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March 30

Things without Mass

These are probably straightforward questions for any physicists out there, but:

• Have we found or identified things without mass (or matter, if you prefer) in the universe?
• If not, are there not-unproven theories that allow for or expect to find massless or matterless things?
• Where do forces and waves (and torques and etc.) fit in? Are they considered things, or effects of things? 207.6.208.66 (talk) 01:35, 30 March 2012 (UTC)

Well, photons have zero rest mass, or at least that's the current thinking, and by default mass means "rest mass" these days.

If you mean that the things have no mass-energy, I suppose that depends on what you mean by a "thing". I am not aware of any way that a physical object can be detected if it has no mass-energy. But the number seven doesn't seem to have any mass-energy, and has arguably been "found", at least if you're a mathematical realist. --Trovatore (talk) 02:06, 30 March 2012 (UTC)

This question might even boil down to metaphysical naturalism, the theory that the only kind of things that "exist" are made of "physical stuff", i.e. mass and energy. In that respect, nothing has been found which conclusivley proves "non-physical" things exist, but a few things, perhaps the most well known being consciousness, present as yet unsolved problems to the position. Vespine (talk) 03:28, 30 March 2012 (UTC)

Maybe just add that it's probably a question of philosophy as much as physics. Specifically metaphysics. Vespine (talk)

Last one. As to the "not unproven theories that allow massless or matterles things"; Cartesian dualism, or more specifically Substance dualism would fall into that category. Vespine (talk) 03:36, 30 March 2012 (UTC)

See, and I thought only the Catholics had a Mass... --Jayron32 03:50, 30 March 2012 (UTC)

Look up wikt:Mass and wikt:Mess. The original meaning is simply the amount of food put out for a meal. (The symbolic wafer has apparently wandered a way from the Last Supper) But Newton's mass has a different etymology entirely. Wnt (talk) 04:25, 30 March 2012 (UTC)

Er, no. The Wiktionary entry for 'mass' clearly indicates that the name for the church ritual derives from the dismissal sentence 'Ite, missa est'. This has nothing to do with amounts of food; it just means "(You can/should) go, it (our prayer) has been sent." AlexTiefling (talk) 13:23, 30 March 2012 (UTC)

Actually, for 'Mass', which I hadn't read (I guess I should have remembered article names over there are case sensitive!). I find myself very skeptical of this interpretation but it's off topic, and not my field. Wnt (talk) 14:28, 30 March 2012 (UTC)

Hmmm, agreed that all particles have a finite mass-energy. Photons would have zero mass-energy if they stood still, while massive particles would have infinite energy at the speed of light, so we don't see these things. Fundamental forces are mediated by particles - with the possible exception if quantum gravity can't be worked out, and anything else a real physicist might think of... These particles, and forces, have mass, or release mass. For example the binding energy of a nucleus makes it less massive so energy has to go in to bring out the protons and electrons doh! neutrons. Or the negative energy of the Casimir effect (maybe lack of virtual particles?). But potential energy e.g. between two charged objects brought together can also have positive mass...

Hypothetically, if you had a "zero mass electron", and you brought it near a regular electron, I'd expect it to go accelerating away at an infinite rate (zero inertia). But with access to a proton I suppose it would zip to it from any distance based on the most infinitesimal force. It would certainly be very weird - I wonder if there's a stronger way to refute the possibility. Wnt (talk) 04:09, 30 March 2012 (UTC)

Thanks for the responses: I suppose I should have clarified that I was considering detectable phenomena rather than concepts or ideas. Are zero-mass electrons possible? I was lead to believe (by a school no less!) that they had a fixed mass. They also decided that Newtonian physics sufficiently explained how things worked, so I wouldn't be surprised that they were wrong.

Interesting point on the rest mass of photons: though I can't find exactly where in the speed of light article it outright states that photons move at a fixed speed, I'm inferring it from everyone above. Theoretically speaking, then, what would happen to photons caught in black holes? Would they no longer be able to move? (Or if preferred, what do current theories stipulate might happen to photons when they meet with a black hole?) 207.6.208.66 (talk) 05:19, 30 March 2012 (UTC)

Emphatically no, no zero mass electrons have ever been suggested. Photons move at the speed of light, always; this is the key axiom underlying the theory of relativity. Even in black holes they move at the speed of light, relative to the space they're in, until the singularity is reached; and the mathematical description just ends there without explanation. Wnt (talk) 05:36, 30 March 2012 (UTC)

Well, I guess that makes sense then. Thanks a bunch to everyone, especially Wnt! 207.6.208.66 (talk) 05:45, 30 March 2012 (UTC)

Flat, empty spacetime is something that physically exists, yet in non-quantum physics at least doesn't inherently have any mass to it, even if you take the ambiguous term "mass" to mean "relativistic mass". However, in quantum physics spacetime does inherently have a vacuum energy (which is equivalent to a "vacuum relativistic mass"), although how much of that "vacuum relativistic mass" spacetime has is unclear, since looking at vacuum energy from the perspective of quantum field theory or from the perspective of the cosmological constant gives wildly different values for that, for reasons that aren't understood. Red Act (talk) 06:59, 30 March 2012 (UTC)

Photons always move at the speed of light, true, but what that speed is depends on the medium the photons are passing through. When people refer to "the speed of light" without elaboration they mean the speed of light in a vacuum. But scientists can slow down light dramatically, using low temperature mediums. Just recently I read (maybe in Scientific American?) that in a recent experiment light was slowed down to about 38 miles per hour.

Is it conceptually possible to slow light to 0 miles per hour? Would it be massless/energyless then? Duoduoduo (talk) 19:43, 30 March 2012 (UTC)

Look at slow light. The distinction between phase velocity, group velocity, and signal velocity gets really tricky. I've seen different takes on the situation with photons subject to a refractive index - as I understand it, though, all the "slowing" of light has to do with properties of the wave model; whenever you're looking at an actual photon that thing should be moving at the honest to God vacuum speed of light. Note I could well be wrong/confused on that point - I've seen explanations up to and including modeling electric field as a curvature of space to explain the slower speed. Wnt (talk) 21:01, 30 March 2012 (UTC)

eco-friendly utopia

Imagine everything about pollution, greenhouse effect, blah blah... is magically fixed. Now how can we start again and build cities that are completely eco-friendly, without giving up all the technology we have now (like, say, going back into caves). I mean do we have a theoretical idea of what an eco-friendly city, town, whatever, looks like? I mean we scream about global warming and pollution all the time, but what is it that we ultimately want to reach exactly?--Irrational number (talk) 07:41, 30 March 2012 (UTC)

If you have a subscription to New Scientist, this article on "Rebuilding civilisation from scratch" explores this. The engineer's dream (if not necessarily the sociologist's) is an arcology - a largely self-contained planned city which attempts to be as efficient as possible to heat and cool, to water and feed, to light, and to provide transport around. "What we want to reach" is more of a political question, and it'll depend on who you ask, but the technological approach is roughly bright green environmentalism. Smurrayinchester 08:31, 30 March 2012 (UTC)

I believe population is the real problem. If 99% of the people would be good enough to disintegrate, we would still have enough to maintain a technological society yet not enough to create ecological catastrophes. StuRat (talk) 08:37, 30 March 2012 (UTC)

So what your saying is, is that we ought to nuke Zuccotti Park? 203.27.72.5 (talk) 21:00, 30 March 2012 (UTC)

There are many who would argue that anything less than giving up all technology is pollution. --145.94.77.43 (talk) 09:35, 30 March 2012 (UTC)

There are lots of positions one could take as to the outcome, but the essential goal of all of them is some form of sustainability — e.g., a system that can continue without eventually destroying itself. The problems with global warming and pollution in their current forms is that they are rapidly becoming unsustainable, and the consequences of that are going to be massive in terms of human health, lived environments, and human economies. There are practical solutions, for example, towards reducing carbon dioxide output substantially that would keep the base temperature deviation within holocene levels, for example. They don't involve becoming an eco-topia or anything like that; they involve capturing emissions, improving efficiency, using other forms of energy generation, etc. (e.g. the Stabilization Wedge Game). --Mr.98 (talk) 13:24, 30 March 2012 (UTC)

Tangential, but I wanted to pull this quote from Mr.98's link (emphasis mine):

• "A final criticism is that the Wedge Game focuses on technological fixes rather than fundamentally challenging the endless growth economy that is at the heart of global climate change.The 2007 IPCC reports state clearly that economic and demographic growth are the fundamental drivers of global climate change." I think this aspect is often overlooked. Our basic economic principles which equate growth with wealth and prosperity inherently lead to unstable, unstustainable resource exploitation. SemanticMantis (talk) 15:09, 30 March 2012 (UTC)

I was looking for something more... detailed. I mean I know an eco-friendly system is such that it won't destroy itself and all of those things, but what exactly would it be like? Do we know enough about what it must look like?--Irrational number (talk) 17:39, 30 March 2012 (UTC)

With a lower population, it would work pretty much like it does now. That is, people would continue to cut down trees to plant crops, but this would now only take a small portion of the total land, because of the much smaller population. Fossil fuels would still be unsustainable, though, in that we are currently using them up at something like a million times the rate at which they are formed, and we can't cut our usage to 1 millionth merely by reducing population. The good news is, renewable energy sources like solar and hydro work far better with lower populations. That is, there's just as much sunlight and water, but far less energy is needed. Creating greenhouse gases would no longer be a concern at 1% of our current population, as that lower rate wouldn't have a measurable effect on the climate. Of course, if population began to grow, then the ecology would again be threatened, so we would need controls in place to prevent that. My point is, that it's fundamentally our current population which is unsustainable, and the effects on the environment are just a symptom of this problem. StuRat (talk) 17:54, 30 March 2012 (UTC)

StuRat, you make a good point about population. However, it's not just our population, it's our population and our rate of resource use. There are indeed two factors that can in principle be adjusted. SemanticMantis (talk) 18:12, 30 March 2012 (UTC)

True, but we often make this mistake of thinking that, say, American Indians, lived in a totally sustainable way, when, if they had the same population we do, they would have had environmental problems, too. For example, all those feces, if not sent through a sewage treatment plant, would pollute all the rivers, lakes, and aquifers. That population would also quickly kill off all the wild game. StuRat (talk) 20:07, 30 March 2012 (UTC)

Then we are agreed. While per capita rate of resource usage is the important factor for a fixed population size, the total resource usage rate goes up linearly with population, if they use resources at a fixed per capita rate. Given a certain total target usage rate (which is really what sustainability is), we must then use less per capita to support more people. SemanticMantis (talk) 20:18, 30 March 2012 (UTC)

Right, but I don't know if it's possible to reduce our level of consumption per capita to be sustainable at the current population, much less with an ever-expanding population. At out current pop, about the best we can do is use up one resource, then move on to the next, like from oil to natural gas, to coal, and hope global warming isn't too bad. There's basically no hope we can maintain our current population without having a major ecological impact. StuRat (talk) 00:50, 31 March 2012 (UTC)

The answers to your revised question have more to do with sociology and culture than engineering and hard science. As such, you're basically asking for a political and cultural philosophy that supports sustainable practices. This is basically the realm of speculative fiction. If that angle interests you, I highly recommend this book [1], which (in great detail) posits a societal system that is truly sustainable. Spoiler alert: it involves no urban/rural divide, no cars or fossil fuels, no beasts of burden, and no eating meat ;) SemanticMantis (talk) 20:26, 30 March 2012 (UTC)

Sustainability depends on time horizon. Ultimately, only maximum entropy condition is sustainable. 203.27.72.5 (talk) 21:08, 30 March 2012 (UTC)

let me put it this way: we leave the earth, and meet a fancy beautiful, non-poluted planet just like our earth, with (for the sake of the question) identical conditions. Now we want to build cities, what would the city be like? I want a picture detailed enough to be able to be analyzed for seeing whether it's practical or not (horrible phrasing, sorry!!)--Irrational number (talk) 21:34, 30 March 2012 (UTC)

It really depends on what your goals are. If your only consideration is minimising pollution, then fairly obviously the solution is not to build a city at all and just leave the planet alone. Clearly, you want the minimise pollution while still having humans living there, but how many humans? In what level of comfort? If you want a similar population and standard of living as we have now, then your main problem really isn't the design of the cities. It's power generation. Your main priority would be to find a means of power generation that doesn't rely on burning fossil fuels. We have plenty of those now (nuclear, solar, wind, tidal, hydro, biomass, etc.), they just tend to be more expensive and less convenient than fossil fuels so they aren't used as much. There are other forms of pollution, but getting rid of fossil fuels would get you 90% of the way there. --Tango (talk) 21:50, 30 March 2012 (UTC)

Well, living underground and leaving the surface alone would be nice. I happen to think underground nuclear power is less destructive to the environment than massive solar farms, wind farms, and dams, so I'd go that way. Mines should be the traditional type, not strip mines or mountaintop removal. Food should be grown hydroponically. Transportation would be subways and elevators. StuRat (talk) 01:08, 31 March 2012 (UTC)

I think that suggestion just belies a subjective preference for protecting superterranean environments. Why is destroying the environment of earthworms, plants' root structures and soil microbes objectively better than destroying that of any other organism? 203.27.72.5 (talk) 04:19, 31 March 2012 (UTC)

The earthworms and roots would stay relatively close to the surface, so would be safe. I'm talking about hundreds of meters/yards down. StuRat (talk) 05:26, 31 March 2012 (UTC)

I think what you really need to define (and not just you, but environmentalists in general) is what you want to avoid. Do you want to avoid causing pain, killing individual organisms, causing the extinction of species, influencing behaviours or evolution? Do you only want to avoid affecting life, or are you also adverse to polluting places that don't host life? If you try to take this all to its logical conclusion, you just end up with a paradox; people ought not to exist (as per the Voluntary Human Extinction Movement ) so as to avoid changing the natural environment, but people exist naturally so removing them would in itself be changing the natural environment. 203.27.72.5 (talk) 06:45, 31 March 2012 (UTC)

If human beings, governed by the laws of nature as are any other form of life, are going to be part of this utopia, the idea of anything being sustained but high population growth is pretty dubious. One would have to modify humans to create a society of people averse to having children above the replacement rate for a society which had a minimal impact on the ecology of this new planet. These modified humans could live in rammed earth and cob houses and bike everywhere, or they could live in brick subdivisions and drive 500cu Eldorados. Regardless of the technology they used, the only thing stopping them from naturally expanding to fill the space available would be genetic engineering or some kind of mandated psychological programming. Even these measures would fail given enough generations of mutation. Nevard (talk) 20:53, 31 March 2012 (UTC)

Ironic how all attempts to preserve nature inherently involve fighting against the natural processes that tend to destroy it. 203.27.72.5 (talk) 02:07, 1 April 2012 (UTC)

Two questions about plastic bag bans

The city where I live banned plastic bags in June last year, and so far it seems to have good public support. Plastic bag bans seem to be all the rage in the Philippines right now, with cities like Angeles, Muntinlupa, Pasig and Makati implementing them. I have two questions about such bans:

• 1. What are the short-term and long-term ecological effects of such bans? (both good and bad)

One of the advantages of plastic bag bans is that it cleans up the environment, or does it? I read somewhere that in the long-term, plastic bag bans are actually harmful to the environment, since making paper bags takes up more water and cuts more trees. However, at least they biodegrade easily. So, have there ever been studies about the effects of bans, both short-term and long-term? I'm not asking for opinions, but actual results from studies or experiences.

• 2. Which was the first location in the world to ban plastic bags, and where?

I know San Francisco was the first city in the United States to ban plastic bags, and Bangladesh was among the first countries to ban their use, but where was the first place in the world to ban them, and when? I could not find enough information about this online, but according to some sources, it was a small village in Australia (whose name I forgot), but is this true?

Narutolovehinata5 ^tccsdnew 13:55, 30 March 2012 (UTC)

I think that more information with regard to the proposed legal status of alternatives, as explained in Oxo Biodegradable, Biodegradable bag, and Bioplastic, is necessary to evaluate the effect of such a ban. Presently it appears that degradability is not well defined with some international standard (and more importantly, there's no clear logical dividing line either), so exactly where the line is drawn is important. Wnt (talk) 14:18, 30 March 2012 (UTC)

I read somewhere (unfortunately I can't remember where) that paper bags take a very long time to degrade, which would mean that biodegradability is at best a long term but not short term advantage of paper over plastic.

My understanding is that paper is made of some pretty nasty chemicals. Would that mean that biodegradability is a bad thing?

A disadvantage of plastic bags is that animals eat them and die as a result of a clogged up digestive system. Duoduoduo (talk) 14:57, 30 March 2012 (UTC)

Paper is made with some nasty chemicals more than of them, depending in part on what is done with it - see environmental impact of paper, bleaching of wood pulp. In landfill conditions, denied access to oxygen and nutrients, biodegradation of many things is difficult.

In practice, I think that the urban dweller's main objection to plastic bags is when they end up in every tree, crinkling away in the wind like the flag of some small trashy nation, or when they appear visibly at the shoreline of a river. Any sort of photo- or bio-degradation is enough to help with that. Wnt (talk) 15:23, 30 March 2012 (UTC)

I think the best idea is to get everyone to use reusable bags. (Of course, if they feel the need to wash them after each use, with detergent, this has a high environmental cost, too.) Brown paper bags aren't bleached, which is better for the environment and they biodegrade quickly. The glossy, bleached and dyed paper bags from high-end stores are more of a problem. One other advantage of paper bags is that they quickly get soaked and flattened and thus stop blowing around, unlike plastic bags, which seems to blow around until they hit a chain linked fence or something like that. StuRat (talk) 15:29, 30 March 2012 (UTC)

Actually guys, I'm asking about the effects of plastic bag bans, not about the bags themselves. I'm asking what are the good and bad short and long-term effects of plastic bag bans, not arguments in favor and against recyclable bags. Narutolovehinata5 ^tccsdnew 15:34, 30 March 2012 (UTC)

Well, to reiterate, the specifics depend on the ban. Will a patchwork of overbroad local bans damage demand for oxydegradable bags and thus create more long-term trash elsewhere? Will they encourage the use of paper which then leads to more deforestation? Even with a single known law in your hands it is very hard to work out what it will do, and when it is unspecified... Wnt (talk) 16:55, 30 March 2012 (UTC)

Belgium banned "free" shopping bags years ago, customers have to ask specifically for them and shops have to charge for them. Not sure if they were the first to do it. Since then most shops only have "reusable" bags, though you could question the reusability of the cheapest ones (0.10€), they're made from that stretchable plastic that you know won't last too long if you carry something heavy. This only appies to supermarkets, other shops like fashion, electronics etc. don't mind paying for the bags themselves. Ever since the Greens were in the government some 10 to 15 years ago (??), an eco-tax was introduced on all "avoidable" non-recyclable packaging. That meant 0.50€ for plastic bottles, since recycled glass is an alternative, high taxes on non-rechargeable batteries, on razors with a fixed handle and so on. Basically, all the cheapest products got much more expensive... Ssscienccce (84.197.178.75) (talk) 21:38, 2 April 2012 (UTC)

Duck with face texture like a chicken's wattle

The other day at a pond in northern Virginia I saw a duck and instead of feathers on the front of its head above its bill and around its eyes there was red bumpy skin like unto a chicken's wattle or a turkey's head. This was only around the face area; it had feathers on the rest of its head. What species of duck have red skin on their faces? 20.137.18.53 (talk) 14:09, 30 March 2012 (UTC)

A Muscovy Duck. (+)H₃N-Protein\Chemist-CO₂(-) 14:16, 30 March 2012 (UTC)

Thank you. 20.137.18.53 (talk) 14:46, 30 March 2012 (UTC)

Or possibly a churkendoose.[2] ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:37, 31 March 2012 (UTC)

Visual range of animals that differs from humans

I understand that some birds can see into the UV spectrum. Do they have a wider visual range than humans, or is their whole visual range just shifted up a bit? To put it another way, can birds see all the same colors as us plus UV light, or are they "missing" some of the colors we can see? I'd also like to know about other animals that have a different range than us. Are there any that go the other way, seeing infrared instead of UV? And what do you think it would look like to have such different kinds of vision? 151.163.2.8 (talk) 15:36, 30 March 2012 (UTC)

You could try to follow references from Bruce MacEvoy,'s Color vision pages, chapter 1. – b_jonas 16:07, 30 March 2012 (UTC)

Humans can see ultraviolet after cataract surgery. Reputedly this was used by British intelligence, which recruited such people to "coastwatch" for signalling by German submarines - but the truth of that story seems to be hard to determine. [3] Wnt (talk) 16:52, 30 March 2012 (UTC)

For infrared, lots of animals can see that, like the pit viper. I imagine it looks similar to what we see when using an infrared scope. StuRat (talk) 19:58, 30 March 2012 (UTC)

I believe pit vipers can just detect what direction heat is coming from in a fairly vague way. They don't have any kind of lens, so they won't get a clear image. --Tango (talk) 21:53, 30 March 2012 (UTC)

Fair web info at [4] [5] . The latter sort of gets my imagination going, because it points out that the pits of a pit viper work more by their structure than any special heat receptors. It makes me wonder whether using a tissue printer and doing nothing more absurd than existing sex-change 'therapy', you might take a sensitive area of a blind person (e.g. fingertip) and replace it with a functioning pit sensor. Not saying that's a winning idea, just interesting. Wnt (talk) 20:30, 30 March 2012 (UTC)

That page says pit vipers have a resolution of about 5 degrees, or roughly the your hand held at arm's length. That kind of resolution would be a lot better than nothing for a blind person, but it wouldn't be any near a replacement for sight. And remember, you're just seeing heat, which isn't a very precise way of seeing things anyway (the long wavelength doesn't help). I don't think a pit-fingered blind person would be able to do much more than identify where doors and windows are, which many of them can do with their eyes anyway (what we call "blind" isn't usually no sight at all, although it can be in some cases). --Tango (talk) 22:03, 30 March 2012 (UTC)

Another thing is that many animals are sensitive to polarization of light, so they can see visual structure that is invisible to us. Looie496 (talk) 21:42, 30 March 2012 (UTC)

Most Birds have four types of pigments unlike the the three that humans have. So they don't have any missing colours. In fact with more receptors, they can distinguish more colours. ( mathematically, i think of it this way: with three receptors, all the colours you can see fall in a 3D space; for a given intensity, all the possible shades will fall on 2D space. see RGB-> HSV coding of colours; with more receptors, you get more shades) As a concrete example, you don't distinguish blue+yellow light from green as they both activate red, green and blue receptors, maximally activating the green receptors. birds will be able to tell apart certain pure shades from a mixture that humans can't tell apart. As an aside, birds have two types of red receptors: both have the same rhodopsin sensor, but each has a differently coloured oil droplet that gives the whole cell a different sensitivity. So the longest wavelength peak for the filtered red cell is around 610nm, a bit lower than ours. But a total of 5 primary colours! hope this answers your questionStaticd (talk) 18:31, 31 March 2012 (UTC)

There are probably human tetrachromats too. --ColinFine (talk) 11:57, 1 April 2012 (UTC)

Bees can see UV light which has led some flowers to evolve patterns which are not visible to humans. Vespine (talk) 23:57, 1 April 2012 (UTC)

Help with bird identification

I saw this bird at a park in Southern California. It was maybe six inches long, and appears to be a juvenile, though it doesn't resemble the juveniles of the other bird species that were around. Can anyone tell what it is? 69.111.79.119 (talk) 15:47, 30 March 2012 (UTC)

What were the other adult birds you saw around? Coots?, grebes? My current best guess is Pied-billed_Grebe, note the distinctive dark patch on the beak. (Yes, I know the chick pictured in our article doesn't look much like yours... but this google image search shows how much the chicks can vary [6]) SemanticMantis (talk) 16:01, 30 March 2012 (UTC)

Heavy water plant

What is the purpose of the pipes running up the hillside at the Vemork plant? I am asssuming they have nothing to do with heavy water production: maybe connected with the factory's original purpose of fertilizer production? SpinningSpark 17:17, 30 March 2012 (UTC)

Its a hydro-electric plant. These are the pipes delivering the water from the reservoir to the turbines.--Aspro (talk) 17:36, 30 March 2012 (UTC)

That is, the Penstocks. DMacks (talk) 19:05, 30 March 2012 (UTC)

second part of question: distributing energy in the world economy via carriers

Guys and gals, thanks for the detailed responses to the first part of my question above. The second part was not addressed at all - could someone answer it? (About depletable/refillable energy carriers for the world economy akin to a biological ATP/ADP energy carrier analogue.) Please assume infinite energy is available at a single point on Earth only, and all these 'atp' analogues have to be routed there to be recharged. what would work in practice? If you have any other ideas for addressing my thinking in the second part of the question, or references, I welcome them. Thank you! 188.157.72.10 (talk) 18:29, 30 March 2012 (UTC)

"Transporting energy from a single point source" is a perfectly reasonable simplification of our present energy infrastructure. Sure, we've got multiple point sources, but you can generalize it as "one to many" and it makes sense. And we transport energy around. And that's without getting into "infinite" energy; once you posit that, you can do pretty much whatever you feel like doing, because waste becomes an utter non-factor. So yeah, stick the infinite energy thingamabob on the coastline, crack water into hydrogen, ship it, and burn it. Nice clean fuel cycle that'll power more or less anything. Note that we're not talking anything fundamentally distinct from my local swap-a-propane-can program that I use to run my grill, so I don't see the need to reinvent much terminology. — Lomn 18:38, 30 March 2012 (UTC)

If you want to transport large amounts of energy around the world, I suggest copper wires carrying electricity (your "ATP" could be converted into electricity first). They do lose a bit over the course of the transport, but, so long as you have an infinite energy supply, that's not a problem. This isn't currently done between continents, but I see no reason why oceanic cables similar to the ones which carry phone and Internet signals couldn't also transfer power. Laying all this cable would cost quite a bit, up front, but then we would no longer need to send oil tankers all over the world, endangering the environment, etc. Electricity is also the most useful form of energy for most applications. For those applications where we still need another form, like airplane fuel, it could be created using electricity, close to where it's needed. See synthetic fuel. Electricity would make more sense in cars than gasoline, if it was almost free. Each parking space could then charge you a few pennies to recharge your car as it sits. StuRat (talk) 19:32, 30 March 2012 (UTC)

having more rem dreaming or at least additive From-dream awakemenment

how could i get this?.. thanks!. 109.65.39.161 (talk) 19:39, 30 March 2012 (UTC)

You need to ask your question more clearly. I have no idea what "additive from-dream awakenment" is. See REM sleep for basics. I wonder if lucid dreaming is what you're really interested in. Note that we can't give you specific medical advice for treating some sleep related illness, but we can answer general questions about such an illness. Wnt (talk) 20:35, 30 March 2012 (UTC)

I believe most people dream about the same amount, the main difference is how much they remember their dreams. Keeping a dream diary is supposed to be a good way of learning to remember dreams. --Tango (talk) 20:48, 30 March 2012 (UTC)

March 31

Pupil Dilation

Is it true that one of the causes of the pupil's dilation is when the subject is romantically interested in another person with whom they are in contact?186.28.49.19 (talk) 00:49, 31 March 2012 (UTC)

See pupil dilation. The first paragraph says it "may indicate interest in the subject of attention or indicate sexual stimulation", and a scientific reference is provided. Relatedly, some cultures have believed that dilated pupils make a person more attractive, perhaps because it creates the impression of increased interest, see Belladonna_(plant)#Cosmetics. Indeed, Belladonna is so named because it was used to make a lady look beautiful. SemanticMantis (talk) 03:43, 31 March 2012 (UTC)

Back pain is common because we sleep on mattresses?

My personal experience and some thinking about this issue suggests that we're better off sleeping on the floor. When I bought a new mattress, I was adviced to get one of soft to medium firmness, these are supposed to be good for the back. I weigh 60 kg, and then a soft to medium mattress should give the best support. Firm mattresses are recommended for people over 100 kg, and super firm mattresses for people who are heavier than 160 kg. I ended up choosing one of medium firmness.

However, after several weeks sleeping on this expensive mattress, I got horrible back pains. I never had back pain before. Since my old mattress was a lot firmer than the new mattress, I was convinced that my back needs to rest on a firm mattress. I decided to sleep on my mattress back and the back pain was completely gone in a matter of a week.

Then, I still needed to buy a good mattress that would fit in my new bed, so I went back to the dealer, explaining the problem. A new super firm mattress was ordered. Unfortunately for me, these are also much more expensive than the medium firmness mattresses. The end result is that I now sleep on a super firm mattress, which also costs a super amount of money :( .

During the time I had back pain while sleeping on the medium matress, I noted the following. If you sleep on a hard mattress or lay on the floor, you can move your hand beneath the lower part of your back. But on a mattress of medium firmness, what happens is that this space is filled up by the mattress, so the mattress keeps your back in that position.

This is the only relevant difference that I noted, so it seems to me that the lower back should not be supported by the mattress. Gravity will pull your back a bit straight and that, I guess, allows the muscles in your back to relax. A medium firmness mattress will, by supporting the lower back in its naturally bent position, prevent the muscles from relaxing properly during sleep.

Now, mattresses are a recent invention, so one has to wonder if the fact that so many people complain about back pain has something to do with sleeping on mattresses. Surely, people living in the Stone Age did not suffer as much from back pain as we do today?

Now, if sleeping on super firm mattresses is indeed good for the back, then you could just as well sleep on the floor. That is only a little less comfortable, but much cheaper. And then I'm only 60 kg, if you weigh 100 kg or more, then a super firm mattress would still be way too soft.

So, should we all do away with beds and mattresses and sleep on the ground? Count Iblis (talk) 02:34, 31 March 2012 (UTC)

There is a problem with firm mattresses, though, in that they can cause pressure points. That is, the bits of flesh squeezed between your bones and the firm mattress can have too much pressure on them. This either results in constant turning, which interrupts sleep, or in damage to those areas, like bruises. It works out that lightweight people can typically tolerate a wider range of mattresses, while the obese may not have any firmness which doesn't cause one problem or another. For a nice compromise on firmness, I suggest a mattress directly on the floor, with no bed-spring underneath. The mattress can be a thin, soft one, making it inexpensive. This also avoids the occasional problem with squeaky springs. :-) Falling out of bed is also less painful, but getting up can be more of an effort.

Note that the underlying cause of back pain in humans is that we are bipedal, but with a spine designed for quadrupedal motion, which has been modified just enough to get it to barely work. Imagine a suspension bridge balanced on it's end, then shored up until it (barely) manages to stay upright. StuRat (talk) 02:43, 31 March 2012 (UTC)

About your last point, did bipedal animals like T-Rex have some special features preventing it from getting back pain that we don't have? Count Iblis (talk) 03:07, 31 March 2012 (UTC)

Yes, their spine wasn't upright like ours, but remained relatively horizontal, with a massive tail balancing against their head and torso. For birds like the ostrich, most of the spine is also horizontal, although it often turns vertical at the head. StuRat (talk) 03:14, 31 March 2012 (UTC)

From my experience, the common types of mattresses varies quite significantly from country to country. It's generally suggested that mattresses in large parts of Asia (e.g. China, India, Malaysia) are much more firm then those in countries like NZ, the UK and the US. And the construction of mattresses also varies (although there's also usually choice), e.g. springs with foam or latex support, all latex or all foam, a combination of latex and foam etc. (With various kinds of springs such as pocket springs and inner springs, foams and fibres.) There's also fancy stuff like memory foam. I don't really understand StuRat's suggestion, if you want to save money or prefer not to use a bed then fine, but if not, just choose a bed with a solid based if that's what you prefer. Also from my experience here in NZ, the recommendations when choosing a bed seem to vary from store to store and probably sales person to sales person, but weight doesn't generally seem to be a big consideration, rather personal comfort and preference as well as sleeping position and spinal alignment when laying down. Of course trying to work out how comfortable a mattress is from lying on it for a few minutes isn't that successful, but I guess it's better then nothing. (And I don't know if it's the same in the US but here most chains including bed shop chains seem to have exclusive mattresses, the range or at least names of the mattresses are completely different between chains even if the brand is the same. It sounds like there's a lot of dodgy stuff going on with warranties in the US [7].) Nil Einne (talk) 05:11, 31 March 2012 (UTC)

My point was that the bed frame, box spring, and mattress are only needed if you want a very soft bed. If you want a firm bed, much of that is unnecessary. Just a comforter on a shag carpet with padding might even be sufficient. But, if you want to spend extra money to be conventional, you certainly can. StuRat (talk) 05:20, 31 March 2012 (UTC)

I don't think it's just a matter of being conventional, having the bed too low down can make it difficult to get in and out of, particularly if you're old. Being so low down also makes engaging with other people in the room that aren't on the bed odd. Elevating the mattress may also allow stuff to be stored under the bed and makes cleaning easier (putting the mattress directly on the floor isn't going to completely stop dust and dirt going under it). In some countries the 'box spring' you keep speaking of is fairy uncommon, possibly present in fold out beds but little else. If you have flexible supports it's usually in the form of flexible slats, but in other cases, any sort flexible base is rare, when people do have mattress on the floor, it's primarily to save money, not because they think it's better. Nil Einne (talk) 08:33, 31 March 2012 (UTC)

If the box spring has a use I have yet to figure it out; I think it's basically cosmetic, a bustle for a piece of furniture. My gut reaction is that having beds raised off the ground at all is an elaborate contraption designed to keep people free from rats and thus the bubonic plague, but I'm unaware of evidence for this. But checking our article on bed it does mention that in Germany raised beds became popular around the 13th century as "luxury increased" (which indeed was a consequence of the plague, which relieved the earth of some of its burden). I suppose it's even possible that the change was accomplished by natural selection, with no conscious awareness of the reason! Wnt (talk) 14:06, 31 March 2012 (UTC)

To solve the issue of the top of the bed being too low, perhaps placing the mattress above a set of drawers would make most sense. This would dramatically increase storage area in the bedroom and not allow dust to accumulate underneath. (You wouldn't want handles that stick out, but rather indentations in the front of each drawer that could be used as hand-holds.) I've seen this approach done with waterbeds, but not with a regular mattress. I wonder why not. Is there a concern that bedbugs would have more places to hide ? StuRat (talk) 14:16, 31 March 2012 (UTC)

You haven't seen this done with a regular matress, Stu? In my (UK) experience it's very common. The base of the bed comprises a fabric-covered wooden frame (usually on castors) with 4 large recessed-handled drawers, two each side, occupying its full volume, and with a solid, fabric covered top about 15" off the floor on which any design of mattress matching its dimensions (which are standardised to the standard bed sizes) can be placed. (Single-bed versions may have only two drawers.)

The frame usually comprises two 2-drawer halves (head and foot ends) for ease of transport, which are firmly attached together in situ; the drawers are commonly used to store spare bed linen and clothes: bedbugs are not very common in the UK, and in any case I understand that they prefer crevices and cracks in walls and furniture rather than clothes, so this type of bed would not increase their prevalence; a bigger worry would be clothes moths, but one takes the usual elementary precautions against them as in any other clothes drawer or wardrobe.

I've been sleeping on such a bed for nearly 25 years (and for several years one of the family cats liked to sleep on my socks in the drawer immediately under my head end/side, which I therefore left open a few inches for her convenience).

Oddly, our main article Bed doesn't describe or name this exact type, but it's briefly described at the end of Platform bed as a "storage platform bed". Examples can currently be seen in any furniture or specialist bedding shop in the UK. {The poster formerly known as 87.81.230.195} — Preceding unsigned comment added by 90.197.66.209 (talk) 17:53, 31 March 2012 (UTC)

Yes, those beds are quite common. The flaw I've found with them is that you need the bedroom to be about twice as wide as the bed if you're going to be able to make full use of the drawers without needing to move the bed in order to get into them. --Tango (talk) 19:03, 31 March 2012 (UTC)

Well, it's only a flaw if you actually have a very narrow bedroom. Obviously, one should always choose furniture suitable for the room in which it's to be used. {The poster formerly known as 87.81.230.195} 90.197.66.209 (talk) 23:13, 31 March 2012 (UTC)

You can also get a storage bed where the mattress lifts up and the whole of the bottom of the bed becomes accessible. This would solve the problem of narrow rooms. --TammyMoet (talk) 08:25, 1 April 2012 (UTC)

Bed#Types of beds includes:

A captain's bed (also known as a chest bed or cabin bed) is a platform bed with drawers and storage compartments built in underneath.

Captain's bed (currently a redlink) may not be the more common name for this, but it is the one I am familiar with. -- ToE 01:11, 1 April 2012 (UTC)

Yes, I noticed that that was the nearest description in the Bed article to what we're talking about, but to me the term implies one side being flush against the wall (or bulkhead), rather than with drawers etc on both sides. When I went shopping today I meant to ask in a bedding shop what they call it, but it being Sunday that particular shop was closed. {The poster formerly known as 87.81.230.195} 90.197.66.103 (talk) 14:41, 1 April 2012 (UTC)

Silicone rubber lids

I have a nice set of cookware from Anchor Hocking. They are glass bowls suitable for storing, microwaving, and serving food. However, one deficiency is that they have plastic lids which warp if subjected to microwaving. This makes it necessary to remove the lids and place paper towels over the top to catch any splashes, and the paper towels sometimes fall into the food. So, I'm wondering why they didn't make the lids out of silicone rubber.

1) Is it prohibitively expensive ?

2) Does silicone rubber have a coefficient of thermal expansion that doesn't work with glass ?

3) I read that fillers used for silicone rubber can emit annoying fumes, would that be the reason ? Are there no fillers which don't do this ?

4) Also, could the lids be made of clear silicone rubber, or would they need to be colored ?

StuRat (talk) 14:28, 31 March 2012 (UTC)

I don't see why it couldn't be done. In fact, if I'm not mistaken, we (meaning my wife and I) have a lid made of silicone for a glass bowl. Dismas|^(talk) 14:41, 31 March 2012 (UTC)

Clear or colored ? Have you microwaved it with the lid on ? If so, were there any fumes ? StuRat (talk) 15:13, 31 March 2012 (UTC)

I looked in our cabinets but can't find the lid right now. But I do remember it being less rigid than plastic. It's blue in color and there were no fumes. When I microwaved with it, I popped the top to keep it from building pressure. But yes, I microwaved with it. Dismas|^(talk) 15:23, 31 March 2012 (UTC)

I have this silicone lid product [8], which has several uses. I use it on the stove top, in the microwave, and as a jar opener. I've never noticed any fumes, and it comes into contact with hot food all the time, yet does not stain or discolor. SemanticMantis (talk) 15:48, 31 March 2012 (UTC)

Interesting, it doesn't fit on a pot, but is placed directly on the food. This makes it seem more likely that there is some problem with getting it to fit, such as a different coefficient of thermal expansion than the pots. StuRat (talk) 06:34, 2 April 2012 (UTC)

Dow Corning claim you can have transparent silicone rubber[9] but it seems to be more commonly colored. --Colapeninsula (talk) 13:41, 2 April 2012 (UTC)

April 1

exponential power of 2 and number of ancestors 30 generations ago

I'm sure the answer to this is simple, but for some reason I just can't think of it.

A person has 2 parents, 4 grandparents, 8 great-grandparents, and so on. So it's 2 to the power of the number of generations. Yet it wasn't until 1800ish that there were a billion people on earth. All you have to do is go back 30 generations for 2-to-the-30th to equal a billion; that's at least 600 years, conservatively. So how could you have a billion great-great-great-to the thirtieth-power-grandparents, when there weren't that many people on earth?

I will feel really stupid when someone explains this, because it must be obvious.

It's March 31st where I am, so, no, this isn't some kind of April Fool's trick.76.218.9.50 (talk) 00:48, 1 April 2012 (UTC)

Inbreeding. At some point in the past, the same person occupies multiple places in your family tree; i.e. one man is your great-9x-grandfather multiple ways (i.e. you decend via his son on one side of your family, and via his daughter on the other side). The further back you go, the more likely this is to happen. It is unlikely that this would work for 3-4 generations, where people generally know each other and try to avoid marrying close cousins. But when you get back 9-10 generations, there's no way that you know who you are related to, so it is quite likely for your parents to have shared some many-centuries-old ancestor, likely several. --Jayron32 00:52, 1 April 2012 (UTC)

Actually, I thought of this right after I posted -- by taking the question from the other direction. I thought, let's say in year A.D. XXXX there were 100 million people on earth. By definition, everyone alive now descends from those 100 million; yet there are many times more "slots" in the family tree than 100 million; therefore, many people occupy many different slots in the genealogical table. Somehow, this is still pretty mind-boggling, but it basically comes down to lots of people marrying their own (say) third- or fourth- (or tenth- or (gasp) first-)cousin. It would be interesting to know at what point the number of people on earth equaled exactly as many as, on average, we all have "slots" in our tree AS OF that date; clearly, it was sometime between 30 generations ago and 1800ish. That would be the tipping point toward less fourth-cousin-marriage, wouldn't it?76.218.9.50 (talk) 01:02, 1 April 2012 (UTC)

Jayron, your link led me to Pedigree collapse. Very interesting!76.218.9.50 (talk) 01:07, 1 April 2012 (UTC)

The classic example of pedigree collapse is Charles II of Spain, who I believe only had two great-great-grandparents (i.e. all 16 possible positions in the family tree are taken up by one man and one woman.) --Jayron32 01:09, 1 April 2012 (UTC)

Family tree of Charles II of Spain

Our article says "The inbreeding was so widespread in his case that all of his eight great-grandparents were descendants of Joanna of Aragon and Duke Phillip of Austria." That doesn't mean he only had two great-great-grandparents, though. His inbreeding involved a lot of uncle-niece marriages, rather than just cousin-cousin marriages, so the generations are all messed up. Joanna and Phillip will have occupied a lot of slots, but they won't all have been in the same generation (they were his great-great-great-grandparents and his great-great-great-great-great-grandparents, I haven't checked the other lines). This is his family tree. --Tango (talk) 01:44, 1 April 2012 (UTC)

In otherwords, FUBAR. --Jayron32 01:50, 1 April 2012 (UTC)

(edit conflict) All his eight great-grandparents were descendants, not children, of Joanna of Aragon and Duke Phillip of Austria; in fact, none were their children, and Charles II had 14 distinct great-great-grandparents. By my best count, Joanna of Aragon and Duke Phillip of Austria did each occupy 14 positions in his family tree: as 2 of his possible g-g-g-grandparents, 6 of his g-g-g-g-grandparents, and 6 of his g-g-g-g-g-grandparents. (Is there an easy of doing this, say by counting the cycles in the tree?)-- ToE 02:17, 1 April 2012 (UTC) (corrected from 2, 6, 5, & 1. -- ToE 02:35, 1 April 2012 (UTC))

You might also enjoy most recent common ancestor. Dragons flight (talk) 01:10, 1 April 2012 (UTC)

If you go back 29 generations, you have 2^29=537 million "slots". If we assume 20 years per generation (ie. women on average have children when they are aged 20, which is a fairly standard assumption and is about right until recently), that is 29*20=580 years ago, or about 1430. According to World population was around 450 million in 1400, but dropped significantly due to the Black Death and didn't recover for 200 years. That means the world population was more than 2^28 and less than 2^29 for around 300 years or so, which means 29 generations is the closest we'll get. I don't think there is anything particularly significant about that, though, especially since that 450 million people includes people in multiple generations - you have to fill slots in the 29th, 30th and some of the 31st out of people alive at that time (and don't forget generations will be different lengths in different parts of your family tree, so someone 20 generations above you and someone 25 generations above you could easily have been alive at the same time). It's an interesting bit of trivia, but that's all! --Tango (talk) 01:30, 1 April 2012 (UTC)

Also significant is that, until quite recently, there was very little mixing between populations. The actual number of generations to actual pedigree collapse needs to be adjudged within a specific population, and not across all humanity. Until fairly recently, for example, it would be unlikely that an Australian Aborigine and an Aztec would have any chance to share an ancestor closer than about 10,000 years to either; so any calculations made across all humanity likely overshoot the actual result by many generations. --Jayron32 01:40, 1 April 2012 (UTC)

I suspect the average generation is greater than 20 years, since women tended to have lots of kids, so their "average" age of childbirth would probably have been somewhat higher. But to return to my question: "It would be interesting to know at what point the number of people on earth equaled exactly as many as, on average, we all have 'slots' in our tree AS OF that date; clearly, it was sometime between 30 generations ago and 1800ish. That would be the tipping point toward less fourth-cousin-marriage, wouldn't it?" It is interesting to think that the "coefficient of inbreeding" (so to speak) would be decreasing with each passing moment from some date in the past, particularly since the invention of the car plus decent roads. This must have some sort of implication greater than trivia? And particularly so after a few more centuries? A good thing, I'd guess, for those of us who love our common race's diversity.76.218.9.50 (talk) 01:57, 1 April 2012 (UTC)

Pedigree collapse is indeed the term that's used. If you know your tree back a ways, you may be surprised at how many instances you'll discover of first-cousins marrying, for example - a common occurrence, at least until around 1900 when genetic issues finally came to be noticed. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:14, 1 April 2012 (UTC)

I was kidding when I used the phrase "coefficient of inbreeding" above -- but it turns out that's exactly what it's called! (Talk about successfully faking it.) Check this out, about Charles II of Spain -- an article that should be added to several different WP articles, but I'm too lazy: http://blogs.discovermagazine.com/gnxp/2009/04/inbreeding-the-downfall-of-the-spanish-hapsburgs76.218.9.50 (talk) 02:26, 1 April 2012 (UTC)

From that link: Charles II's coefficient of inbreeding was (F = 0.254), where the higher the number, the more inbreeding. "What [you are] doing here is summing up through all of the distinct paths to common ancestors. You weight this by the number of individuals between the common ancestor and the individual whose inbreeding coefficient you are calculating (note that, for example, some of Charles’ lines up ancestry back up to his common ancestors have different numbers of generations). Finally you have to include in the inbreeding coefficient of that common ancestor. Let’s play this out for a brother-sister mating. Assuming that the grandparents, of whom there are only two, are unrelated. So FA = 0. Then, it simplifies to: FI = (0.5)3 X ( 1 ) + (0.5)3 X ( 1 ) = 0.25 In other words, Charles II was moderately more inbred than the average among the offspring from brother-sister matings!" Please, somebody, start the inbreeding coefficient article! (And again, this isn't a joke -- just a coincidence; I didn't notice the April 1 thing till after I first posted above, and the link is a real one.) Start here!: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005174 76.218.9.50 (talk) 02:38, 1 April 2012 (UTC)

I don't think anyone needs to start a new article: see Coefficient of relationship - Nunh-huh 03:14, 1 April 2012 (UTC)

... to which Inbreeding coefficient redirects. -- ToE 03:19, 1 April 2012 (UTC)

Well, thanks for patronizing me ... that's always fun. But where exactly in that article do you find THIS?: FI = Σ (0.5)i X (1 + FA) 76.218.9.50 (talk) 03:28, 1 April 2012 (UTC)

Nowhere, until you add it. Wikipedia never gets better until you make it better. --Jayron32 15:24, 1 April 2012 (UTC)

Our current theory of Common descent basically means that if you go back far enough, all life can trace its ancestry back to just one common progenitor. That article actually touches on the OP's question. Vespine (talk) 04:48, 2 April 2012 (UTC)

fine structure

why is it important to know the fine structure of an element when its wavelengths are unique to identify it. — Preceding unsigned comment added by 197.255.118.206 (talk) 01:30, 1 April 2012 (UTC)

Fine structure has technical details, but basically it allows one to probe the detailed organization of the electrons in an atom, in general all of the quantum numbers more specific than the principal quantum number. In other words, though you can identify the element by its gross structure, you can know how the electrons of that specific element are organized via the fine structure. Fine structure allows one to know specific information about a specific state of that element, for example, to distinguish between triplet oxygen and singlet oxygen. --Jayron32 01:32, 1 April 2012 (UTC)

In Situ Generation of Acid Catalyst in Esterification

Hello. Why does the acid catalyst in esterifications need to be generated in situ? If I directly add the acid, what side reactions can occur? Thanks in advance. --Mayfare (talk) 03:58, 1 April 2012 (UTC)

It doesn't need to be generated in situ. What makes you think it does? 203.27.72.5 (talk) 04:18, 1 April 2012 (UTC)

(edit conflict) Most acid catalysts used for things like Fischer esterification are common mineral acids, like sulfuric acid. Those acids are significantly hygroscopic to the point where the water they bring to the party is enough to mess up the slow, and highly equilibrium dependent, esterification process. Remember that ester hydrolysis directly competes with esterification: they both function in acidic environments, and the factor which controls the equilibrium shift is the relative amounts of alcohol to water; too much water and the reaction system won't produce good yields of ester. Indeed, in esterification you need to find ways to deal with the fact that it produces water as a byproduct, and that water itself has to be dealt with. Introducing additional water that comes with, say, the water always present in a bottle of sulfuric acid which is sitting on the shelf (so-called "concentrated sulfuric acid" is 98% acid and still has 2% water in it; lower water concentration spontaneously lose excess SO3 until the concentration settles back to a 98/2 ratio). You can use enough dessicant in the reaction to deal with the produced water, and I suppose that can also deal with the water present in the acid; but generation in situ removes one of the sources of water, which helps drive the reaction to make more product more efficiently. --Jayron32 04:22, 1 April 2012 (UTC)

Considering how exothermic the reaction of concentrated sulfuric with water is, I doubt using sulfuric acid is going to add any significant free water to the reacting mixture. According to Esterification#Preparation, sulfuric acid helps the reaction by sequestering water. 203.27.72.5 (talk) 04:51, 1 April 2012 (UTC)

Indeed. Good point. --Jayron32 04:53, 1 April 2012 (UTC)

So coming back to the OP's question, you can use externally sourced acids provided they are either also a dehydrating agent or aren't in aqeuous solution. Sometimes those acid aren't suitable e.g. if acid sensitive functional groups are present, so in that case you can get a mild acid without adding any aqeuous solution by in situ generation. If you want to get an idea of the side reactions, look at the reactants' functional groups. 203.27.72.5 (talk) 05:57, 1 April 2012 (UTC)

Say I had a simple carboxylic acid and a simple alcohol. --Mayfare (talk) 17:08, 1 April 2012 (UTC)

Regarding Common Emitter BJT configuration.

I recently came across a paragraph in an engineering textbook regarding BJT transistor configuration. According to the text, when a BJT is in common emitter configuration, and operating within the active region of the output characteristics curve , the current at the output or collector terminal is not affected by any resistor placed in series with the output. Instead, the collector current is only affected by a change in the current at the base terminal. However, according to my logic, if the resistance at the collector is increased to a large enough value or a small enough value, the collector current should change. I am really confused on this one and I just don't know how changing resistance level of a pathway doesn't change the current in the pathway. Please help. — Preceding unsigned comment added by 210.4.65.52 (talk) 09:11, 1 April 2012 (UTC)

In the active region, the transistor collector is a high impedance terminal and thus acts like a current source. The collector current will be relatively constant (assuming Vbe is constant) as long as the collector resistor does not go too high. The collector resistor and the output resistance at the collector (in h parameter terms: hoe) can be considered to be in parallel as far as the signal is concerned so there will actually be a division of current between the two resistances, but as long as Rc is small compared with hoe, essentailly all the current from the internal current source will flow through the load resistor. If the load resistor goes high enough, then the transistor will enter the saturation region. In the case of very low resistances down to zero, most of the current from the internal current source will go through the load as it is much smaller than hoe.--92.28.78.70 (talk) 13:26, 1 April 2012 (UTC)

FYI, the above user 92-something has been blocked as a sock of a banned user. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:47, 1 April 2012 (UTC)

Is the information he provided accurate? It sounds plausible if memory serves. Edison (talk) 00:09, 3 April 2012 (UTC)

Banned users are not allowed to edit, regardless of the alleged quality of their edits. In the case of 92-something, he won't be back under the current address for awhile. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:05, 3 April 2012 (UTC)

Canning timing

Hi. I'm making a mango-lime chutney. The recipe fills 8 jars. When it comes to the processing part of the recipe, I can only fit 5 jars into my big pot of boiling water. The processing takes 15 minutes. I need to find out if it's ok to fill the other three jars at once, then wait 15 minutes for their turn to process, or wait and fill and process them, or something else. I need a source that answers this question. Thank you for your help.

I would fill 4 jars, process them and keep the chutney warm while that's happening. Then process the others. However, I wouldn't bother processing a chutney mix. When I make chutney, I sterilise the jars using a microwave method and fill them, seal and store. The vinegar and sugar in the mix preserves and prevents mould forming. My guide in all things preserving is Marguerite Patten's books. You must have a recipe there for your chutney: if it's in a book, what does the book say? --TammyMoet (talk) 12:43, 1 April 2012 (UTC)

Thank you for your advice. I appreciate that it is sourced. I have no book; the recipe was in my aunt's papers.

I would appreciate any other sources that I could read. Thank you again. — Preceding unsigned comment added by 184.147.123.69 (talk) 17:53, 1 April 2012 (UTC)

This might help, looks worth reading the linked page and other linked articles. --TammyMoet (talk) 20:09, 1 April 2012 (UTC)

Thank you for your efforts! It seems this is not an issue that comes up often. Maybe other people have bigger pots. — Preceding unsigned comment added by 184.147.123.69 (talk) 11:43, 2 April 2012 (UTC)

Metallicity of stars

Will the increasing metalicity of stars mean the formation of earthlike planets, and eventually life, is inevivitable.? — Preceding unsigned comment added by 99.146.124.35 (talk) 14:27, 1 April 2012 (UTC)

It has already happened once. Depending on how you read the Drake equation, it is may have already happened many times. --Jayron32 15:22, 1 April 2012 (UTC)

Sound c ards

Are we really to believe that an ordinary sound card costing say $100 with some cheap spectrum analyser software, can out perform an expensive Audio analyser like the Audio Precision 1?--92.28.78.70 (talk) 16:24, 1 April 2012 (UTC)

As always, one needs specific data and details of the experiment to support any claim. Look at exactly what was compared and what the specific results were to see if "out perform" is a valid conclusion or is specific to certain types of performance in certain situations. But note well that cost is not a good measure of quality (or more precisely, that only expensive things can be good or that cheaper things cannot also be good). To a certain approximation, RedHat's made a fortune selling software that is available for free:) DMacks (talk) 16:57, 1 April 2012 (UTC)

Another important point to keep in mind: audio electronics are quite simple, compared to other domains of electronic engineering. Audio signals have very narrow bandwidths - only a hundred kilohertz will provide very sufficient oversampling. The signal center-frequency is at 0 hertz... when digitized, a low bit-rate is sufficient... even in the analog domain, the signal to noise ratio need not be any better than the perceptual noise sensitivity of the human ear - only maybe 40 dB (about 10 or 12 bits, for the digitally-minded; though CDs and others use 14- or 16-bit samples)... so from this standpoint, every specification is trivial to design. It doesn't take cutting-edge analog or digital hardware; and with today's computer horsepower, even a naively-implemented software can trivially process the entire data representation of the human audible range. A general-purpose, low-end $200 oscilloscope will probably outperform an "audio analyzer" on most technical specifications.

The "value-add" of professional audio equipment tends to come in the form of other intangible goods: reliability; brand-recognition, and the consequent perceived status for the bearer. In the case of audio processing systems, usability and user interface may be better on a high-end system. While I know how to perform any arbitrary signal processing on my audio-card in C, most people find the process a bit convoluted, and prefer user-interfaces with things like sampling loops, sliders, and one-button effects processors. So, there's a market for expensive audio hardware. Even I have been known to spend more money than I should on a tube-amp, whose complicated, "nonlinear" analog system-response is easily modeled by a cheap DSP; I bought my tube-amp "because it was awesome," not really because its tone was any better or different than my mini DSP-driven practice amp. It's 2012; the MOS transistor is, for all purposes, perfectly ideal in the kilohertz range; and the microprocessor can run arbitrarily complex software on each sample; so these facts really invalidate the old-fogey claims about tube tone quality. The same logic applies to sound-cards. Nimur (talk) 17:50, 1 April 2012 (UTC)

FYI, the OP has been blocked as a sock of a banned user. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:44, 1 April 2012 (UTC)

Dogs who howl at the phone

Why do dogs sometimes have a tendency to howl when the phone rings? 98.235.166.47 (talk) 22:14, 1 April 2012 (UTC)

Because it startles them. --Jayron32 22:55, 1 April 2012 (UTC)

Cats and dogs have much better hearing than humans, and as annoying and ear-piercing the phone may be to us, it can be downright scary to the animal whose ears it has assaulted. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:45, 1 April 2012 (UTC)

According to an urban legend, some dogs who are tied to (or standing in a puddle of water adjacent to) telecommunications poles, will bark or yelp when the phone is ringing to indicate an incoming call, because the electricity involved stimulates them in a painful way. In theory, this should only be the case if the telecommunications equipment is faulty or unsafe in some way. --Demiurge1000 (talk) 23:50, 1 April 2012 (UTC)

I have a cat who hates when my wife's cell phone is on speaker. I had a dog who went absolutely berserk if the smoke alarm went off. There's something about those sounds that the animals react to. Humans have similar reactions such as fingernails on a chalkboard. Some people hate while it doesn't matter to other people. Dismas|^(talk) 10:56, 2 April 2012 (UTC)

As this is a Reference Desk, here is a reference from Canada's Pet Information Centre; "Sometimes dogs will howl when they hear sirens or other loud higher pitched sounds like clarinets and flutes. These sounds may even come from a television set. Dogs do this as an instinctive response to hearing what they interpret to be another howl (dog in the distance). They are not doing this because it hurts their ears." This site, ALL ABOUT DOGS and CATS - Resource Center for Canine & Feline Lovers, agrees; "Howling may be triggered by sirens, singing or other noises the dog finds similar to howling, says Dan Estep, Ph.D., a certified applied animal behaviorist in Colorado and co-author of Help! I'm Barking and I Can't Be Quiet (Island Dog Press 2006)". Alansplodge (talk) 19:04, 2 April 2012 (UTC)

Subjective biking effort

Is there any rough metric of the subjective effort required to bike X meters up a slope of Y degrees? From experience, I know the metric must be highly nonlinear in Y, because a 20 degree slope feels much more than twice as hard as a 10 degree slope. --140.180.39.146 (talk) 22:36, 1 April 2012 (UTC)

Subjective effort is unique and subjective, by definition. It sounds like you want us to quantify your unique experience. Simple trigonometry can be used to calculate the effect of different slopes on the objective (i.e. actual) force you need to exert in order to move your bike a certain speed. It sounds like your asking for the effect on how hard it feels like to you; quantifying your "feelings" is only something you can do, since this is a highly individualized effect. --Jayron32 22:54, 1 April 2012 (UTC)

It's partly a function of gearing. My experience is that with a normally geared road bike, I can handle a slope of up to 6 degrees with the same effort as flat riding, by going slow enough. If the slope is steeper, I can't go slow enough to equal the effort level of flat riding without having trouble staying upright. A slope of 10 degrees is hard work, and 15 degrees takes major fitness to sustain for more than a few minutes -- 20 degrees is deadly. With a mountain bike, though, the equation changes, because the gearing is different and it's easier to stay upright. Looie496 (talk) 00:31, 2 April 2012 (UTC)

The question as you posed it, "the subjective effort required to bike X meters up a slope of Y degrees", can't be answered, because said subjective effort depends on speed and gearing. A much more answerable question would be, what is the subjective effort (or time to exhaustion) required to bike at a cadence X RPM at the power output of Y% of your "functional threshold power".

You can compute the power output needed to go at a given speed up a given slope using this calculator. Your functional threshold power can be estimated using a stationary bike or the same calculator. For an untrained adult male it should be on the order of 150 W. At high percentages of FTP, subjective effort is minimized at a cadence of 70 to 90 RPM. Power output below 100% of FTP at a cadence in this range can be sustained for many hours (essentially, until you deplete your carbohydrate reserves.) If the cadence is too low or the attempted power output is too high, you eventually run into exhaustion.

Road bicycles are typically geared in such a way that an untrained, non-overweight rider can pedal on the grade of up to ~5% without getting out of the optimal region. Any steeper than that, and there's no gear low enough to maintain even 70 RPM. Mountain bicycles have more generous gearing.--Itinerant1 (talk) 08:39, 2 April 2012 (UTC)

April 2

What is the effective population size of the United Kingdom?

And what about the world? Anyone who fails to read the question properly and gives me the population will be shot. Egg Centric 01:19, 2 April 2012 (UTC)

Maybe you could read the cited article and provide the answer yourself? :) ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:15, 2 April 2012 (UTC)

I doubt it. I seriously doubt that the data exist to allow the question to be answered. Even if that's wrong, the answer would require fancy analysis of genetic frequency databases for the UK population. Looie496 (talk) 03:11, 2 April 2012 (UTC)

The data is there, someone just needs to crunch it. The number is much lower than you'd expect. Tenesa et al (2007) estimate the effective population size of the entire non-African human population at 3100 individuals.--Itinerant1 (talk) 08:48, 2 April 2012 (UTC)

I bet it's low for the UK. ;-) Ssscienccce (84.197.178.75) (talk) 20:26, 2 April 2012 (UTC)

The standard stereotype is that everyone in the UK is gay. How they manage to reproduce at all is downright miraculous. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:50, 3 April 2012 (UTC)

Glow leak

How do you break open a glow stick? I couldn't find any information in Wikipedia's article about breaking open a glow stick. Also—when broken—are the chemicals toxic? If so, in what way (e.g., when touched, inhaled, swallowed, etc.)? 71.146.8.88 (talk) 06:08, 2 April 2012 (UTC)

From the article you've already linked to....

"Glow sticks contain hydrogen peroxide, and phenol is produced as a byproduct. It is advisable to keep the mixture away from skin and to prevent accidental ingestion if the glow stick case splits or breaks. If spilled on skin, the chemicals could cause slight skin irritation, swelling, or, in extreme circumstances, vomiting and nausea. Some of the chemicals used in older glow sticks were thought to be potential carcinogens.[12] The sensitizers used are polynuclear aromatic hydrocarbons, a class of compounds known for their carcinogenity."

So, I wouldn't recommend breaking one, but if you must, it's presumably just a matter of brute force, or a sharp blade. HiLo48 (talk) 06:19, 2 April 2012 (UTC)

Put it in a sealed plastic bag, on the sidewalk, and pound on it with a hammer, I suppose. This should minimize your exposure. Wear goggles, just in case. StuRat (talk) 06:31, 2 April 2012 (UTC)

Thanks. 71.146.8.88 (talk) 02:17, 3 April 2012 (UTC)

Thanks. 02:10, 3 April 2012 (UTC)

Why would we provide information on how to break something that A) isn't meant to be broken, B) is potentially hazardous if broken, and C) is easily broken open by any sharp kitchen knife? We don't provide instructions on how to cut open a baseball to see what is inside that... Dismas|^(talk) 10:52, 2 April 2012 (UTC)

Exactly Dimas. I'm amazed such a trollish question was answered Richard Avery (talk) 15:30, 2 April 2012 (UTC)

Seriously? Now people are accused of trolling for asking how to break open a glow stick? What's next - accusing OPs of trolling for wanting to do anything you haven't done before and don't personally see a reason for? --140.180.39.146 (talk) 16:36, 2 April 2012 (UTC)

I didn't say they were trolling. I just didn't (and still don't) understand why we would cover something like that in an encyclopedia. Dismas|^(talk) 19:11, 2 April 2012 (UTC)

I've opened maybe 100 glowsticks, use a razor blade or other sharp knife, cut off the top 10 mm, careful not to break the glass tube inside. Then turn around, hold above container, cut other end, once air can come in, the liquid and the glass tube will slide out. Don't do it above the container holding all the (colorless) liquid, one glass tube breaking will ruin all of it. The glass tubes are really thin an break easily. As far as I know, the description given in the article does not fit current glowsticks, there does not seem to be any reaction with hydrogen peroxide (tested up to 30% concentration). Also, I doubt here in the EU carcinogenic liquids would be allowed in childrens toys, and that's what they are, according to te packaging. Not posonous, not for children < 3 years. "Can cause discolorations on clothing and furniture." Now that clearly talks about the inside! I complained on the talk page about the inaccuracies, seems no one knows what's in todays glowsticks.... Ssscienccce (84.197.178.75) (talk) 18:25, 2 April 2012 (UTC)

May I ask in what way was my question “trollish”? 02:11, 3 April 2012 (UTC)

Thanks. 02:12, 3 April 2012 (UTC)

Why did you open 100 glowsticks ? StuRat (talk) 18:50, 2 April 2012 (UTC)

Wanted to make something out of it. Under UV light the glass tubes light up brightly like thiny neon tubes; you can mix the chemicals and put them in other stuff (with a syringe), heating the mix gives really bright light, the empty glass tubes can be bent in whatever shape you want by heating them with a lighter and filled with the mix (looks much better than glow sticks because the glass is transparent, not just translucent)... Just messin' around, you know... :-) Ssscienccce (84.197.178.75) (talk) 19:59, 2 April 2012 (UTC)

Heh, that sounds quite fun, to tell you the truth. :) (I noticed that you use emoticons.) 02:13, 3 April 2012 (UTC)

100 glowsticks!!! Then you will love this [10]. I'm just glad your not my next-door neighbour – no offence :-)--Aspro (talk) 21:16, 2 April 2012 (UTC)

The nurdrage vids are quite entertaining :-) I used to enjoy chem vids on youtube, after Myfanwy94 I had enough for a while... Ssscienccce (84.197.178.75) (talk) 01:34, 3 April 2012 (UTC)

Having just had a quick look at Myfanwy_94 I'm now hoping we live on different continents. Hope you don't have any access to glow-in-the-dark radio active isotopes – no ! don't tell me, or I wont sleep tonight ;-) --Aspro (talk) 01:52, 3 April 2012 (UTC)

Myfanwy94 died, probably from pulmonary edema. I think he was 16 :-( Ssscienccce (84.197.178.75) (talk) 03:27, 3 April 2012 (UTC)

Totally OR, but when I accidentally ate a naan bread with glowstick juice on it (long story but basically one of my housemates had decided to paint our kitchen with them, rave-style) I spent the next 24 hours throwing up. I'm assuming the two are linked, though it's always possible it was a coincidence. I didn't die at least, so unless I was very lucky, the lethal dose for the stuff is very high. Smurrayinchester 22:15, 2 April 2012 (UTC)

Very high?! °O° How much did you ingest then  ?? Ssscienccce (84.197.178.75) (talk) 03:27, 3 April 2012 (UTC)

Um, I would think this is obvious **but if you almost died ingesting glowstick stuff please at least ASK a doctor if you're going to be alright**. Not here. Call up a doctor, any doctor, and ask. Tell them how sick you were and for what period of time and why. Toxicity is not just instant death, and the adage "anything that doesn't kill you can only make you stronger" is obviously false, as people die from wounds later on all the time. 94.27.233.109 (talk) 18:46, 3 April 2012 (UTC)

Well, the adage doesn't say it has to kill you immediately. But, some things which don't kill people definitely leave them weaker, such as the quadriplegic. StuRat (talk) 20:39, 3 April 2012 (UTC)

Load in motor and current

Hi guys, one of the students I homework help got this (quite confusing, in my opinion) question for their test. I must admit I'm having a bit of trouble with it. Electricity was never my strong suit, unfortunately.

"A DC motor with a load attached is at full rpm. A voltmeter attached to the motor gave a reading of 400V. The load is then detached and a reading of 415V is then observed. Explain the change in voltage. In which case will the current be higher or lower, and explain."

So I think that the change in voltage would be caused by the back EMF. But it wouldn't explain why the drop is so low. "Load" is an ambiguous term to- does it have an innate resistance? I thought for a while and I'm just confusing myself now.

For the second part, wouldn't the back EMF in both cases limit EMF to close to 0? So torque 0, and by t=nBIA since B, A, constant, I=0 in both cases. Or have I just completely forgotten my high school physics somewhere. But then I think of Power in case 1=Power in case 2, so VI case 1 = VI case 2, so the 400V one would have a higher current. Then I think of V=IR and a load would give more resistance. Yeah I'm confused.

Sorry for this huge mess of random musing, it's late and my brain is tired. Hopefully I can get a definite answer

Thanks in advance, 27.32.104.185 (talk) 12:41, 2 April 2012 (UTC)

An ideal voltage source has zero internal resistance, so you wouldn't measure any difference between load or no-load operation. Since no source is really ideal, you measure some voltage drop when load is attached, caused by the current flowing through the resistance of the power source... Ssscienccce (84.197.178.75) (talk) 18:41, 2 April 2012 (UTC)

This is obviously a homework question. So I don't think we will answer it. As a clue though. This is inductive not a 'pure' resistive load. So it not right to think V=IR but V= back EMF + I R. Once the load is taken off the back EMF drops and thus the kilo-wattage drops. So you have to add something back to the other side of the equals sign. That's the terminal voltage returning upwards towards the supply voltage at open circuit less the voltage drop due to the conductance through the motor... Depriving oneself of sleep it not a good thing.--Aspro (talk) 19:56, 2 April 2012 (UTC)

"Once the load is taken off the back EMF drops and thus the kilo-wattage drops" : that seems to fit with $P=I\cdot V_{cemf}$ in Brushed DC electric motor, but in my opinion, that formula is wrong; back-emf rises so P drops, I think ... Ssscienccce (84.197.178.75) (talk) 20:21, 2 April 2012 (UTC)

That article's talk page is the place to query that formulae. --Aspro (talk) 21:06, 2 April 2012 (UTC)

Thanks for the responses. I did say it was a homework question, and I agree that the resistance of the load would cause the voltage drop. But I don't understand why "once the load is taken off, the back EMF drops". The second case without the load is still at maximum rpm, so wouldn't the back EMF almost cancel out the input? Also, how do you know the kilowattage drops (is this just power)? 27.32.104.185 (talk) 22:17, 2 April 2012 (UTC)

Assuming you've read through the various article and your mind is still 'stuck' I will have a go at explaining it verbally – but be warned – over simplification can often lead to buggering the whole thing it up. So lets take your second question first: “The second case without the load is still at maximum rpm, so wouldn't the back EMF almost cancel out the input?” In a perfect motor, with no bearing friction, wind shear dragging the rotor, and an infinite amount of poles; the conductors will not be cutting any lines of force, as they will be in perfect synchronization with the magnetic fields – so there will be no back, forward or any other EMF. OK so far? As soon as some load is applied , the rotor retards (with regard to the fields) and some EMF is induced in the conductors. Yet, the more it retards the more each brush cover more than one commutator section. Now the motor winding has current flowing through more than one winding – in other word there is more copper to carry the current – more conductance – more 'adjacent' magnetic flux to attract the poles – i.e., more juice going through to drive it round - more juice less voltage (and if some now asks about 'Q' at this point, I shall scream because were talking DC but Henry's are OK because that's valid). Also, due to the retardation, back EMF is now produced due to them cutting lines of force as they are now out of perfect synchronization. The formula’s explains the relationships far as the units of V, I, R, P etc. are concerned but they don't explain how these ruddy things goes round and round. --Aspro (talk) 23:31, 2 April 2012 (UTC)

The question's phrasing is a bit vague and puzzling. "Full RPM" might mean a motor running at its rated speed with nominal voltage and no load, as opposed to the highest RPM at which it could turn without self-destructing when run at a very high voltage and minimal load. If you then apply a load to a DC motor, the speed typically drops, along with an increase in current, So perhaps the question writer meant that the motor is running under nominal conditions: rated voltage, rated load, rated speed. The question then says the load is "detached," so the motor goes from some load to no load other than air resistance and bearing friction. I would expect its speed to increase compared to the initial condition while the current decreases. The voltage measured at the terminals increases from 400 to 415. The loaded voltage was 3.61% less than the no-load voltage. This change would be due to the source impedance (in this DC case, the source resistance) of the battery, power supply, or generator plus the resistance of the power system and conductors connecting it to the motor, assuming that there is no voltage regulation at work to increase and decrease the supply voltage. Why all the discussion about "synchronization" and "impedance?" Direct current was specified. DC is not AC, although sometimes pulsating unidirectional power sources are loosely termed "DC." What is to be "synchronized" if the energy is supplied by a battery or its equivalent in the form of a good power supply power supply? Edison (talk) 00:01, 3 April 2012 (UTC)

? If your replying to me, If the speed increases if the load is taken off, isn't that the same thing as the 'rotor retarding' when load is applied? You don't seem to have read my posting properly. I pointed out that we're talking about DC. The synchronization I spoke of is in a 'perfect motor' where the angle of the live commutator segments are in phase (match) with the sum of the vectors of the magnetic field at max speed. What has AC got to do with that? Also I don't remember bringing in impedance – were talking DC – as you pointed out.--Aspro (talk) 00:50, 3 April 2012 (UTC)

The comment was a general one on the thread. "Synchronization" with respect to motors and generators generally implies AC , and seems meaningless when the source voltage could be constant, as from a battery. Maybe the term was used in your classes on DC motors, but I do not recall it being an aspect of DC motor operation. With AC there are synchronous motors and induction motors which run under load at a lower RPM than a synchronous speed would indicate. Synchronization of AC generators is a job to be done when one is brought online, but is pretty meaningless with DC generators. Edison (talk) 02:09, 3 April 2012 (UTC)

Are you being recalcitrant for the sake of it? This has nothing to do with AC motors.--Aspro (talk) 02:49, 3 April 2012 (UTC)

Since it is not related to AC, there is little reason for you to talk about "synchronization." Edison (talk) 14:39, 3 April 2012 (UTC)

Thanks Aspro and Edison. I have read through the responses and although I don't totally get it, (never did physics past high school), I think I understand most of it now. But I find it quite surprising that they would this confusing question in an exam for high school students. It's hard to believe how you can explain it in a paragraph. In regard to the 'perfect motor' with infinite poles, I think their syllabus only covers simple DC motors (i.e. two N/S magnets and a square coil with power), so I don't think that was the answer they wanted- so there will be a change of flux and induced EMF. And I do think the 'full RPM' needs to be disambiguated. Now I'm really interested in seeing their answers. 27.32.104.185 (talk) 10:56, 3 April 2012 (UTC)

You wrote Load is an ambiguous term too - does it have an innate resistance? I was puzzled by this. In the question, load is not referring to an electrical load - it is referring to the mechanical load being driven by the DC motor. For example, in an automobile the windshield wiper is operated by a DC motor. The load is the reciprocating mechanism plus the resistance to motion as the wiper blades move back and forth across the windshield. Many variable-speed cranes and large machine tools such as lathes and boring machines have DC motors. Dolphin (t) 02:34, 4 April 2012 (UTC)

fine -structure

can the value of the fine structure of an atom placed in an external magnetic field be the same as the value measured in the absence of an external magnetic field ? What is the literature value of the fine-structure(splitting) of the mercury yellow spectral line ? — Preceding unsigned comment added by 197.255.118.206 (talk) 16:54, 2 April 2012 (UTC)

Please do your own homework.

Welcome to the Wikipedia Reference Desk. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. Nimur (talk) 19:36, 2 April 2012 (UTC)

You may wish to read Zeeman effect and fine structure. You should find the literature for yourself if that is your assignment. Graeme Bartlett (talk) 06:39, 3 April 2012 (UTC)

April 3

Gravity

Why does gravity keep the Earth going around the sun and not pull the Earth towards the Sun? 86.41.181.188 (talk) 02:16, 3 April 2012 (UTC)

Gravity does pull the Earth toward the sun. It is the conservation of angular momentum that keeps Earth at a fairly constant distance from the sun. Physicists say that there is an "effective radial potential energy field" as a result of the initial angular momentum of the Earth. In simple terms, we can say that this means an "effective force," centrifugal force, counteracts the inward pull of gravity; but if we want to correctly solve the equation of motion for Earth's orbit, we must be very careful in treating this "effective" force - it is a bit of a mathematical abstraction that only applies if we are considering just the radial component of motion. If we properly solve the movement of Earth in three dimensions, we do not need the "effective" centrifugal force; in fact, we see that it's just a mathematical trick to simplify a treatment of inertia. Nimur (talk) 02:26, 3 April 2012 (UTC)

No offence Nimur, but that's a heck of a confusing answer! ;) If the earth was standing still, it would get pulled into the sun, the reason it doesn't is beause it isn't standing still but has velocity in an orbit, which is a balancing act of sorts. The Earth's momentum is essentially what keeps it from falling "into" the sun, and the sun's gravity is what stops it from flying away from the sun, the two balance out and the earth keeps flying around in "roughly" the same path. The counter intuitive part might be that if this was happening in air, the earth would quickly loose velocity due to wind resistance and WOULD therefore spiral into the sun, but in space of course there is no air, so due to conservation of momentum the earth is speeding along now, not much slower then when it formed billions of years ago. Vespine (talk) 02:32, 3 April 2012 (UTC)

Actually, the orbit article has a section called understanding orbits which might be more help. Vespine (talk) 02:37, 3 April 2012 (UTC)

Sometimes, the explanation that is most simple to a physicist can be quite confusing to a non-physicist, because of the mismatch between each person's previously-acquired knowledge and baseline of understanding. Sorry if my answer wasn't very easily understood. At the very least, I hope it will encourage the OP to think in terms of energy, inertia, and the many different ways we can represent an object's position. Energy and inertia always follow the same rules, no matter how we measure the position. Nimur (talk) 03:29, 3 April 2012 (UTC)

If the OP attaches a ball to a piece of string and whirls it around his (or her) head he will have his answer. The tension in the string pulls the ball towards his hand, but the ball doesn't fall inwards so it collides with his hand! The inward force on the ball exactly matches the force required by the ball to travel in a circle - this required inward force is called the centripetal force.

In situations like this I find explanations relying on man-made concepts such as momentum and angular momentum are unsatisfactory. Eminent scientists have developed concepts such as momentum to help them calculate the magnitude of forces and the duration necessary to match their observations, but it cannot be said that the physical world acts in the way it does because of momentum. I concede the same can be said of fundamental concepts such as force and time, but I think a question of the kind we are dealing with is better answered with fundamental concepts, and the more fundamental the better, rather than more complex concepts such as conservation of angular momentum.Dolphin (t) 04:06, 3 April 2012 (UTC)

It seems like a man-made concept, and yet - angular momentum is the thing quantized by Planck's constant. Photons carry it in discrete amounts as particles go up and down in their orbits. In a sense, it's actually more fundamental than position or velocity, and someday, somehow, I think someone will create an intuitive model that explains why. Wnt (talk) 04:10, 3 April 2012 (UTC)

(ec - believe it or not this was written at the same time as the above) The cannon figure is a classic and useful example. But Nimur's explanation could be expressed more simply. Think of spinning a ball around on a string over your head. The string pulls straight in on the ball, so it doesn't do anything to keep it from going around and around. Only air resistance stops the ball from going around, and there's nothing like that in space, so ... a planet just goes around and around, even though gravity, like the string, constantly pulls in on it. Wnt (talk) 04:08, 3 April 2012 (UTC)

I've personally always thought the "ball on a string" analogy was terrible; sure the sum of forces is similar, but conceptually it's completely different. In my opinion, a better way to think about it is with those wishing well coin funnels that you see at malls and the like. Gravity is always pulling the coins downward towards the center, so if you just dropped the coin straight down it would fall right in. However, give the coin some tangential (sideways) velocity, it will "orbit" around the hole; if you somehow got rid of friction, the coin would never fall into the hole, but just keep "orbiting" around it, because the downward force is balanced out by the upward force provided by the curved side of the funnel. It's just like this picture of bikes on a slanted race-track; gravity may be pulling them down towards the bottom (center) of the race track, but their sideways velocity is great enough that gravity can't pull them down fast enough. If they stopped, of course, they would fall to the bottom of the track. (And if you've never played with one of those coin things, I feel sorry for your childhood). -RunningOnBrains^(talk) 07:08, 3 April 2012 (UTC)

I concur, isn't centripetal/centrifugal force a red herring in this? I'd always thought that, in reality, the Earth (along with all other bodies in the solar system) is continuously falling towards the centre of the Sun but has enough angular velocity to be continually missing the target. Blakk and ekka 13:37, 3 April 2012 (UTC)

Most physical phenomena can be explained satisfactorily using two or more explanations. I am not aware of any physical phenomenon that has only one correct explanation. In the case of the question above, I prefer an explanation using centripetal force and you prefer an explanation using conservation of angular velocity. The User who asked the original question will be able to comment on which explanation he prefers, but neither explanation is incorrect and neither is a red herring. Dolphin (t) 22:59, 3 April 2012 (UTC)

Newton's cannonball describes how a mass can be "missing the target". One can even demonstrate this orbital effect given a steel ball and a strong magnet. But as our article on gravitation touches upon, the Earth's orbit is not ascribed to a force, but to spacetime curvature due to the Sun's mass-energy. Thus, with Einstein's theories, gravitational forces are only analogous to the known centripetal forces due to boson interactions. But if Einstein's postulates are completely wrong (FWIW I believe I can demonstrate why these can be wrong or I would not bother with this line of reasoning), then gravity is simply another quantum matter phenomenon with extensive near-field-like bonds such that it is also attributable to a centripetal force interaction. --Modocc (talk) 16:46, 3 April 2012 (UTC)

Underwater hand stand

When you do a hand-stand on dry land, the blood rushes to your head. Does it also do this if you're entirely submerged at the time ? If not, why not ? StuRat (talk) 04:48, 3 April 2012 (UTC)

Yes, it will. Gravity is exactly the same underwater as it is on dry land. The buoyancy from the water isn't going to have a significant effect on what happens inside your body. If you were deep enough underwater then the pressure might be a significant factor - fighter pilots wear g-suits which apply pressure to the legs and prevent blood from pooling there due to the high g-forces (standing upside down is essentially -1g, so it has a similar effect). If the water is applying pressure to your whole body, that may impact the flow of blood in a similar way (although it would be evenly applied, so it might all end up cancelling out). --Tango (talk) 05:48, 3 April 2012 (UTC)

The pressure gradient is greatest close to the surface which makes me wonder if it would then have a significant effect on the blood rushing down to your arms and head if you were doing a hand stand in a swimming pool, for example. I've done hand stands in pools before, but between trying to hold your breath and keep water out of your nose, it's hard to tell whether the blood is rushing to your head as much or not.. Vespine (talk) 06:07, 3 April 2012 (UTC)

Why is the pressure gradient greater close to the surface? I thought it was pretty much constant (it should increase slightly as you get deeper due to the increased density of the water, but that will be a very small effect). --Tango (talk) 11:19, 3 April 2012 (UTC)

For all practical purposes, the pressure gradient is constant. A scuba diver should worry just as much about nitrogen narcosis, lung hyperinflation, and other pressure effects, whether they rapidly ascend from 100 to 50 feet or from 50 feet to the surface. (Actually, you should worry more in the former case, because you're still far from help!). While water density can vary across thermoclines, haloclines, and a tiny tiny variation with depth due to fluid compressibility, these effects are all essentially negligible for practical purposes; the pressure of water is well-modeled by the linear fluid head equation. Our article is Pressure head, but I always use the baby-physics version: pressure = density × gravitational acceleration × depth. Assume constant g and density for all practical cases: = ρg = constant. Nimur (talk) 15:00, 3 April 2012 (UTC)

You are correct in pointing out that the pressure gradient is constant, but lung hyperinflation (or air embolism due to pulmonary barotrauma) is much more of an issue while ascending in shallow water because of the much more rapid volume changes in a given amount of gas. Yes, dP/dh is constant, but for a given amount (moles or mass) of gas V ∝ 1/P so dV/dh ∝ P^-2 dP/dh. This is also why it is more difficult to maintain neutral buoyancy during a shallow dive than a deep one, as during a shallow dive a small change in depth results in a larger change in volume of the air in the diver's BCD, making the unstable equilibrium which is neutral buoyancy harder to maintain. Nitrogen narcosis here is a red herring, as it is not brought on by ascent. -- ToE 20:43, 3 April 2012 (UTC)Note that the "-2" exponent in dV/dh ∝ P^-2 dP/dh isn't quite fair when discussing lung hyperinflation, because while we are talking about a constant mass of gas expanding during a given portion of ascent, we are not dealing with an under-inflated lift bag (a dangerous thing due to runaway near the surface) where that same mass of gas makes the entire ascent, but instead with divers who may be intermittently holding their breath (a dangerous thing during ascent) so that there is a smaller mass of gas involved in the shallower depth ascent than there would be had they held their breath the whole way (an even more dangerous thing). This resetting of the volume brings the effect down to ∝ P^-1 due to the relative (as in fractional) pressure changes being greater at a lesser pressure for the same absolute pressure change. -- ToE 22:17, 3 April 2012 (UTC)

Sorry. "Narcosis" was incorrect. I meant "outgassing." It can be a problem for any ascent at any depth. Nimur (talk) 21:03, 3 April 2012 (UTC)

Ascent rates are limited at all depths due to DCS concerns, but while the old U.S. Navy tables were built around a maximum ascent rate of 60 ft/min, modern models often incorporate variable ascent rates, with slower ascent rates near the surface and more rapid ones permitted at depth, because while the rate of change of pressure with respect to depth is constant, it has a greater effect on off gassing at shallower depths because. For many of the mechanisms involved, such as microbubble growth, the issue is not the absolute rate of pressure decrease as much as it is the relative rate of pressure decrease. From a depth of 30 meters a diver must ascent 20 meters to reduce the absolute pressure in half, but need only ascent a further 10 meters (to the surface) to reduce the absolute pressure in half again. -- ToE 21:25, 3 April 2012 (UTC)

No, it will not. The issue of blood running to your head during a hand stand or the issue of a someone's feet swelling while standing at work all day is not so much that of the differing blood pressure between the head and the toe as much as it is the difference, between the head and the toe, in the relative pressure between the tissue and its surrounding environment. The atmospheric pressure does not differ sufficiently over a 2m change in altitude for this to be a factor on land, but in the water, which has roughly the same density as your blood, the pressure differential will be the same at your head as at your toes (regardless of your orientation), so you feet will not swell if you stand up and the blood will not run to your head if you do a head stand. This is the same mechanism which allows for compression stockings to alleviate swollen feet. They certainly don't work by lowering the blood pressure in the feet, but by compressing the feet they do lower the relative pressure between the blood in the feet and their surroundings (the atmospheric pressure augmented by the compression of the socks). -- ToE 21:53, 3 April 2012 (UTC) I do find that unless I exhale slowly through my nostrils during an underwater hand stand or somersault, water will run into my nose and on to my sinuses, resulting moderate discomfort.

We seem to have a disagreement. Shall we adjourn to the pool for a test ? StuRat (talk) 04:41, 4 April 2012 (UTC)

Some people already have. Telling which way is up can be a life and death thing for a scuba diver ... and they can have trouble doing so. Wnt (talk) 05:52, 4 April 2012 (UTC)

Sprained pelvises

How does one sprain one's pelvis? Human pelvis and sprain don't mention it, and the diagrams given at the human pelvis article don't show anything that I can recognise as ligaments or anything else (if such exist) that would be sprainable. Never heard of the idea until reading this article. Nyttend (talk) 05:22, 3 April 2012 (UTC)

That just says he injured his pelvis, not sprained it. It might have been a hairline fracture, for example. StuRat (talk) 05:29, 3 April 2012 (UTC)

Maybe not in this article, but it does indeed seem possible to sprain your pelvis. I believe it's the Sacroiliac joint that gets sprained. Vespine (talk) 06:13, 3 April 2012 (UTC)

I read it in a printed newspaper and linked to the story without reading the online version in detail; apparently the AP wire stories get slightly modified from newspaper to newspaper. Now I'm curious how my Google search terms found this story in the first place... Nyttend (talk) 11:14, 3 April 2012 (UTC)

One possibility is that the original version of the article at that link did say "sprained", but they have since corrected it. Unfortunately, they seem to be quite lax about online corrections, and often seem to fix problems without notification, in order to mask their incompetence. StuRat (talk) 18:15, 3 April 2012 (UTC)

Pencil sharpener

I have a metallic pencil sharpener on my desk made from magnesium (Checked by Energy-dispersive X-ray spectroscopy). I looked and found that the machining properties of extruded magnesium are a little better than that of aluminium. Is this the only cause to use the more expensive magnesium than aluminium? The people in the machine shop told me that turning housings for electronic devices is due to the higher risk of fire more complicated for magnesium. So you buy the better machining properties by safety problems.--Stone (talk) 08:09, 3 April 2012 (UTC)

Magnesium is also quite a bit less dense than aluminium (1.74 vs 2.70 g/cc). This is why it's used (alone or in alloys) where low weight is important: see this section of the article. I guess a light pencil sharpener is useful? --Colapeninsula (talk) 09:11, 3 April 2012 (UTC)

It is not a space flight pencil sharpener but a very ordinary one. So weight is no topic for that thing.--Stone (talk) 09:23, 3 April 2012 (UTC)

this video at 1:34 says magnesium is harder - http://www.youtube.com/watch?v=7Gco191pEag - I would think since you're constantly abrading against it with both carbon (inside the pencil) and wood, magnesium is simply higher-quality than aluminum. (I would think the softer it is, the easier/faster it wears away). Naturally it can be brittle etc and break off as well, but as a rule in such a situation I would think hardness would be the determining factor. The whole linked video is interesting btw and points to the fact that there are both aluminum and magnesium pencil sharpeners (in the same form, prob. same company). It also shows the fire risk you mention. 188.36.162.23 (talk) 12:14, 3 April 2012 (UTC)

The knife in the sharpener is made from steel, so the advantage is not that big.--Stone (talk) 13:39, 3 April 2012 (UTC)

I would think the rest of the groove wears away as well, since you're sticking the pencil in there with pressure and turning it around corkscrew like. After a while, you would think a harder material would retain the crisp groove better and just have a tighter sharpening experience. I mean, imagine it's wood. after a while the groove would be too big/loose for the pencil, and pushing it and turning wouldn't be eough, you would also need to angle it up toward the blade, and it would get closer and closer to feeling like you're just using a straight blade without the groove in the pencil sharpener holding the pencil tight and in form for you... --188.36.162.23 (talk) 13:43, 3 April 2012 (UTC)

I remember hearing about (but cannot find in our articles) NYMTA farecard readers having their metal groove worn down so much by rubbing of the paper(?) cards that the magnetic stripe tracks no longer aligned well with the read/write head. DMacks (talk) 15:21, 3 April 2012 (UTC)

You can see KUM's PR reason for using magnesium [11]. Someone here [12] suggests that perhaps magnesium is used as a sacrificial anode to keep the blade from rusting. This site [13] seems to have an entirely different reason for wanting magnesium pencil sharperners. Nil Einne (talk) 14:28, 3 April 2012 (UTC)

Male emotion

Why is it that males don't express emotions other than happiness or anger? Clover345 (talk) 20:31, 3 April 2012 (UTC)

They do. If you've never seen a male cry with sadness then you've never been to a funeral. If you've never seen a male worried then you've never been to court. If you've never seen a male compassionate then you've never been to a homeless shelter. Seems like you've led a pretty lucky life, congratulations! -RunningOnBrains^(talk) 20:43, 3 April 2012 (UTC)

Wow, what eloquence. Probably the best response I've ever rread on reference desk. 188.157.251.150 (talk) 21:11, 3 April 2012 (UTC)

Sarcasm meter is definitely spiking... If the asker wanted to know why males have a tendency to show their emotions less often than females, it is generally thought to be mostly societal pressure: men who have strong feelings of some emotions, particularly sadness are perceived as less masculine. In addition to social pressure, there is the simple factor of societal norms: children grow up mimicking those around them, and if they don't see other males crying then they certainly would be less likely to do so. It is thought that biology does not play as important a role as these factors (source). This should not be surprising, as the number of "feminine" men who frequently express emotions other than anger and happiness has certainly been on the rise due to the metrosexual movement and increasing societal acceptance of homosexuality, and I doubt this is due to some new wave of human evolution. -RunningOnBrains^(talk) 00:35, 4 April 2012 (UTC)

(same poster). No sarcasm, you handled the question incredibly well and eloquently. 188.157.46.184 (talk) 08:36, 4 April 2012 (UTC)

They definitely do. I definitely do, and my friends definitely do (male and female). Falconus^{p t c} 21:10, 3 April 2012 (UTC)

Males experience all of the same emotions females do, but they can be trained by their culture to avoid displaying them. Wikipedia has an article, Emotions and culture which may give some leads for further research. --Jayron32 22:13, 3 April 2012 (UTC)

Can't find it online, so I'll have to recite from memory, from a Shoe comic strip. Two birds at the lunch counter. One says, "How about the Cubs?" The other says, "Mmmph." The waitress says, "You men are all the same. You never talk about your feelings!" First bird says, "How do you feel about the Cubs?" Second bird says, "Mmmph." ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:48, 3 April 2012 (UTC)

Males can be just as affected emotionally by a "chick flick" as women. That is, they can be angry, confused, sad, and even deeply depressed. The only difference is that they experience these emotions prior to the actual movie, when first informed that they must attend. :-) StuRat (talk) 04:37, 4 April 2012 (UTC)

Is the outside of single cell algae lipophilic or hydrophilic?

Or, are there algae strains that are lipophilic on the outside? If you would spray water with lipophilic algae in it on water that is covered with a layer of oil (perhaps a few inches thick), I guess all the algae would be trapped in the oil layer, making it easy to pump out algae-free water below the layer. Harvesting them would be a lot easier and cheaper than for example centrifuging. As algae don't form "drops" in water like fat does, the answer is probably simply "No", but then again there are algae containing very high oil percentages (like 70% of dry weight). They might be forced to leaking a little of it? Joepnl (talk) 22:04, 3 April 2012 (UTC)

AFAIK, all living cells, including algae, are encased in a phospholipid bilayer called the cell membrane, and I don't think there is a sufficient distinction between any species of algae, or indeed any eukaryote life, with regards to this level of structure. --Jayron32 22:08, 3 April 2012 (UTC)

Thanks, that's the (interesting) dissapointment I was expecting :) Joepnl (talk) 22:21, 3 April 2012 (UTC)

A formal symbol for a postulated absolute rest frame?

What symbol or symbols, in a formal way as possible, would be best identified with the antithesis of Einstein's relative reference frame postulate? This absolute rest frame is not necessarily equivalent with the CMB rest frame, although it may very well be. With my work, the absolute rest frame is identified as the inertial frame for which identical clocks (that are distant from strong gravitational influences) tic with a maximal observed rate or, for unstable identical matter, decay with the fastest rates. When studying the observable clocks or matter with different velocities, matter with these maximum rates should be a subset of the entire lot. In my work, I need to reference this absolute rest frame quite a bit, and so if there is some symbolism in use that I could use or borrow from that would be helpful, otherwise I'm open to suggestions. --Modocc (talk) 22:34, 3 April 2012 (UTC)

When you're making your own stuff up, you can use whatever symbols you want for it. There are fairly standard ways of expressing special relativity, but if you're hypothesizing an alternative in which an absolute rest frame and time dilation coexist, you're really on your own as to how you want to express that. There is no widely-agreed-upon standard way to express that, because that's not a widely-agreed-upon idea. The harsh reality is that you aren't going to get any physicists to take your ideas seriously regardless of what symbols you use to express them, so you might as well just use whatever symbols make the most sense to you for your own exploration and enjoyment.

And there is no special frame of reference in which clocks tick at their fastest rate. Clocks are always measured to run the fastest in an inertial frame of reference in which the clock is at rest. A "moving" clock may run slowly as measured in your "lab" frame, but the clocks in the lab frame are running equally slowly as measured in an inertial frame of reference in which that "moving" clock is at rest. Red Act (talk) 23:57, 3 April 2012 (UTC)

For any given precision of measurement, such as incremental differences of, say, a hundred meters per second and directions of one arc secs, there are a limited number of possible velocities that can be measured. One of those velocities will be maximal with respect to rates when the observers compare them, such as with the Twin paradox such that one ages the fastest (only two velocities are compared, so its not very precise, but one can add more velocities with incrementally different directions and speeds... and one of these clones will age the fastest). For my model, I assume the existence of a classical Euclidean geometry for space (its not a Minkowski space) as well as an invariance for the simultaneity of events such that events that are simultaneous in one inertial frame are also simultaneous in other frames. --Modocc (talk) 00:28, 4 April 2012 (UTC)

There is no frame in which clocks tick the fastest. If you believe that there is, please describe in detail an experiment which could detect that frame, and I (or someone) will tell you what's wrong with the experiment. The experiment has to be one that could actually be performed, given an unlimited budget. I'll get you started: you probably want to launch a bunch of clocks at different speeds in deep space and measure their tick rates. The one with the fastest tick rate is the one closest to your state of absolute rest. What you need to do now is tell me how to measure the tick rates of the clocks. -- BenRG (talk) 00:55, 4 April 2012 (UTC)

There is already plenty of matter out there that is already moving at different velocities, and which do not tick any faster/slower because observers have accelerated their reference frames (thereby changing their own clock rates). Comparing observers that have different velocities after they have measured that matter with their different reference clocks you will find which observers have the fastest clocks relative to each other, based on measurements of that unaltered matter, for there is only one and absolute reality. With my model, there is no need to worry about seeming to measure a universe which is magically seeming to become an awesome pancake either, and the time dilation that we measure is entailed, as well as the inertial frame Lorentz corrections to the Doppler relation (which is necessary to accurately model the differences in measured wavelengths, frequencies and energies) and the transverse Doppler relation. I've got the basics pretty much covered or else I wouldn't be here asking this question about referencing the absolute rest frame which, with a couple of other reasonable assumptions, gives these results. Since only with my model, as oppose to relativistic models, this inertial reference frame is absolutely unique, I have used the capital U to represent it, and the U prime for any other inertial frame, but I'm not sure if U is the best available choice (and barring any alternative suggestions, I am thinking it is the best choice though). --Modocc (talk) 01:57, 4 April 2012 (UTC)

April 4

Black hole gravity

A black hole has gravity so strong that light can't escape. How does its gravity escape? 71.215.74.243 (talk) 03:49, 4 April 2012 (UTC)

Well, only objects with mass is trapped by the gravity well; grivity does not have mass. Plasmic Physics (talk) 03:55, 4 April 2012 (UTC)

Agreed. If gravitons exist, they must be massless. StuRat (talk) 04:32, 4 April 2012 (UTC)

Photons are also massless.--Modocc (talk) 04:48, 4 April 2012 (UTC)

See gravitational wave. In M theory gravitons are not stuck to the brane (like particles with mass are) and so are free to radiate into higher dimensional space. SkyMachine ⁽⁺⁺⁾ 04:45, 4 April 2012 (UTC)

(ec) That doesn't help — photons are also massless, but cannot escape a black hole. It's well known that there are lots of difficulties reconciling quantum mechanics with general relativity; I wouldn't be surprised if this were one of them (or at least one problem with describing gravity in terms of virtual-graviton exchange).

Photons have no rest mass, but that seems rather an esoteric thought, as they are never at rest, and do have mass when in motion. By comparison, I believe that gravitons never have any mass. Any physicists here to verify this ? StuRat (talk) 04:51, 4 April 2012 (UTC)

Photons and gravitons both have zero invariant mass, which is what physicists usually mean by the word "mass" these days when not otherwise specified. I am reasonably sure that gravitons, assuming they exist, do indeed have nonzero mass in the sense of mass-energy — if they didn't, how could they affect anything else? You also skipped over my second paragraph, which shows that the same problem exists with photons and electric forces. --Trovatore (talk) 04:57, 4 April 2012 (UTC)

Note also that black holes can definitely have charge, and their charge can be felt via electrostatic forces, which according to QED are mediated by the exchange of virtual photons, even though real photons cannot escape the black hole. I don't know what the resolution of this problem is, or even whether there is an agreed resolution, but it does appear to show that the problem is not unique to gravity. --Trovatore (talk) 04:47, 4 April 2012 (UTC)

See General relativity. Gravity is not what you think it is. --Jayron32 04:25, 4 April 2012 (UTC)

For black holes to gravitate, their gravitons, if these exist (they have not yet been measured (perhaps due to extinction) and are only theoretical) would need to "escape"... in which case, the gravitons could not be interacting with themselves very often. Analogously, photons that get absorbed/emitted are able to form waves with minimal interactions with each other. --Modocc (talk) 04:45, 4 April 2012 (UTC)

Try Google for "how does gravity escape a black hole?" You get loads of links, like [14] [15] [16][17] etc. Some points from these:

• How does the electric field escape a charged black hole?

• Gravitons don't have to escape; the information needed is all present in the original collapsing star.

• Virtual particles can do all sorts of unreasonable things, like escape the event horizon. They can't carry information, but they effectively mediate the force...

Wnt (talk) 05:50, 4 April 2012 (UTC)

You say virtual particles can't carry information (and I've heard it said before), but what does that actually mean? It is those virtual particles that tell us the mass and charge of the black hole - isn't that information? --Tango (talk) 06:06, 4 April 2012 (UTC)

Any information released would be random and unrelated to the original information of particles that fell into the black hole, appart from only a few sum total values such as total mass (which you infer from size of the event horizon), sum charge. SkyMachine ⁽⁺⁺⁾ 07:19, 4 April 2012 (UTC)

Quantum entanglement is handy of course, especially whenever information is lacking..., --Modocc (talk) 08:44, 4 April 2012 (UTC)

I like the string ball theory of black holes: where everything is crushed into a quivering, tiny non-zero yarn ball of superpositioned superstring where entanglement means nothing. Plasmic Physics (talk) 08:58, 4 April 2012 (UTC)

Why is pressure a scalar?

Consider this figure, which shows the forces due to pressure on an infinitesimal quantity of fluid. It's clear that p_y(y)=p_y(y+dy)+ (infinitesimal), or else there'd be an infinite acceleration in the y direction. Similarly, p_x(x)=p_x(x+dx) + (infinitesimal). But pressure is treated as a scalar, which implies/assumes that p_x=p_y. What's the justification for this assumption? It's not surprising, physically, that this would be the case if the fluid were incompressible, but is there a more rigorous justification for the claim that pressure is a scalar? 65.92.5.132 (talk) 04:21, 4 April 2012 (UTC)

In the general case, you have a stress tensor. Intuitively, I think it's the case that the stress tensor has to reduce to a single number, the pressure, in equilibrium conditions in a material that has zero tensile strength, but I can't say I'm sure about that. --Trovatore (talk) 04:26, 4 April 2012 (UTC)

Almost. In the most general case (nonzero viscosity and/or nonequilibrium system), you can have shear stresses, too. In a static system and the case of zero shear modulus, shear stresses must vanish and the stress tensor has to be diagonal in all reference frames. The only rank-2 tensor that is diagonal in all reference frames is a multiple of Kronecker delta (a tensor with all diagonal components equal to each other), $p\delta _{ij}$ where p is pressure. This fact is known as Pascal's law (though he obviously discovered it experimentally long before people knew about tensors or shear moduli.)--Itinerant1 (talk) 05:35, 4 April 2012 (UTC)

And to put this into more intuitive terms, consider the figure from the original question, and draw a plane at a 45 degree angle through the box. If $p_{y}(y)\neq p_{x}(x)$, you can calculate that the force exerted by the fluid on one side of the plane on the fluid on the other side is not perpendicular to the plane. --Itinerant1 (talk) 05:43, 4 April 2012 (UTC)

This is why I visualise pressure as equivalent to energy density, which is a scalar quantity. I used to think that since force is a vector quatity, pressure must also be a vector quantity; that pressure is the average quantity of force applied at a right angle a surface. Plasmic Physics (talk) 06:19, 4 April 2012 (UTC)

there are 4 kinds of topology in Set{1,2},and 29 in Set{1,2,3}, is exist a Formula in a Set {1,2,3,4,5,6...n}?

there are 4 kinds of topology in Set{1,2},and 29 in Set{1,2,3}, is exist a Formula in a Set {1,2,3,4,5,6...n}? — Preceding unsigned comment added by Cjsh716 (talk • contribs) 04:34, 4 April 2012 (UTC)

A topology τ on a set X is a subset of X which satisfies a number of rules. Thus any valid τ must be an element of the power set of X, and thus the number of topologies must be less than or equal than 2ⁿ where n = |X|. So how can there be 29 possible topologies of the set {1,2,3} as 29 > 2³ = 8? (Or am I misunderstanding something about your notation?) -- ToE 06:37, 4 April 2012 (UTC)

The 29 appears in our article Finite topological space#Number of topologies on a finite set, so I struck my comment above and will withdraw from this discussion until I better understand what is going on. -- ToE 06:42, 4 April 2012 (UTC)

A topology is a set of elements of the power set, not just an element of it, so the number of topologies is bounded by 2^(2^n), not 2^n. AndrewWTaylor (talk) 08:19, 4 April 2012 (UTC)

29 doesn't look much like it would be 2^(2^n) of anything. I guess 2^(2^3-3)-3....--188.157.46.184 (talk) 08:28, 4 April 2012 (UTC)

try http://oeis.org/A000798 --Digrpat (talk) 09:13, 4 April 2012 (UTC)

In most cases nNot every subset of pow(X) is a topology, so 2^(2^n) is an upper bound, not the actual number of topologies. AndrewWTaylor (talk) 09:32, 4 April 2012 (UTC)

How is heat energy converted to mass?

Mass is converted to energy in nuclear fusion and fission. Energy can be converted to mass, too. I see that Mass–energy equivalence#Practical examples says that each kilogram heated 1°C gains 1.5 picograms of mass. Where and what is that new mass from heat? 71.215.74.243 (talk) 08:09, 4 April 2012 (UTC)

It is in the motion of the particles, because heat energy is (mostly random) motion. A fast-moving particle simply has more relativistic mass than a slow-moving one. There are no additional particles making up the mass difference. 157.193.175.207 (talk) 09:09, 4 April 2012 (UTC)

can some women not orgasm from intercourse, but only clitorally

or if they are unable to do the former, is it considered a dysfunction, what is the cure or can be done (other than obviously wearing socks). --188.157.46.184 (talk) 09:29, 4 April 2012 (UTC)

It amuses me that some men seem to think that the only acceptable orgasm is a vaginal one. Oh, and the reference to socks. --TammyMoet (talk) 09:31, 4 April 2012 (UTC)

It's an easy mistake to make - after all, reproduction is through intercourse and men orgasm that way. Having just read http://www.scarleteen.com/article/advice/the_great_no_orgasm_from_intercourse_conundrum however ,I must apologize Tammy, I didn't know this fact. Specifically, my partner thought of herself as having a 'dysfunction' in this way, causing emotional problems like she was 'broken' in some way. To me, I love getting her off orally, and after she has come intercourse is also good for her, but she is upset that she can't come from that. Actually I just learned this would put her in the majority. What do you recommend, if she has come orally and we are having intercourse? Should I touch her as well, or have her touch herself, or use a vibrator or what.. Or is intercourse just a chore in reality for her, despite what she might say about liking it (even if she doesn't get off from it). 188.157.46.184 (talk) 09:37, 4 April 2012 (UTC)

Eating pussy aint so bad. Some women have their orgasm hang ups too. But you should probably address this question to Dan Savage if you want the expertize on this, or atleast subscribe to his Savage Love podcast. SkyMachine ⁽⁺⁺⁾ 09:44, 4 April 2012 (UTC)

Um, thanks for the recommendation, but I think this place can answer the question fine. I do love going down on her, she comes very hard like that, then asks me to put it in, and after a while she is upset that she isn't coming that way too. What do you recommend? One thing would be, we could just stop after a couple of minutes of intercourse... or, use a vibrator/finger her clit as well during it. I don't know, nothing works especially great, and more importantly she has hangups about it and will go as far as to start crying. But as I've just found out, this is perfectly normal (and doesn't bother me at all - the fact that she comes clitorally). Recommendations? 188.157.46.184 (talk) 09:48, 4 April 2012 (UTC)