::::Some relevant articles: [[Electromagnetic radiation and health]], [[Wireless electronic devices and health]], [[Mobile phone radiation and health]]. As far as I understand, for common household devices (mobile phones, wifi routers,...) there are many studies, but no generally accepted view, also due to the fact that people are exposed only for a couple of years, making the discovery of long-term effects difficult. As the situation is not clear, personally I'd minimize exposure, by switching of wireless devices when not needed, not sleeping near wireless sources, etc. [[User:Bamse|bamse]] ([[User talk:Bamse|talk]]) 08:52, 22 December 2012 (UTC)
::::Some relevant articles: [[Electromagnetic radiation and health]], [[Wireless electronic devices and health]], [[Mobile phone radiation and health]]. As far as I understand, for common household devices (mobile phones, wifi routers,...) there are many studies, but no generally accepted view, also due to the fact that people are exposed only for a couple of years, making the discovery of long-term effects difficult. As the situation is not clear, personally I'd minimize exposure, by switching of wireless devices when not needed, not sleeping near wireless sources, etc. [[User:Bamse|bamse]] ([[User talk:Bamse|talk]]) 08:52, 22 December 2012 (UTC)
:::::There's an awfull lot of rot published about the effects of electromagnetic radiation, and cell phone emission in particular. The simple fact is this: for any medical/physiological effect at all, at least one of two things must be satisfied: a) the field strength is so intense that heating of body parts occurs above that which the bloodstream (acting as a heat conductor) can cope with. This requires close proximity to quite high power radio transmitters well and truely beyond what a cellphone can put out. b) the radiation is of sufficiently high frequency to be ''ionising'' radiation, eg X-rays. Cell phone radiation is too low in frequency to be ionising. If neither of these two conditions are satisfied, then chemical bonds cannot be broken, and if chemical bonds are not broken then there cannot be any effect on the body. End of story. The trouble lies in how research grants are granted, and how editors select articles for publication. If yoiu are a researcher, and ask for a grant to show that everything is hunky dory and nobody should worry, a grant committee, who probaly don't really understand it anyway, will not give you a grant. But if you write out an application stating that a possible hazard needs to be investigated, you may get a grant. And if you beaver away spending teh grant money and finally conclude that while you could not definitely establish cause and effect, there should be more research done, well, you might get more grant money. And editors want to sell journals, so even in a professional journal, a bit of sensationalism helps. ''Ratbone'' [[Special:Contributions/124.182.16.117|124.182.16.117]] ([[User talk:124.182.16.117|talk]]) 10:33, 22 December 2012 (UTC)
== Could the hand have a more ideal anatomy? ==
== Could the hand have a more ideal anatomy? ==
Revision as of 10:33, 22 December 2012
Welcome to the science section of the Wikipedia reference desk.
The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.
I occasionally spend my own money on items for a voluntary organisation I'm part of, then bundle a heap of receipts together and seek reimbursement from the organisation's treasurer. My problem is that one receipt I currently have was one printed on that glossy paper with less than permanent "ink", and it has faded so much that it's almost unreadable. (Is that Thermal paper?) One bit that's definitely missing is the amount.
Yep, that's thermal paper all right. Holding it up to a light might help you read it, but actually getting a copy you could use for proof of purchase will be more of a challenge. I've also found that highlighters wipe the ink right off and darken with time, so avoid those. StuRat (talk) 03:14, 16 December 2012 (UTC)[reply]
You should find various suggestions online (I know because I've seen them when I looked). One risk is most of them could potentially make things worse. One option if everything is still visible but some are so faint to be bearly readable is to try scanning the receipt (scanning is likely better then photographing) which generally should not significantly damage/ruin any chances of recovery. From experience it's always best to make a copy, photograph or scan any thermal receipt ASAP to avoid such problems if you want to need them for a while. Particularly if you're not storing them out of sunlight, heat and away from moisture or particularly oils (including skin oils) such as in a wallet. Although even if stored well they could easily fade within a year. Nil Einne (talk) 06:58, 16 December 2012 (UTC)[reply]
"Bearly" is a perfectly good word: "I bearly ate dinner" (meaning I broke into my neighbor's home, ripped the door off the refrigerator, stuffed my head inside, and ate everything). StuRat (talk) 16:54, 17 December 2012 (UTC)[reply]
"Barely" is a perfectly good word, too: "I barely went to work last week, but they complained that I'm expected to wear clothes, even on casual Fridays". StuRat (talk) 16:54, 17 December 2012 (UTC)[reply]
I'd scan the image at the highest res I could then manipulate it in an image processing program to increase the contrast. If it's legible, you might also take it to the vendor and ask him to give you a replacement for it. μηδείς (talk) 21:57, 16 December 2012 (UTC)[reply]
I think the best advice is to go to the customer service desk of the vendor, and ask for a handwritten receipt to replace the original. Very few businesses are actually hoping to dick you out of a few bob because your receipt is fading. μηδείς (talk) 03:20, 18 December 2012 (UTC)[reply]
Microfibre 8 848 metres long weights 0.4 grams?
Hi all,
The swing tag on a waterproof bicycle jacket I bought a little while ago claims
"(name) is a sportswear fabric woven from a fine, light microfiber - so light that the amount needed to reach from sea level to the top of Mt Everest would weigh only 0.4 grams."
I assume the the claim was about the height of Everest, not its distance from the nearest ocean. So I sat down do the math:
Let's assume the jacket just barely floats on water, i.e. Density = 1 g/cm^3. For a filament 884,800 cm long, a total volume of 0.4 grams is accomplished by 0.4 cm^3, which is a cube of 884800 cm * sqrt (0.4/884800 cm^2) = .000672 cm = 6.72 nmum (square) [or 5.96 nmum (circle)] I suspect this is worked backward from a "6 nm micron filament" figure. Wnt (talk) 09:52, 16 December 2012 (UTC)[reply]
Erm, yes, rectangular prism and cylinder. The correction I applied to circle the square was the square root of pi*1^2/4. Haste, waste, etcetera. Wnt (talk) 03:37, 18 December 2012 (UTC)[reply]
Thanks for the answers so far. Lets make this more specific. Consider not a very thin fibre of those that are woven together to make the fabric of the jacket, but at the atomic level. Would a single strand of polymerised (aliphatic, to be most generous to the claim) ester monomers 8 848 metres long weigh 0.4 grams? I lack the vocabulary to describe this in other than laypersons terms: would a chain of ...=<ester polymerisation monomer>=<ester polymerisation monomer>=<ester polymerisation monomer>=... 8 848 metres long weigh 0.4 grams? --Shirt58 (talk) 09:22, 17 December 2012 (UTC)[reply]
Let's take poly-(R)-3-hydroxybutyrate (P3HB) (an aliphatic ester). This article, on page 26 gives the length of two repeat units (in stretched configuration) as 0.92 nm, so 0.46 nm per repeat unit. If we had a completely stretched single P3HB molecule with a length of 8 848 m, it would contain (8848 m)/(0.46 * 10-9 m) = 1.92 * 1013 units. Each unit contains 4 C's, 2 O's and 6 H'2, thus 4*12+2*16+6 = 86 atomic mass units (u). So the total mass of the chain is 1.92 * 1013 * 86 u = 1.65 * 1015 u, which is (1.65 * 1015 u)/(6.02 * 1023 u/g) = 2.7 * 10-9 g = 2.7 ng. In practice, of course such a long molecule would never be in a completely stretched configuration, but even if one would need 1000s of times the number of repeat units to reach the required length, its mass would still be far below 0.4 g. - Lindert (talk) 13:22, 17 December 2012 (UTC)[reply]
To cross-check these two methods, 0.4 g/ 2.7 ng = 1.48 x 10^8 parallel filaments in a bundle. So if the bundle has a 6.72 um square cross-section, and the filaments are packed in a grid fashion, then there are 12172 filaments along a side, each 0.55 nm (really) = 5.5 Angstroms apart. Which seems a little far (especially since the packing could be better) but we seem in the right ballpark, anyway. Wnt (talk) 03:45, 18 December 2012 (UTC)[reply]
(xkcd) why is text in the mirror backward, but not upside-down? what's so special about the horizontal axis?
(xkcd) why is text in the mirror backward, but not upside-down? what's so special about the horizontal axis?
This is the question. I wonder if the TRUE answer isn't any of the baloney that you Google but instead (struck out to avoid spoiler, if you're just now thinking about this or googling it, read following when finished) The mirror doesn't reverse text in any direction. However, if you see a page you can't see it in the mirror in front of you, since it's facing you, not the mirror. - so, to see it, you would have to turn the page toward the mirror. If you turn it around horizontally the text becomes flipped horizontally, and if you turn it around vertically the text becomes flipped vertically. You're the one who flipped the image, so you could view it in the mirror. This seems to me to be a far better answer than the baloney I read while googling it. What do you guys think? Have I unlocked a deeper answer? 178.48.114.143 (talk) 18:27, 16 December 2012 (UTC)[reply]
So you can think about it and/or google it before reading "my" answer. I've updated "(spoiler)" to "(struck out to avoid spoiler, if you're just now thinking about this or googling it, read following when finished)". As you can see from Googling it, most answers are far more complicated and, in my opinion, wrong. 178.48.114.143 (talk) 18:46, 16 December 2012 (UTC)[reply]
Something else that might support this way of thinking about it is to remove the mirror entirely. That is, just hold up a transparency so it's facing away from you. Now, depending on how you rotated it to get it there, the text should either appear backwards or upside-down (but not both, as rotating it both ways brings it back facing you). StuRat (talk) 18:54, 16 December 2012 (UTC)[reply]
The mirror doesn't reverse right-to-left. It reverses your view of things front-to-back. Just write a word on a transparency and look it from behind. You'll see the same as the image in the mirror. OsmanRF34 (talk) 18:56, 16 December 2012 (UTC)[reply]
The question is, specifically, "Why does text appear flipped on the horizontal but not vertical axis in a mirror?" Are you saying that this experience is not descriptive of most people's experience of viewing text in a mirror? (That it so appears?) I offer the answer to that "question" but you seem to be questioning its premise. In fact, in common experience I would say most people know what I'm talking about, as they do have that visual recollection of their interaction with reading things in mirror :) 178.48.114.143 (talk) 19:00, 16 December 2012 (UTC)[reply]
As mentioned elsewhere in this thread: a transparency held up in front of you (normally) and seen through a mirror looks fine. This is an experience of looking identical. You've never had the "experience" of "mirror" writing looking, instead of this, like a picture that in Microsoft Paint you choose "flip horizontally" on? (Note: we are only describing the experience, not its explanation.) 178.48.114.143 (talk) 19:51, 16 December 2012 (UTC)[reply]
Osmond! I just realized that where you write "The mirror doesn't reverse right-to-left. It reverses your view of things front-to-back. Just write a word on a transparency and look it from behind. You'll see the same as the image in the mirror" your suggestion is NOT well-defined! After "writing a word on a transparency" to follow the step of "looking at it from behind" you must now get around it. There are an infinite number of simple ways to get around it - i.e. one for each arc that your face can follow from the point in front to the point behind. Each one shifts the image in a DIFFERENT way!!! It is not at all true that "you'll see the same as the image in the mirror" as that image might have been created following a different flipping-path. The REASON, then, that we think of mirrors as flipping things horizontally is because THIS IS THE AXIS OF GRAVITY: out of the infinite number of arcs that your face can follow to get around to the other side, the only two that make sense are the arc heading 'due' left and the arc heading 'due' right: any other arc (such as the arc in between, heading due 'up') is not a very practical way to get around to the other side. As for why we flip OBJECTS such as a book, along the plane perpendicular to gravity, hence causing an apparent flip over pure horizontal (as oppoesd to the infinite otherp ossibilities) it's because, where this another person in the mirror, they could then see it along the same straight orientation that everyone is accustomed to reading. So, it would seem, the FULL answer is this: mirrors flip on the horizontal because we turn objects toward them on the horizontal. We do this because, where the person in the mirror a separate person instead of ourselves, this is the orientation in which they would be accustomed to reading. What do you think 178.48.114.143 (talk) 21:31, 16 December 2012 (UTC)[reply]
Another factor is how we rotate our heads. We don't stand upside-down to see what's behind us. This might come into effect when they write the word AMBULANCE "backwards" on the front of ambulances, since they assume we will have rotated our heads in the usual way to see it in the rear-view mirror of our cars. If we all rode in our cars upside down, facing forward, then they would need to change how they write AMBULANCE. StuRat (talk) 19:01, 16 December 2012 (UTC)[reply]
It simply depends on the location of the text in relation to your orientation to the mirror, which can be demonstrated quite easily. Place an object like a license plate or bumper sticker oriented so that you can read it normally on the counter in front of you in your bathroom, and then look in the mirror. Your line of sight is going through the letters from top to bottom to the mirror, so the text itself will be flipped top-o-bottom: i.e., upside down. Now place the plate on the wall in the normal reading position to your right between yourself and the mirror. Now the line of sight is going along the plate from side to side in the direction of the mirror, so now the text will be reversed side to side: i.e., backwards. Now, rotate the plate on the wall 90% counterclockwise so that the text is facing you running up the wall instead of parallel to the floor. Now the line of site will be going along the letters from top-to-bottom again, so the letters will once again appear upside down in the mirror. μηδείς (talk) 19:37, 16 December 2012 (UTC)[reply]
Osman has it. Richard Feynman explains the question and answer in his characteristic style here: [1]. He also gives a psychological interpretation for why he thinks people come to the wrong answer. (His answer starts at ~0:45, but it's only a 3 minute clip). SemanticMantis (talk) 19:39, 16 December 2012 (UTC)[reply]
I don't think Feynman addressed why having your face inside out appears to cause a left-right reversal, but not an up-down reversal. StuRat (talk) 20:01, 16 December 2012 (UTC)[reply]
He does, although he might be more explicit. The core cause is that left and right are usually understood as relative directions, while up and down are absolute directions. If you wave your right hand, the hand on the right hand side of the mirror waves back. But from the perceived position of the person in the mirror, that is his (or her) left hand. This becomes clear if you, as Feynman does, go to completely absolute coordinate system. You wave the east hand - so does your mirror image. No flipping. --Stephan Schulz (talk) 20:15, 16 December 2012 (UTC)[reply]
This reminds me that there is (I've read) a language in northern Australia where absolute directions are strongly preferred: "John passed me the gourd with his west hand." —Tamfang (talk) 19:45, 26 June 2013 (UTC)[reply]
This is precisely my contribution: a transparency viewed from in front appears neither flipped vertically nor flipped horizantally. Viewed from behind, however, it depends on how you got around it: did you follow a 180 degree arc around horizantally or fly up over vertically? So it is not at ALL true that the "core cause is that left and right are usually understood as relative directions while up and down are absolute positions". That's not it AT ALL, whatsoever, even a little bit. Whether you flip horizontally, vertically, or a different amount if you end up taking a different path depends on just how you get around to the other side. 178.48.114.143 (talk) 00:04, 17 December 2012 (UTC)[reply]
Your head doesn't appear upside down because, like in my experiment, your head would have to be above or below you between your eyes and the mirror, like the license plate on the counter in front of you. It's impossible. (You could place a dummy head on the counter facing upwards and the face in the mirror (which would have to be titled) would appear to look down out of it.) My suggested experiment explains this in full. I am not sure what Osman means by no flipping, just projection, unless he's making the same sort of point one might make who said there are no colors, just wavelengths of light. His claim a mirror only reverses things front to back is also wrong, as my experiment shows, unless I misunderstand what he means to say. The bottom line is that whether an image appears upside down or backwards to an observer depends on the placement and orientation of all three objects with one another. μηδείς (talk) 20:34, 16 December 2012 (UTC)[reply]
I would agree with Osman that it's a matter of front-back reversal (i.e. mirroring). Imagine that Bob's mirror image were a physical (3-dimensional) object. Now if I as an outside observer would see both Bob and his mirror image, the difference between the two would be a front-back reversal, i.e. to make Bob and his image identical, I would need to bring both of Bob's eyes to the back of his head, moving them exclusively on the front-back axis, and likewise for all body parts. Now I could get the same effect by first rotating Bob 180 degrees around a vertical axis, and then mirroring him in a left-right direction, but the same is achieved by first rotating him 180 degrees around a horizontal (left-right) axis (so that his head pointed down), and subsequently mirroring him in an up-down direction. In fact, I could conduct this procedure with any two axes that are perpendicular to one another and to the front-back axis, so there is nothing special about the left-right direction. I sincerely hope this makes sense to anyone but myself. - Lindert (talk) 00:34, 17 December 2012 (UTC)[reply]
Calling what a mirror does "mirroring" is a neat trick. Pointing out that whatever is closest to the mirror will appear closest in the reflection to the observer is also true. But the fact remains that some text appears upside down but not backwards, and some text backwards but not upside down, and this is explained by the relation of the observed object and the mirror to the line of sight. Osman hasn't addressed the position relative to the observer so far as I understand, and instead of explaining why certain text appears upside down, and other backwards, seems to have denied the difference exists. μηδείς (talk) 04:51, 17 December 2012 (UTC)[reply]
That's because the relative position of the observer and the mirrored object is irrelevant. No matter if you see text in a mirror that is above you, below you, beside you or even between your eyes, it always seems to be backwards but not upside down, but this is not actually the case. Scratch that, I think 91.120.48.242 explained it best (see below). - Lindert (talk) 10:15, 17 December 2012 (UTC)[reply]
Lindert, how about this. Normally it is NOT possible to see an object in a mirror: the text you're reading, or Bob, is facing you and so is the mirror. If you want to see it in the mirror you have to (relatively) turn it toward the mirror. The direction you relatively turn it toward the mirror in determines the axis of reflection. 91.120.48.242 (talk) 08:34, 17 December 2012 (UTC)[reply]
So I think Feynman isn't exactly right in his explanation. If we happen to notice text in a mirror, it can be flipped along one of many directions. It's only when we start to fumble with turning something toward the mirror that we select a "preferred wrong axis" (the left-right flip that keeps up the same direction), perhaps because people rarely flip head-over-heels (so if we show something to someone else we do so by keeping it right-side-up) and also our horizontal symmetry means that if we catch a glimpse of ourselves it's easy to assume there's a mirror person who got there by turning. If we were as asymmetric left-right as we are up-down we might not prefer to flip text horizontally by that trick of the eye. But make no mistake (as Feynman does) that this perception causes the horizontal flip: it's just our reaction to it, turning objects preferably horizontally only. 91.120.48.242 (talk) 12:12, 17 December 2012 (UTC)[reply]
For those wondering about xkcd in the heading, the question is based on http://xkcd.com/1145/ which was published December 10. Hover your mouse over the drawing to see the alt text "Feynman recounted another good one upperclassmen would use on freshmen physics students: When you look at words in a mirror, how come they're reversed left to right but not top to bottom? What's special about the horizontal axis?" xkcd forum thread about the comic: [2]. PrimeHunter (talk) 00:36, 17 December 2012 (UTC)[reply]
So there is no left-right preference in the mirror or your eyes or your brain. Arguments that start with: "You have two eyes that are horizontally placed" fail because you still see the reversal with one eye closed.
For proof of that, do an experiment. Write the word "MIRROR" onto a piece of paper and rotate it 90 degrees so that the first letter is at the top and the last is at the bottom. Now look at that in the mirror - what you get is shown in the image at right. You can see that it's messed up - but now you can tell that you can fix it so that you can read the word EITHER by a left-right flip OR a top-bottom flip. There is no "preferred direction" here.
The thing is that in order to see text in the mirror, the real-world object already has to be being viewed from behind. So the mirror doesn't flip it either left-right or top-bottom - it flips it near-far...in depth. Holding the paper backwards so you can see it in the mirror is what flips it. The mirror flips near-to-far, so the thickness of the paper is what is reversed, effectively moving the text from the side of the paper that's furthest from you to the side that's nearest. What makes text flip horizontally but not vertically is simply our choice as humans as to how to rotate the paper so we can see it in the mirror...the fact is that we seem to always choose a left-right rotation and not a top-bottom one when we do that!
My God, SteveBaker!!! You have PRECISELY described how DEEPLY rooted your IMPLICIT step is, which is the ONLY one that causes the flip!!!! The IMPLICIT step happens RIGHT HERE: "For proof of that, do an experiment. Write the word "MIRROR" onto a piece of paper and rotate it 90 degrees so that the first letter is at the top and the last is at the bottom. Now look at that in the mirror - what you get is shown in the image at right" -- WRONG!!!! There are TWO ways (that I list) to "now look at that in the mirror". You can flip the paper around horizontally (top of page remains top of page, bottom of page remains bottom of page) or you can flip it around head-over-heels (left side of page remains left side of page right side of page remains right side of page). Your idea of "LOOKING AT IT IN THE MIRROR" so deeply includes the IMPLICIT step of flipping it horizontally that you think it's part of the definition of "looking at it in the mirror." NOPE! I have bested you and Feynman! :) :) :) :) :) 91.120.48.242 (talk) 15:12, 17 December 2012 (UTC)[reply]
OOps. Tricky guy, you do mention that in the final sentence. But, trickily, you pretend that "now look at it in the mirror" is as well-defined as "rotate it 90 degrees so that the first letter is at the top and the last is at the bottom". That's a clear direction. "Now look at it in the mirror" is NOT. :) The ONE AND TRUE answer to this riddle or conundrum is:
“
The reason mirrors prefer to flip text and images horizontally but not vertically - an empirically true characteristic everyday mirrors "actually" possess - is that to look at items in the mirror, we flip them around horizontally, instead of vertically or around some skewed axis. Mirrors don't flip the items - we do.
Interesting article. For this picture - http:/upwiki/wikipedia/commons/5/52/Mirror.jpg - how would you check how the front of that jug actually looks, so you can compare it with what we see in the mirror... would you walk around it? Not me, I'd fly up over it, ending up with my head on the ground, since that way I can see what it really looks like. So you see, that mirror flips everything vertically!!!! 91.120.48.242 (talk) 15:50, 17 December 2012 (UTC)[reply]
I still don't get what the continuing problem is here. The text does appear upside-down when the reflected object is below and between you and the mirror, not backwards. A book held oriented so that you can read it in the normal reading orientation as you look at a mirror that reflects both you and the book will show the text upside down, not backwards. Is this unclear, or does anyone dispute it? The same holds with a mannikin head. Place it before you on a bathroom counter on its back peering up into a mirror; its reflection will seem to gaze downward out of the mirror. Does anyone dispute this or find it confusing? Objects above or below the line of sight of an observer will appear reversed upside-down, not backwards in a mirror. Is this confusing or disputed? μηδείς (talk) 03:12, 18 December 2012 (UTC)[reply]
I am amazed by the length of this discussion. Have none of you actually tried going to your bathroom mirror, and holding a page of wrinting so that you first see the writing reversed, and then manipulating the page so the that the mirror image is upside down (it will then not be transposed left to right). Trust me, if you did, you would instantly understand what is going on, and realise that Poster 91.120.48.242 had it right, as did Steve Baker, and Medeis/μηδείς in his last post is off in fantasy land. Ratbone 60.230.205.13 (talk) 01:31, 19 December 2012 (UTC)[reply]
I have suggested the experiment that you just suggested twice already and given an explanation. (Search the word bathroom and see if ayone besides myself in this thread has used the word.) I have done the experiment. Hold a book in front of you, open on the counter and facing up the way you would normally read it, and the text in the mirror in front of you will appear upside down. If you hold the text beside you with the words running parallel to the floor, or tilt up the book so that the cover text faces the mirror, the image of the text will be backwards, not upside down. That amounting to "fantasy land" sounds like crazy talk if you ask me. Perhaps I confused you? μηδείς (talk) 03:40, 19 December 2012 (UTC)[reply]
I was well aware of the previous suggstions to go to the bathroom and try it. I posted because it strongly seems that none of the other posters actually went and tried it. Its one of a multitude of things that are a bit difficult to reason out in one's mind, but a few seconds manipulation in front of an actually mirror makes it immediately clear. In your explanantion in your post (that I referred to) you have it all confused - did you actually try it with a mirror? I think not. Your second and penultimate sentences are wrong - it depends on orienation of the object, not on whether it is positioned to the left (or right), or below (or above) your line of sight. Enough of this. If anyone still hasn't figured it out, go to a mirror now and try it. Like another poster said, hold teh text in front of you so you can read it directly. Now, rotate it about a vertical axis so you can read it in the mirror. Viola! It looks transposed left to right. Then, rotate it back again, and now rotate it about a horizontal axis so yoy can again see it in the mirror. Viola! Its now not transposed left to right, its upside down. Move it about left to right, up and down, round about, without changing the orientation. Stays upside down, doesn't it? Nothing to do with lines of sight. Ratbone 124.178.37.15 (talk) 05:45, 19 December 2012 (UTC)[reply]
Yuck. You can illustrate these things without images.
Has anyone considered that the answer is because of how our brains process visual information? What we see is divided into a right and left visual field, but does not have an "up" and "down" visual field. Mirrors reflect the light the way mirrors do, but once the light hits our retinas the brain takes over. There's nothing special about mirrors, or any axis in particular, it's all a quirk of our nervous system.Quietmarc (talk) 14:59, 23 December 2012 (UTC)[reply]
Just having scanned through quickly, I don't see anyone mentioning one arbitrary choice of symmetry breaking, namely that the mirror is implicitly assumed to face horizontally rather than vertically? Place a mirror flat, face-up on your desk (or face-down on the ceiling). Hold a book oriented upright the opposite side of it, and inspect the writing in the reflection. It appears to be flipped vertically, not horizontally. This doesn't answer anything, just though I'd mention the oversight. — Quondum14:36, 24 December 2012 (UTC)[reply]
Evolutionary fate of siblicide
Is it reasonable to expect that throughout evolution siblicide and similar behavior in animals will disappear as disadvantageous traits by means of producing a single offspring (so that Nazca Booby for example would lay only one egg instead of two)? I also wonder whether such behaviour (including eating of males after copulation in spiders) creates a gender disbalance in population, ultimately leading to difficulties in finding a mating partner. Brandmeistertalk20:12, 16 December 2012 (UTC)[reply]
Yes, as said it will depend on the selection pressure. As it is, birds with this egg laying pattern lay two because it is a small investment (mass of the egg and difference in labor warming two eggs--which is negligible) compared to the risk of laying one egg that is sterile or cracked or lost before hatching and losing a whole breeding season. One egg inevitably hatches first, and since the math shows that one fat healthy chick is more than twice as likely to flourish than two underfed chicks, the inevitable happens.
Were there, however, to be a long string of really good years where those rare nests with two surviving chicks did well, then the trait that leads one sibling to oust the other will slowly be ousted by the friendly-chick trait. Just a few chicks with the friendly trait would do so much better over time that in a colony of a million birds it could become the dominant pattern over a geological instant. There seems to be some constraint with ground laying colony birds, however, that favors the one chick. It's probably lack of real estate.
In any case, whichever trait leads to better survival in the long run will win out, since it does a friendly chick no good if he and his sister both die before reproducing. You should read Richard Dawkins's The Selfish Gene. which makes this all quite clear. Just don't fall for his notion that genes, and not organisms are the objects of selection. Ernst Mayr's What Evolution Is also covers the topic more briefly but with a much more accepted viewpoint on selfish genes. μηδείς (talk) 21:47, 16 December 2012 (UTC)[reply]
I think one of the best examples where siblicide has been apparently selected for is the case of oophagy, or intrauterine cannibalism. Basically, the first embryo to develop to the point it can eat will consume the remaining embryos, potentially including other embryos that have also developed that far. From an evolutionary perspective, it seems like a very uncomplicated way to provide one's offspring with extra nutrition so it can develop further before leaving the womb. Someguy1221 (talk) 02:17, 17 December 2012 (UTC)[reply]
It should be noted in oophagy that most are simply unfertilized eggs intentionally produced for the first fertilized egg to consume. But yes, this is pretty radical siblingslaughter when it happens. μηδείς (talk) 03:09, 18 December 2012 (UTC)[reply]
December 17
Doppler Phase and Frequency shift
Hi,
Suppose I am having a stationary receiver and a moving (say approaching) transmitter, there is a frequency shift due to the Doppler effect, what will be the effect on the phase? Will there be a phase shift as well?
The phase difference will continuously change. When the relative distance changes by half a wavelength you will get a π change in phase. You can think of phase as the integral of frequency over time if you are into calculus. Graeme Bartlett (talk) 20:37, 17 December 2012 (UTC)[reply]
Strictly speaking, doppler effect IS primarily a continuous phase shift. Moving the transmitter means moving the transmitter waveform - this is readily visuallised as changing the phase. Changing the phase means changing the frequency as the wave has to "catch up" or "slow down" as the case may be. Mathematically, the differential of a continuous phase shift is a step change in frequency - this fact is often used in frequency modulated radio transmitters as phase modulating a fixed frequency is easier than precisely modulating an oscillator directly. Keit 58.167.244.186 (talk) 00:15, 18 December 2012 (UTC)[reply]
To add a bit more clarity to the question: If a pulse was transmitted from the moving transmitter, and received at the stationary receiver, the received pulse would have a phase shift, or a frequency shift or both?
Gulielmus estavius (talk) 18:18, 18 December 2012 (UTC)[reply]
The concept of phase is inappropriate to a pulse. Phase actually is a concept applicable to continous waves. If, say, the transmitter is moving toward the reciever, the pulse width will be shortened, if the transmitter is moving away, the pulse width will be lengthened. This is the simplest and best way to think about it.
However, by fourier analysis, any periodic waveform, including pulses assumed to be periodic, can be shown to comprise of a number of superimposed frequencies that are harmonically related. See http://en.wikipedia.org/wiki/Fourier_analysis. You can calculate the frequncy shift and phase rotation for each component frequency and then recombine them all. The resulting new pulse shape will then emerge, shortened or lengthened as I mentioned above. In practice, such a calculation woulkd be lengthy and not show anything you didn't already know, unless the medium (ie the air or whatever that the wave is travelling through) offers a significantly different attenuation to the different frequencies. If this is the case, as it is for humid air and rain at microwave frequencies, the pulse shape will be distorted. Keit 124.178.37.15 (talk) 02:13, 19 December 2012 (UTC)[reply]
Electromagnetic waves and bathroom tiles
Do bathroom tiles or any other tiles have an effect on radio waves? Specifically, would a wifi router placed in tiled room perform worse? bamse (talk) 23:46, 17 December 2012 (UTC)[reply]
They will have some effect, but probably not much, unless they contain metals (which is possible, in the glaze). However, it could either magnify or disburse the signal, depending on where the router is placed. StuRat (talk) 00:13, 18 December 2012 (UTC)[reply]
Stu likes to dash off answers without first thinking. To "magnify or disburse the signal" is presumably StuRat's way of saying the tiles could affect the direction of propagation of the signal. This can only occur if the tiles contain electrically conductive channels or a suitably graded (as in shaped like a lens) dielectric constant - neither is very likely. However, tiles are ceramic, and as a rule any ceramic that is not specifically designed for radio applications or is designed as microwave oven safe cookware will absorb the microwave frequencies used in routers. The absorbsion does not require the presence of metals as it comes from inconvenient molecular resonance, and many oxides show absorption at microwave frequencies. For example, water does not contain any metal, but absorbs the 2.5 GHz radiation used in ovens and earlier types of router very well.
Whether or not the tiles have any noticeable effect depends on other factors. For instance, even if the tiles "shadow" the signal, it may still reach the shadowed area by reflection off other metal structures. The moisture in timber can absorb the signal. If you have brick internal walls (common where I live), the bricks, another form of ceramic, will absorb some of the signal. In other words, the bathroom tiles are just one of a multitude of structures in a building that will reduce the signal strength. That's why the rule of thumb for the undegraded range of typical home routers is given as (typically) "8 m or two walls". Keit 58.167.244.186 (talk) 00:33, 18 December 2012 (UTC)[reply]
At any rate, the moral is that it's quite pointless to try to figure out which objects will have what affect, as you'd need to know the exact composition of everything in the room and then run some complex software simulations. Just move the router around and check the reception at each point. If you've ever dealt with rabbit ears, it's a similar process. StuRat (talk) 00:49, 18 December 2012 (UTC)[reply]
Speaking of which, does anyone know to what degree can you image the inside of a house simply by moving around the perimeter and/or watching the variations in signal from a moving source such as a cell phone? I imagine there must be some "top men" who have worked on that one. Wnt (talk) 15:53, 18 December 2012 (UTC)[reply]
Since there could be any number of materials and objects, some of which reflect the signal, amplify it, or absorb it, it would be a very difficult problem. I believe there have been methods developed for detecting airplanes by how they affect cell phone transmissions, but an airplane in an empty sky is a far easier problem than a crowded houses. StuRat (talk) 19:45, 18 December 2012 (UTC)[reply]
The output from a cellphone is not very suitable - not in spectrum, modulation, or in sufficient power. I would be very hessitant in saying it would be difficult though. If using a more suitable energy source, this is readily possible. Just as doppler effect and time domain reflectometry can be used to image the body's internal structures (http://en.wikipedia.org/wiki/Doppler_ultrasound#Doppler_sonography), time domain domain reflectometry (http://en.wikipedia.org/wiki/Time-domain_reflectometry) can be, and is, used to image the internal structures of a building, as where there are transitions in the dielectric constant (http://en.wikipedia.org/wiki/Dielectric_constant), a tiny amount of energy is reflected. The fraction reflected is very small, and no way large enough to concentrate the signal in the way apparently expected by StuRat in his use of the words "magnify" and "amplify", but it is still measurable, particularly if the spectrum and pulse shape of the energy source is chosen appropriately. In fact, since the variation of dielectric constant in building materials and air doesn't vary as much as the acoustic properties of the internal structures of the body, it can work quite well.
Such building penentrating radar devices are available commercially. They are used in law enforcement (looking for evidence concreted in for example, searching buildings without breaking in, in order to justify a search warrant) and in civil engineering (examining building structures for hidden defects). Looks like StuRat did a quick post on something he knows nothing about again.
In a previous career posting, I was responsible for buildings owned by my employer. One of the buildings showed some concrete spalling and cracking - a sign of overloading (or concrete cancer), even though what was in the buiding was not especially heavy. I watched the consulting engineers use one of these radars to figure out why. It showed up the steel reinforcement buried deep within the concrete quite well, along with coke cans and other rubish the builders had apparently tipped into the pour during construction. It also showed where the concrete floors transitioned into the floorspace below. If I manage to remember what it was called, I'll post the manufacturer's website.
Looks like you posted on something you know nothing about yet again, Ratbone. See [4]. It's not an individual cell phone power we're talking about, but the summation of the power in all the cell phone towers in the area. StuRat (talk) 20:49, 19 December 2012 (UTC)[reply]
Your link returns Error 404 Page Not Found. And you have again failed to read what was posted. Wnt clearly mentioned a cell phone, not a tower. Ratbone121.215.50.184 (talk) 02:03, 20 December 2012 (UTC)[reply]
Link works for me. And I expanded on Wnt's comment by indicating that while a single cell phone can't map out a house, a cell phone network can map out planes in the sky. StuRat (talk) 05:19, 20 December 2012 (UTC)[reply]
"As irregular as the light curve of Alpha Ceti"
What is this referring to? Full sentence: "Or was the mentality of an average girl in her first year of high school supposed to be as irregular as the light curve of Alpha Ceti?" from The Disappearance of Haruhi Suzumiya. --107.207.240.46 (talk) 23:55, 17 December 2012 (UTC)[reply]
At a guess, and without any familiarity with the work, I imagine the sentence should be read for meaning as "...supposed to be irregular?" Light curves can be irregular in the case of variable stars, but Alpha Ceti is not one. So a literal reading would be "...supposed to be completely regular?", but that strikes me as unlikely. Rather, I suspect the author was going for the first meaning I suggest, with the Alpha Ceti phrase chosen for atmospheric, rather than astronomic, effect. — Lomn01:56, 18 December 2012 (UTC)[reply]
Our article on Minkar refers to the likelihood that it will become a Mira-type variable. That seems clear enough if you have a basic knowledge of stars. It's unclear whether the OP even realizes Alpha Ceti is a red giant. Some clarification would be helpful, otherwise we're going to end up speculating all over the place. The OP should explain in his own words what he understands and doesn't at this point. μηδείς (talk) 03:07, 18 December 2012 (UTC)[reply]
Doesn't the reference Peter gave settle it? [7] "MENKAR (Alpha Ceti) ... The star has a variety of characteristics that rather set it apart. It is a light irregular variable, changing its brightness erratically by about six percent. (It is technically known as an "Lb," or "giant irregular," variable). Menkar is also notably deficient in carbon, having less than 20% the amount found in the Sun. More interesting perhaps is that it is also observed as a source of radio waves, the radiation coming from a cool wind that blows from the star's surface. Though its exact evolutionary status is uncertain, it is surely dying. The best estimate is that it began life with about three times the mass of the Sun, has quit fusing hydrogen to helium in its core (as the Sun is doing now), has already passed the stage in which it fuses the helium to carbon, and is now entering the stage in which the carbon core contracts. Menkar is then on its way to becoming another great unstable variable like its constellation-mate Mira." I don't know the story, but perhaps the author was looking for something like a six percent variation rather than a werewolf. Wnt (talk) 03:52, 18 December 2012 (UTC)[reply]
Did you read our article on werewolf? If so, do you still need the allusion explained? Alternately, do you recognize just how useless this response is? — Lomn05:03, 18 December 2012 (UTC)[reply]
I mean that if the girl had the personality of Mira she would be varying from catatonia to the most intense sort of mania. Perhaps the author's deliberate intent was to represent a girl with noticeable, but not overwhelming changes of mood. And Lomn, remember Nietschze and monsters. Wnt (talk) 15:52, 18 December 2012 (UTC)[reply]
I have an old cell phone that works just fine, except that the 6 year old battery doesn't hold much of a charge (I have to recharge it every night). I ordered a "new" cell phone battery, and they sent me one which is 7 years old. I am displeased. However, is it possible, that if this battery has been on a shelf all this time, it will hold a charge better than the newer battery I'm using now, which has been in continuous service for 6 years ? I'm wondering if I should test it out first, or send it back right away. StuRat (talk) 22:28, 14 December 2012 (UTC)[reply]
Found this thread which specifically says Avoid purchasing spare lithium-ion batteries for later use. Observe manufacturing dates. Do not buy old stock, even if sold at clearance prices. So I think I'd be trying to return it, say you weren't informed it was old stock. Vespine (talk) 02:31, 18 December 2012 (UTC)[reply]
Some technicians can "rebuild" an old battery by cracking it open and putting in freshly-manufactured lithium-ion cells of the appropriate size. Then the battery should work like new. If you insist on using such an old phone, perhaps getting your old battery rebuilt would be best. Note: If the phone's paint is very scratched, you may want to get the phone's plastic housing replaced. You may also want to keep the phone in a carrying case. Unforgettableid (talk) 07:38, 18 December 2012 (UTC)[reply]
That quite unhelpful, but, since you bring it up, I will explain why I kept such an old phone. It's for my Mom, who doesn't want any fancy features like cameras, texting, voice recognition, touch screens, etc. If you buy a current model without any of those features, you get a piece of junk, which doesn't have individual buttons, and is thus hard for an elderly woman to use. (Well, the Jitterbug, which she had before, was OK, but rather expensive for someone on a limited budget who only wants a cell phone for emergencies.) So, I reactivated my old Motorola V170, which was a quality phone, at the time, with separate buttons, but no unwanted features, and costs under $7 a month on TracFone, and intend to give it to her for Christmas. StuRat (talk) 04:35, 22 December 2012 (UTC)[reply]
Time travel paradox name?
What is the name for the paradox below?
A catastrophe occurs. A scientist spends 20 years making a time machine to erase this catastrophe from history. He succeeds, goes back in time, and prevents the catastrophe. But now, the event prompting him to build the time machine never occured, so he never built the machine. Because he never built the machine, the catastrophe occurs, prompting him to build the time machine...and so on forever. NealCruco (talk) 03:25, 18 December 2012 (UTC)[reply]
Yes, strings of words placed together without any connection to reality or evidence are nonsense, also known as flatus vocis, and so forth. If there were evidence that something like backwards or discontinuous causation had occurred, addressing it might be helpful. But pretending that nonsense is real or worth entertaining is at best enabling, which is, itself, evil. The science ref desk no more deals with science fantasy than it deals with theology. μηδείς (talk) 04:49, 18 December 2012 (UTC)[reply]
Would you prefer that questions about theoretical physics be asked at the humanities desk, and consider all references to such to be works of fiction? Someguy1221 (talk) 04:56, 18 December 2012 (UTC)[reply]
Not as science fiction, but as faith in the supernatural, along the line with, "Do people stay married in heaven?", or, "Can God create a rock so heavy ha can't lift it?" μηδείς (talk) 17:39, 18 December 2012 (UTC)[reply]
There's a proof in formal logic that any single contradiction implies every single contradiction, the same essentially as using the little assumption that 2+2=5 to prove 2=3 from 2=5-2 and to prove 16=25 from squaring both sides of (2+2)^2=5^2. These Kartz-Reimer conjectures always sneak in an infinity somewhere, and end up exploding in a basement on the south of Spain. Every time you read one of these supposed means of travel, the essay ends with, "But there's a catch; achieving this would require more energy than exists in the entire universe." And your all the way back to talking about the rock he created so heavy that even God can't lift it. μηδείς (talk) 17:39, 18 December 2012 (UTC)[reply]
I have read one theory that even if time travel were possible we would never invent it, because whenever we did people would go back and change the past accidentally or otherwise. This would continue until a "stable history" in which time travel was not invented was hit. -- Q Chris (talk) 09:10, 18 December 2012 (UTC)[reply]
While this sort of paradox isn't possible now, that doesn't mean the original question is only meritless nonsense. The fact that we can link to a wikipedia article about it means that somewhere, someone thought such an idea had enough momentum to justify writing about it. While original author might have done better to place his question in the entertainment reference desk, without knowing his intentions I can easily see the original author wishing to investigate the scientific validity of such a paradox. Merely criticizing and admonishing him does little for anyone. Glosser.ca (talk) 21:19, 18 December 2012 (UTC)[reply]
Ok. OP here. I merely wanted to see what this paradox was called. I knew that time travel is not yet scientifically possible. However, I remembered reading about this paradox on Wikipedia. I couldn't remember its name, so I asked here. NealCruco (talk) 23:23, 18 December 2012 (UTC)[reply]
There is no problem with your question, my first answer was snippy, and the paradox is a concept and does have a name, which has been given. No different from a lot of theological concepts one could throw out which are nonsense, but the object of great study. μηδείς (talk) 03:26, 19 December 2012 (UTC)[reply]
As far as I can see, no one here has linked to the article Time travel, which says
it is currently unknown whether the laws of physics would allow time travel into the past. Such backward time travel would have the potential to introduce paradoxes related to causality, and a variety of hypotheses have been proposed to resolve them, as discussed in the sections Paradoxes and Rules of time travel below.
The article has sections called "Theory", "In physics", "Time travel to the future in physics", and "Other ideas from mainstream physics". This was not a question for the entertainment desk.
As for Jayron's well-known point that if time travel into the past were possible, why hasn't anyone shown up yet?, there are at least two potential answers that I've heard put forth as possibilities (not that I'm endorsing them!): (1) Maybe the equilibrium is such that everything happens in such a way that no paradoxes occur, and people have shown up from the future but the no-paradox equilibrium prevents that from being apparent to us (a variant of Q Chris's point above). To best understand this one should refrain from trying to ask "what happens next?" and then "what happens 'after' that?", etc., and instead step back from the 4-dimensional space-time diagram and view it as a fixed diagram. (2) Maybe time travel to the past is possible only to the extent that you can travel back as far as the time when the time-travel device was first created, and maybe none has been created yet.
It is also possible that every time someone attempts to time travel backwards a new parallel universe is created. Also, even if time travel was consistent (no parallel universes), wouldn't it create an ideas paradox? For instance, you from the future tells you how to build a new invention, and then you tell the same thing to your past self once you go into the future a specific amount. However, where would the info about the design of this invention originally come from? It would be a never-ending loop. Futurist110 (talk) 20:42, 21 December 2012 (UTC)[reply]
It's been pointed out here that there's no evidence that travel to the past can occur; but sometimes theory precedes empirical observation: e.g., there was no evidence that the Higgs boson exists, until we found evidence for it. Duoduoduo (talk) 15:22, 19 December 2012 (UTC)[reply]
It's also currently unknown whether the laws of physics allow the Flying Spaghetti Monster to inhabit a base on the dark side of the moon, or one of the five suspected planets of Tau Ceti. Wikipedia shouldn't be filled with such crap. But it is. μηδείς (talk) 03:32, 20 December 2012 (UTC)[reply]
How dare you insult His name in vain like that! The FSM will make you burn in hot chili sauce in the afterlife unless you repent your blasphemy immediately and avoid saying offensive things to Him ever again. ;) Futurist110 (talk) 20:43, 21 December 2012 (UTC)[reply]
Our articles thixotropy and shear thinning are bit redundant, and each could use some work. I believe that most, if not all honey is thixotropic at room temperature. If you've ever played with one of these honey spoons [8], it's fairly obvious that the viscosity changes at different flow rates and stream diameters. Thixotropy is why the last little threads dripping off behave so differently than the first big glob. So-- in short: make some bubbles in any honey (at say, 1 atm, 70 F) to see how they propagate :) SemanticMantis (talk) 16:51, 18 December 2012 (UTC)[reply]
I want to learn how bad the optics of a -13.50 polycarbonate lens are. How can I?
In a 2011 study, four researchers (Citek K, Torgersen DL, Endres JD, and Rosenberg RR) ordered lots of prescription glasses online. They ranged in price from US$8 to US$400. Among their online purchases, "in 22.7% of spectacles, at least 1 lens" failed the standard impact-resistance test used throughout America. And spending more didn't get you better impact resistance. In comparison, "internal test results of over 53,000 [traditionally-dispensed] lenses by Walman Optical Company [...] find that <0.5% failed impact testing." Karl Citek, the lead investigator, "does not recommend buying eyeglasses online". My glasses are old and somewhat scratched; I'd like to buy new ones. I bike a lot; I don't want glasses with subpar impact-resistance. But I still want to buy online, to save money. How can I avoid putting my eyes at risk? Unforgettableid (talk) 07:31, 18 December 2012 (UTC)[reply]
When buying prescription glasses online, for guaranteed shatter resistance, order polycarbonate lenses. The researchers found that "all of the 28 lenses known to be polycarbonate passed impact testing, with [center thickness] as low as 1.26 mm." But note that if you order polycarbonate, it's crucial to order a scratch-resistant coating. Also note that "most opticians recommend avoiding polycarbonate in a prescription that is higher than a +3.00 or lower than a ‑3.00." If you use it in a stronger lens, the optics won't be so clear. Unforgettableid (talk) 07:31, 18 December 2012 (UTC)[reply]
I live in a major metropolis in Canada. Is there any reasonable way for a Canadian or an American to buy ‑13.50 eyeglasses, without spending hundreds of dollars, and without exposing oneself to undue risk from possible subpar impact-resistance? Unforgettableid (talk) 08:24, 18 December 2012 (UTC)[reply]
It is most unwise for a high-minus person to buy over the internet, as the supplier won't be able to invite you to try things in a temporary frame, and won't be able to examine/test your old glasses to see what suits you. But if you must, then this becomes an issue of understanding what the differences are between lenses of the same nominal prescription, and specifying everything.
It happens to be that I am nearsighted and have required progressively-stronger lenses over the years: ‑6 dioptre as a teenager, ‑8 dioptre at age 50, and back to ‑5 now, at age 70. In my experience things start to get really critical once you get over about ‑6 to ‑7, so I sympathise with you.
The main problem with high-minus prescriptions is chromatic aberration: with a bad lens material, you will see thin black lines on a white background separate into red and green lines. For most scenery, things will just look blurry. Lenses, whether glass, polycarbonate or other plastic, are available in a range of refractive indexes (not to be confused with the lens power, i.e. dioptre). People tend to go for high-refractive-index lens materials: this results (for nearsighted folk) in thin edges, which makes the spectacles look attractive cosmetically. But, as a rule, a material with a low refractive index will give lower (better) chromatic aberration. If you don't ask, you'll get the common stuff, which nowadays is not necessarily what you want. Chromatic aberration is measured by the Abbe number. Find out the Abbe number for the lenses you are considering. Choose the lens with the best Abbe number for lenses of equal refractive index and identical prescription. This will give you the sharpest vision. Some suppliers over the years have tried to tell me that over a few weeks the brain gets used to chromatic aberration and compensates, but I've never believed it.
Another factor is more critical for high-minus people. At one time, spectacle lenses where made with a high front curvature. The modern trend is to use flatter front curvature as it looks nicer. However, if you have been wearing spectacles for decades, the modern low curvature lenses take some getting used to.
Be sure that your spectacles are made to the correct pupillary distance (PD; also known as interpupillary distance or IPD). For most people, PD is pretty non-critical, and some lens fitters just use 68 mm for everybody. That's slack and unprofessional, but they do it anyway. For high-minus people, PD is especially critical. Your prescription should say something like: "PD: 67/71". That means your left eye centreline is 67/2 mm from the centre of your nose, and your right eye centreline 71/2 mm. For low-minus people, taking facial asymmetry into account is less important. A single number (e.g. 68, meaning 68 mm between the centrelines of the two eyes) is often quoted. But for high-minus people it must be got right.
To Wikipedian UnforgetableID: Please don't edit the posts of others on Reference Desk, especially without leaving a signature. I know you meant well, and your changing of links to Wikipedia form is an improvement, but if you have changed the meaning in any place, folk would blame me and might want to post corrections. Wickwack 124.182.169.44 (talk) 01:56, 19 December 2012 (UTC)[reply]
Like I said in my edit summary, I believe all my changes were copyediting, and none changed your meaning. But you make fair points: no matter how careful an editor is, they risk accidentally making significant changes to the poster's meaning. As for you: Why not log in? :) I am grateful for your kind reply to my glasses question. But I didn't bother going to your user page to thank you: you weren't logged in, and it's often fruitless trying to write on the talk page of a logged-out user. Cheers, Unforgettableid (talk) 07:09, 19 December 2012 (UTC)[reply]
I don't log in because certain admin folk decided to block me. Judging by the chat in the admin talk page, they think I am someone else that they took a dislike to. By using variable IP I have largely stopped their nonsense. I made some suggestions some time ago in good faith for changes that will make managing trolls and other undesirables easier and better targeted but that seemed to inflame them. Wickwack 120.145.70.101 (talk) 11:16, 19 December 2012 (UTC)[reply]
How long would the sun last if a small black hole was placed inside it?
And what about the Earth? Furthermore, do black holes experience friction differently than regular objects? If you dropped a sand-sized one in the Earth, how long would it take before it settled at the core (or would that even happen)? Goodbye Galaxy (talk) 19:04, 18 December 2012 (UTC)[reply]
It depends greatly on what you mean by "small", as the rate at which a (very small) black hole interacts with surrounding matter will, more or less, increase exponentially with respect to the black hole's mass. This site at Cornell suggests that a black hole with an event horizon of roughly atomic radius, or about a billion tons mass, would never consume the Earth, or at least not on any meaningful time scale (and it further notes that it doesn't consider Hawking radiation working to evaporate the hole first). This paper suggests that black holes of less than a trillion tons might in fact create stable pockets of matter outside their event horizons, preventing any real growth. But there remains a black hole of a given mass that would consume Earth in a day, or another in a century, or whatever other data point you elect to choose. As for friction -- friction is ultimately an electromagnetic effect, and as black holes can have an electric charge, they should experience friction. However, I strongly suspect that that sort of atomic-level interaction won't bear much resemblance to macroscopic everyday friction as we conventionally think about it. — Lomn19:25, 18 December 2012 (UTC)[reply]
(ec) I think this would be extremely difficult to work out, because material near the event horizon would become extremely hot, and would make it hard for other material to reach the event horizon. It isn't even clear to me that the black hole would remain inside the Sun or Earth, or whether it would be perturbed by other planets such as Jupiter into a distinct orbit. Looie496 (talk) 19:27, 18 December 2012 (UTC)[reply]
Keep in mind that a Black Hole of about lunar mass would be about the size of a grain of sand. That'll be an awful lot of gravity near the event horizon. I doubt microscopic effects would play a deciding role with anything that heavy.
If I got that right, the shell theorem applies to Black Holes as well, if you're not too close to the event horizon. While the Moon has only .18g at surface level (~1700km from the center), a Black Hole of equal mass would have the same at that radius. The gravitational pull would increase when you got closer - 1.8 * 1011g at a radius of 1.7m or very roughly 5.2 * 1017g at a radius of 1mm. The latter is probably correct to the leading digit only, because 1mm isn't much more than the Schwartzschild radius itself. - ¡Ouch! (hurt me / more pain) 07:27, 19 December 2012 (UTC)[reply]
Wasn't it said that a small black hole would bounce back and forth from crust to crust (the Earth feeling as vaporous as air to it) eventually coming to rest at the core and eating it away till there's nothing left? I wonder what the first effect that would cause problems would be. Loss of planetary magnetic protection? Sagittarian Milky Way (talk) 03:10, 20 December 2012 (UTC)[reply]
Is there any online gizmo or app that I can use to determine the exact apparent position of the Sun against the sky on a specific date? I'd prefer to know in map form as I'm placing the image of the sun on a flat representation of the northern celestial sphere that will have no grid lines. The date I'm most interested in is March 29 (when it is in Pisces, but I don't know where in that constellation it would be), but if there's an app out there for any date I'd like to know. Thanks! --NellieBly (talk) 01:44, 19 December 2012 (UTC)[reply]
Thanks. I did google and found a number of calculators that purported to be useful but only gave me a numerical answer, which is basically 100% useless for my purposes. Thanks again! --NellieBly (talk) 04:43, 19 December 2012 (UTC)[reply]
If you want an "exact" position, then a numerical answer is the way to go. Get a star map that shows the degrees of the ecliptic (the path of the sun). Then you can plot the position on there before transferring it to your own map.--Shantavira|feed me08:37, 19 December 2012 (UTC)[reply]
And if you want an image readily made for you, try one of the many planetarium software freely available on the web (Including the excellent Stellarium mentioned above). Dauto (talk) 14:44, 19 December 2012 (UTC)[reply]
Furthest object not moving away from us
What is the furthest object that is not moving away from the Milky Way galaxy? I read that the Andromeda galaxy will eventually merge with us, so I guess it's not moving away like the rest of the universe. Is the rest of the Local Group also gravitationally bound to us? What about the Virgo Supercluster?
I found this, and the abstract says that there are blueshifted galaxies in the Virgo supercluster, and that this may be due to their falling towards the area surrounding M86, or to "being repelled from the local cosmological void". I suppose it stands to reason that there will be movement both towards and away from any given point, and that this will not necessarily be due to any gravitational attraction by that point. --TammyMoet (talk) 09:10, 19 December 2012 (UTC)[reply]
There are long-period comets that may have spent time outside our galaxy before we detect them entering our solar system. Some have orbits that take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have barycentric apoapsis distances of nearly 70,000 AU with orbital periods estimated around 6 million years. DreadRed (talk) 12:23, 19 December 2012 (UTC)[reply]
(1) The solar system includes things not on the ecliptic that are orbiting the sun. (2) Someone might define the extent of the solar system as being the extent of the heliosphere, within which the solar wind dominates the interstellar wind, and the Oort cloud of comets extends out beyond that. (3) But still the solar system and the Oort cloud are just a tiny corner of our galaxy, the Milky Way galaxy, which includes at least 200 billion stars. "Outside the galaxy" means beyond that. For comparison, the Oort cloud may extend one light year from the sun, whereas the Milky Way has a diameter of 100,000 light years. Duoduoduo (talk) 14:55, 20 December 2012 (UTC)[reply]
I'm planning to cold concentrate sulfuric acid by using a dessicating agent. The idea is to add a hygroscopic anhydrous sulfate salt to 30% sulfuric acid. Leaving it for a few minutes, and deccanting of the the liquid fraction. Repeating it until the system has reached an equilibrium state where the salt reached its dessicating potential. Which salt would be most suitable? Plasmic Physics (talk) 10:05, 19 December 2012 (UTC)[reply]
I thought about using copper sulfate to use the colour change as an indicator, but the copper would foul the acid and interfere with my nitric acid production. Magnesium sulfate has a greater hydrate capacity, but it lacks the colour change. Plasmic Physics (talk) 10:19, 19 December 2012 (UTC)[reply]
One way to get an idea is to look at the vapour pressure of water over these various substances. I suspect that you will find that concentrated sulfuric acid will draw water from your copper sulfate. Graeme Bartlett (talk) 11:47, 19 December 2012 (UTC)[reply]
Yes, so the substance with higher vapour pressure will put out water that the thing with lower vapour pressure will absorb. As you say when that pressure is very low, the rate will also be low. But the gas phase will isolate the salt hydrate from the aic. But I think the sulfuric acid will absorb water from your copper sulfate. After all in low humidity copper sulfate dehydrates. You may be better off using silica gel, which will cause less damage to sulfuric acid. Graeme Bartlett (talk) 12:19, 21 December 2012 (UTC)[reply]
I have been having this strange thought lately... Could it be possible, that everything is just a pigment of my imagination, and I'm all but living a weird dream, and will soon wake up to a new reality? Imagine, when you cease to exist, everythng in your perspective would be nothing, as if we did nothing all. Could it really be? Life is an enigma..... BonkersThe Clown(Nonsensical Babble)11:28, 19 December 2012 (UTC)[reply]
Flemish painter Jan Van Eyck, working in the 15th century, did not ordinarily include blue in his paintings because of the non-availability of an affordable blue pigment. Thus for him blue was a pigment of imagination. DreadRed (talk) 12:39, 19 December 2012 (UTC)[reply]
Dolittle is talking to the computer on a massive planet-busting bomb that has incorrectly started to count down towards zero. He hopes to persuade it to not blow up: Dolittle:"Hello, Bomb? Are you with me?" ... Bomb:"Of course." ... D:"Are you willing to entertain a few concepts?" ... B:"I am always receptive to suggestions" ... D:"Fine. Think about this then. How do you know you exist?" ... B:"Well, of course I exist." ... D:"But how do you know you exist?" ... B:"It is intuitively obvious." ... D:"Intuition is no proof. What concrete evidence do you have that you exist?" ... B:"Hmmmm... well... I think, therefore I am." ... D:"That's good. That's very good. But how do you know that anything else exists?" ... B:"My sensory apparatus reveals it to me. This is fun!"....and so on.
See also: Simulation hypothesis - which I'm beginning to suspect might explain a lot about the nature of our universe - and could even be true.
Sure, it's possible that we're all a part of your dream (or that you are a part of ours) - but you have to consider how it would be that the "real" you could construct such an incredibly detailed dream - something so incredibly self-consistent and full of billions of fully fleshed-out characters? That seems very unlikely to me - unless the "real" you is a much more capable being than a mere human. You'd have (for example) to be able to dream the whole of Wikipedia - stuffed with more knowledge than a human brain could possibly hold. Since you couldn't have all of that information in your head - you'd have to invent most of it as you dream of clicking on links...but invention of something that rich and complex also seems beyond human mental capacity.
So we're now left with the option that "you" are some kind of vastly more capable being (one would think to apply the word "god-like") who is merely dreaming of being human. If this being is not human then how do you know that the "you" in your dreams is that being? If it's not human isn't it capable of constructing "you" in it's thoughts too? Now you're in the realms of religion - that some godlike being is dreaming (or perhaps just mentally constructing) everything we see, know and experience. So now you just invented God...or maybe just an amazingly powerful computer (hence Simulation hypothesis).
It boils down to this: Yes, you might be right. But should that cause you to act or behave any differently? If I was utterly convinced that what I experience was merely a dream, I might be more strongly tempted to push the limits and see what happens - knowing that the worst that could happen would be that I would wake up, with no actual consequences whatever. That's a very dangerous view to take on life - and could easily result in self-destructive behavior.
So the most rational view is to presume that this is actual reality...and if it turns out to have been nothing more than a dream, you're not any worse off.
I am still waiting for "Bonkers" to email me offering to transfer all his wealth to me and releasing me from any damages incurred if I punch him in the face or strangle him to prove whether he'll wake up in an afterlife (or whatever) once I do so. Waiting.... μηδείς (talk) 17:57, 20 December 2012 (UTC)[reply]
promising technologies in natural resource management
machines used in agriculture
machines used in fruit processing
machines used in processing of precious stones
machines used in forest management
machines used in water treatment and management
machines used in air quality management
machines used in fish treatment
machines used in food processing — Preceding unsigned comment added by 39.54.129.90 (talk) 16:19, 19 December 2012 (UTC)[reply]
I saw a TV program about mapping the universe, out to more than 5 billion light-years. But aren't they assuming something? They are seeing galaxies where they were 5 billion years ago - they will be somewhere else "now". (I know there is no universal now.) Bubba73You talkin' to me?16:23, 19 December 2012 (UTC)[reply]
But as you've just noted, there is no "universal now". "As currently observed from here" is as good as it gets in that department. That said, anybody engaged in such an effort is undoubtedly aware of that caveat. Further, it's not as if we need a map for traveling several billion light years (which is one of the few areas where that discrepancy really matters) -- but even if we did, we'd just do the same thing we do for all other travel: observe current position, velocity, acceleration, and such, and estimate where it will be when we arrive. It's the same thing space agencies do on every rocket launch, just on a bigger scale. — Lomn16:48, 19 December 2012 (UTC)[reply]
To what end? "As currently observed from here" is hard fact, and from any bound-by-relativity perspective, is the only meaningful "now" with respect to Earth, and is not blurred by the estimates and errors produced by extrapolation. In terms of a scientific survey of the universe, it's entirely appropriate. Even some crazy-hypothetical billion-light-year spaceship mission doesn't care about the "now" you're referencing but the "where when we get there". — Lomn18:36, 19 December 2012 (UTC)[reply]
What I mean if the map shows a large structure, say 500 million light years across, that may never have existed because parts of it that we see now were a long way away from each other. That is, we might be looking at part of it were it was 2 billion years ago and the other part were it was 2.5 billion years ago. Bubba73You talkin' to me?02:21, 20 December 2012 (UTC)[reply]
Because of the speed-of-light limit, structures half a billion light years across are not coupled at time scales shorter than that anyway. It exists if anything of that size can be said to exist. In spacetime we see a "diagonal" slice of the object instead of a "horizontal" slice, and that doesn't really matter. -- BenRG (talk) 04:47, 22 December 2012 (UTC)[reply]
Medeis' example is fairly apt, though -- stellar structures, as a rule, don't move with respect to nearby structures at anything approaching the speed of light, and so a picture pieced together over a relatively short time frame remains accurate at broader scales. We might note that Google Maps isn't capturing the appearance and disappearance of potholes, and this would be analogous to individual supernovae and the like within astronomic megastructures, but those supernovae don't impact superclusters any more than potholes alter the route of I-40. — Lomn03:38, 20 December 2012 (UTC)[reply]
I think that if present observations from Earth show a collection of stars 500 million light years across, and let us say that their observed arrangement is globular, it is possible that they never really had the arrangement that we are observing, because the light reaching Earth from the most distal of those stars, is reaching Earth 500 million years after the light from the most proximal of those stars. I think I am re-stating what Bubba73 is saying. I agree with it. Bus stop (talk) 03:57, 20 December 2012 (UTC)[reply]
The example I gave was a structure about 1/10 the size of the map. So consider something roughly 1/10 the size of the Earth - the distance from North America to Europe. Now suppose the Google Earth map of America was made 400 million years ago and the map of Europe was made 500 million years ago. It probably wouldn't show the two continents in their proper relationship. Bubba73You talkin' to me?04:46, 20 December 2012 (UTC)[reply]
But now you've badly mixed timescales. Sure, over 100 million years, the continents can massively shift -- because the Earth is far, far less than 100 million light years in scale. Take your 100 mly superstructure, though: there is a 100 my lag between observations, for certain not-entirely-useful definitions of "between". However, no physical object can possibly cross that gap during the course of the observation lag, because of light speed limits. Practically speaking, most of those objects are probably moving at no more than 1% or so of light speed. Thus, motion over the observation period is, at the large scale those astronomers are interested in, insignificant. To return to your Atlantic discussion, it's like asking whether the distance from New York to London is 5500 km or 5600 km. Sure, that's some room for distortion, but a map with that error range is still perfectly useful for discussing the size and shape of the Atlantic. And again -- all this is predicated on assuming some notion of distant simultaneity which does not exist. What we observe, for that distant superstructure between 400 mly and 500 mly away, is also what the near edge of that superstructure observed 400 my ago, and that observation is in accordance with all forces acting from that superstructure. It is the real thing. — Lomn14:23, 20 December 2012 (UTC)[reply]
Ultimately, most of this question seems to boil down to the fundamental linkage of space and time (the whole "looking in a telescope is like looking back in time" thing). I'll pull this closer to home and illustrate: Alpha Centauri is 4 light years away. Suppose that, for an assumed "simultaneity", it went supernova 3 years and 364 days ago. It is the Mayan apocalypse, and will destroy all life on Earth (this is all bogus, but that's not the point). Is it meaningful to talk about that supernova as having already happened? Or, more specifically, is it any more meaningful than to talk about our present observations? No. A Cent is still a normal star. Some photons reach the earth, and its radiation does not sear us to our bones. No information about what A Cent might have done over the past four years, from the perspective of an A Cent observer, can reach us ahead of the speed of light observations of A Cent itself. That "past four years" phrase is no more meaningful than (in fact, it means the same as) discussing what A Cent will do for the next four years, starting now. A Cent is either four light years away from us or four years away from us, but those numbers can't be added together. And A Cent is a high-proper-motion star -- over a millennium, it moves twice the width of a full moon relative to background stars. That's enough motion to observe over a four-year span with good equipment. But all interactions that A Cent has with us -- light, gravity, radiation, etc -- comes from where I look up and see it right now, not from some projected "where it will be in four years" point. That point, that star, won't happen for four years. It's not real yet. — Lomn14:40, 20 December 2012 (UTC)[reply]
It may also be worth noting, though, that some other observer somewhere else won't see the same thing. Returning to the large-scale superstructures, an observer on the other side of the Sloan Great Wall won't see what we do, because different objects are at relatively different distances (and velocities) from his perspective. And his view is also the real thing, because of the relativity of simultaneity. Both observers are right, and no observer is privileged or "more right". What we see is real, but what someone else sees -- though it be different -- is also real. — Lomn14:48, 20 December 2012 (UTC)[reply]
Lomn says to Bubba "But now you've badly mixed timescales." But that's exactly Bubba's point: that when we make a map of the universe consisting of current observations, we're mixing time scales to a very large extent (to Bubba's chagrin), since the photons from one star we see now were emitted at a very different time than were those from another star we see now. Lomn's point is that astronomers are aware of that, they take it into account as needed, and they don't need to have a map that doesn't "badly mix timescales". Duoduoduo (talk) 15:23, 20 December 2012 (UTC)[reply]
Well, no -- that's a secondary point at best. The main point is that "mixed timescales" as Bubba discusses are a flawed understanding of how the universe works. Current observations, regardless of distance, represent the real universe that we interact with. — Lomn15:34, 20 December 2012 (UTC)[reply]
That's only true if we stay pretty much stationary with respect to the faraway stars and galaxies. It becomes a problem when we start traveling. On the moon, for example, there was a delay of a second or two between transmitting and receiving, between the astronauts and the space center. I don't think anyone would argue that there was some unique "now" at each end, as each was aware of the delay. "Now" was the same for both. Whether it's a light-second, a light-year, or a light-billion-years, it's the same principle. Obviously, though, a star map for someone planted on earth needs to match what we can see, not what it actually looks like a billion years later. But it would be intersting. Perhaps what the crab nebula looks like "now" could be extrapolated somehow. ←Baseball BugsWhat's up, Doc?carrots→ 06:12, 21 December 2012 (UTC)[reply]
No, that's not true (in no small part because we are not pretty much stationary wrt distant astronomical objects). There is no meaningful absolute position or speed, and so there is no "only true if" privileged position as you posit. Apollo astronauts on the moon observed a different universe than did earthbound Mission Control (specifically, the two parties would disagree about the order of an event on the moon and an event on earth occuring within 1 second of each other. And each would be correct). Everybody understood the time gap, and how to adjust for it if necessary, but that's not at all the same as saying it didn't exist. — Lomn14:22, 21 December 2012 (UTC)[reply]
To be fair to the original question: many popular science documentaries and "poster charts" of the universe do badly mix timescales and blur facts into interesting sound-bytes and colorfully incorrect diagrams. I hope that if you're planning an interstellar journey, or even just undertaking any learning about any astronomical observations, that you find some fairly rigorous sources of knowledge - just as you wouldn't use a cartoon-ish novelty-map to navigate across an ocean. Professional astronomers spend great effort to make sure that their work is well-informed by the appropriate relativistic corrections, correct type of distance measurements, and so forth. Consequently, their work, and the maps of the universe they produce, tend to be much more difficult for the average person to consume - exactly like if you tried to read an aviation chart or a navigational chart for piloting a busy shipping channel. For example, you might start with the Sloan Digital Sky Survey, which is a scientifically rigorous "general purpose" astronomical map. Specialized astronomical observations use different types of references. Nimur (talk) 19:17, 20 December 2012 (UTC)[reply]
There's a chronology of the universe since the big bang which is basically the same everywhere (because the universe is homogeneous) and when you see something "as it was 5 billion years ago" you're seeing it at around the 8.5 billion year point in that chronology. The fact that we can see earlier eras this way is useful; it wouldn't be better to extrapolate the images to the present day, even if we could. -- BenRG (talk) 04:47, 22 December 2012 (UTC)[reply]
Perpendicular force
For what reason the force component which is perpendicular to the object's velocity, changes its direction but does not change its speed? I know the argument that perpendicular force does not do work and therefore can not change the kinetic energy, but I am looking for more straightforward explanation. Thanks in advance, 94.159.217.166 (talk) 19:00, 19 December 2012 (UTC)[reply]
Because a perpendicular (sometimes called orthogonal) force has no component which is either working to move the object forward or backward. That is, this perpendicular force doesn't make the object move faster or slower (in the initial direction of travel), and as such, it doesn't affect the velocity in that direction. --Jayron3219:12, 19 December 2012 (UTC)[reply]
If a force does not change an object's velcity, it is because the object is constrained not to move in the direction of the force. For example, a downward force on a shopping trolley does not move it becuasue teh ground under the wheels stops it moving downwards. Therefore no kinetic energy is imparted. Quite different to a horizontal force for which the wheels allow acceleration. Ratbone121.215.50.184 (talk) 01:57, 20 December 2012 (UTC)[reply]
The OP's premise is "the force component which is perpendicular to the object's velocity, changes its direction but does not change its speed". Jayron interpreted this to mean "its speed in its initial direction", and Ratbone interpreted it to mean that its velocity -- speed and direction combined -- was not changed, implying some countervailing force. To clarify for the OP, assuming no countervailing force canceling out the exterior force, the addition of the perpendicular force does not change the object's speed in its initial direction; but it does impart speed in the perpendicular direction. So now the velocity of the object consists of its new direction combined with its speed in the new direction; the speed in the new direction is found non-relativistically (someone correct me if this is wrong) as the square root of the sum of the square of the (unchanged) speed in the original direction and the square of the imparted speed in the new direction. Duoduoduo (talk) 15:35, 20 December 2012 (UTC) And the speed in the new direction is greater than the speed in the original direction, since the hypotenuse of a right triangle is longer than either of the legs. Duoduoduo (talk) 15:54, 20 December 2012 (UTC)[reply]
If I understood you properly, I don't think you are right. The force component which is perpendicular to the velocity does not change the speed at all. Only the tangent component changes the speed. 94.159.217.166 (talk) 21:31, 20 December 2012 (UTC)[reply]
Illustration of tangential and normal components of a vector to a surface. Again, you have to distinguish between speed in the original direction, which is not changed but which is only one component of after-the-fact speed, and speed in the new direction, which is a combination of the unchanged speed in the original direction and the imparted speed in the perpendicular direction.
You can visualize it using vectors. The original motion is depicted by a vector going in one direction, say horizontally, with speed indicated by the length of the arrow. The imparted motion is depicted by a vector going perpendicularly, say vertically, again with speed indicated by the length of the arrow. The combined motion is given by completing the rectangle and drawing a vector from the starting point to the far corner of the rectangle. By the Pythagorean Theorem, this diagonal vector is longer than either of the two vectors giving rise to it. Euclidean vector contains this diagram. You can think of the motion as occurring from the origin to the endpoint in one unit of time. Duoduoduo (talk) 22:07, 20 December 2012 (UTC)[reply]
You appear to be assuming that a force is applied for a while such that the force is always perpendicular to what the object's initial velocity was. But what the OP is asking about is the case where a force is applied for a while such that at any instant, the applied force is always perpendicular to the object's velocity as of that instant. In that situation, the object's speed remains constant, with only the direction of the object's velocity changing. Red Act (talk) 00:05, 21 December 2012 (UTC)[reply]
I think the OP is thinking about circular motion, where a constant centripetal force changes the direction of motion but not the speed. Otherwise, Duoduoduo's description is correct - a force, even in a perpendicular direction, will usually change the speed. As an example from orbital mechanics if you have a satellite orbiting the Earth, it has a constant speed, but a changing velocity caused by the gravitational attraction of the Earth. If an additional force pushes the satellite towards the Earth (perpendicular to the direction of motion), the orbital radius decreases and the speed increases (if I recall correctly).--Wikimedes (talk) 00:22, 21 December 2012 (UTC)[reply]
No, a total force that's always perpendicular to an object's velocity at all times will not change the object's speed under very general conditions, not just in the case of circular motion.
Let be an object's velocity, a time-varying vector. Let be the total force applied to the object, also a time-varying vector. In Newtonian mechanics, at any instant, .
and are perpendicular only if , which implies that .
An object's speed is , where is a time-varying scalar. Taking the derivative, how an object's speed changes with time is in general given by . But in the special case of force and velocity being perpendicular, , so in that case , i.e., the speed does not change. Red Act (talk) 02:41, 21 December 2012 (UTC)[reply]
I stand corrected. Good answer to the original question as well. (I'm still curious why my orbital mechanics example doesn't apply....)--Wikimedes (talk) 13:29, 21 December 2012 (UTC)[reply]
It doesn't apply because to move into a lower orbit the satellite must spiral inwards and during the spiral motion Earth's gravity is not exactly perpendicular to the motion. Dauto (talk) 15:09, 21 December 2012 (UTC)[reply]
Red Act has shown correct mathematical steps, but under what circumstances does is a force perpendicular to velocity? When the motion of the body is constrained, of course. This can only be satisfied if the constraint is to move in a circle with finite radius, or a circle with infinite radius (a straight line perpendicular to the force). Ratbone 121.215.30.187 (talk) 15:38, 21 December 2012 (UTC)[reply]
@Dauto - I understand so far, but what if the additional force acts in a direction so that the total force on the object (additional + gravitational) remains perpendicular to its velocity?--Wikimedes (talk) 16:17, 21 December 2012 (UTC)[reply]
In that case than, as Red Act skillfully demonstrated, the speed remains constant, but the additional force will eventually be a large force equal and opposite to gravity's force leading to uniform motion That's not the outcome you described. Dauto (talk) 18:50, 21 December 2012 (UTC)[reply]
No, Ratbone, the total force on an object being perpendicular to the object's velocity does not at all imply that the object must be moving in a circle or along a straight line. Indeed, as long as you always move an object at a constant nonzero speed, you can move the object along any path you want, and the total force being applied to the object will always be perpendicular to the object's velocity.
Your math essentially states that if an object has a certain speed, and a force applied to it does not change that speed, then that force must be orthogonal to the direction of velocity. Well and good. But if the object is unconstrained, the force will accelerate it in the orthogonal direction. And the scalar (absolute) speed |V|, being a positive function of the velocity in all three coordinates, must increase, and kinetic energy increase, as the component speed in the original direction cannot change without a force component in the same direction. Therefore, the object is constrained. Ratbone121.221.223.13 (talk) 08:54, 22 December 2012 (UTC)[reply]
I was assuming a billiard ball moving left to right a fixed speed, which is then hit exactly perpendicularly by another ball whose mass and speed are such that it stops or bounces back at the point of impact (or continues forward but more slowly than the original ball) so that the contact between the two balls is instantaneous only. How does this fit in (or does it fit in) with Red Act's analysis? Duoduoduo (talk) 16:30, 21 December 2012 (UTC)[reply]
The problem with the situation you describe is the word instantaneous. In reality there is no such a thing and the contact force acts during a small - but finite - time interval. Even though the force is initially perpendicular to the motion, by the end of the short interval they won't be perpendicular any more. Conversely, if you postulate an instantaneous contact, the acceleration will be infinite, and the dot product will be indeterminate. Zero times infinite is indeterminate. Dauto (talk) 18:45, 21 December 2012 (UTC)[reply]
Then your original answer using the Pythagorean theorem is the correct one, but, again, because of the caveats I pointed out, that solution doesn't really correspond to the question asked in which the force is always perpendicular to the velocity. Red Act's answer is the more appropriate answer here. Dauto (talk) 20:33, 21 December 2012 (UTC)[reply]
I understand that, outside the body, HIV is a very wimpy virus (it doesn't survive very long, you can't get it from the toilet seat, etc.) But what about other well-known viruses? If someone with a cold sneezes on a door knob, how long will those pesky cold cooties be viable? In general, are most well-known viruses more on the HIV side of survivability or more on the Superman side? TresÁrboles (talk) 21:10, 19 December 2012 (UTC)[reply]
Well, a virus, by definition, is a parasite on live cells. So, unlike bacteria, they aren't going to thrive when away from live cells. The might lie dormant, though. StuRat (talk) 21:14, 19 December 2012 (UTC)[reply]
From Common cold#Transmission: The viruses may survive for prolonged periods in the environment and can be picked up by people's hands and subsequently carried to their eyes or nose where infection occurs. It's footnoted to a reference if you're interested in more details. Duoduoduo (talk) 22:15, 19 December 2012 (UTC)[reply]
You might find the stuff at this article interesting. It's not a direct answer to your question, but it does delve into some of the possibilities. And what better bedtime story could you ask for than finding out that one of the greatest killers of all time was able to be resuscitated after 80 years? Matt Deres (talk) 01:55, 20 December 2012 (UTC)[reply]
Mayo Clinic suggests cold virus can last 48 hours or so, but I kind of think this assumes indoor.[9] Exposure to the sun would certainly lessen the time. Some other online sources of less certain authority say hepatitis can live outside the body for more than a week. I remember reading something similar about the filoviruses (like ebola virus). Shadowjams (talk) 02:59, 20 December 2012 (UTC)[reply]
Why have some countries chose the ±220 V and others the ±110 V? Big countries can have 110 like the US or 220 like Russia. Why not something in the middle like 170 V? Or 400 V? And how can it be that countries like Japan have both 50 Hz and 60 Hz? Or Saudi Arabia 127 V and 220 V? I know that it has historical reasons, but one country should be able to unify internally its frequency. I also know that power lines transmit at a different voltage that what we get at home. OsmanRF34 (talk) 22:27, 19 December 2012 (UTC)[reply]
Well, a lack of standardization comes about when systems are set up with little thought for existing standards. I believe it's easier to double or halve the voltage than to change it in other ways, so you tend to get multiples of 2. In the US for example you get both 110-120 V (most things) and 220-240 V (used for heating, major appliances, etc.). StuRat (talk) 22:33, 19 December 2012 (UTC)[reply]
Yes, it's not surprising that different countries have different standards, but no country has a mix of the two, in contrast to volts and frequency. OsmanRF34 (talk) 12:52, 20 December 2012 (UTC)[reply]
Driving on the same side of the road as you have the steering wheel is difficult and dangerous. It might be an arbitrary decision as to which side of the road you're going to drive on - but having the steering wheel on the same side is a poor choice. SteveBaker (talk) 20:24, 20 December 2012 (UTC)[reply]
It's due to a conflict between history and geography. Historically there was a British influence, hence the driving on the left. But the cars are imported from the US, hence the steering wheels on the left. Duoduoduo (talk) 21:50, 20 December 2012 (UTC)[reply]
Japan has both 50-Hz and 60-Hz because of the history of electrification: one electrical grid grew around Osaka Electric Lamp's American 60-Hz equipment, while another grew around Tokyo Electric Light Co.'s European 50-Hz equipment. By the time that people thought about setting up a national standard, there was so much installed equipment that would need to be changed that it didn't happen. See Energy in Japan#National grid. --Carnildo (talk) 00:50, 20 December 2012 (UTC)[reply]
It doesn't just say "pressure dependent", the entry in the infobox is a link to Carbon#Characteristics. There it says At atmospheric pressure it has no melting point as its triple point is at 10.8 ± 0.2 MPa and 4,600 ± 300 K (~4,330 °C or 7,820 °F), so it sublimates at about 3,900 K.TonywaltonTalk01:40, 20 December 2012 (UTC)[reply]
The scientist who wanted to find this out made a 6 foot (~2 meters) magnifying glass filled with alcohol (why? to reduce the weight? to reduce the cost?) to focus sunlight. He put a diamond in front of it (bought with personal money) and it disappeared. For some reason I remember it turning into carbon dioxide under the glass, though.?? And what substance managed to hold it long enough for the diamond to be destroyed? Platinum? How did he not go blind, at least temporarily, or did he look away? Maybe the diamond was quite small. And it must feel like an oven next to that thing. Sagittarian Milky Way (talk) 02:52, 20 December 2012 (UTC)[reply]
I've seen an experiment where someone heated a diamond until it was white hot (or close to it) using a blow torch and then dropped it into a container of liquid oxygen, after a while of bubbling there was no diamond left. I'm sure if you googled it you could find a video on youtube. Expensive way to prove diamonds are not indistrucable. Vespine (talk) 03:00, 20 December 2012 (UTC)[reply]
The diagram is good, and goes to show that room temperature/room pressure diamond is essentially metastable, as it isn't the ground state phase at those conditions. --Jayron3204:42, 20 December 2012 (UTC)[reply]
Graphs like that are always a bit inaccurate and fuzzy. As SGM notes above, it isn't "pixel perfect". There's probably a time-dependency on certain phase transitions: a slow, gradual heating at low pressure may provide the ability for diamond to rearrange into the more stable graphite form before melting/subliming while a sudden burst of extremely high heat may cause it to essentially instantly vaporize without waiting to rearrange to graphite, for example. --Jayron3214:01, 20 December 2012 (UTC)[reply]
The problem is all in the kinetics of the reaction - the change just happens so slowly at even fairly high temperatures that it's difficult to get it to happen. These researchers tried their hardest using both high temperatures (~1000°C) and bombarding with Argon ions, but only managed to observe what they interpret to be the first stages of such a change. Mikenorton (talk) 19:02, 20 December 2012 (UTC)[reply]
Although it has been claimed (by e.g. Paul Davies) that an intelligent caveman could have theoretically derived the equations of General Reletivity and electromagnetism by demanding covariance under general coordinate transforms and local gauge transforms. Count Iblis (talk) 15:01, 20 December 2012 (UTC)[reply]
But to make that very straightforward demand, one would need a well-developed wave theory of electromagnetism as a prerequisite, which would require the preceding discoveries about electrostatics. Otherwise, we must ask: "what" remains constant under the general transform? Not everything; not length; not mass. The answer, of course, is that the most fundamental constants remain constant: the electric and magnetic permissivity/permittivity of free space, which in tandem define the speed of light (rather, the constant of propagation in the wave theory of electromagnetism). I have no doubt that an (arbitrarily-intelligent) caveman could deduce such facts, but reasoning alone would not substitute for the necessary intermediate experimental discoveries. Particularly, the experimental evidence that ε0 and μ0 are in fact perfectly constant regardless of frame speed requires some very well-calibrated machinery that is easier to build after the cavemen invent machine-tool industrial society. After these substantial developments to validate this not-so-obvious fact, the next caveman could state with confidence what every earlier caveman might have already speculated about the very strange nature of our universe. Nimur (talk) 21:02, 20 December 2012 (UTC)[reply]
Relativity, the speed of light and gravity's effect on spacetime are much more to do with physics and astrophysics rather than astronomy, which deals with the actual objects in the universe. So probably not. douts (talk) 15:47, 20 December 2012 (UTC)[reply]
Count Iblis (or anyone else), could you elaborate on Paul Davies' claim or provide any references? I can't see how that statement means anything other than the rather self-evident claim that we have derived those things and we are just latter-era cavemen. 87.112.53.136 (talk) 17:23, 20 December 2012 (UTC)[reply]
What, exactly, do you think they predicted? The answer is probably 'no', but people seem to be talking a lot about things that Mayans did not, in fact, predict at all, so it's best to be sure. AlexTiefling (talk) 16:06, 20 December 2012 (UTC)[reply]
I'm not really concerned or know too much about it but I thought that they claimed that there would be no more earth after tomorrow? Or is that just when they got tired of chiseling stone for the time being?165.212.189.187 (talk) 16:15, 20 December 2012 (UTC)[reply]
They could accurately predict the movement of stars and planets in the night sky as seen from earth, based on how they had moved in the past. Knowledge of gravity and relatively are not required for that. Greek, Persian, Egyptian, and Chinese astronomers, among others, did the same thing back in the day. thx1138 (talk) 16:49, 20 December 2012 (UTC)[reply]
Why would someone need to know all this stuff? And where do you get the idea that the Mayas were accurate? They were unique among all people who didn't have access to a telescope, but did they know something that we didn't know? OsmanRF34 (talk) 18:34, 20 December 2012 (UTC)[reply]
Where in the world will you find an analogue calendar that goes on forever? Answer: nowhere. The rules for the calculation of such a calendar are available, but when people write down the actual calendric details for the next X years, X is always a finite number. Why would the Mayans have been any different? They had to stop somewhere, and here we are. When you look in your desk diary, and see the calendars for the next 3 years but no further, does anyone ever think this means the world is going to end in 3 years? Obviously, no. -- Jack of Oz[Talk]07:46, 21 December 2012 (UTC)[reply]
I know that this is a government picture, and not just some hobo trying to prank people, but still, why in the name of our savioris a footprint doing directly to the left of the closest rover wheel? Tread marks? Buggie111 (talk) 23:10, 20 December 2012 (UTC)[reply]
There is something that looks vaguely like a shoe print immediately to the left of the wheel in the low resolution version, but downloading the highest resolution version makes it clear that it is merely an artifact of a wheel being steered. Floda 60.230.197.188 (talk) 00:24, 21 December 2012 (UTC)[reply]
This reminds me that if you take any image of Mars or Moon from wikipedia and zoom in, at the end you see rectangular structures like blocks of a street map. There are people beliving that this is the prove for intelligent life in both places.(I know that this is called a pixel)--Stone (talk) 07:11, 21 December 2012 (UTC)[reply]
Wow! I'd forgotten all about that post. I was definitely on a rant that day! The "footprint" in the OP's photo is more a case of Pareidolia than of inappropriate image enhancement...but User:Stone's point is very true. You can produce all sorts of interesting images that look like alien cities or whatever just by the process of repeatedly loading and saving the image in a lossy image format like JPEG or GIF and/or by using image "enhancement" tools. In my post that User:Someguy1221 linked to, I deliberately mis-used these techniques to "find" a convincing photograph of an alien city from a screen-shot of the OP's post here on Wikipedia! The alien city map was "hidden" in a period at the end of his post. The resulting conspiracy theory is pretty much on a par with what most web sites who deal with these kinds of claim would have said. SteveBaker (talk) 14:33, 21 December 2012 (UTC)[reply]
Yes and no. Clearly we can (and do) produce ozone. That "metallic" smell you get when a cheap electric motor is running is ozone produced by electrical arcing. But there are two problems with using this to mitigate the depletion of earth's ozone layer:
Ozone is only a good thing at very high altitudes (20 to 30km). Down here at sea level, it's a nasty pollutant. So it's no use just setting up an ozone factory down here on the surface of the planet. The gas would have to either be transported up there - or made up there in some kind of flying factory. Both are exceedingly difficult things to do.
Producing ozone requires energy - and at the scales required to match the ozone that's naturally produced up there, we'd need an ungodly amount of energy. Right now, the odds are good that we do vastly more damage to the atmosphere from CO2 produced as a by-product of all of that energy usage.
No, ozone is constantly being simultaneously created and destroyed in the stratosphere creating an stable equilibrium. Any added ozone would quickly be destroyed returning to the same equilibrium point. Ozone depletion is caused by the presence of some pollutants in the stratosphere which change the chemistry and shift the equilibrium towards a smaller amount of ozone. Dauto (talk) 14:57, 21 December 2012 (UTC)[reply]
Well it's really more accurate to simply say that the pollutants (or rather their eventually chemically-unbound constituents) simply act to break down the reactive O3. The construct of a stable equilibrium state is an artificial one in this context and confuses the issue a bit. In any event, more to the point, there really isn't much need to produce ozone for this purpose; since the implementation of more significant controls of CFCs, ozone depletion has stabilized and even begun to be reversed in the most potentially ecological hazardous regions. As noted above, there are other pollutants which are more significance issues at present. Snow (talk) 07:56, 22 December 2012 (UTC)[reply]
Eight compartments in the rectus abdominis - which comprise the six pack?
So I'm not entirely sure whether it's rows 1-3 or rows 2-4 of the rectus abdominis that count as the six pack. By that I mean is the pair of somewhat angled compartments at the top of the muscle considered part of the six pack (or does it have some other colloquial name?) or is the pair of long compartments at the bottom of the muscle - below the navel - the bottom of the six pack (if no then does it have some other colloquial name?)?
Well, given that "six-pack" is a colloquial term and not a clinically anatomical one, it really has no absolute definition of which superficial features of the abdomen belong to it. That being said, in a person with well-defined-enough musculature, the outline of all of segments of rectus abdominis would be obvious, though which would be more prominent would, of course, vary by individual. I don't know that people are really used to defining the "six" of the six-pack so precisely so much as using the expression idiomatically to refer to the whole of the mid-abdomen in a fit person, but I suspect if you asked people to make a call, they would mostly select rows 1-3 as the main constituents. Snow (talk) 08:07, 22 December 2012 (UTC)[reply]
Wireless energy and magnetic field
So, I was watching this video documenting wireless energy and how it would use magnetic fields to get the energy from one object to another. In it described some concerns with safety (e.g. cancers), and someone briefly described how we've been bathing in a magnetic field for thousands of years anyway, so there would be no real issue.
Though, two questions come to mind: would the magnetism from the wireless energy "outputs" be much higher than the Earth's? And second, why not just harness the magnetic field that we've been living in? Thanks! 174.93.61.139 (talk) 21:13, 21 December 2012 (UTC)[reply]
If you look at Orders of magnitude (magnetic field), you'll see that the earth's magnetic field is incredibly weak. (25 to 65 micro-tesla). By comparison, a small refrigerator magnet is about 200 times more powerful and a small neodymium magnet (such as you find in many modern products) could be around 100,000 times stronger. So while we're pretty sure that the earth's magnetic field is "safe" - it's by no means obvious that much higher magnetic fields would not have health risks. SteveBaker (talk) 21:44, 21 December 2012 (UTC)[reply]
A steady magnetic field does not transmit energy. It has energy, but the energy just sits there. So there is really nothing to "harness" from the Earth's magnetic field.
To transmit energy, what you need is a changing magnetic (and electric) field; that is, you need electromagnetic radiation. Such radiation has known, and probably some unknown, health effects, all of which vary widely depending on frequency/wavelength, but which are in general quite distinct from (and likely to be more serious than) any dangers from a static magnetic field (of the same average intensity as the changing one, say). --Trovatore (talk) 00:23, 22 December 2012 (UTC)[reply]
As for known health risks, those with metal implants must avoid strong magnetic fields, while extremely powerful magnetic fields can interfere with anyone's blood flow by affecting the iron in hemoglobin. StuRat (talk) 04:11, 22 December 2012 (UTC)[reply]
You wanna pop up a cite for that last statement? Ferromagnetism as I understand it is a bulk property of iron; it isn't usually seen in iron compounds (except maybe magnetite), where the iron atoms are dispersed from one another. (I don't doubt that sufficiently powerful magnetic fields are dangerous, but I'm skeptical that the mechanism you propose is a particularly important one.) --Trovatore (talk) 06:08, 22 December 2012 (UTC)[reply]
Some relevant articles: Electromagnetic radiation and health, Wireless electronic devices and health, Mobile phone radiation and health. As far as I understand, for common household devices (mobile phones, wifi routers,...) there are many studies, but no generally accepted view, also due to the fact that people are exposed only for a couple of years, making the discovery of long-term effects difficult. As the situation is not clear, personally I'd minimize exposure, by switching of wireless devices when not needed, not sleeping near wireless sources, etc. bamse (talk) 08:52, 22 December 2012 (UTC)[reply]
There's an awfull lot of rot published about the effects of electromagnetic radiation, and cell phone emission in particular. The simple fact is this: for any medical/physiological effect at all, at least one of two things must be satisfied: a) the field strength is so intense that heating of body parts occurs above that which the bloodstream (acting as a heat conductor) can cope with. This requires close proximity to quite high power radio transmitters well and truely beyond what a cellphone can put out. b) the radiation is of sufficiently high frequency to be ionising radiation, eg X-rays. Cell phone radiation is too low in frequency to be ionising. If neither of these two conditions are satisfied, then chemical bonds cannot be broken, and if chemical bonds are not broken then there cannot be any effect on the body. End of story. The trouble lies in how research grants are granted, and how editors select articles for publication. If yoiu are a researcher, and ask for a grant to show that everything is hunky dory and nobody should worry, a grant committee, who probaly don't really understand it anyway, will not give you a grant. But if you write out an application stating that a possible hazard needs to be investigated, you may get a grant. And if you beaver away spending teh grant money and finally conclude that while you could not definitely establish cause and effect, there should be more research done, well, you might get more grant money. And editors want to sell journals, so even in a professional journal, a bit of sensationalism helps. Ratbone124.182.16.117 (talk) 10:33, 22 December 2012 (UTC)[reply]
Could the hand have a more ideal anatomy?
Ignoring any sentimental feelings we might have for the current configuration, or it feeling unnatural if you were suddenly given it (if you were born with 4 fingers and 2 thumbs, those hands you can control like second nature and the stock one would feel wrong) What could we do to better serve 21st century first world humans, more than hunter-gatherers? How about more fingers? Longer ones? Thinner ones? Extra joints? Of course extra length means the palm has to be longer to close them/make fists. More fingers probably means thinner and therefore weaker and easier to break. And forget opposable everything, there's no room for enough muscles. Really, I want longer thumb, though. (Added bonus: if we have 12 fingers, we get to use the superior base-12) Sagittarian Milky Way (talk) 21:45, 21 December 2012 (UTC)[reply]
The problem is that we humans modify our environment to suit our bodies - and on a longer timescale, evolution has modified our bodies to suit our environment. If our hands were larger, our cellphones would have bigger touch-screens. The tools we use like screwdrivers are optimized to produce the maximum amount of torque for the hands we have. Everything we do is built on a scale where we can interact with it. So I suspect that most changes that you could imagine would be worse than what we have. Sure, you could get more dexterity at the cost of less strength - or vice-versa - you could imagine hands that were more able to cup liquids or hands with a hard pad to allow us to use them as hammers. But for every improvement you could come up with, there would be some kind of downside - because we're adapted to our environment - and vice-versa. SteveBaker (talk) 21:57, 21 December 2012 (UTC)[reply]
Clearly, some of these things might not have realistic evolutionary paths. And you can't seperate cause and effect completely, but if intelligent designers had adapted us for this technological level instead of cave-man, our chimp-men ancestors might've had a somewhat harder life but we'd have an easier one and our technology would be suited to that (different game console controllers and screwdrivers maybe?) I don't know about the hammer pad though, rocks are everywhere. And even hand bone can get injured from punching a forehead hard enough, much less nails. But screwdrivers are not optimized to produce the maximum amount of torque for the hands we have. Assuming enough room, that would be something like this or that: . Sagittarian Milky Way (talk) 22:39, 21 December 2012 (UTC)[reply]
When working inside PC cases and around car engines I've often wanted a longer thumb, and sometimes finger joints that bend both ways. HiLo48 (talk) 04:11, 22 December 2012 (UTC)[reply]
If we started all over again, and ignoring the issues I mentioned above, which number base is best to use?
12? 16? 20? If we go too high (like the Babylonians' base-60) there are too many symbols and practical scale numbers of the current era which aren't outrageously long in the first place (US national debt, 7 billion people) don't get much shorter. Lets say we don't wan't to just have base 26 (or 25, 24) by appending a mark to each letter. There are enough umalauts and diacritic marks in the world already. Sagittarian Milky Way (talk) 21:59, 21 December 2012 (UTC)[reply]
My opinion, first of all, is that base 12 is overrated. Sure, you can more easily divide by three. How often is that a big issue? It's not that hard in base ten. And in base 12, you can't divide as easily by five.
Too small a base and numbers become unwieldy; too large a base, and you have to memorize big addition and multiplication tables. Other than that I don't think it makes that much difference. Base 8 or 16 would have some minor advantages for the man–computer interface. --Trovatore (talk) 22:16, 21 December 2012 (UTC)[reply]
The Babylonians used a base that could more accurately be described as mixed 6 and 10 so it didn't require all that many symbols. Dmcq (talk) 01:38, 22 December 2012 (UTC)[reply]
From a computational standpoint, the ideal number base would have as many small factors as possible. Base 60, for example, is good: the prime factorization is 2*2*3*5, making division by 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30 easy. From a usability standpoint, the ideal number base would have as few symbols as possible. Binary is ideal, with only two symbols. The "perfect" number base is a balance between the two goals (base 10 is fairly lousy from a computation standpoint, since dividing by 5 (easy) is less common than dividing by 3 or 4 (both hard)). --Carnildo (talk) 01:47, 22 December 2012 (UTC)[reply]
Also, in the earliest stages, there weren't distinct numbers, but rather a series of repeated objects. So, in a base-10 system, for example, 9 dots followed by a space, followed by 6 dots, followed by a space, followed by 2 dots would represent the number 962. In such a system, it's best to use a small base, limited by how many dots most people can recognize instantly without having to count them. Always placing the dots in the same pattern, as on dominoes, allows for a somewhat larger number. StuRat (talk) 04:04, 22 December 2012 (UTC)[reply]
December 22
Generating electricity from a hypothetical magnetic monopole
Hypothetically, suppose I have a stable magnetic monopole and I want to generate electricity from it, according to the modified Faraday's_law_of_induction from Magnetic_monopole:
Assuming that it's a permanent magnet, then B is constant w.r.t. time and it simplifies to:
According to this, all I would have to do is wound enameled wire around the magnetic monopole to generate electricity, does that sound about right?
The article on dry ice is rather weak on the detail of how dry ice + water = clouds of "smoke". I gather the "smoke" is condensed water vapour, but does this come from the surrounding air or from the water? What is the exact mechanism of its production? Also, why does the "smoke" sink? Is it because carbon dioxide is heavier than air (as I read in one place) or because the vapour is cold (as I read in another)? — Preceding unsigned comment added by 86.160.86.1 (talk) 03:09, 22 December 2012 (UTC)[reply]
What sort of earthing do you have in mind? Electrical HV distribution earthing? Domestic residence electrical earthing? Radio station earthing? Telecommunications (phone company) eathing? Earthing for other purposes such as corrosion control? I have been involved in earthing for HV substations, radio stations, and phone company infrastructure, and I have not seen nor heard of charcoal being used. I have not seen, but have heard of salt being suggested, but it is not generally used, as it will give only a temporary improvement.
The main purpose of added substances around electrical earthing electrodes in the ground is to improve conduction to earth by making the surrounding matetial hygroscopic, as the mineral components of sand and soil are electrical insulators. Electrical conductivity in sand and soil is provided by interstitial water (in desert areas with very little moisture in the soil, getting a decent earth is extremely difficult, at least unless you can drive earth electrodes down into the water table, which, if the soil is dry, will probably mean drilling down great depths and going thru a rock layer). A secondary factor in some areas may be to reduce soil chemical action/corrosion of the earth electrode(s). Bentonite is the usual material used. Sometimes a small amount of salts in a proprietry mix is added to the bentonite when the bentonite is sold specifically for eathing purposes, and is claimed by suppliers to improve conduction still more. However, any benefical effect must be short term, and in any case it can be shown by mathematical analysis that any such improvement over damp bentonite is marginal anyway. The improvement for added salts comes from improving conduction in the water in teh soil/bentonite, but any added salts, not already in the soil, will gradually leach away until diffusion equilibrium is reached. Conversely, if you back fill with pure bentonite or sand, over time salts and other conduction-promoting substances in the soil will gradually diffuse in. So if your electrical earth is good enough electrically upon installation, over time it will typically get a bit better, providing you do NOT add salts.
With radio transmitter earthing, usually no soil treatment is done, as it is more effective to just bury sufficient radial wires to make a ground plane.
With domestic electrical earthing in the Multiple Earth Nuetral system, no soil treatment is done. The safety and operation of the MEN system does not depend on any single residence earth - it depends on a multitude of earths working together, so the performance of any one earth is unimportant.
You will get a better answer if a) you make your question more specific, and b) you append your question with a pen-name. Its a free world in Wikipedia, and you don't have to, but you will get better answers if you do. Keit121.221.223.13 (talk) 10:01, 22 December 2012 (UTC)[reply]
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