere is definitely a better analogy. The Friedman equations, which are solutions of Einstein's field equations for a homogenous and isotropic energy distribution (aka our universe), predicts a constant radius of curvature throughout the universe. That's the definition of a hypersphere; a torus has positive curvature at some places along its surface, negative curvature at others. The real reason we don't see far-away objects being magnified is because the universe is flat to the limits of measurable accuracy. If it were closed (and hence sphere-like), it would have an enormous radius of curvature whose effects haven't been detected yet. If it were actually open, it would have negative curvature throughout. That's a lot harder to imagine because no 2D surface of constant negative curvature can be embedded in a 3D space, but mathematically it represents an infinite universe. --Bowlhover (talk) 05:49, 17 May 2013 (UTC)

Don't know if this helps but consider this. The big bang (if such a thing happened) created time, mass, distance and everything. So, to answer the OP question: there is nothing existing (nothing at all) outside the distance that light has travel since the big bang. Beyond that horizon not even time exist. No laws of physics – nothing. The out side of the know universe isn't there. It does not exist yet -until the universe expands there. So the universe is not expanding in to anything – its creating space. I took my head a while to get around this concept but I haven't found a better way to capsulize what is known. The shape of anything can only be defined by its boundaries with something outside -but in this case there is no outside. So consider this analogy: A primitive person may gauges distance by how far people can walk in a day. If people in mounting villages can arrive in in two days (walking down hill) and others from the deep valleys take two days (walking up hill) will he realize that they both have traveled different distances - if they both took two days to arrive?. He will not be able to see the whole shape of the planet from this local traffic and likewise we can only see a little of the outer reaches of the universe. So I don't think any one can comprehend the shape because as yet we don't know – but we are probably 2.5 % of the way there.Aspro (talk) 00:01, 16 May 2013 (UTC)

Sucrose gradient

Why doesn't the sucrose in a sucrose gradient diffuse to reach equilibrium and thereby eliminate the gradient? — Preceding unsigned comment added by 129.215.5.255 (talk) 15:32, 14 May 2013 (UTC)

Doesn't it? Dauto (talk) 15:34, 14 May 2013 (UTC)

The diffusion equation can reach steady state with a non-zero gradient only in certain conditions - when there is a source or sink external to the region in consideration. In your application, do you have a source or sink (inflow or outflow)? Nimur (talk) 15:43, 14 May 2013 (UTC)

I was curious about this myself long ago and forgot to look it up - good question! In a sucrose gradient centrifugation, in biology, different concentrations are layered on top of each other in a closed tube. The point is that diffusion is slow: specifically, the diffusion coefficient is 520 um^2/s [8] or 5.7 x 10^-5 cm^2/s [9]. (The precise value depends on your conditions but let's take the second 0.0057 mm^2/s since the first looks crowdsourced) According to Fick's laws (the first), the rate of flow is proportional to the gradient in space - if you have a 30% difference in concentration over a distance of, I dunno, say 10 cm, then you dphi/dx = a constant (you hope) 0.3% / mm difference . J = -D dphi/dx = 0.0017 % mm/s, i.e. 0.0017% crosses a mm distance in one second. In order for the entire gradient to break down to 15% across the board, 15% has to cross the centerline, which would take 147 minutes ... except the gradient, and so the motive force, gets weaker and weaker and so it trails off exponentially over time. But this gives a first-order sense of how the gradient can last long enough. Wnt (talk) 16:30, 14 May 2013 (UTC)

This is something I had wondered about in the past as well (despite having done sucrose gradient centrifugation myself). Just a follow on from your excellent answer, surely the fact that you're centrifuging these gradients at 10.000+ g will affect the mixing? Or not, since the acceleration is approximately uniform over the length of the tube? Fgf10 (talk) 07:04, 15 May 2013 (UTC)

It should have an effect. In cesium chloride gradients, the force is enough to create a gradient. But cesium is very dense, and sucrose is not. I think the sedimentation coefficient is actually the relevant number, and that is quite low. [10] Wnt (talk) 23:27, 15 May 2013 (UTC)

Salt fingering is an interesting phenomenon that happens because of the slowness of diffusion. Dauto (talk) 18:22, 16 May 2013 (UTC)

a Brinicle is another interesting formation that depends on the slowness of salt diffusion in order to occur. Dauto (talk) 18:51, 16 May 2013 (UTC)

3 core 0.229in diameter wire vs 7 core 0.229 in diameter wire pure copper

How do these wire compare ? which one is better for upto 20 amp DC current ?

I don't think there will be much difference in conductive properties, but the 3-core wire will be quite a bit stiffer -- harder to break, but also harder to bend. Looie496 (talk) 17:08, 14 May 2013 (UTC)

There would be a significant difference in conductive properties at high frequencies because of the skin effect - but nothing really measurable at DC. That said, if the overall diameter is the same (0.229"), can you get more cross-sectional area with three circles or seven? Gut feel says that you get more copper and less air with seven cores than with three...but I'm not enough of a math whiz to know for sure. If you really care (and I doubt you need to!), get a length of each and weigh them...since the plastic sheath isn't likely to weigh much, whichever wire is heaviest should have more copper and hence less resistance at DC. SteveBaker (talk) 20:12, 14 May 2013 (UTC)

Sorry, I didn't mean to say that the conductivity is the same for both, just that it's so high as not to matter except in some sort of extreme application such as wires that are miles long. For either one of them, a kilometer of wire has a resistance on the order of 1 ohm. Looie496 (talk) 21:08, 14 May 2013 (UTC)

Not necessarily true. The OP asked about DC. If the source of DC is a 12 V lead acid battery, and the 20 A load is 25 m (~ 75 feet; taking into account go and return, routing within and around buildings etc, this could be almost just the next room), then the voltage drop in the cable would be 1 V, about enough to loose 8% of the power, and seriously compromise the operation of the load. Wickwack 121.215.78.174 (talk) 00:28, 15 May 2013 (UTC)

Yes, there's 20.367% more copper in the 7-strand case than in the 3-strand case. In the 3-strand case, the ratio of the two radii involved is 1 + cos 30° + (sin 30°)(tan 30°), and the rest of the calculation is easy. After I did the calculation, I learned that I could have gotten the answer a lot quicker if I'd known about the Circle packing in a circle article. Red Act (talk) 02:44, 16 May 2013 (UTC)

Did Prafulla Chandra Ray discovered Mercurous nitrite?

Can any expert in chemistry explain me if Prafulla Chandra Ray discovered Mercurous nitrite. I am really confused reading the wikipedia article and The Telegraph article 1. Solomon7968 (talk) 17:34, 14 May 2013 (UTC)

See http://www.ias.ac.in/resonance/Jan2001/pdf/Jan2001p42-49.pdf for a full account. Looie496 (talk) 17:45, 14 May 2013 (UTC)

What do you call the inverse of speed?

Hi,

Speed is (distance travelled)/(time taken) . Is there a specific word for the inverse of speed that is (time taken)/(distance travelled) and would have units of seconds per meter.

Thanks, Gulielmus estavius (talk) 18:11, 14 May 2013 (UTC)

Sloth? μηδείς (talk) 18:23, 14 May 2013 (UTC)

Stay on topic, please
The following discussion has been closed. Please do not modify it.
(Could you please use a <small> tag - or some other means to indicate when you're joking - this is a potentially confusing answer...I'm fairly sure sloth isn't the scientific term for inverse speed - and certainly your link doesn't indicate that) Could you stop using Wikipedia as an alternative to Facebook or Twitter please Medeis? You may find your own jokes amusing but most of the people who don't know your "style" have no idea what you're talking about. Thanks. The Rambling Man (talk) 20:39, 14 May 2013 (UTC) I am not joking. If you bothered to look at the source, rather than your own emotional content, you'd see the word sloth means slow-th, i.e., slowness. μηδείς (talk) 22:25, 14 May 2013 (UTC) I would call sloth or slowness the opposite of speed, using speed as a relative term. Speed as a measurement is distance over time, so the inverse would be time over distance. ←Baseball Bugs What's up, Doc? carrots→ 23:08, 14 May 2013 (UTC) It doesn't matter why you choose to call it that. Here on the reference desk, we work hard not to invent our own answers - which is what you (evidently) did. If it's a joke - then it's essential that you make that very, very clear...if it's not a joke then it's irrelevant that you've invented your own personal word for this...nobody gives a damn. Either way - bad answer. SteveBaker (talk) 19:37, 15 May 2013 (UTC) Quite. Medeis, please use Twitter or Facebook, rather than abuse Wikipedia (which is, after all, an encyclopedia, should you wish to contribute to content) to publish your "humorous" responses to Reference Desk questions. The Rambling Man (talk) 21:30, 15 May 2013 (UTC)

This abstract kinda suggests that "slowness" has been used - but there clearly isn't a formal SI derived unit like the "siemens" or "mho" is sometimes used as a reciprocal ohm or "hertz" as reciprocal seconds. SteveBaker (talk) 18:58, 14 May 2013 (UTC)

Hertz is NOT a unit for reciprocal seconds. Hertz dimensionally is 1/T, but Hertz is reserved for expressing the frequency of a periodic function that can be measured by cycles per second. There are other phenomena that are dimensionally 1/T that are not cycles per second and are thus not expressed in Hertz. For example, angular velocity (expressed in radians per second) also has dimensions of 1/T. And, obviously, if it takes me (say) 40 seconds to type this, and I express that as (0.025 Hertz)^-1, you'll rightly think I am a complete idiot. Also note that under SI, Hertz is defined in terms of atomic phenomena. If for some reason you are measuring the frequency with reference to sidereal time, the correct way to express it is as "cycles per second". Wickwack 121.215.78.174 (talk) 00:45, 15 May 2013 (UTC)

The perm (unit) coincidentally cooks down to (some multiple of) the same base units – if you cancel all the units in kg/s/m^2/Pa, then you get s/m left over – but obviously isn't quite what the OP is looking for. TenOfAllTrades(talk) 19:19, 14 May 2013 (UTC)

No scientific term exists, but there's one in popular culture: the "minute mile". See four-minute mile, for example. --Bowlhover (talk) 19:52, 14 May 2013 (UTC)

OK so the inverse of speed is journey time. Sussexonian (talk) 21:48, 14 May 2013 (UTC)

No, that has the dimensions of time: it's actually got to be journey time per distance: units of time-per-distance, in order for things to work dimensionally. -- The Anome (talk) 22:42, 14 May 2013 (UTC)

I think Journey time works, journey implies distance, a journey of zero distance is not a journey. Vespine (talk) 22:49, 14 May 2013 (UTC)

I'd think of journey time as being along the lines of "I live about an hour away from London". Not something with units of time/distance, but rather using time in place of distance (with an implied reference speed, in this case the speed of a car) 91.208.124.126 (talk) 08:19, 15 May 2013 (UTC)

The inverse of Electrical resistance as measured in Ohms is conductance, as measured in Mhos (Mho is Ohm backward, of course). So by that logic "Speed" could be "Deeps". There doesn't appear to be a standard unit for this, so maybe you could invent your own. As a matter of pure nosiness, why do you ask? Tonywalton ^Talk 01:01, 15 May 2013 (UTC)

Well, the invent-your-own answer is obviously superseded by sloth, but the "deeps" idea is worth extra credit! μηδείς (talk) 02:00, 15 May 2013 (UTC)

Enough. If anyone is interested in continuing this, take it to a talk page
The following discussion has been closed. Please do not modify it.
Why on Earth would anyone choose sloth, which to just about anyone means a certain South Amaerican animal, or a person behaving like one, makes no sense in any other context, and not choose slowth, which atleast is a new word in English, & seems to have something to do with being slow? Of course, the right wword for the property of being slow is the existing word slowness - which is why neither slowness, slowth, or sloth is ever used for the purpose of naming time per distance. Now, enough of this nonsense, Medeis. English is a very good language, and a lot of experince has gone into its use. While there is once in a while a need to invent a new proper nown (as otherwise we would be going around with cellular radio linked portable telephones stuck to our ears instead of cellphones (USA), or mobiles (non-USA)), if you feel the need to invent a new word otherwise, stop. There is almost always an existing word in either English or another European language to do the job, or a good reason why there isn't. Wickwack 121.215.37.165 (talk) 07:09, 15 May 2013 (UTC) The animal was named for its characteristic "sloth" or "slowness" or "laziness", not the other way around. ←Baseball Bugs What's up, Doc? carrots→ 15:34, 15 May 2013 (UTC) I have no idea what Wickwack's point is. It seems to have been written without reading past the third line of this thread or the link on the third line. μηδείς (talk) 17:51, 15 May 2013 (UTC) I think most readers have no idea what your point(s) mean and whether they actually contribute to embellishing the encyclopedia. You also need to consider that many visitors to the reference desks are new editors and your "style" is entirely inappropriate and discouraging. It seems clear you're happy to use this forum as some kind of substitute for Twitter or Facebook, and most of your edits don't actually provide any encyclopedic value whatsoever. The Rambling Man (talk) 21:34, 15 May 2013 (UTC) The OP is clearly asking for a scientific term. "sloth" is clearly not a relevant answer. This kind of "clever" reply can be quite annoying. 86.176.211.20 (talk) 00:29, 16 May 2013 (UTC) Feel free to look for information that might actually answer the OP's question. Ironically, "sloth" is probably the best answer posed so far. ←Baseball Bugs What's up, Doc? carrots→ 00:40, 16 May 2013 (UTC) After immediately thinking of hertz and wavelength I could not come up with an existing term that answered the OP's question from within the perspective of physics, so my mind immediately went to etymology, which is a very rigorous science. Certain editors' unfamiliarity with the fact that the word precedes the animal is not surprising. The OP may be curious why this immediately elicited Rambling Man's hostility, given he's an admin. Unfortunately, the latter's inability to deal with me and other editors objectively has been documented on his talk page and various other places such as the ITN talk page where he has apologized for such behavior. I really would appreciate it if someone would hat the string of comments terminating with this one as off-topic as well; it's tedious being attacked for no reason and feeling you have to explain yourself or stand offended. μηδείς (talk) 03:56, 16 May 2013 (UTC) Medeis, you got "attacked" by several people because you are wrong - very definitely wrong. Whether the meaning of sloth was first a person who is slothful, or the sloth animal, matters not. What matters is that the dictionary meaning of sloth is a sluggish arboreal tropical American edentate; inactive; lazy (Source: Chambers Dictionary). You cannot just appropriate a word to mean something totally different. The inverse of speed or distance / time is not an animal, nor is it lazyness. You might just as well declare that the opposite of boyancy is swimming. For a person who claims to be accustomed to precise scientific language, you show an alarming lack of familiarity with language concepts. Ratbone 120.145.139.227 (talk) 08:20, 16 May 2013 (UTC)

In SI, if you want to quote something as a reciprocal of speed, the convention is to just express the the numerical value with the units, as in 20 seconds per metre. Note that SI does not have names for everything (that would be impossible), and actually has a unit name for only relatively few things - generally things that had a unit name in the MKS, CGS, or Ft.Lb.Sec systems. For example, absolute viscosity is quoted in SI as Pascal-seconds (Pa.S). Wickwack 121.215.78.174 (talk) 00:56, 15 May 2013 (UTC)

you can "return back" or at distance or at time . thanks Water Nosfim --81.218.91.170 (talk) 04:53, 15 May 2013 (UTC)

I have asked this specifically since I was working with some code, which was dealing with some computation which made much use of the value inverse of the speed of light, and I was just wondering what would be a proper and clear name for a variable that stored that value. Many have here talked of Ohm and Mho, but what I was interested was not the units but the quantities ie, resistance and conductance. I imagine the inverse of speed could be of use in some situations, akin to how wavelength and wavenumber quantities are in use while describing waves. Gulielmus estavius (talk) 13:56, 15 May 2013 (UTC)

From my Google search for "inverse of velocity", I found some interesting results, including one which mentioned the expression "LoadFactor".

—Wavelength (talk) 22:27, 15 May 2013 (UTC)

Careful. Load factor is an existing term in many fields that has meanings not the slightest bit to do with inverse velocity or distance / time. The general concept is like this: If an automobile is in actuall use for 20 hours per week, the Load Factor is 20/(7x24) = 0.12. Whether it is stuck in a slow freeway clog up, or is going a top speed has nothing to do with it. The site that comes near the top when googling inverse of velocity seems to be about some sort of project mamagement approach, where load factor is the amount of useful work done by someone as a function of their time employed. Wickwack 124.182.175.128 (talk) 00:13, 16 May 2013 (UTC)

How about calling it 'progression'? Plasmic Physics (talk) 08:56, 16 May 2013 (UTC)

Once again, we are not here to invent terms for our OP - we're here to find terms already in widespread use. Calling it "progression" (or "sloth" or "slowness" or "deeps") is about as useful as calling it "wibble" or "qwertyuiop" - those are not terms in actual use, so it's pointless to invent them in answer to an actual question here on the RefDesk. SteveBaker (talk) 16:49, 16 May 2013 (UTC)

The term "slowness" is used in well logging with a sonic tool which measures the time between a sonic source and a receiver on the same tool through the rock formation in terms of slowness measured in units of microseconds per foot (see footnote on page 40) - it is also known as "interval transit time". Mikenorton (talk) 17:17, 16 May 2013 (UTC)

What does slowness have to do with it? 'Speed' doesn't have an innate 'fast' or 'slow' quality to it. All of the answers relating to slow make no sense at all. Speed =/= Fast. Speed = distance/time. This can be very very fast or very very slow or anywhere in between. Speed =/= Fast. --Onorem♠Dil 17:46, 16 May 2013 (UTC)

Colloquially an increase in speed would be described as going faster, so a decrease in speed (inverse) would be seen as going slower. But I do see your point. The problem seems to be that time/distance makes no intuitive sense. Wouldn't time normally be an independent variable? Or is that irrelevant? μηδείς (talk) 18:26, 16 May 2013 (UTC)

Similarly, "inverse frequency" is frequency, not infrequently, but a small value of frequency is a large value of infrequency (if you guys can follow what I just said then you're doing fairly well already :-)). Consider also, that the OP is referring to the inverse speed of light which is about (1/3)*10^-8 s/m therefore, unsurprisingly, light itself is still quick! But with slower objects we obtain larger values for inverse speed, thus this parameter measures the degree of "slowness"; large values indicate slowness, conversely the smaller values for photons indicating these are not slow. Google Scholar [11] brings up a few sources for its use (for instance, the concept is used in geophysics as mentioned above). -Modocc (talk) 18:47, 16 May 2013 (UTC)

That's what it is called, slowness, so it has quite a lot to do with it. It's a fact that sonic log interval transit time, the reciprocal of velocity, is also referred to as slowness. That is just how it is. Think of it as an indirect measurement of porosity, so higher interval transit times or "slowness" indicate higher porosity values. Sean.hoyland - talk 18:52, 16 May 2013 (UTC)

Well, it is defintely called the rate of change in time according/with-respect to distance. Plasmic Physics (talk) 23:04, 16 May 2013 (UTC)

Exactly, time/distance is called rate. I see no problem here and nothing to do with slowness. Let's say it takes me 5 mins to cover 1 km. So my rate is 5min/km. Yes, value representing my min/km rate would get lower if I get faster. time/distance rate would be useful if we seek to find what time it would take to cover distance. To cover 10km, it would take 50min = 10km * 5 min/km, so units also behave: time/distance * distance = time. Viva la algebra! AgadaUrbanit (talk) 00:03, 17 May 2013 (UTC)

Reread your link: rate = distance/time and distance = rate*time. Most rates, such as speed, are changes per time. The inverse velocity or speed is still a rate though, as it is a ratio between two measurements! But calling it a rate is vague to the point of being too ambiguous (which rate does someone mean, because it matters), and there are sources for "slowness". Modocc (talk) 00:44, 17 May 2013 (UTC)

OK. AgadaUrbanit (talk) 01:04, 18 May 2013 (UTC)

My slow car has a great insurance rate and better loan interest rate

May 15

Is this an artifact?

Rock may be an artifact

Could this rock be an artifact? Note the hole through it. It is almost 7cm long. Bubba73 ^{You talkin' to me?} 00:06, 15 May 2013 (UTC)

I'm absolutely not qualified to say, but look at kettle hole. I think you can have hydraulic drilling on a very small scale under certain circumstances, but certainly I cannot tell you this is the case here. Wnt (talk) 01:22, 15 May 2013 (UTC)

Holes caused by dripping are much wider than this due to splashing. μηδείς (talk) 01:57, 15 May 2013 (UTC)

The hole is about 3.5mm wide on one side and nearly 4mm on the other side. Bubba73 ^{You talkin' to me?} 02:03, 15 May 2013 (UTC)

The hole is through the thinner part of the rock and it is nearly 1 cm thick there. Bubba73 ^{You talkin' to me?} 01:27, 15 May 2013 (UTC)

I noticed that there are two small indentions on both sides next to the hole - those must be signs of how the hole was made by people. See File:Rock hole back.jpg and File:Rock hole front.jpg. Bubba73 ^{You talkin' to me?} 02:39, 15 May 2013 (UTC)

Where is it from? HiLo48 (talk) 02:50, 15 May 2013 (UTC)

I don't know. My wife found it but she doesn't remember where. We live in the state of Georgia, so I asked her if it was in Georgia, but she doesn't remember. Bubba73 ^{You talkin' to me?} 03:55, 15 May 2013 (UTC)

• I'm no expert here, but I doubt the hole was created naturally. I'd lean towards something Native American, but that's a guess. I found a similar item online here but the description given isn't helpful. Have you contacted your local historic society, they'd probably have some better answers. Hot Stop 04:05, 15 May 2013 (UTC)

I have not contacted a historical society. I ran across it cleaning out a box of stuff today. I don't remember it, but my wife says that she found it. I suspect native American too - there are such artifacts around here. It looks like the hole was to put a cord through it. So maybe it was worn, used as a toy, or a weapon. Bubba73 ^{You talkin' to me?} 04:16, 15 May 2013 (UTC)

And she doesn't remember anything about where she found it. What state? She doesn't remember if she found it in the ground or in a museum gift shop. Bubba73 ^{You talkin' to me?} 05:30, 15 May 2013 (UTC)

Is there any darkening or discoloration around the hole (either side)? 64.235.97.146 (talk) 13:29, 15 May 2013 (UTC)

I think it may have been overlooked Hot Stop, but the similar holed object on that website is a weight. Maybe that's what this is? Thanks ツ Jenova20 ^(email) 14:20, 15 May 2013 (UTC)

Perhaps there is a little darkening, but not much. See the close-up photos linked above. The "weight" description has a question mark after it. Bubba73 ^{You talkin' to me?} 16:44, 15 May 2013 (UTC)

Some molluscs will bore into rock. See Pebbles with holes made by sea creatures. Alansplodge (talk) 17:52, 15 May 2013 (UTC)

This is one small hole all of the way through the rock, and it was unlikely to have been in the ocean. Also there are two marks (slight indentions) on the sides of the hole on both sides of the rock that may be signs of a tool being used. Bubba73 ^{You talkin' to me?} 18:03, 15 May 2013 (UTC)

If you put a string through it and hung it around your neck could it work as a stone gorget/throat armor ? Sean.hoyland - talk 18:30, 15 May 2013 (UTC)

If you wore it like a necklace it would be too low. If you tied it up higher - maybe. Bubba73 ^{You talkin' to me?} 18:41, 15 May 2013 (UTC)

• Wild guess here, but possibly a sinker used for fishing. Hot Stop 00:32, 16 May 2013 (UTC)

It does look a lot like that. Bubba73 ^{You talkin' to me?} 02:34, 16 May 2013 (UTC)

• We need a petrologist here not an archeologist. To my eye, it looks like just sandstone or arkose. Both can contain iron oxides inclusions. Second: from its smooth appearance this stone is water tumbled (iron oxide, water, dissolve...?) Third: man made holes are round (except in higher cultures). Fourth: the hole's diameter differs in stepwise in dimension from face to obverse. Have not seen this in man fashioned objects but in natural holes -frequently found. Natural.--Aspro (talk) 00:36, 16 May 2013 (UTC)

I thought it was sandstone, but I'm not sure. It is smooth. Bubba73 ^{You talkin' to me?} 02:34, 16 May 2013 (UTC)

Sandstone abrading sandstone of the same hardness will leave a smooth surface. Also, if you notice, that at the ten o'clock and four o'clock positions to the hole, there appears to be some iron staining. Testing for iron with what you have in the kitchen is not easy. Yet, if you have one of those super powerful rare earth magnets. Then, by suspending the rock from a thread and after waiting for it to settle. The magnet may show that you have and iron rich river pebble.Aspro (talk) 14:28, 16 May 2013 (UTC)

FWIW, this rock File:Rock - another one.jpg was next to it in the box, but my wife doesn't remember if it has any relationship to the other one. Bubba73 ^{You talkin' to me?} 02:45, 16 May 2013 (UTC)

Office plant identification

Unknown typical office plant

A colleague has been looking after this plant for months in my office, and it is doing very well, but we have no idea what it is. It is not important at all, but if someone has 10 seconds to identify it for us, it would be great, we are very curious. I can provide a close up picture of a leaf if required. --Lgriot (talk) 10:12, 15 May 2013 (UTC)

Looks like Dieffenbachia possibly Dieffenbachia_seguine. Use Google images and search for Dieffenbachia. 196.214.78.114 (talk) 11:08, 15 May 2013 (UTC)

BTW FYI http://www.hoax-slayer.com/killer-house-plant-warning.shtml and Dieffenbachia#Toxicity 196.214.78.114 (talk) 11:15, 15 May 2013 (UTC)

Thank you very much, I think you are right.--Lgriot (talk) 13:42, 15 May 2013 (UTC)

Resolved

Weak mixing angle

Which values of the weak mixing angle would leave the electroweak force unbroken?

This image leads me to believe that a weak mixing angle that is a multiple of 90 degrees would leave the electroweak force unbroken. Is that correct? Am I mistaken in thinking that the weak mixing angle a free parameter? Baconmancer (talk) 17:08, 15 May 2013 (UTC)

A weak mixing angle that is a multiple of 90 degrees wouldn't be a mixing angle. Note that the weak mixing angle is determined by the position of the Higgs field labeled with an H at the picture. Note that the line connecting the origin to the Higgs field is perpendicular to the electric charge axis (labeled with a Q). The mixing angle is basically a measure of the ratio between the vertical scale (Labeled with a Y for weak hypercharge) and the horizontal scale (labeled with a T for weak isospin). You would need one these scales to be reduced to zero in order for the angle to be a multiple of 90 degrees. That can't be done without drastically changing the nature of the theory. Dauto (talk) 00:35, 16 May 2013 (UTC)

The number of microbes in the throat in case of cough

Hello, I have some questions about I have read (in one of my exercise in Math) that the pain in the throat is caused by found of 10^9 microbes (which is called "streptococcus"). First of all I would like to know if it truth. Second, I would like to know what to do if I want to check the air that I breath out of my mouth for looking for microorganisms like streptococcus what is the tool that I need for that? thank you :) {I hope that I've expressed myself properly... because English it's not my mother language}מוטיבציה (talk) 19:51, 15 May 2013 (UTC)

The tool you need to test for streptococcus in your throat is a physician or other medical professional. We don't have any of those at Wikipedia. See Wikipedia:Medical disclaimer. --Jayron32 19:59, 15 May 2013 (UTC)

• If you read about streptococcus in a math exercise, the biology was probably highly simplified, if not wrong. Likely, the people who wrote the exercise were just using bacteria as a way to introduce exponential growth. Anyway, testing for microbes in exhalation might be done for research purposes, but that is not how doctors test for strep throat. As our article explains, this is usually done via throat culture. And note that we have strict rules about requests for medical advice, so please don't ask us for medical advice, because we will not give it. However, I think you are looking for information and references, and your question passes Kainaw's criterion. SemanticMantis (talk) 20:34, 15 May 2013 (UTC)
• There is a nice review article on diagnosis of respiratory infections (not precisely the same thing) here: PMID 23415152; unfortunately that does not directly address the OP's question about diagnosing Streptococcus pyogenes as a cause of pharyngitis; it does include Streptococcus pneumoniae as a cause of pneumonia. -- Scray (talk) 22:55, 15 May 2013 (UTC)

off-topic meta-discussion. This sort of this belongs on the Talk page
The following discussion has been closed. Please do not modify it.
Tell me, what is the point of citing PUBMED articles here? Not only is a PUBMED article likely to be over the head of an OP asking a question like this, the article can only be accessed if you pay for it, or you are attched to a university of institution who has paid the subscription. Wickwack 124.182.175.128 (talk) 00:22, 16 May 2013 (UTC) While I could have taken time to summarize that article, as I said it doesn't directly answer the question. The point of citing (aside from it being an excellent practice for answering biomedical questions) is that for those interested, they can read the abstract for free, and (contrary to your assumption) a large number of Pubmed entries link to the free full text. For articles not available in full text for free, some questioners are students or otherwise have access. If you don't find any of that worthwhile, feel free to ignore my answer and cut my Wikipedia paycheck by 80%. -- Scray (talk) 01:24, 16 May 2013 (UTC) You know, Wickwack, this is a reference desk. The last the we should be doing is berating someone for at least attempting to provide a high quality reference. Too many times, we treat this as a "give an answer I pulled out of my ass desk" and Scray should be applauded for providing sources that the OP may be interested in. --Jayron32 01:38, 16 May 2013 (UTC) I agree that Ref Desk answers are better if they cite a relevant reference, preferably a creditable online reference, if not then a standard textbook. As far as I know, PUBMED, which is part of the Elsevier publishing empire, only makes articles available for free if they are judged to be of signifiant historical interest. If you are able to access articles without charge, it's probably because your employer is an educational, research, or scientific organisation that has paid for access - just as I can access similar journals in my field. Citing a source that the OP, and the rest of us, cannot access or are not likely to access because of the cost, or is likely to be above the OP's head, is not just as bad as not citing any source, it is worse. It is worse because it gives sense of authority which the source may or may nor support. Too many Ref Desk posters seem to think that scattering a few hot links around, whether directly relavent or not, satisfies the Ref Desk policy. It only does that if the cited source is a) available, b) is directly relevant, and c) supports what the poster's answer, or answers the question. More importantly, when answering questions, we should endevour to assess the OP's competence and educational level, and answer accordingly. Not always easy to do this, but we should always try, and mostly it is easy. I've been writing papers and technical manuals for decades - the first one learns is to write for your reader, and to help him/her, not to impress. Incidentally, a Ref Desk answer is still a good answer without any citation if what it says is self evidently true, for example by using logical inference. E.g., if a question can be answere with some simple algebra, there's no need to cite an algebra source. What I try to do as well, subject to Rule 1 Write For Your Audience, is use/explain the correct terminology, so that the OP and anyone else can google the terms and learn a lot more. Wickwack 124.182.35.52 (talk) 03:32, 16 May 2013 (UTC) No wonder you are so confused - Pubmed is a tool provided by the United States National Library of Medicine, specifically the National Center for Biotechnology Information. I suggest you take your poorly-informed criticism of my answers elsewhere. -- Scray (talk) 03:40, 16 May 2013 (UTC) I'm not confused, you are. Public internet access to PUBMED articles is managed by Elsevier Science Direct, and has to be paid for - $31.50 in this case. Log in as an ordinary private internet user and try it. Check you facts before critising others. Wickwack 124.182.156.152 (talk) 05:25, 16 May 2013 (UTC) Let's continue this discussion in a more appropriate place. -- Scray (talk) 05:50, 16 May 2013 (UTC)

An Agar plate containing blood agar can be used to culture streptococcus, but culturing dangerous micro-organisms probably should not be attempted by untrained/unqualified personnel. A sterile swab would generally be used to obtain a sample from the throat, rather than trying to get microorganisms from the breath. Edison (talk) 02:50, 16 May 2013 (UTC)

True, but that's not what the OP asked. They asked how to test their breath. While the title mentioned cough (and I now realize they might have meant to suggest tests of sputum rather than just breath), sputum culture is not a reliable test for pharyngitis (as you say, throat swabs are standard, and that's not what they were asking about). -- Scray (talk) 03:23, 16 May 2013 (UTC)

Nuclear reaction

If a really big asteroid smashed into Earth (like the one that is supposed to have finished off the dinosaurs, for instance) then would there be enough energy in the resulting explosion to cause any nuclear reactions (fission/fusion)? If not, how far away from the required energy would it be? Fairly close? Vastly too weak? 86.176.211.20 (talk) 20:29, 15 May 2013 (UTC)

No, but it has nothing to do with the energy involved. As a practical matter, only certain elements are capable of nuclear fission or nuclear fusion. This section explains nicely how fusion can be accomplished outside of an active star, which is basically restricted to a small number of carefully controlled reactions involving a limited number of nuclei. The earth is, by and large, not made of these nucleii, so forget that. Likewise, nuclear fission requires highly concentrated collections of fissile material. While that stuff does exist on earth, it doesn't exist in concentrations necessary to support nuclear fission; that's why we need to enrich it. Basically, the only fissile nuclide native to earth in any sizable quantities is Uranium-235, any other fissile nuclide you'd need to make via other means, and there is nowhere on Earth where U-235 exist in anything except very trace amounts, many many many orders of magnitude too small to support sustained chain reactions. So no, no impact is ever going to start a nuclear reaction of any sort. --Jayron32 22:13, 15 May 2013 (UTC)

The above answer is a little hasty in dismissing fusion, as that happens often in hydrogen, of which there is plenty on the Earth's surface in the form of water. The question of whether we would get either fission or fusion does, of course, depend on both the energy dissipated in the impact and what materials are actually present at the impact site. I agree that there is very little chance that fission will occur, purely since the sort of materials that undergo fission at reasonable temperatures (uranium and plutonium)are incredibly rare. Fusion, on the other hand, we have plenty of hydrogen for. The question is then one of energy.

The asteroid which is thought to have killed off the dinosaurs was probably about 10km across. Doing some ballpark calculations, we find that a 10km sphere of rock falling onto the Earth's surface has to dissipate about 10²⁴J of energy. If you assume that about 10% of that energy remains within that 10km across volume at impact and assuming it hits water, you would end up with a transfer to each water molecule of about 10^-17J or, in units beloved of nuclear physicists, about 100 - 1000 electron Volts. Now, to get hydrogen to undergo fusion, you need to give each atom about 700,000 electron volts, so we find that we only have about 1000th of the energy density required to cause hydrogen to undergo fusion. Of course, this is a very approximate calculation, but 1000 times the energy is still a long way off. So probably no nuclear reactions, I'm afraid. Eaglehaslanded (talk) 22:20, 15 May 2013 (UTC)

My question was kind of based on the premise that any nucleus could potentially disintegrate (and/or fuse) if you smashed something into it hard enough. However, clearly if the most easily fusible / fissile nuclei won't do that at the relevant energies then the more difficult ones won't either. 86.176.211.20 (talk) 23:08, 15 May 2013 (UTC)

The problem is sustainable fusion or fission as well. While, hypothetically, you can "force" any two nuclei to collide and do something, given enough energy, practically only those reactions which are highly exothermic will be self-sustaining. And known self-sustaining fusion and fission reactions are very limited in number, and cannot occur using random bits of the Earth, but rather require certain very carefully controlled types of matter and environments which simply do not exist naturally on earth. --Jayron32 23:34, 15 May 2013 (UTC)

I wasn't suggesting that such an event could trigger any kind of self-sustaining nuclear reaction, but wondering whether it could cause any nuclear reactions to happen due to the energy of the impact itself. 86.176.211.20 (talk) 00:23, 16 May 2013 (UTC)

Assuming that the calculations of Eaglehaslanded are plausible, that "1000th of the required energy density" would be on average, and actual energies of individual atoms would be higher or lower, right? Given this distribution of energy, is it possible that some atoms could still undergo fusion? 86.176.211.20 (talk) 00:47, 16 May 2013 (UTC)

No, even on the long tail of the Maxwell–Boltzmann distribution, I'd expect to find no measurable number of atoms with enough energy. --Jayron32 00:56, 16 May 2013 (UTC)

XKCD's "what if" section is very keen on suggesting that the pressure wave in front of a sufficiently fast moving object could be enough to cause fusion in the atmosphere (e.g. what-if.xkcd.com/1/‎ and what-if.xkcd.com/12/ ) Much as the examples are probably well beyond any practical asteroid (well, at lest one that we're likely to be abel to comment on afterwards), is there actually any possibility of this? MChesterMC (talk) 09:31, 16 May 2013 (UTC)

Depends on what the definition of "is" is. Seriously. It is, of course, quite trivial in any of these cases to mathematically calculate exactly how much energy is needed to do whatever it is you want to do. The math is easy, trivial, and can be done by anyone with a calculator. The difficulty is in envisioning a real scenario where such events could occur. So, when you ask if some bizarre scenario is possible, my first inkling is not merely to do the trivial math, but to try to envision a scenario in the real, physical universe where such math would come into play. I simply can't see it. --Jayron32 12:02, 16 May 2013 (UTC)

• I'd like to put this into some sort of context. Consider an Apollo Capsule re-entering Earth's atmosphere at a horrendous rate of knots. The plasma created at the shock-front is only a few inches thick. The temperature only gets up to about 5,000 degrees in this instance, because the kinetic energy that is converted into heat, get radiated away at the speed of light. The OP is speaking about a ruddy great lump of cosmic material of much greater mass (and thus kinetic energy) than this. As the OP question's: This issue about whether the 'dynamic' confinement time at X Kelvin at Y density for Z seconds is enough to trigger enough fusion reactions to account for what is observed in meteor encounters. Re: The nuclear and aerial dynamics of the Tunguska event. S. J. D. D’Alessio and A. A. Harms McMaster University, Hamilton, Ontario, Canada : and Natural low energy nuclear fusion reaction. Boris A. Andrianov. National Research South Ural State University, Chelyabinsk. What is interesting, is that numerous (an as yet, anecdotal) reports from witness of the of the latest big Russian meteor, is that they experienced radiant heat in excess of what Newtonian physics can account for. Aspro (talk) 13:38, 16 May 2013 (UTC)

As an aside: Found a non pay-wall reference to Natural low energy nuclear fusion reaction by Boris A. Andrianov, here at > http://www.google.com/url?q=http://www.intellectualarchive.com/getfile.php%3Ffile%3DHvKjaN0om65%26orig_file%3DAndrianov.pdf&sa=U&ei=6eKUUbn2As6KhQeT74CIBQ&ved=0CBgQFjAA&usg=AFQjCNHLlNCPO-302RDitKKzu3JEmO4R-g < . Why can't science (that we pay for in our taxes) be free for anyone to access?Aspro (talk) 14:00, 16 May 2013 (UTC)

The atmospheric heating is never going to be enough to cause nuclear reactions - there are only a very few materials that remain solid at more 5,000 degC and that don't boil at more than 6,000 degc. As soon it gets hot enough they boil or melt, they carry the heat away and the object doesn't get much hotter from that point onwards. So temperatures just don't get high enough to get nuclear effects during entry into the atmosphere. Any effect that might happen would most likely have to be on impact with the ground or ocean. SteveBaker (talk) 19:10, 16 May 2013 (UTC)

I'm not entirely clear on whether the question is, would there be any noticeable incidence of fusion (answer is clearly no), or would there be likely to be even a single fusion reaction? I suspect the answer to the latter question is still "no", but it's not quite as trivially disposed of.

The biggest problem with Eaglehaslanded's analysis is that ordinary hydrogen cannot fuse in a two-body collision, except in the very rare case that the resulting diproton decays to deuterium (see proton–proton chain reaction). I don't know how rare "very rare" is, but let's suppose it's so rare that we wouldn't expect it to happen even once in this scenario (anyone know if that's true?). I can't find out much about proton-deuteron reactions in a quick search, but I gather that they're not very likely either. (Three-particle collisions are so unlikely that I feel pretty confident that we can exclude them, though I haven't tried to calculate it.) So probably we'd need to get lucky and have, at least, two deuterons collide, or a deuteron and a Helium-3, or a proton and a Lithium-6, or a proton and a Boron-11. All of these involve species that are much much less abundant than ordinary hydrogen.

Anyone feel like putting these things together and estimating the probability of even a single fusion reaction in the scenario described? My guess is it's going to be negligible — but I haven't done the math. --Trovatore (talk) 02:26, 17 May 2013 (UTC)

Explosions...IN SPACE!

How big would an explosion on Earth have to be to be visible to the naked eye in space? How much TNT/how big a nuke would this equate to? --iamajpeg (talk) 22:17, 15 May 2013 (UTC)

You would have to specify how far away in space. Possibly you are thinking of low earth orbit? 86.176.211.20 (talk) 23:09, 15 May 2013 (UTC)

Brightness matters, too. It doesn't have to be big so much as bright. Mingmingla (talk) 23:13, 15 May 2013 (UTC)

In fact, an explosion doesn't have to be very big at all to be seen from LEO. One of the early ISS occupants reports that he could clearly see American ordnance from space during the 2001 invasion of Afghanistan. "One night, I looked down on Afghanistan and I saw these big, bright explosions ... I was witnessing the invasion of Afghanistan in pursuit of Osama bin Laden and the Taliban." The article states that he, "was able to identify what he saw as explosions from cruise missiles and bombs being dropped from B-52s." Evanh2008 ^{(talk|contribs)} 00:42, 16 May 2013 (UTC)

(EC) Where in space? The Kármán line? The lower boundary of the exosphere? Geosynchronous orbit? Half way to Proxima Centauri? Red Act (talk) 23:15, 15 May 2013 (UTC)

The human eye has a visual acuity of about one arc-minute. If you're looking from the International Space Station (about 400km above the Earth's surface) during the daytime, that corresponds to being able to distinguish things about 120 meters across. That's a dust plume from a reasonably large controlled demolition, or blasting in an open-pit mine or construction project.

If you're looking at night, it's a different matter. You're looking for the flash of the initial explosion, so the size of the explosion doesn't matter, it's the brightness that determines if you can see it or not. If your eyes are fully dark-adapted and you know where to look, you can see amazingly dim things. --Carnildo (talk) 00:43, 16 May 2013 (UTC)

The distance is only part of the picture here. The ISS orbits at about 250 miles up. That's around ten times further than the distance to the horizon when you're standing on the top of the Empire State Building...so things look ten times smaller. That's really tiny! However, when you look out at the horizon from a tall building, there's lots of air and mist and pollution between you and what you're looking at. But the vertical view from the ISS is looking though only maybe 20 or so miles of that 'stuff' - so unless there is a cloud in the way, the view is actually clearer even than looking at the horizon from the top of the Empire State building. Better still is that the shimmering of the air is present only in the lower altitudes - and you're looking though a lot of that when you're on top of that building - but hardly any of it from the ISS. So this winds up being a trade-off between size and optical clarity. For small objects that don't have much contrast against the landscape, the view from the ISS would make them hard to see. For large objects or things that are very bright or very dark compared to the rest of the landscape, they are likely to be easier to see from the ISS than from the top of a tall building. Fires and explosions at night should be easily visible from space - even if they are quite small - but a cloud of smoke drifting over the landscape would be much harder to see. So for a modest daylight explosion that produces much smoke - I doubt you'd see it from orbit - but even a fairly small one should be clearly visible at night. SteveBaker (talk) 16:38, 16 May 2013 (UTC)

Where an explosion makes a visible flash, Carnildo is on the money, SteveBaker is close. Some idea of the visibility of visible flashes at night can be obtained by considering aircraft navigation lights. Nav lights don't flash one a and go out, but their power will give us a rough guide. Nav lights for light aircraft have generally been incandescent globes (LEDS are taking over) about 25 mm diameter rated at 28V about 5 to 6 W. Max altitude for unpressurised light aircraft is about 4000 m, where the distance to horizon is 225 km. The nav lights in airliners are usually rated at 26 W. Airliners can reach ~13,000 m, at which the horizon is 412 km away. In both cases on clear nights (and for ailiner altitudes all nights are clear) pilots can see the nav lights of other aircraft when they come over the horizon, commercial aircraft very noticably. Wickwack 120.145.81.185 (talk) 00:02, 17 May 2013 (UTC)

Amateur (or professional) mycologist wanted

Unfortunately, I don't have a picture, but perhaps someone can put me in the right ballpark as to what I have seen. While watering the impatiens and petunias this evening, I noticed that in some specific patches of mulch, when I hit it with the hose, a puff of what looked like dust or smoke would appear. Upon investigating one of these spots, I found, buried just under the top layer of mulch, a very fine orange-tan powder, roughly the color of my wife's foundation powder (this color) and what looked like smallish blisters just underneath it (maybe the size and shape of the tip of my pinky finger), which were very sensitive, even a gentle prod with a stick caused the blisters to burst, and spread a fine cloud of what I assume were mold spores of some sort. A few pokes, and the patch dissipated entirely, leaving almost no evidence of its prior existence. If it helps, I live in Raleigh, North Carolina. Any clues? --Jayron32 22:57, 15 May 2013 (UTC)

Certainly sounds like a puffball, but you probably know about those and want the genus/species. I wonder whether you might have missed the rest of the spore sac (the "blisters" being the tip; though perhaps they were the entire spore sac), and an example that can grow low to the ground in decaying matter is Geastrum triplex, fairly ubiquitous in distribution. The puffball mode of spreading spores is pretty common, and not monophyletic (has probably arisen more than once in fungal evolution). -- Scray (talk) 23:21, 15 May 2013 (UTC)

No, I'm familiar with puffballs. This wasn't them. These were clusters of small blisters, not large fruiting bodies like puffballs. Immature puffballs are firm as well, unless these were a very different type of puffball. The ones we have around here tend to grow into largish (baseball-or-orange sized) balls which send off purple-black clouds of spores, and until they mature, are fairly firm, and even when mature, aren't this fragile. This stuff literally almost evaporated at the slightest provocation. I'm not denying this might have been some very different kind of puffball-type fungus than I am used to, but it definitely didn't look anything like Geastrum triplex or any puffball I am familiar with. After the spores dissipated, I looked around in the mulch, and nothing particularly stood out: no residual stalks or scraps of the bodies left over, as I usually see when I run over a puffball with the lawnmower.--Jayron32 23:29, 15 May 2013 (UTC)

Going purely from the habit and location, you may be the proud owner of a slime mold! See [12], and our page on Fuligo_septica (of course not necessarily that species) The so called "dog vomit" need not be so bright as the picture in our article, see the 10th pic here [13], which is pretty close to your makeup photo. I believe the slime mold genus Fuligo matches all your characteristics, down to the color, growing in mulch, and the powdery spores. SemanticMantis (talk) 23:37, 15 May 2013 (UTC)

I did not realize the slime molds could produce such a puff - thanks for the well-crafted answer. -- Scray (talk) 23:53, 15 May 2013 (UTC)

Ooh, I like this! I'll go with this. Does this present any sort of problem for my garden, or is it just a harmless annoyance? --Jayron32 00:20, 16 May 2013 (UTC)

The one complaint I'm aware of is that some spores may try to take hold on your house. I've heard the little dots/failed colonies can be tough to scrub off of vinyl or aluminum siding. Of course they won't do much there but leave a tiny dot. You'd think it might conceivably try to eat wood siding, but I've never heard of that, and you can google around these issues if you're concerned. I suppose paint and other sealants keep them out pretty well. As for your garden, I wouldn't worry. It's just a decomposer of sorts that does well in mulch. Me, I'd be more interested in putting some under my microscope, or seeing if I could get it to solve steiner trees or rail networks! (see [14], or google /slime mold city planning/ for videos and other fun stuff). SemanticMantis (talk) 02:04, 16 May 2013 (UTC)

May 16

Use of degree symbol with Fahrenheit

I was taught (told) in college that it was not proper to use the degree symbol with temperatures in Fahrenheit. In other words, you'd say -40°C but -40F. Now, I can't seem to find any credible (or non-credible) resource to tell me that that might be true.

I know Kelvin is abbreviated "K" only, but why would my professor teach that Fahrenheit was, too? I'm not dismissing the possibility that he was an idiot, but I also don't want to switch back without knowing that there's no legitimate source claiming "F" is the right way to go. Please give me your scientific/engineering input. Jared (t)  18:30, 16 May 2013 (UTC)

See Degree symbol#Typography.—Wavelength (talk) 18:33, 16 May 2013 (UTC)

(edit conflict) ACS Style Guide (3rd ed, 2006) editorial-style guideline states "degree (use °B, degrees Baumé; °C, °F, but K)". DMacks (talk) 18:34, 16 May 2013 (UTC)

All right, I kind of knew that, I'm more looking for an explanation as to why "F" was taught in lieu of "°F." Jared (t)  18:36, 16 May 2013 (UTC)

Your professor was mistaken. Dauto (talk) 19:11, 16 May 2013 (UTC)

Maybe so, but he is not alone. I remember being told the same thing, and it even made it into a Thermodynamics textbook from Cambridge University Press: "degrees Celsius (°C) ... degrees Fahrenheit (F) ... the degree symbol is used only with the Celsius unit to avoid confusion with the coulomb". - Lindert (talk) 21:14, 16 May 2013 (UTC)

Well, people can publish any random words they want into a book of any sort, but the fact that it is published doesn't make it authoritative. Most sources that discuss this issue note that the degree symbol should be used with Celsius/Centigrade and Fahrenheit and no degree symbol for Kelvin. This style guide from Penn State University and The National Geographic Style Guide and The U.S. Metric Association and The (U.S.) National Institute of Standards and Technology, which quotes the General Conference on Weights and Measures, which in 1967 established the current practice of NOT using the degree symbol with kelvins. The CGPM (the french acronym for this group) is the international body which maintains the standards for measurement worldwide; it is non-binding of course, but still considered the premier authoritative body on these issues, and many national standards-making bodies conform themselves to their standards. If the CGPM says its so, it's so. That's probably as authoritative as we need here. --Jayron32 23:57, 16 May 2013 (UTC)

Check out how a major newspaper reports its temperatures.[15] The "F" is understood, of course. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:51, 16 May 2013 (UTC)

What American institutions do is not a good guide except in so far as they may reflect Amaerican common practice - the USA is notorious for either ignoring international standards, or claiming that international standards are a wicked European plot to reduce American competitiveness. What newspapers do means nothing - the average journo and average sub-editor is pretty ignorant of science and standards. (The Sunday Times, a major newspaper, consistently printed silicone every time for years whenever talking about computer chips (silicon). And every newspaper I've ever seen, when mentioning the poles that carry electricity distribution, calls them "telegraph poles", as in A fatal accident occurred in Black Stump last night when a car collided with a telegraph pole). What matters is what is standardised by SI and IUPAC. See Quantities, Units and Symbols in Physical Chemistry (any edition, current edition is 3rd), International Union of Pure and Applied Chemistry (IUPAC). The standard is use the degree symbol with F, R, or C, but never K. That school teacher had it wrong (not that uncommon with school teachers, and most school texts are written by teachers) - or maybe the OP Jared remembered it wrong. Wickwack 120.145.81.185 (talk) 00:22, 17 May 2013 (UTC)

Fahrenheit is superior to Celsius, as it's a finer gradient. And I don't recall my teachers ever saying that the degree symbol shouldn't be used with Fahrenheit. Maybe the OP had a teacher with his own idea of how things should be. It happens. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:25, 17 May 2013 (UTC)

"A finer gradient?" I'm not sure what your teachers meant there, but if they meant what I think they meant (that it provides a finer gradation), then why don't they post highway speeds in inches/year? Also, large parts of the world now make use of the amazing new concept of the decimal mark that allows us to specify quantities with arbitrary precision quite independent of the concrete unit used. --Stephan Schulz (talk) 07:27, 17 May 2013 (UTC)

That's what I meant by a finer gradient: In everyday usage, there's no need to specify the temperature in decimal points. If today's high is 71, that's close enough - as opposed to somewhere between 21 and 22. ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:07, 17 May 2013 (UTC)

That's just nonsense, as it's false precision. There is no need whatsoever in for instance a weather forecast to have decimals, even in Celcius, as nobody can feel the difference between 21, 22 or 23. In field where such accuracy matters (science, medicine) they use..., yep you've guessed it, Celcius or Kelvin. 131.251.133.27 (talk) 10:54, 17 May 2013 (UTC)

• Unsourced attacks on various ethnicities are not helpful. μηδείς (talk) 01:37, 17 May 2013 (UTC)

Just to be picky, the ISO 31-0 specifies that you should leave a space between the number and the unit. Aaadddaaammm (talk) 06:13, 17 May 2013 (UTC)

Dimensional analysis of a punch

I was reading about the awesome Shi_Yan_Ming, but I'm confused by the units given in the article. It says "Shi's punching power has been measured at 772 lbf (3,430 N) of force" -- so far so good, (though I might have expected units of impulse). The article continues "...while his one-inch punch was measured to have 1.78 vC" What is a vC? I can't see how one-inch punch could be measured in Coulombs, and v isn't an SI prefix anyway. The sentence cites a TV show as reference. Does this make sense to anyone, or is it just some weird typo? SemanticMantis (talk) 21:09, 16 May 2013 (UTC)

One-inch punch makes a similar claim, but without units, and comparing to car crashes... SemanticMantis (talk) 21:12, 16 May 2013 (UTC)

No idea what a vC is, but that portion of the article was added on 31 August 2010 by an anon editor (69.160.245.131) in this edit: http://en.wikipedia.org/enwiki/w/index.php?title=Shi_Yan_Ming&diff=382143742&oldid=370427550 . They quoted as a reference a History Channel "Stan Lee's Superhumans" show aired on 12 August 2010. I can't find any other reference to a "vC" in this context anywhere, apart from this, which from the date comes from WP. Looks to me like utter nonsense to be honest, but going back far enough that nobody can be asked about it. Treat it like those hair product adverts about pro-trimethoxygoomtribble liposides, ie a complete bunch of old tosh. Tonywalton ^Talk 23:38, 16 May 2013 (UTC)

A punch delivers kinetic energy, not force - it ought to be measured in Joules (or watt-seconds). Wickwack 120.145.81.185 (talk) 00:28, 17 May 2013 (UTC)

Well, not strictly true. It does deliver a force and kinetic energy, insofar as kinetic energy is a force which is applied over a distance. --Jayron32 02:54, 17 May 2013 (UTC)

And it also has impulse, as force applied over time. So what it ought to be measured in will depend on what we're trying to assess. SemanticMantis (talk) 03:47, 17 May 2013 (UTC)

• Alright, that bit is clearly a mess. This is outside ref-desk domain, but since we're here: should I/we just delete all this nonsense? I don't have the time/interest to make it better, but is this a case where no info is better than poor quality/wrong info? SemanticMantis (talk) 01:51, 17 May 2013 (UTC)

Shadows cast by moonlight

Can moonlight shadows be seen in urban areas or can they only be seen in open countryside with no other light source? Also in such countryside areas, how much lighting does a full moon provide in an area with no other light source? Does it become as light as it does in a city at night with the street lamps? Clover345 (talk) 23:27, 16 May 2013 (UTC)

They can be seen in cities. The Moon provides for a lot more light than street lights, if you walk outside during a full Moon it feels almost like daytime, you don't get that with street lights. Believe it or not, well away from cities during clear Moonless nights, you can even see the shadow cast by the Milky Way, see here. Count Iblis (talk) 23:41, 16 May 2013 (UTC)

Does this depend on where you are on the planet though? Clover345 (talk) 00:15, 17 May 2013 (UTC)

(ec) I'm not sure what sources Count Iblis is relying on, but the full Moon is not brighter than typical streetlight illumination. Typical moonlight provides around 0.2-0.3 lux at full Moon; that can get as high as perhaps 1 lux under ideal conditions (perfectly clear sky, tropical latitude with the Moon directly overhead). If you Google for street lighting standards or similar keywords, you'll find a number of different local and national standards from various jurisdictions. Typical recommended minimums I've seen are no less than 2 lux for low-traffic residential streets and footpaths, on up to as much as 50 lux for high-traffic, complex intersection (see, for example, the British standards in table 2.1 of this document).

In practice, that means you might see a faint shadow cast by moonlight in an urban area if you're in a dark alley, a poorly-lit park, a sheltered backyard, or otherwise protected from normal street and building lighting, but it would take a bit of doing. Under no circumstances should you expect the Moon to offer as much light as even minimal street lighting. TenOfAllTrades(talk) 00:24, 17 May 2013 (UTC)

Don't you guys every go out at night? Air pollution must be realy bad where you live. Wickwack 120.145.81.185 (talk) 00:26, 17 May 2013 (UTC)

Yeah, and a lot of it depends on whether you are a vampire. μηδείς (talk) 01:32, 17 May 2013 (UTC)

May 17

Cold myth

If its a myth that cold weather or being cold makes people catch a cold, then why do so many people get colds in such conditions? Clover345 (talk) 00:13, 17 May 2013 (UTC)

You don't get automatically get a cold from cold whether, although, you are more prone to catching cold in cold weather. Plasmic Physics (talk) 01:04, 17 May 2013 (UTC)

I don't know if I would call it a "myth" per see, more like "it's a complicated situation where the evidence is so overwhelming that there must be some kind of relationship, even if it's merely a correlation and not a causative one". Our article talks about it fairly briefly, but there are a number of links to more in-depth sources. Matt Deres (talk) 01:13, 17 May 2013 (UTC)

See Correlation does not imply causation and Post hoc ergo propter hoc. It is true that "cold and flu season" is the winter in many places, and there are higher incidents of cold and influenza in the winter months, but there is little evidence that the cold temperatures themselves are directly to blame. The closest connection I have heard is that cold weather encourages people to spend more time indoors in close quarters with other people, which tends to exacerbate transmission of such illnesses, but that isn't something that's caused by cold temperatures per se. See this article from WebMD. --Jayron32 02:51, 17 May 2013 (UTC)

I admit it goes against the mainstream, but I personally believe that cold weather does play a direct role. The mechanism is that when you are out in the cold, the temperature of the body surface drops, including the nasal mucous membranes. This drop reduces the efficiency of immune responses pretty dramatically, and makes it easier for viruses to take hold. There is literature supporting the existence of this process, but so far no really strong evidence that it plays a major role. Looie496 (talk) 03:15, 17 May 2013 (UTC)

The Science desk is no place for spreading common superstitions that fly in the face of established science. You pointed out, correctly, that there's no strong evidence to support the purported mechanism. On the other hand, there's very strong evidence indicating that the flu spreads more easily when people are in close proximity in an enclosed area, and very strong evidence that people spend more time indoors during the winter. There's no need to fish around for complicated explanations when the obvious one works just fine. --Bowlhover (talk) 03:47, 17 May 2013 (UTC)

Louie, please, . --Jayron32 03:48, 17 May 2013 (UTC)

As I said, there isn't a great deal of supportive literature, but see for example PMID 12357708, PMID 17705968, and PMID 18977127. Looie496 (talk) 06:09, 17 May 2013 (UTC)

Our article flu season discusses this a little and lists some common potential explanations.Phoenixia1177 (talk) 04:03, 17 May 2013 (UTC)

Influenza and the common cold are unrelated diseases. HiLo48 (talk) 04:06, 17 May 2013 (UTC)

True, but they both have increased incidences at the same time of year, and similar symptoms, which is why they are often discussed together. --Jayron32 04:08, 17 May 2013 (UTC)

Sorry about that, I'm a little foggy since I actually have the flu right now; anyways, I read that as flu not cold for some reason. My mistakePhoenixia1177 (talk) 04:26, 17 May 2013 (UTC)

Flu and colds don't occur much mid-winter, at least here in Australia with mid-winter temperatures around 0 to 15 C diurnal cycle. They occur more at the beginning or end of winter. So that suggests that low temperatures affecting the immune system is not the complete answer. My own experience, which my GP has confirmed, is that there is a flu peak in February, ie summer. There has been some coverage of this in the news media in the last year or so. The Public Health authorities have surmised that it corresponds to flu peaks in Europe, carried to Australia by tourists and business travellors. In Autumn here, the weather tends to rapidly fluctuate. In the last week or so we've had daily maximums of 20 C and nice sunshine clear days, but yesterday it changed to wet and 25 C. I think this confuses the body somewhat - 25 C today seems pretty warm, but mid-summer it would fell damm cold. Probably tomorrow it will drop back to 20 C. Wickwack 121.221.228.142 (talk) 06:14, 17 May 2013 (UTC)

Type 1a supernova with a neutron star instead of a white dwarf? (a twofer for that star)

Hi, could a neutron star accrete mass and blow apart roughly like a white dwarf is believed to in a type 1a? I suppose the nuclear combustion and light curve would be different, if it even could happen. Thanks199.33.32.40 (talk) 00:39, 17 May 2013 (UTC)

Going by neutron star#Binary neutron stars, yes. A neutron star in a binary system can (potentially) accrete sufficient mass from its companion star to collapse into a black hole; I don't have the reference listed in the footnote handy, so I can't comment on just how violent the process would be or how exactly its appearance would be expected to differ from a Ia supernova. You might also be interested in the (plausible, but as-yet hypothetical) Thorne–Żytkow objects: red giants which have collided with neutron stars, and which may for some hundreds of years have a neutron star slowly spiralling in towards their cores. If the pair of stars are sufficiently massive, a supernova explosion and black hole may result; if the stars aren't massive enough, then you just end up with a heavier neutron star. TenOfAllTrades(talk) 02:24, 17 May 2013 (UTC)

A Type Ia supernova occurs when accretion ignites nuclear fusion of carbon on a white dwarf. Nuclear fusion is impossible on a neutron star because there are no nuclei--the entire star is made up of closely packed neutrons, without any protons or electrons. So no, a neutron star can't undergo a Type Ia supernova. --Bowlhover (talk) 04:04, 17 May 2013 (UTC)

Toluene

Is toluene used in consumer products anymore? 24.23.196.85 (talk) 06:26, 17 May 2013 (UTC)

It is sold as a solvent for use in arts and crafts. Plasmic Physics (talk) 07:06, 17 May 2013 (UTC)

If you're looking at making TNT, some printeries sell nitric acid. Plasmic Physics (talk) 07:08, 17 May 2013 (UTC)

Let's not forget sulfuric acid, you'll need that for a catalyst. It is sold as battery acid, however, you'll need to remove impurities before you can use it. Plasmic Physics (talk) 07:22, 17 May 2013 (UTC)

Toluene's availability as a consumer product may be more strictly regulated in your country. Plasmic Physics (talk) 07:23, 17 May 2013 (UTC)

You'll find all these useful tips in 'The Terrorist's Handbook', made infamous for being officially blacklisted by the US government for obvious reasons. That shouldnt stop you from finding a copy online. It has several versions and comes under various titles. Plasmic Physics (talk) 07:29, 17 May 2013 (UTC)

Will the US govt blacklist you soon? HiLo48 (talk) 07:40, 17 May 2013 (UTC)

What Plasmic left out is that one of the steps in the recipe will result in immediate vaporization of the person attempting to make the bomb. Exactly which step that is, of course is classified info. ←Baseball Bugs ^{What's up, Doc?} carrots→ 07:56, 17 May 2013 (UTC)

Aren't you confused with nitroglycerine? Plasmic Physics (talk) 08:00, 17 May 2013 (UTC)

No, the so-called "Terrorist's Handbook" has a step which will destroy the bomb-maker instantly. They don't want you to know that, of course. As regards nitroglycerine, it was first created by a Nobel experimenter, whose test ended tragically when he tossed a bottle of the stuff to his assistant, Dr. Klutz. ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:31, 17 May 2013 (UTC)

I wasn't recomending that particular recipe. In any case, it's not so much a step, as it is a missing step that makes it so dangerous. Plasmic Physics (talk) 09:36, 17 May 2013 (UTC)

Toluene is mainly used as an octane booster additive in petrol/gasoline (especialliy since lead-based additives were banned), and as paint stripper and paint solvent. It is used in a multitude of less important consumer products. It is the main reason why dopey street kids sniff paint and petrol, thereby making themselves even more dopey. See "Uses" in the Wikipedia article. Wickwack 121.215.70.116 (talk) 10:05, 17 May 2013 (UTC)

Special Relativity: follow up

In order for Special Relativity to develop its equations (e.g. Lorentz transformations), it's essential to assume that the speed of light does not depent on the inertial system measuring that speed; Is it essential to assume that the speed of light does not change over time/space either? HOOTmag (talk) 19:21, 12 May 2013 (UTC)

It's not essential to start from the constancy of the speed of light. Einstein did it that way in 1905 because at that time everyone knew that the speed of light was c but couldn't figure out what that speed was relative to, and he wanted to point out that you can just take the speed to be c, full stop, not relative to anything in particular, without any logical contradiction. There are other ways of motivating special relativity, though. For example, you can derive it from the reciprocity of redshifts: if two rocket ships move inertially away from a common starting point, each one sees the other redshifted by the same factor. That gets you a theory with the same mathematical structure as Einstein's theory that doesn't say anything about the speed of light as such. You can then, in the course of defining your system of units, take the speed of light to be constant. You don't have to, but you can without any contradiction—that's what distinguishes special relativity from Newtonian physics.

So it doesn't really make sense to ask whether the speed of light varies with position or time since that depends on how you define your units of measurement. A real time variation of physical constants would show up as a change in some other measurable quantity, such as the electron-proton mass ratio. -- BenRG 21:02, 12 May 2013 (UTC)

Admittedly, I'm rather surprised by your response. As far as I understand, Lorentz transformations are results of Special relativity, aren't they? Whereas assuming them is mathematically equivalent to assuming that the speed of light does not depend on the inertial system measuring that speed, isn't it? Hence, assuming that the speed of light does depend on the inertial system, contradicts Special Relativity, doesn't it? I'm just asking whether assuming that the speed of light changes over time - contradicts Special Relativity. HOOTmag (talk) 21:20, 12 May 2013 (UTC)

On the first point, it would be more correct to say that special relativity is a result (or an application) of the Lorentz transformation, rather than the other way round - see History of Lorentz transformations. Tevildo (talk) 23:24, 12 May 2013 (UTC)

From a historical retrospective point of view - you're right, but I'm talking from a relativistic point of view. Einstein concluded Lorentz transformations from the constancy of speed of light, not vice versa. HOOTmag (talk) 07:20, 13 May 2013 (UTC)

Minkowski spacetime is homogeneous in space and time, and you need some assumption to distinguish that from some other spacetime geometry that isn't, but I'm not sure that gets at the core of your question. The meaning of "speed" in Einstein's postulate is not obvious a priori, since the paper argued that the seemingly obvious notions of distance and time that physicists had had until then were actually wrong. Einstein uses the postulate to justify his method of synchronizing clocks, and it's not until the synchronized clocks are introduced that the inertial reference frames are defined and the "speed" in the original postulate has a clear meaning. That's okay because all physical theories are circular in that way (see this thread, the final reply beginning "There is an unavoidable circularity...", where I think I explained this better than I'm doing it here). The postulates in the original paper are a jumping-off point for the argument, but because of the circularity they aren't really postulates in a formal mathematical sense, and there's no useful distinction to be made between assumptions and conclusions. -- BenRG 07:33, 13 May 2013 (UTC)

Logically speaking - every assumption is a conclusion, while for the practical purpose of the current thread - I don't distinguish between assumptions and conclusions. By saying that A is an assumption/conclusion of B, I just mean that the negation of A contradicts B. As for Minkowski spacetime: My original question refers to Minkowski's relativistic equations as well. HOOTmag (talk) 08:23, 13 May 2013 (UTC)

I think the word for which you're looking is consequence. Conclusions are whatever propositions that are designated as conclusions (i.e, meant to be proved). If an assumption is not designated as a conclusion (and they usually aren't), then it isn't one. --Atethnekos (Discussion, Contributions) 17:23, 13 May 2013 (UTC)

As explained here, the speed of light is not a real physical constant. Count Iblis (talk) 23:51, 12 May 2013 (UTC)

Other users are giving IMO unnecessarily complicated answers, so to keep it simple the answer to OP's question is yes. In his 1905 paper, Einstein mentions explicitly (albeit briefly) that "it is clear that the [Lorentz transformations] must be linear on account of the properties of homogeneity which we attribute to space and time", which basically implies that the speed of light doesn't change in space or time. 65.92.6.9 (talk) 03:01, 13 May 2013 (UTC)

And, by "homogeneity," Einstein means to say that the value of the permittivity of free space and the permeability of free space - commonly, ε₀ and μ₀ - are well-defined and always constant at all positions. From this postulate, the equation of retarded time is trivially found by solving Maxwell's equations. The Lorentz transform is a mere algebraic simplification of the more general form.

By coincidence, I had been reading The Sign of the Four this evening - published 1890 - and it referenced (very indirectly) the Elements of the Philosophy of Newton by Voltaire (Holmes is quoting a pithy bit of French, and alluding to shedding some light on the case). Naturally, my inclination was to track down the text, and read as much of it as I could... it is available online (but not at Project Gutenberg, unfortunately, nor in English translation - here it is at Elementi della filosofia di Newton). It is absolutely amazing! To read Voltaire succinctly express Isaac Newton's optics - to talk about the constancy of the speed of light, and to talk of light as both a ray and as a particle... published in the year 1738 as a regurgitation of Newton's earlier and far more technical writings on optics - a thought crossed my mind, which I will summarize here... "those who think Einstein's work was really amazing have not spent enough time reading the works of his predecessors." Nimur (talk) 04:04, 13 May 2013 (UTC)

See Standing on the shoulders of giants, a quote not created by, but often attributed to, Newton on his own work. It applies to every scientist in history since the first caveman tried to make fire. --Jayron32 04:28, 13 May 2013 (UTC)

The giants on whose shoulders the cavemen stood were, of course, the nephilim. I'm not exactly sure how standing on a nephil's shoulder helps you make fire, but there you are. --Trovatore (talk) 08:11, 17 May 2013 (UTC)

OP's comment: I still wonder which relativistic equation must result in the constancy of speed of light in space/time. The equation of retarded time - just results in the equation: $c={\frac {|\mathbf {r} -\mathbf {r} '|}{t-t_{r}}}$, which does not tell us whether the very value: ${\frac {|\mathbf {r} -\mathbf {r} '|}{t-t_{r}}}$ is constant. Note that Lorentz transformations do tell us that the speed of light does not depend on the inertial system measuring that speed. HOOTmag (talk) 07:20, 13 May 2013 (UTC)

You are confusing a result with a derivation. The derivation proceeds along a line of mathematical reasoning that starts by solving Maxwell's equations for a moving source. This gives a description of the electric and magnetic fields at all points in space and time. From that, a lot of algebraic manipulation gives you a wave equation with a propagation speed, independent of the motion of the source. Our articles cover these topics, but this is fairly advanced mathematics and physics. My recommendation is to begin studying the wave equation in its classical form, until it is so intimately familiar to you that you recognize it, even when obfuscated by multiple independent variables. Then you will be able to identify propagation velocity by inspection. More bluntly: even if you are an autodidact, you require a many years of mathematical preparation before you can reasonably interpret and use the equations that govern the relativistic behavior of light. Commonly, this means three to five years of rigorous study of introductory calculus and physics at a university level. It's a bit unreasonable to think that an encyclopedia can expedite that process to just a few days or hours. Nimur (talk) 14:56, 13 May 2013 (UTC)

No, I was not confusing a result with a derivation: I just wondered "which relativistic equation must result [by a mathematical derivation] in the constancy of speed of light in space/time". Anyways, as opposed to what you've claimed, I don't think one needs "three to five years of rigorous study of introductory calculus and physics at a university level" in order to answer my original question. HOOTmag (talk) 08:22, 16 May 2013 (UTC)

I said that mathematical preparation is usually necessary. You might be a genius beyond everyone's wildest expectations. But you're still asking the same question, which has already been answered incredibly thoroughly, leading me to believe that you don't have the prerequisite context so that you can understand the answer. Which part are you still stumbling on? A constant speed of light is a direct consequence of the assumption that the permittivity of free space and the permeability of free space - commonly, ε₀ and μ₀ - are well-defined and always constant at all positions. These parameters define the speed of light in our best theories of electromagnetics, and this premise matches physical experiment. Are you unable to see the link between physical observation and the equation that models it? If so, I suggest you start reading about electrostatics, and then electrodynamics. Nimur (talk) 13:58, 16 May 2013 (UTC)

The well-known constants ε₀ and μ₀ are really assumed to be constant at all positions, and we undoubtedly shouldn't forget that Einstein was highly inspired by their constancy when he developed his relativistic mechanics, even without us mentioning the important role played in electromagnetism by Special Relativity (e.g. by its motivating the "manifestly covariant" tensor form, and by giving formulas for how the electric and magnetic fields are altered under a Lorentz transformation from one inertial frame of reference to another, and by showing that the frame of reference determines whether the observation follows electrostatic or magnetic laws). However, I was not asking about relativistic electromagnetism, nor about the psychological inspiration of the constants ε₀ and μ₀ in Einstein's mind when he developed his relativistic mechanics. I was just interested in relativistic mechanics per se, i.e. in its explicit assumptions (e.g. the constancy of speed of light) and in its results (e.g. Lorentz transforms). Relativistic mechanics involves concepts like: time, space, mass, force, momentum, energy and the like, yet not concepts like electric charge or magnetic field. Relativistic mechanics assumes/concludes some claims, e.g. the equation of time dilation (and likewise), yet not any claim about ε₀ and μ₀, so that one can study relativistic mechanics, without having studied Maxwell's theory, and still wonder whether - one must assume/conclude the constancy of speed of light in time/space - in order to fully grasp the fundamental principles of relativistic mechanics.

As for mathematics: as a mathematician I can assure you that one needs no advanced mathematics for understanding whether the constancy of speed of light in time/space is needed for relativistic mechanics. HOOTmag (talk) 17:51, 16 May 2013 (UTC)

ds^2 = dx^2 + dy^2 + dz^2 - c^2 dt^2. It's really not different from the factors of 1 multiplying dx^2, dy^2 and dz^2, which you can relate to Pythagoras' theorem (and which you can change to arbitrary values by using different units for measuring distances in the x, y and z directions). Count Iblis (talk) 13:12, 13 May 2013 (UTC)

The invariant ds^2 = dx^2 + dy^2 + dz^2 - c^2 dt^2, only shows that c can't depend on the inertial frame. However, this invariant may vary in time, hence - one can suppose c itself varies in time - without contradicting the very invariant mentioned above. HOOTmag (talk) 08:22, 16 May 2013 (UTC)

To address the OP's original question, if you require that a Lorentz boost has the same mathematical structure that it does now, and that boosts be invertible such that a boost by velocity ${\vec {v}}$ followed by a boost by velocity $-{\vec {v}}$ should return the original coordinate system then it follows that the speed of light field must be a Lorentz invariant, i.e. $c(x,y,z,t)=c(x',y',z',t')$ measures the same value for all possible Lorentz transforms. This generally implies that the speed of light is a constant independent of space and time. Now, one could replace a Lorentz boost by an integral composition of differential boosts in such a way that one could self-consistently describe a world where the measured value of the speed of light varied in space and time; however, the math then describing a change of inertial reference frame would be more complicated then the current Lorentz transformation. Dragons flight (talk) 19:44, 13 May 2013 (UTC)

On reflection, one can also have a solution where only allows the speed of light field to transform with the boost. That version is also fairly natural, but would require forgoing the idea that all observers can agree on the way the speed of light changes in space and time. Dragons flight (talk) 23:18, 13 May 2013 (UTC)

I still wonder about how you derive that $c(x,y,z,t)=c(x',y',z',t')$ in the same inertial frame. HOOTmag (talk) 09:31, 16 May 2013 (UTC)