Wikipedia:Reference desk/Science

Welcome to the science section
of the Wikipedia reference desk.

skip to bottom

Select a section:

• Computing
• Entertainment
• Humanities
• Language
• Mathematics
• Science
• Miscellaneous
• Archives

Want a faster answer?

Main page: Help searching Wikipedia

   

How can I get my question answered?

• Select the section of the desk that best fits the general topic of your question (see the navigation column to the right).
• Post your question to only one section, providing a short header that gives the topic of your question.
• Type '~~~~' (that is, four tilde characters) at the end – this signs and dates your contribution so we know who wrote what and when.
• Don't post personal contact information – it will be removed. Any answers will be provided here.
• Please be as specific as possible, and include all relevant context – the usefulness of answers may depend on the context.
• Note:
  • We don't answer (and may remove) questions that require medical diagnosis or legal advice.
  • We don't answer requests for opinions, predictions or debate.
  • We don't do your homework for you, though we'll help you past the stuck point.
  • We don't conduct original research or provide a free source of ideas, but we'll help you find information you need.

Ready? Ask a new question!

How do I answer a question?

Main page: Wikipedia:Reference desk/Guidelines

• The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.

See also:

• Teahouse
• Help desk
• Village pump
• Help manual

January 16

Ice fossils

Can fossils be formed in ice? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 02:29, 16 January 2012 (UTC)

I don't think so, if by "fossils" you mean where the bone is replaced by minerals. That requires that the minerals be transported in by a liquid. Of course, an animal can be frozen as long as the ice remains, so that's even better than a fossil, but not likely to be as old. StuRat (talk) 02:36, 16 January 2012 (UTC)

I meant ice as the minerals, not the medium. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 18:28, 16 January 2012 (UTC)

I don't understand. How can ice be minerals ? StuRat (talk) 22:53, 16 January 2012 (UTC)

Maybe minerals embedded in the ice. Though I would think they are too diluted to do anything. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:51, 17 January 2012 (UTC)

I think that water serves as the solvent for minerals that replace the structural details of a once-living organism in the process of fossilization. I would think that pure ice would not be able to contain the detail of fossils as normally understood. I think it may be theoretically possible to have impure water containing any type of contaminant participating in a variation on fossilization in which contaminants serve to record detail, and which would have to be maintained at a temperature that will keep it in its solid state. But that makes me consider yet another difficulty—how would molecules of water move into place in a fossilization process if they were frozen? Bus stop (talk) 01:16, 17 January 2012 (UTC)

You mean permineralization, but with ice crystals instead of minerals filling in the spaces between tissue? In a way. Freezing is pretty much the same process as permineralization. The water is trapped within the tissues that then solidify. These can form casts. But they're quite useless aren't they? as the original tissue still exists anyway.

Ice (and water) is too plastic and ephemeral. Global temperatures fluctuate too much. By the time the organic tissues begin to disappear for the ice cast to be of any value, the ice itself will have long rearranged themselves again or have gone.

While you can technically have more macroscopic trace fossils preserved in ice (like a footprint), I doubt it would also last for more than a few years. You cant exactly overlay it with another "stratum" of ice and still expect it to be recognizable either.-- Obsidi♠n Soul 01:31, 17 January 2012 (UTC)

Well, they can identify individual yearly layers in the ice, and count them to tell how old a given sample is. So, theoretically, they could also make out a footprint, but the problem is they wouldn't know to look for it. Perhaps if there was something that would reflect a signal between the layers, like a thin layer of ash from a volcano, the footprint might be a bit more visible. Also, in places where interesting footprints are expected, like by a cave entrance, the scientists might make the effort to search for them. StuRat (talk) 05:43, 17 January 2012 (UTC)

True, but then you'd have to rely on more serendipitous factors as well. Like the ice having to maintain constant temperature - unlikely when talking in geochronological terms. They can't be buried too far or compacted - pressure = heat. While stratigraphic sequence is still preserved even after that, the trace fossil is not. They most likely wouldn't survive in interglacials as well. Not to mention collection problems. The only places where I can reasonably expect these criteria to be met is in space. If ever exopaleontology/exoarcheology become valid sciences, ice fossils would probably be common. On the other hand, if the ash falls on the footprint and then solidifies, that can theoretically make a perfectly recognizable trace fossil - a mold. Again with the stipulation that the ash wasn't warm in the first place.-- Obsidi♠n Soul 06:14, 17 January 2012 (UTC)

Information transfer Faster Than Light - DIY

Not a physicist, so maybe this is an old question, but I didn't find a good saerch query response.

There is an insanely simple method for communicating faster than light, and I wonder whether it violates some obscure physics rule or equation.

It's this: run a string that cannot stretch or break (hypothetical material) out a REALLY LONG WAY, like a billion miles, or maybe Earth to the Moon. Put a person on each end. Send signals in real time, with zero delay, along the string by a series of tugs, like Morse Code, only by pulling on the string. That's it. If it really can't stretch, then the far end will feel the tug from the near end at precisely the moment it is pulled. There will be no delay for signal transmission regardless of the distance between the two points.

I know - it is technically ridiculous. But, purely from an idea standpoint, wouldn't it in fact constitute sending information faster than light?

If not: why not?

Dave — Preceding unsigned comment added by 24.113.99.57 (talk) 02:38, 16 January 2012 (UTC)

Nope, those type of waves don't move faster than the speed of light, they may be more like the speed of sound. Any real-world string will have some elasticity, so it will stretch when you pull on it, until eventually the far end catches up. StuRat (talk) 02:44, 16 January 2012 (UTC)

A similar thing has occurred to me too in the past, except I envisaged pushing and pulling a long rigid rod. I concluded, as you say, that far from being instantaneous, the transmission of information would be at the speed of sound in the material, just like if you whack an iron girder then the vibrations would travel along it at the speed of sound. But what about some hypothetical absolutely rigid material, with no little atomic "springs" inside it? Does the restriction about speed of information preclude such a material from existing (independently of other objections)? 86.181.172.222 (talk) 03:02, 16 January 2012 (UTC)

You have to consider carefully what 'rigid' means. The forces that bind atoms together in a solid material are electromagnetic in nature, and therefore mediated by photons—and so limited by the speed of light. In other words, if I wiggle an atom at one end of the rod, the next atom down the line has no way of 'knowing' what I did to the first atom until my wiggle is transmitted – no faster than the speed of light – through electromagnetic interactions. Your hypothetical 'absolutely rigid' material is an impossibility. TenOfAllTrades(talk) 03:10, 16 January 2012 (UTC)

Sorry, but you're ignoring my provision "independently of other objections" which I expressly put in to ward off this kind of answer. Say there was some other novel type of matter that was not composed of atoms but of some completely rigid stuff. Can we say that this substance can't exist solely because it would violate faster-than-light transfer of information? 86.181.172.222 (talk) 03:16, 16 January 2012 (UTC)

Restating your question as "If superluminal communication were possible, would superluminal communication be possible?", the answer would be yes—but not terribly informative. Once one starts invoking non-physical imaginary materials, we're out of science and into science fiction. In the real universe, forces are mediated by force carriers limited by the speed of light. TenOfAllTrades(talk) 03:24, 16 January 2012 (UTC)

Yeah, I guess .... but I was hoping there might be a more interesting answer! 86.181.172.222 (talk) 03:40, 16 January 2012 (UTC)

I think the answer is quite interesting! Prior to twentieth century understanding of physics, who could have predicted that material properties like tensile-strength and elasticity would have hard upper limits? And... that these limits would be governed by properties of electricity and magnetism? It's a fair bet that Archimedes' lever gedankenexperiment never took account of this limitation - even though he probably knew a lot about electrostatics! The really interesting part of the answer you received is that we now understand this connection. Nimur (talk) 05:20, 16 January 2012 (UTC)

Science Fiction has suggested other forms of "matter" (sometimes from an alternative "super-dense" universe) with "magic" properties, but such inventions really don't belong in the universe as we know it. We know about Supersolids and Bose–Einstein condensates, where atoms don't behave conventionally, but "super-rigidity" has never been found or postulated by experts. The nearest anyone has found to super-rigidity" is possibly on the surface of a cool Neutron star, and light certainly behaves strangely there, but this is because of the high gravity, not because shock waves are transmitted instantaneously. The only other forms of instantaneous communication that I've heard suggested are quantum entanglement (where the "effects due to entanglement travel at least thousands of times faster than the speed of light", but it doesn't actually work, even in theory, as faster-than-light communication), and telepathy (believe whatever you wish about that). Dbfirs 09:10, 16 January 2012 (UTC)

It seems others have already provided an answer, but allow me to link to the same question in the Physics FAQ which gives the same answer, hoping you or other readers will find the FAQ useful. – b_jonas 12:11, 16 January 2012 (UTC)

I will just note that when people say "real world substance," they don't mean, "oh, this is just an engineering problem." They mean, "any substance that acted in such a way would violate the rules of physics." It violates non-even obscure physics rules and equations for the reasons given above. It does so by deliberately ignoring what rigidity means on a physical level. --Mr.98 (talk) 14:26, 16 January 2012 (UTC)

Another answer is based on the relativity of simultaneity. If a force were to be able to act instanteneously accross a finite distance, then in your frame momentum would be consererved, because the change in momentum of one object would be the opposite of the change in momentum of the other object, and his would happen at the same time. However, in the frame of someone moving relative to you, the two mometum changes will happen at different times, so momentum would not be conserved. The only way to conserve momentum in all frames, is to have momentum conserved locally, at each point in space-time. Count Iblis (talk) 13:44, 16 January 2012 (UTC)

From Quantum entanglement, "Quantum entanglement is a form of quantum superposition. When a measurement is made and it causes one member of such a pair to take on a definite value (e.g., clockwise spin), the other member of this entangled pair will at any subsequent time[7] be found to have taken the appropriately correlated value (e.g., counterclockwise spin). Thus, there is a correlation between the results of measurements performed on entangled pairs, and this correlation is observed even though the entangled pair may have been separated by arbitrarily large distances.[8]" so if you did the following experiment: 1) Entangle two pairs of atoms, pair #1 and pair #2, 2) Carry one atom from pair #1 and one atom from pair #2 on a spaceship out 10 light minutes away from the laboratory on Earth where the other atoms from pair #1 and pair #2 are 3) Out at the remote location on the spaceship, flip a coin in your floating spaceship and slap it against the wall. If it comes up heads, cause the spin of your pair #1 atom with you to change and shine a red light back to Earth. If it comes up tails, cause the spin of your pair #2 atom with you to change and shine a blue light back to Earth. 4) Back on Earth, where the atoms are being watched and a telescope is pointed in the direction of the spaceship, the observers on Earth will know the result of the coin flip by way of the atom, practically 10 minutes before they get the message by way of the light, no? 69.243.220.115 (talk) 16:54, 16 January 2012 (UTC)

This idea has been bandied about since Bell's early work, but I don't think it works out. The problem, as I understand it, is that you can't arbitrarily decide what state to collapse it into, nor can you know if the other person has already collapsed the state. So you can't send a signal this way; you can't even send an "empty" signal (you can't say, "when I collapse the state, then you'll know the deed has been done" — because you can't detect if the state has already been collapsed, or if your measurement collapsed it). You can compare the states and find they are the same — but that requires slower-than-light communication. But I'm not a physicist, so someone surely can elaborate more on this, or correct me... --Mr.98 (talk) 17:44, 16 January 2012 (UTC)

I'd say you pretty much covered it. Truthforitsownsake (talk) 18:19, 16 January 2012 (UTC)

Is the FTL nature of quantum entanglement not confirmed experimentally? If so, then that (set up the test the way they did that does work to confirm it). If not, why is it believed by scientists to be true? I haven't carefully read the entire article and am not highly trained in physics, and so might not even be able to understand it all. 69.243.220.115 (talk) 19:20, 16 January 2012 (UTC)

From the article, "Experimental results have demonstrated that effects due to entanglement travel at least thousands of times faster than the speed of light," with a ref to this article in Nature, which seems pretty unambiguous about it. But again, I'm not a physicist... --Mr.98 (talk) 19:42, 16 January 2012 (UTC)

So if the effect is experimentally confirmable, use the same method they used to confirm it to send information FTL. 69.243.220.115 (talk) 19:47, 16 January 2012 (UTC)

It doesn't work that way, for the same reason I explained. The experiment looks roughly like this: we separate entangled particles. We both agree to check it at the same exact time. We check them. Then we telephone each other and confirm that the states are the same. That is, it takes slower-than-light communication to know what the states are (we can't set them), and it takes slower-than-light communication (the telephone call) to know that we did it at exactly the same time, and using our very precise time measurements regarding when we checked the state to calculate the speed. There's no FTL information passing there. If I screwed it up and checked it a week earlier, the person on the other end would have no way to know that unless I told them... via something slower than the speed of light (e.g. the telephone). We cannot arbitrarily set the state, we can only read it. There's no information traveling FTL here. The key distinction you have to make here is whether information is being exchanged FTL, not whether there is FTL phenomena. The latter is not necessarily prohibited by SR, but the former is, and there isn't currently a way to do it within the current laws of physics.--Mr.98 (talk) 19:51, 16 January 2012 (UTC)

So you on Earth and the guys out 10 light minutes away have really good clocks synchronized and agree to look at your atoms at a specific time. You look at the appointed time and see that yours is "up" or whatever state your equipment can measure, and that "information" leads you to believe that at that point in time, the state of the atom 10 light minutes away is also "up" and then 10 minutes later you get the confirmatory color of light beam, but you knew that 10 minutes earlier than the light beam told you because you looked at your entangled atom. True, it didn't involve any setting by the guys in the spaceship. They couldn't tell you anything they wanted, but it still seems like a kind of information, you knowing what the state of an atom 10 light minutes away was before any slower-than-light means could tell you. 69.243.220.115 (talk) 20:13, 16 January 2012 (UTC)

There's no information communicated FTL; that's the key part of SR. Zero communication. Case in point: if a meteor destroyed the other station, you'd have no way of knowing unless some STL communication took place as well. Another way to think about this, is how your situation above is any different than if instead of entangled particles, you simply have duplicate photographs in sealed envelopes. You can both open the envelopes and say, "oh, look, it's a photo of a dog," and you can say, "well, according to our system, he has a photo of a dog, too." But neither of you get to choose what the photos are (nobody does, technically), and there's no way to use them to pass any information between the two of you. The photo is itself a "piece of information," as it is, but you can't pass information this way. If all you're saying, at this point, is, "isn't it cool that particles are entangled?," then I agree, but trying to find a convoluted way to argue that this allows FTL communication is just not going to pan out, sorry. This stuff has been hashed over at length by more clever people than you or I, including folks who had strong vested interests in trying to come up with FTL communication schemes. :-) --Mr.98 (talk) 20:25, 16 January 2012 (UTC)

I see now. Your envelope analogy is very good. 99% of the time, I remember to just not bother trying to think of ideas that work and that someone else will always have thought of it first if it is a good idea :) 69.243.220.115 (talk) 20:58, 16 January 2012 (UTC)

• I was interested above to read "the effects due to entanglement travel at least thousands of times faster than the speed of light". Although my understanding of this is very hazy to say the least, I'd always imagined that these effects did not actually entail anything "travelling" anywhere, and that they were therefore instantaneous. The Wikipedia article says that the "thousands of times faster" is experimentally confirmed. Is it still an open question whether the effects are instantaneous? But, while writing this, another thing has occurred to me, which is that there is not actually any such thing as simultaneity, right? So how do we determine this question anyway? Now my head hurts. 86.181.206.2 (talk) 12:49, 17 January 2012 (UTC)

My reading of that statement is that there are limits to the precision of our ability to measure simultaneity. Simultaneity is when we say two events occur at the same time. It's a non-trivial thing to measure precisely, and there are technical (and theoretical, but I don't think those experiments approach those) limits to our ability to measure precise times. (Relatedly, see relativity of simultaneity.) Saying it propagates at least 1000 times faster than the speed of light is highly encouraging to the idea that it propagates instantly. The deeper question is whether the property (e.g. spin) was the same all along — e.g. is it like an envelope, which has a photo in it even if you haven't opened it. This is a deeper quantum question about the nature of the uncollapsed wave function. Too deep for me, but my understanding of the Bell test experiments is that it has been more or less confirmed that the properties are not there until you look for them (there are no "local hidden variables" — the particles don't just secretly have the information with them). But there are still multiple interpretations of that data available. This is exactly where the analogy with the envelope breaks down — in the quantum case, there isn't a fixed photograph inside the envelope until somebody looks. Then both envelopes have the same photographs inside of them. This is what Einstein thought was "spooky." It doesn't make a lot of sense in a macroscopic world, but that's true of most of quantum mechanics. Being spooked by it is a sign that you actually are taking it seriously, I think. --Mr.98 (talk) 13:13, 17 January 2012 (UTC)

Thanks! The "relativity of simultaneity" thing is puzzling me still. If there are events A and B, then according to the article you linked (and according to what I previously understood), some observers may see A before B, some may see B before A, and, presumably, some special-case observers will see them both happen at precisely the same time. So in the case that A and B happen to be quantum-entangled events, how does the question of whether they are simultaneous even make sense? 86.181.206.2 (talk) 14:05, 17 January 2012 (UTC)

The relativity of simultaneity should not come into effect unless the two observers are in different inertial frames. Given that they are both on Earth and their labs are presumably "still" I don't think it really comes into play. If one of them was in a very fast traveling rocket then you'd have a much harder time saying which even happened before the other. Generally these sorts of SR effects don't come into effect unless one of the frames is moving at a significant fraction of the speed of light. But they are interesting to think about, nonetheless. What the relativity of simultaneity emphasizes (and the only reason I linked to it here as food for thought) is that the definition of simultaneity is based on two events happening at the same time. The problem is, according to relativity, what time it is varies according to your inertial frame. In a Newtonian view, saying something happened "at the same time" implies that there is one universal time somewhere out there, but that isn't the case. There are local times, and they can vary from other local times. In the case of this experiment, though, this probably doesn't matter too much except maybe in a few decimal places far at the end of the time measurement. But I'm not 100% sure of this; knowing where you are and when something happened very precisely can be very tricky in physics. --Mr.98 (talk) 14:22, 17 January 2012 (UTC)

Do you mean to imply that there is some theoertically sound and exact way of establishing simulaneity by ensuring that everything is "still" (in some appropriate sense), and this obviates all the relativistic objections that simultaneity is actually not well defined? On a more practical level, though I can't do the maths, I would have thought that even the slow speeds of normal experience might be relevant when distinguishing between something travelling, over a short distance, thousands of times faster than light, and something instantaneous. The difference must be only some vanishingly small fraction of a second. But I am speculating because really this is well beyond my knowledge. 86.181.206.2 (talk) 14:38, 17 January 2012 (UTC)

Well, my knowledge of SR and inertial frames isn't perfect, so I could be pretty off on that. This also strikes me as a not-totally-normal use of SR since even if the two observers are not in the same reference frame, they are not "communicating" in the same way you would in a typical SR situation. But the more I ponder over it, the less confident I am that you could, even theoretically, distinguish between absolute simultaneity and very-very-very-very close simultaneity. Even if you could measure time to arbitrarily low values (which you can't, not even theoretically), being able to distinguish between entanglement acting instantly or ridiculously fast is probably not possible. Even if you had one experimentalist on the other side of the universe from the other, you probably couldn't distinguish it from being many billions times the speed of light and being actually instant. (Before one rushes off to make that experiment, keep in mind it would take billions of years for the results to be received from each station.) --Mr.98 (talk) 18:53, 17 January 2012 (UTC)

As far as I know, simultaneity is perfectly well-defined within a single reference frame. As long as you both have good clocks you can synchronise them by relying on the speed of light being constant. Just send a pulse of light from A to B, where it hits a mirror and goes back to A. A times how long it takes the light to get there. Halve that and you have the time it takes light to travel from A to B (1 minute, say). B then says to A "According to my clock, it's 12:00am". When A receives that message it knows that, according to B's clock, it is now 12:01am so A can set its clock to 12:01am too and then you have an easy way of knowing when events are simultaneous. (Note, you have to do it like that rather than just having A and B starting off in the same place, setting their clocks, and then walking away from each other because as soon as they start moving they aren't in the same reference frame and all bets are off [unless you measure the movement very carefully and do some relativistic calculations and compensate for the error, I suppose].) --Tango (talk) 00:05, 18 January 2012 (UTC)

"simultaneity is perfectly well-defined within a single reference frame". So, for example, if I have two alarm clocks not moving relative to each other, then will all observers also not moving relative to the clocks agree that the alarms go off at the same time? Does that not mean that the two events really are simultaneous, in some absolute sense, and anyone disagreeing because they are moving is just looking at things from "a distorted perspective"? Like I could say two lines really are parallel, but you might not see them that way. 86.181.206.2 (talk) 00:28, 18 January 2012 (UTC)

Persistent BO

I have some cotton T-shirts that are getting a few years old now but still in pretty good condition so still wearable. The problem is that within only a couple of hours of putting them on they can start smelling of BO from the armpits, usually as soon as they get the slightest bit of sweat in them. It doesn't matter how they're washed or how often they're washed it still happens. I don't have bad BO in general and other T-shirts I wear don't have this problem even older ones. What would cause this and why are only some T-shirts affected? Any suggestions about how it could be fixed? Dob in a Nerd (talk) 03:19, 16 January 2012 (UTC)

Is it possible it's tighter than the other T-shirts you wear ? I find having good air flow to the pits is essential to them keeping dry. If they stay wet, then they start to stink. StuRat (talk) 04:33, 16 January 2012 (UTC)

Interesting. Well I would say that they are probably fairly tight ones and I don't think I have this problem in any looser fitting ones, however I do have as old or older ones just as tight that don't smell either so it can't be just that. Dob in a Nerd (talk) 04:47, 16 January 2012 (UTC)

I've also noticed that certain fabrics tend to cause BO more than others. Cotton usually isn't bad (presumably because it wicks away moisture), but nylon and other artificial fabrics are. Are you sure there's no nylon in it ? Also, I suggest you wash them with bleach, as plain detergent will probably remove most of the stink, but leave enough bacteria to "seed" your pits and start them stinking quickly. StuRat (talk) 04:54, 16 January 2012 (UTC)

Yes agree that the synthetics tend to smell more quickly in general. But these are definitely cotton, just checked two of them. Can't use bleach cos they're colours and specifically say not to bleach but I have tried those stain removal sprays with little effect but will try again. Dob in a Nerd (talk) 05:06, 16 January 2012 (UTC)

There is "color-safe bleach", although perhaps "oxidizer" is a better name than "bleach". You could also pour alcohol directly on the pits, but be sure they are rinsed thoroughly before drying, or they could catch fire. StuRat (talk) 05:14, 16 January 2012 (UTC)

OK thanks for your suggestions I might give it a try. If anyone else has got any ideas still happy to hear them. Dob in a Nerd (talk) 06:55, 16 January 2012 (UTC)

It might be that there is odorous material trapped in the fabric, and when it heats up (as a result of your body heat) the smell comes out. So it's not you that smells, but the t-shirt. Soaking in vinegar is sometimes suggested to remove smells from fabrics. --Colapeninsula (talk) 11:06, 16 January 2012 (UTC)

Changing to a biological washing powder should solve the problem for you. You may need to make a solution of the powder first and paste it onto the affected areas, let it soak for a while (not dry out though) and then wash. Using washing powder that has enzymes in it (biological washing powder) will prevent this problem happening. --TammyMoet (talk) 12:06, 16 January 2012 (UTC)

I do use an "enzyme powered" detergent possibly the dearest one on the supermarket shelf that usually tops comparative independent tests of washing powders in this country so it's not that and that's not preventing it, however I haven't tried making the concentrated paste thing first and letting that soak in so I will give that a try thanks. Dob in a Nerd (talk) 03:25, 17 January 2012 (UTC)

Time is stopped

What is the time? That is a big question.The matter move from a place to another place,but time never change.The change just a feel people thought,because you are you in this situation.Time like space and place .We just a geust in the situation of time.So giving up the concept of thime.You will be liberated by the new opinion.That is my view about time. — Preceding unsigned comment added by 123.138.31.86 (talk) 04:15, 16 January 2012 (UTC)

What's your question? We do have an article called Time, if that would help. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:32, 16 January 2012 (UTC)

We also have an article on the human perception of time, which is probably less well-understood than the actual physics-descriptions of time. In physics, definitions of time as a dimension or as a variable in an equation tend to be axiomatic, so there's really not much to explain. Nimur (talk) 05:25, 16 January 2012 (UTC)

The nature of time can also be a very interesting, complicated and challenging topic in physics. See List of unsolved problems in physics#Arrow of time and Arrow of time. Red Act (talk) 05:58, 16 January 2012 (UTC)

Hyperbolic lines around radio towers

While walking through the National World War II Museum today, I saw a map in one of the exhibits. The caption said something about planes finding their way to France for the D-Day bombings and such. The map, besides having an outline of the coasts of England and France, had various radio navigation towers on it with hyperbolic lines drawn around them. The caption didn't mention them or explain in any detail how the pilots used them. So what were these lines and why would they be hyperbolic and not circular? Thanks, Dismas|^(talk) 05:37, 16 January 2012 (UTC)

See Decca Navigator System. AndyTheGrump (talk) 05:45, 16 January 2012 (UTC)

G was the World War II phase navigation system used by the RAF, that later evolved into Decca (post-war)... but I don't know what the actual chart you saw was, so all this is a little speculative.

I don't think that Decca was ever used in the United States (at least, non-experimentally). 1940s-vintage American aeuronautical NAVAIDs were usually nondirectional beacons, LORAN, and eventually VOR. I presume in Occupied Europe, the navaids were German, not British; but the physics is the same for all Hyperbolic navigation navaids. (NDBs would have circular radiation patterns, so iso lines aren't drawn on sectional charts). RAF and American AAF pilots probably used British or American equipment dialed to German stations. One of the most important post-War technology development thrusts was spurned by the realization that enemy bombers could use our own NAVAIDs, resulting in the technology that we now call the Emergency Broadcast System. (The beeping noise on television was just for testing. In an actual bomber-air-raid emergency, the TV station would be spoofing as a radar tower or malformed NDB in the wrong state). Nimur (talk) 06:13, 16 January 2012 (UTC)

Would that spoofing actually be a part of the Emergency Broadcast System, or just something else that was also implemented (or considered)? I can't find anything about it in the EBS article, but it would be neat to cover if it we had reliable sources for the design, testing, or implementation of such a system. TenOfAllTrades(talk) 23:18, 16 January 2012 (UTC)

Some more information at CONELRAD. I'll see if I can dig up a more authoritative reference. Nimur (talk) 23:38, 16 January 2012 (UTC)

Official handbook for station-operators (2007): AM & FM Emergency Alert System Procedures. Steps 5 through 9, for participating stations, to enable your tower to transmit data from the National Activation system. The FCC and FEMA will jointly decide, under direction of the White House, what signals to transmit out of your tower. Additional information, Emergency Communications, from the FCC.

And, Special Temporary Authority, for ad-hoc, emergency use of radio towers. Nimur (talk) 23:57, 16 January 2012 (UTC)

Tau-catalysed fusion

Wouldn't tau-catalysed fusion be even more efficient than muon-catalysed fusion, because taus are over fifteen times more massive than muons and would therefore bring two deuterium or tritium atoms over fifteen times closer to each other than muons do? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 19:15, 16 January 2012 (UTC)

I don't know, but it's worth noting that a tau's lifetime is seven orders of magnitude shorter than a muon. So that's lowering the threshold of usefulness substantially, no? --Mr.98 (talk) 19:49, 16 January 2012 (UTC)

The tau appears to have a 17.39% chance of decaying into a muon anyway when it finally does decay though. The alpha sticking problem seems to be more of a critical issue with muon-catalysed fusion, not sure if taus will stick to alpha particles as often as muons do. If they don't, then it would make more sense to produces taus. I guess it all depends on which can produced more efficiently, muons or taus. ScienceApe (talk) 11:02, 17 January 2012 (UTC)

If taus are 15 times more massive than muons, presumably that means they take 15 times more energy to make. If muon-catalysed fusion uses more energy than it produces, then tau-catalysed fusion would be even worse. --Tango (talk) 12:18, 17 January 2012 (UTC)

Is there anything that decays and produces taus? ScienceApe (talk) 12:57, 17 January 2012 (UTC)

Yes, of course. Anything that is heavy enough to produce a tau particle in its decays will occasionally produce them. But other things like protons and neutrons are much more likely to be produced. Dauto (talk) 19:47, 17 January 2012 (UTC)

To clarify Dauto's "of course": it is a general principle in particle physics that any way of decaying that doesn't violate any conservation laws will sometimes happen (it may happen with an incredibly low probability, but it will happen sometimes). It's worth noting that a tau is about twice the mass of a proton, though, so there aren't many particles heavy enough to decay into a tau on their own (but you can certainly make them by colliding particles together so that the kinetic energy contributes as well). --Tango (talk) 21:27, 17 January 2012 (UTC)

Didn't realize Taus were actually bigger than Protons. Curious, could anti-protons work? ScienceApe (talk) 03:20, 20 January 2012 (UTC)

I can't find any mention of a particle made up of a proton and an anti-proton, which makes me think it would be incredibly unstable. I would expect the proton and anti-proton to annihilate each other before either of them could fuse with anything. --Tango (talk) 16:03, 21 January 2012 (UTC)

If you are capable of producing antiprotons efficiently enough to make them useful for fusion, you may as well just use antimatter annihilation as your energy source. It's way more efficient. Law of Entropy (talk) 01:47, 22 January 2012 (UTC)

No it's not. Producing antiprotons will never be an energy source since it will always take much more energy to produce antiprotons than you will get out of proton-antiproton annihilation. ScienceApe (talk) 02:00, 23 January 2012 (UTC)

Note: "If you are capable of producing antiprotons efficiently enough..." Point being, instead of getting the comparatively tiny energy from a catalyzed nuclear reaction, just completely consume the antiproton, if you are generating them efficiently. If making the antiproton is inefficient, then you shouldn't be using it to catalyze fusion in the first place.Law of Entropy (talk) 09:15, 24 January 2012 (UTC)

The tau requires more energy to produce, has a much shorter lifetime and suffers from a more serious alpha-sticking problem (Being heavier it moves more slowly making it easier for the alpha particles to capture it). Dauto (talk) 19:47, 17 January 2012 (UTC)

climate data

Anyone know where on the internet I can get climate data for Hampshire, England, preferable Petersfield. I need temperatures, rainfall and wind direction averages for each month, and within the next few hours if at all possible. The internet only gives me tomorrow's weather and climate change stuff.

148.197.81.179 (talk) 22:19, 16 January 2012 (UTC)

Have you tried searching pubmed? Might be your best bet. N^oformation ^Talk 22:23, 16 January 2012 (UTC)

That's a collection of biomedical journal references, apparently. i can't see the connection myself. 148.197.81.179 (talk) 22:42, 16 January 2012 (UTC)

Derp, I meant google scholar. Sometimes I forget that science exists outside of biology :). N^oformation ^Talk 22:43, 16 January 2012 (UTC)

This site gives temperatures, rainfall, sunshine hours but no wind speeds I'm afraid. Mikenorton (talk) 22:57, 16 January 2012 (UTC)

I've just found the temperature and rain data, half an hour of searching on the Met Office site. Now all I need is the wind data. 148.197.81.179 (talk) 23:01, 16 January 2012 (UTC)

6 hours left to find it... 148.197.81.179 (talk) 01:57, 17 January 2012 (UTC)

January 17

Order of chemical reaction

It is said by many references (e.g., http://en.wikipedia.org/wiki/Reaction_order) that the rate at which a chemical reaction of reactants A and B to give products X, Y, ... occurs is given (approximately) by: r = [A]^n [B]^m k where r = reaction rate, [A] & [B] are the reaction concentrations, n & m are "reactant" orders, and k = the arrhenious factor, modified arrhenius factor, or eyring factor to taste. The sum of n amd m is known as the reaction order and is generally 2 or 3. I gather that depending on the actual reactants, n & m may or may not be equal, and are usually but not always are equal to 1 or 2 (may be fractional vaules]. Chemical databases often seem to give only the reation order, and not the individual n & m values. What I would like to know is this: Q(a) how do you apportion the order between [A} and [B]? Q(b) The Wikipaedia at http://en.wikipedia.org/wiki/Reaction_order implies that if the reaction is elementary, the reaction order is related to the stoichiometry. What is the relation? Some simple reactions I have checked e.g., O+C>CO (monatomic O & C) are order 3 if there is one product, and order 2 if there 2 products (eg O2>O+O or O2+C>CO+O) - is this the rule? None of this makes much sense to me - one would think that doubling the concentration of of either reactant would double the rate of particle collisions & therefore the probability of a molecular change. So for O2>O+O (which would actually be O2+O2>O+O+O+O i.e., two oxygen molecules colliding) and ought to be proprtional to the square of the O2 concentration, ie order 2 - but this is apparently wrong! Why? — Preceding unsigned comment added by 124.182.32.40 (talk) 03:05, 17 January 2012 (UTC)

Can you give an example of a chemical database that only gives the overall order? Often times, a reaction will be described as "2^nd order in "A", or something of that nature, meaning that [A] has an exponent of 2. As for why a reaction is a particular order, it has to do with the reaction mechanism. You cannot simply look at the overall reaction and predict the order (i.e. your "rules" for one or two products will not hold up in general). This is not a subject that is easily learned from an encyclopedia such as Wikipedia - it is best to learn from a competent teacher, or at least a textbook. Even many chemists do not really fully learn about why reaction orders are the way they are (though many do - it depends on the person and the sub-field) Buddy431 (talk) 03:34, 17 January 2012 (UTC)

One major flaw in your (IP 124.) thinking is the "therefore..." of "doubling the concentration of of either reactant would double the rate of particle collisions & therefore the probability of a molecular change". The actual change from reactants to products might involve several discrete steps (the "reaction mechanism" Buddy431 mentions), not just a single unified change as suggested by the equation. One key is to look at the rate limiting step and the identities and numbers of molecules involved in it, definitely not the collisions of reactant molecules for the net reaction. DMacks (talk) 03:45, 17 January 2012 (UTC)

Buddy431 may be correct, but weren't the examples given elementary (single hump) reactions? The database at http://kinetics.nist.gov/kinetics/index.jsp is an exapmple that gives the overall order only. So unless there is a way of working out how to allocate the reaction order, this database would seem to be near useless. Surely, if it is not just a matter of particle collision rates, the additional factor(s) can be at least named, if not described? I have some chemistry textbooks of undergraduate & graduate level, and the most intelligent thing they say on the matter is "the order can be determined by experiment". Is this because the subject is outside the syllabus, or because it is too complex, or because it is not understood by even experts? I would suspect the first. Meanwhile, I obtained data for all the reactions involving monatomic and molecular C & O and H & O, and the Inquirer's "rule" holds up in every case. Keit124.178.159.199 (talk) 04:25, 17 January 2012 (UTC)

In an elementary reaction, with no intermediates, the reactant order for liquids and gases is generally the same as the stoichiometry. For example n A + m B -> C is generally governed by a reaction rate like $r=k[A]^{n}[B]^{m}$, where the exponents are the same as the reduced stoichiometric coefficients. There are lots of exceptions however. For example, reaction involving solids (or solid catalysts) are generally zeroth order. In addition, many reaction rates are controlled by non-obvious intermediate phases that may have unexpected reaction orders. So in general one usually has to look up the reaction rate (or at least the reaction mechanism) to know for sure what the rate is. Dragons flight (talk) 06:02, 17 January 2012 (UTC)

So, taking C+O>CO as an example, DragonsFlight's n and m are both 1 and thus the reaction order is 2. Isn't there no intermediate phase? But this reaction is listed (http://kinetics.nist.gov/kinetics/index.jsp ) as Order 3, not 2. Why? Keit124.178.159.199 (talk) 07:34, 17 January 2012 (UTC)

Monoatomic oxygen isn't stable. 2 C + O₂ -> 2 CO is a third order reaction that would seem more natural. The intermediate in this case would then be 2 O -> O₂ via fast reaction. Dragons flight (talk) 07:42, 17 January 2012 (UTC)

Yup, again there is an assumption that the given net equation is actual reaction (atomic/molecular species as written, single mechanistic step, no other catalytic components, etc.). The reaction proposed in the ref given in the database is:

C + O + M → CO + M

all taking taking place in the gas phase as part of a rapid high-energy sequence of reactions (so single-atom oxygen is correct--actually they studied the dissociation of CO and extracted this reverse-reaction as part of a complex series of reactions under steady-state or equilibrium conditions). But notice "M"--an unidentified "collision partner" involved in the reaction! If I understand the details correctly, it could just be a sink for excess energy or balance the electronic states or something like that. So its chemical identity is not changed or specifically identified, so a "net equation" analysis might ignore it on both sides. But there it is, now that the actual reaction is given for the specific mechanistic details, rather than the net chemical one--a third chemical entity to correlate with it being third-order. DMacks (talk) 08:05, 17 January 2012 (UTC)

Yep, that's a correct interpretation for "M"; it's an energy sink. You'll quite often see something similar whenever you have two monatomic species coming together to form a diatomic product in the gas phase. (Consider what happens when, for example, the independent C and O atoms meet. They start out as free atoms, then when they get together, they form a chemical bond...which releases energy...which has to go somewhere...and the only place that energy can go is back into the kinetic energy of the two atoms, which just pulls the bond apart again. The only way that the bond can be stable is if a third atom or molecule is present right when the C and O get together; one or both atoms can then collisionally transfer some energy to the third party, leaving the newly formed CO without enough internal energy to break itself apart again.) TenOfAllTrades(talk) 15:33, 17 January 2012 (UTC)

Heterogeneous catalyst anyone? Gas-phase reactions involving three reactants with no intermediates don't really happen in real life. "2 C" would simply be part of a solid carbon substrate, oh hey like they form syngas.

and the only place that energy can go is back into the kinetic energy of the two atoms, which just pulls the bond apart again.

Not necessarily. See rabi frequency. I wonder if it is possible to do stimulated emission with photochemical reactions; it would be so cool. 137.54.17.9 (talk) 20:23, 17 January 2012 (UTC)

It's not clear they are proposing exactly "no intermediates" for the C+O→ CO (with M involved), just that the concentrations of all three are involved in the rate-limiting part of the pathway. However, they do propose and then argue against:

C + O → CO*

followed later by a discrete reaction of decay of that excited state via collision with M using data from their experiment. DMacks (talk) 20:38, 17 January 2012 (UTC)

I had thought that when two particles collide, then there is a probability of forming a new complex, depending on whether their kinetic energy and their orientation (steric factor) permits it. The new complex then spontaneously decays back into new particles,which may or may not be the same molecules as what collided (and they could be of the same molecular formula but different atoms paired/joined up. In the case of two atoms such as C & O coliding (as distinct from two molecules colliding) there are no bonds to break, but the resulting CO holds less (bond) energy - this just means that the CO complex will form at lower kinetic energies than otherwise. In other words, TenOfAllTrades is incorrect - the resulting CO molecule will carry on with kinetic energy greater than the summed kinetic energy of the reactant C & O - hey - this means a temperture rise, just what we get! Is this wrong? I read the wikipedia article on rabi frequency - I'm none the wiser for it. Keit

Incidentally, I think this is Wikipedia Reference Desk at it's best. Right or wrong in theory, each contributor, and my own effort in explaining it, has improved my undertsanding better than hours of pooring over textbooks. Please keep it up! Keit121.221.234.228 (talk) 02:51, 18 January 2012 (UTC)

Energy density greater than diesel petrol

Are there any liquids that have an energy density that is greater than diesel petrol? ScienceApe (talk) 10:41, 17 January 2012 (UTC)

I think some thicker fuel oils are higher energy density than diesel. This slightly dubious chart claims that #6 oil has significantly more energy than diesel. (However, #6 fuel oil is not something you would put in your car.) APL (talk) 11:35, 17 January 2012 (UTC)

Would #6 fuel oil be useful as a napalm ingredient? ScienceApe (talk) 12:38, 19 January 2012 (UTC)

The list cited by APL gives energy density per volume. The energy in a liquid fuel comes from the hydrogen and carbon atoms. It follows that liquids with a higher density (mass per unit volume) have proportionally higher energy value, and meassured on a energy per unit mass basis, all hydrocarbon fuels have the same energy value, within an insignificant variation range. But measured on a volume basis, the energy content can vary a little bit, as carbon is a heavier atom than hydrogen, so an increased carbon conent leads to greater mass per volume. Keit124.178.159.199 (talk) 12:23, 17 January 2012 (UTC)

Doesn't the energy come from the hydrogen and carbon bonds? (Also, what is "diesel petrol"? "diesel" and "petrol" are two different things.) 86.181.206.2 (talk) 12:51, 17 January 2012 (UTC)

According to diesel fuel diesel is any fuel that can be used in a diesel engine. Diesel petrol is just a way to be more specific about the petroleum derived substance. ScienceApe (talk) 12:59, 17 January 2012 (UTC)

Perhaps this is a regional thing. Are you American? In the UK, "diesel" and "petrol" are two different types of fuel. What we call "petrol" over here is called "gasoline" in the US, so perhaps "diesel petrol" would not sound so odd there. 86.181.206.2 (talk) 13:55, 17 January 2012 (UTC)

No, it still sounds pretty odd. We mostly don't use the word petrol at all; most people probably recognize it, as the British way of saying "gasoline". To the best of my recollection I have never heard anyone refer to diesel petrol. --Trovatore (talk) 17:08, 17 January 2012 (UTC)

That's because no one does. I got the name wrong. According to the article, it should be called petrodiesel. ScienceApe (talk) 19:43, 17 January 2012 (UTC)

Oh, I see — sounds like a back-formation, like snow skiing, motivated by the advent of "biodiesel". --Trovatore (talk) 20:44, 17 January 2012 (UTC)

In USA, it's relatively common to hear people refer to "diesel gas", even though that's technically incorrect. (I guess on the idea that anything you get at a gas station and then put in your car must be a type of gas.) I assumed that the commonwealth speakers did the same with their words. APL (talk) 21:20, 17 January 2012 (UTC)

The chart in the Energy density article is very relevant. It visualizes the distinction that Keit above emphasize. From the chart you can see that diesel beats liquid hydrogen on density per volume, while it beats aluminium and anthracite on density per mass. Considering fuel for transportation, it's appaling to see how bad lithium ion batteries are doing. The chart also seem to indicate that laptops (where volume is key) would not run significantly longer if they were fuelled by hydrogen fuel cells. EverGreg (talk) 12:56, 17 January 2012 (UTC)

Could I burn "aluminium" in an aluminum engine block? What does it cost per gallon? Edison (talk) 14:42, 17 January 2012 (UTC)

A mixture of water and molten aluminium is highly explosive and has been theorized to have played a part in the WTC collapse [1]. Aluminium powder can be used as Thermite, which produces extremely high temperatures. So the energy is there, but nobody has built an engine that runs on it. Yet.. EverGreg (talk) 14:58, 17 January 2012 (UTC)

One of the early internal combustion engines could run on coal dust. It burns at a significantly lower temperature than aluminum, though.

Actually, Space Shuttle Solid Rocket Boosters ran on aluminum. It's a pretty standard material for solid rockets. --Itinerant1 (talk) 20:30, 17 January 2012 (UTC)

How does constructive/destructive interference work in a chorus?

Hi all. When a single voice is singing in a chorus, from a given distance back you hear it at a given volume. When another voice joins in, singing at the same volume, you the listener hear it grow louder. As more voices join in, it becomes louder still. I assume that this is because of interference — the different waves are contributing to make bigger waves. But why is this always the case? Shouldn't the additional waves be just as likely to cause destructive interference? Shouldn't the net effect of a dozen voices cancel each other out? Why does adding another voice (and note, they could be singing something completely different) always make it louder? Thanks! — Sam 63.138.152.219 (talk) 14:53, 17 January 2012 (UTC)

Great question! I'll give it a first try. :-) Each singer produces a lot of different frequencies, so some will cancel destructively and some constructively with other frequencies from other singers. This also depends on where you and the singers are standing. It can be constructive in one place and destructive in another. But your issue remains. This should even out, so that two singers sound as loud as one on average.

That is, if the amplitude of one singer is 2, you get constructive interference with 2+2 = 4 or you get 2-2=0, with the average (4+0)/2 = 2 or one singer. So what's wrong here? Judging from the decibel article, the power is proportional to the square of the amplitude. So an amplitude of 2 results in a volume from the singer of 2*2 = 4. While the amplitude of two singers in constructive interference gives a volume of 4*4 = 16. If you then average constructive and destructive interference you get (16+0)/2 = 8. Then we're back with our initial intuition: Two singers produce a volume twice as high as one.

But since decibles involve a logarithm, this isn't quite right, nor is our intuition. The relevant formula, with a handy calculator, can be found here: [2] EverGreg (talk) 15:28, 17 January 2012 (UTC)

Very interesting. So is this saying that, while the average amplitude of the waves that hit our ears will still be the same ("2", in your example), the average power will be greater? That's pretty surprising. — Sam 63.138.152.219 (talk) 16:09, 17 January 2012 (UTC)

No—the amplitude does vary from 0 to 4, but the average isn't 2 because it isn't symmetrically distributed. For incoherent superposition the average works out to 8/π ≈ 2.5, but I don't think this is physically meaningful. The square of the amplitude is proportional to the energy of the vibration and simply adds, as you'd expect. -- BenRG (talk) 01:20, 18 January 2012 (UTC)

Yes, but on top of that, a doubling in energy does not double the perceived volume, because our ear (and the decibel scale) has a logarithmic relationship between sound energy and volume. EverGreg (talk) 14:21, 19 January 2012 (UTC)

Interference only happens when waves are coherent. Sound from two different singers is never coherent, therefore neither destructive nor constructive interference occurs. The intensity of the waves adds linearly: two singers produce twice the intensity of one. In theory, if two singers sang exactly the same frequency to much better precision than the human ear can distinguish, their voices could interfere. This isn't going to happen in practice, though. --Srleffler (talk) 17:48, 17 January 2012 (UTC)

That doesn't sound like a correct explanation. Humans can most certainly perceive interference that occurs between two humanly distinguishable pitches. Humans can hear pitch differences down to about 6 cents, which for example for a 440 Hz pitch (A above middle C) is 2.6 Hz, well below the roughly 15 Hz cutoff below which audible beats can be perceived. For a pitch difference of considerably more cents (and/or between higher pitches), the interference can instead be perceived as a difference tone, but it's audible interference occurring between distinguishable pitches either way. Red Act (talk) 19:03, 17 January 2012 (UTC)

Relatedly

I don't want to hijack the above question (please feel free to reply to the section above!) but I had a related question that came to mind. When I was a child (well before puberty), I could at will make a kind of high pitched ringing voice in the back of my throat. It was quite loud. I can't quite describe how I did it — it wasn't just screaming or shrieking, but sort of interior whistle. It wasn't just me that could do it, there were a few other boys who could as well. Anyway, if two of us did it at the same time, it would produce a very strange sound, almost electrical sounding in nature (like the sound a rippling electrical transformer would make in movies), or something somewhat similar to what a cicada sounds like. (I'm having trouble describing this, like most qualia.) My real question is what caused this very strange and really quite distinct sound. My guess is that the frequencies being emitted by me and the other boys were more or less fairly close to being pure tones of some sort, and the differences between them were somehow creating some kind of dynamic interference. Does that make any sense? (Is there a name for this kind of sound that young children can produce?) --Mr.98 (talk) 19:23, 17 January 2012 (UTC)

If the frequencies were close enough, probably they could produce beats. Have you tried it out as an adult? :P Lynch7 19:31, 17 January 2012 (UTC)

Agreed that beating of two similar frequencies is a key player. Note that the beats will drift dynamically. I had a very similar experience to Mr.98 as a child, with the exception that my high-pitched sound was not especially different than a normally-voiced falsetto. The phenomenon does sound rather different than say, listening to someone else tune a flute to a tuning fork. I think this is because, if you are producing one of the two voices, then, in addition to the beats in the sound waves carried by the air, your skull is beating too :) SemanticMantis (talk) 20:09, 17 January 2012 (UTC)

Good point. Your voice will be sorta interfering with the beats as well, so all in all, quite a weird sound. Lynch7 20:39, 17 January 2012 (UTC)

Re: "Is there a name for this kind of sound . . .", Overtone singing and its links may be of relevance/interest. {The poster formerly known as 87.81.230.195} 90.197.66.192 (talk) 09:22, 19 January 2012 (UTC)

Gravitational time problem.

On earth I am attracted by a force = G * m1m2/r^2

However what happens if there is say a globular cluster 100 light years away. I did not exist 100 yrs ago so It cannot be the above formula there must be a time element. — Preceding unsigned comment added by 92.30.204.80 (talk) 17:25, 17 January 2012 (UTC)

The formula you gave applies only for two masses at a single instant in time. If there is another mass (your cluster) or a mass which is changing over time (you?) or relative movement of the various objects, then things get more complicated. Basically a time element will appear because m1, m2, and r will not be constants but each will be a function of t. Staecker (talk) 17:38, 17 January 2012 (UTC)

You're quite correct, there is a time element to gravity. Gravity is most accurately modeled with general relativity, which does take speed-of-light considerations into account. The simple Newtonian gravity model expressed by the equation above can be viewed as an approximation to general relativity, that only works well under a certain set of circumstances. Red Act (talk) 17:51, 17 January 2012 (UTC)

[edit conflict]You are correct that the formula you give is not appropriate for that situation. You need general relativity to deal with gravitational forces between very distant objects. --Srleffler (talk) 17:52, 17 January 2012 (UTC)

If you don't like Einstein tensors, it's possible to use a formulation analogous to the relativistically-correct Liénard–Wiechert potential for electromagnetism. You simply replace time with retarded time in all the relevant places in your equations, and presto - a general, relativistically-correct formulation of gravity. (Not to be confused with a general-relativistic correct formulation of gravity). There's no shortage of research on the applicability of this approach: Google found almost 2,000 research papers; and I'm pretty sure I saw this worked out to completion in a physics course-note some time ago. The moral is, you don't need general relativity, unless you need it. Nimur (talk) 19:41, 17 January 2012 (UTC)

In addition to the Newtonian model, there are dozens of other alternatives to general relativity, some of which are consistent with special relativity, and some of which avoid the use of tensors. One historical example that uses retarded potentials is Whitehead's theory of gravitation (although that one does use tensors). Like the Newtonian model, the other alternatives all work well in some circumstances, and give wrong answers in other circumstances. Only general relativity always gives the right answer (although it has problems dealing with quantum mechanics, and it has awkward singularities). Red Act (talk) 01:34, 18 January 2012 (UTC)

I'm not sure if everyone's got what the OP is on about. I think the question is really about how the gravity formula can even make sense, given that there is an attraction between two things that are so far apart, the concept of their simultaneous existence is mucked up. In fact, if I've understood rightly, it's not that complicated. Whatever general relativity involves, it reduces to Newtonian physics in most cases (I don't know how big the corrections are for a distant star). So the gravity that you feel is from the star as it was 100 years ago, and in simple calculations, you don't need to know anything more. If it got destroyed about 50 years ago, that will have no effect on the current gravitational pull. Same for your gravitational pull on the star, ie. at the moment, it doesn't sense any pull from you, since your existence is outside its light cone. IBE (talk) 20:11, 17 January 2012 (UTC)

When I was in school ca. the early 2000s, our astronomy professor taught us that it wasn't entirely clear how fast gravitational force propagated. That is, if the Sun blinked out of existence right now, how long until we felt the gravitational effects on Earth? At the time I recall some science writer (Brian Greene?) suggesting it would be instantaneous, while my professor (a serious astronomer) said that this was wrong, that it would propagate at c. I see we have an article on speed of gravity which more or less agrees with my professor. --Mr.98 (talk) 20:33, 17 January 2012 (UTC)

School in the 2000s? Then you must be fairly young, and I don't know where (or rather when) you cram this knowledge in. I doubt it was Brian Greene, since The Fabric of the Cosmos explains quite carefully the speed of gravity is exactly c (p72, 2004 ed). IBE (talk) 20:50, 17 January 2012 (UTC)

By the early 2000s, I think it was pretty well established that gravity didn't act instantaneously. Reference 17 of the article you link to is dated 2001 and describes experimental evidence for gravity travelling at approximately the speed of light. --Tango (talk) 21:10, 17 January 2012 (UTC)

It was probably in 2000 or 2001, so that probably would have been pretty fresh stuff at the time. (It was right around when all the evidence for inflation came out — the professor in question had some small role in one of the experiments involved in detecting that.) I think it must have been Greene's Elegant Universe. The preview pages I can see online don't have the right pages in them (I think it is 73 or 74? He talks about time and GR there.) so I can't recall who was right and who was wrong — the positions might have become switched in my mind. (And I've long since given away my copy at a yard sale.) But I remember it being a subject of contention. --Mr.98 (talk) 01:21, 18 January 2012 (UTC)

It was controversial once, but I thought it was settled by the observations of PSR B1913+16 in the 1970s. I feel obliged to point out that general relativity requires energy conservation, so you can't extract any prediction about what would happen if the sun blinked out of existence. It has to go somewhere, and whatever causes its sudden departure has to come from somewhere, and whatever that is, it gravitates too, because everything gravitates. That's why the whole thing was controversial: in contrast to the electromagnetic case, you can't reach in with an uncharged manipulator arm and push your charged particles around. Gravity "knows" about everything, and that made it plausible that there might be no need for gravitational waves to correct the long-distance field when the gravitating object's state of motion changed. The other problem was/is figuring out what it even means for a gravitational wave to propagate at c, given that a gravitational wave is a distortion of the spacetime geometry that gives meaning to the speed c in the first place. -- BenRG (talk) 02:27, 18 January 2012 (UTC)

'Blue' Veins in the Heart, elsewhere?

So I understand that blood in the human body is never blue. However, veins do look blue from the skin. The wikipedia article claims that this is a side effect of light absorption through the skin. However why does the human heart have visible blue veins? See this photo for an example. --188.220.46.47 (talk) 19:38, 17 January 2012 (UTC)

Veins are not blood, veins are vessels that carry blood. If I have a green hose, it does not make the water inside the hose green. --Jayron32 16:44, 19 January 2012 (UTC)

lens as spatial filters

A focusing lens focuses light focused at infinity into some image 100 cm away.

Does it matter if a diffraction filter is placed before, or after the lens? I mean, a lens is an inverse fourier transform right? And a diffraction screen forms a fourier transform? Do the operations commute, so long as the diffraction screen is placed well before the image plane? 137.54.17.9 (talk) 19:49, 17 January 2012 (UTC)

A lens is a lens; it's not a fourier transform. The operation that a lens performs can be modeled by a fourier transform.

As far as commutation of optical elements: to first order, some optical elements commute; but you should be aware of the real-world effects that countermand that. All (passive) optical elements attenuate the light somewhat. Imperfections in the optics are usually unpredictable and therefore irreversible in practice. Changing the optical path changes the scale sizes you need. (For example, if you magnify an image, your next optical element needs a larger aperture; in other words, aperture is non-commutative if you have magnifying optics).

I don't know how you would place a diffraction grating in the optical path. How would the light go through it? Nimur (talk) 20:18, 17 January 2012 (UTC)

Wouldn't the collimated beam get diffracted and then get focused later? 137.54.17.9 (talk) 20:52, 17 January 2012 (UTC)

Most classical theory considers infinitely thin apertures. What happens when the spacings have thickness, as in real life? Do you get a 3D fourier transform? Do you get optical aberrations because now different orders of light are diffracted and focused differently? 137.54.17.9 (talk) 20:32, 17 January 2012 (UTC)

What you get is "a thing that can't be accurately modeled with the thin lens equation." For this reason, we have optical ray tracing and full wave equation modeling. Such mathematical models take into account the way that waves interact with sophisticated material properties and complicated geometries. Nimur (talk) 22:45, 17 January 2012 (UTC)

Searching for medical term (not advice)

From personal experience, when I go to the doctor for a regular check-up, one of the first things they do is check my blood pressure. It is invariably always a little higher when they check it at the office, as opposed to when I check it at home. I attribute this to just being nervous- or a little high strung- about the whole doctor's office experience. Is there a specific medical term for this? Note: I am looking for a specific medical term (if it exists). I realize that "false-positive" or "placeabo effect" might loosely apply...but I imagine there is an actual term found in medical texts. Anyone know? Quinn ^➳WINDY 21:00, 17 January 2012 (UTC)

White coat hypertension. DMacks (talk) 21:01, 17 January 2012 (UTC)

Excellent, thank you. The converse term Masked hypertension (found in the article) is also very useful info for my project. Cheers! Quinn ^➳WINDY 21:10, 17 January 2012 (UTC)

My own doctor has told me that blood pressure can vary quite a bit during any given day. What they're looking for is trends. There are various tests your doctor can schedule for you if your blood pressure is consistently elevated or high. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:06, 18 January 2012 (UTC)

Well, no matter how many BP tests are scheduled at the doctor's office, if every one makes you nervous, then that approach is no good. Perhaps you might get used to them over time. If not, home testing would seem to be the only alternative. No doubt the docs are worried those aren't accurate enough, though. Some way to do highly accurate home tests is apparently needed for these patients. StuRat (talk) 00:27, 18 January 2012 (UTC)

That isn't necessarily true. As our article notes and should be obvious, most trials regarding the risks of high blood pressure are based on blood pressures recorded in clinical settings. So most of the values we have for risks are based on the value recorded in clinical settings, not any lower values you may record at home. Furthermore, if you're having your blood pressure recorded regularly, your still likely to detect changes (they may be slightly masked, although that's unclear to me from the article).

Of course since the effect varies between individual, it's likely best (as our article also hints at), for further studies to be done on the risks associated with blood pressure based on values not recorded in a clinical setting. But even this doesn't mean the clinical value is useless once we have such information, and it would only make sense to consider non-clinical values. In fact our article appears to suggest it may provide additional useful information about risks, which isn't that surprising.

Nil Einne (talk) 05:46, 19 January 2012 (UTC)

Also, the blood pressure meter you are using at home could be faulty in a way that it consistently measures lower than real pressure. – b_jonas 16:35, 21 January 2012 (UTC)

Deepest drilling

Apologies for the multiple questions (I'm trying to get through some of my to-ask list before the big black banner at the top of the page reaches the Zero hour), but both this one and the above are actually related to a project I'm working on...I'll spare you the boring details. So, the question is, what is the deepest anyone has drilled into the Earth? To clarify, I am looking for "closest to the core" as opposed to "from sea level". And, if possible, I'd like to know what, if any, technical limitation prohibit someone from drilling deeper. Is it just because there has been no reason to go deeper...or is it temperature...pressure...or perhaps the drill itself become too weak after a certain point? Anyway, any links to articles or outside resources about this are appreciated. Quinn ^➳WINDY 21:17, 17 January 2012 (UTC)

The Mohorovičić discontinuity article lists several attempts (each in turn with their own articles linked) to drill down that deep. -- Finlay McWalterჷTalk 21:20, 17 January 2012 (UTC)

Take a look at the articles that are linked to in the Mohorovičić discontinuity article as well. Project Mohole, Kola Superdeep Borehole and Sakhalin I. However, the Sakhalin I wells are "extended reach boreholes" which means the well path turns underground and goes horizontal. So, the Sakhalin depth of 12,345 meters (40,502 ft) is actually only about 1000m deep.

As for the problems, everything you mentioned above, is part of the reason. The conventional procedure to drill a well uses a motor on surface to turn the drill bit. The drill bit is connected to the surface by drill pipe, which comes in about 10m/30ft joints, which can be combined into 30m/90ft stands. As the bit drills through the earth, fluids called drilling mud are pumped though the drill pipe and come out drill bit. The mud cools the bit and helps wash cuttings (ground up rock) up hole. You have to remove the cuttings to keep drilling. Mud also helps to prevent blowouts, which happen if the pressure of the fluids in the rocks exceed the pressure in the borehole. Blow outs are bad. Once you've drilled 30m/90ft, you have to stop drilling for a minute and connect another stand of drill pipe, using the derrick and another motor to lift and move the stand. Each stand is heavy. Drill pipe can weigh over 100lb/ft (130kg/m) so a stand can easily weigh as much a couple cars. So, the deeper you go, the more powerful your drilling motor and derrick motor have to be and the stronger your derrick has to be. So the first reason is every bit of depth you drill adds a lot of more weight.

While you drill the bit will start to wear out. It wears out faster at higher temperatures. While mud can help with this, there are problems with mud at very deep depths as well. Once the bit wears out, you have to pull out the drill bit and replace it. A good crew can pull a stand (move the drill string up by 90ft) about every 5 minutes. So at 40,502 ft, it takes at least 37hours to pull the bit and 37 hours to run back in. So if it takes 3 or more days every time you have to change the bit, and you have to change it pretty often that deep (due to temperature and other factors) that really adds up and costs a lot of money. A deep water drilling rig may cost $2-3 US per second to operate. So the second reason, bits wear out faster due to temperature and take longer to replace and cost a lot of money.

Finally, the mud system gets more complex. You have design muds that won't boil at temperatures over 400F and will be viscous enough at those temperatures to bring up cuttings, but not be too viscous at surface temperature. Exotic muds are very expensive. Then you get to the point where it's impossible to get the properties you need with conventional fluids, so you have to go to experimental fluids, which cost even more. Eventually you get to the point where you can't get a fluid that does everything you need, and have to design much more complex systems. You also need a lot of fluid moving very fast to cool the bit, so you have to have larger and more powerful mud pumps to get flow rate necessary to cool the bit. Which adds more cost. The third reason is the cost and complexity of the drilling mud.

Even with an infinite amount of money, conventional drilling eventually hits a point where it becomes technologically impossible to go any deeper due to the temperature, ie steel or titanium melt, fluids break down from temperature and pressure, derricks and motors can't be made any stronger.

the ;tl:dr version. The deeper you go the hotter it is, the longer it takes and the more it costs until the equipment just can't do it. And the cost and time don't increase linearly, but maybe exponentially. Tobyc75 (talk) 22:01, 17 January 2012 (UTC)

What a nice and clear explanation. Thank you. --Mr.98 (talk) 12:58, 19 January 2012 (UTC)

I concur. Just got back after the black out, and was pleasantly surprised to find this very informative and detailed answer. Thank you for the time you spent on this! Quinn ^➳WINDY 16:53, 19 January 2012 (UTC)

Thanks, for the kind feedback. Tobyc75 (talk) 22:41, 19 January 2012 (UTC)

If you launch unmanned probes in space, you get to show nice close-up photos of Jupiter moons to show to the public. But how would you convince a senator to approve money for a project to drill down deeper than ever? According to Irregular Webcomic strip 2835, geology has always been the abandoned foster child of science. – b_jonas 16:34, 21 January 2012 (UTC)

January 18

What was "opening medicine"?

I heard an Australian author on the radio today talking about her books, based around the life of her convict ancestor, William Wiseman. She asked the audience if anyone knew what 'opening medicine' was. There were groans and laughs from the audience and she asked those in the know to explain, on the quiet and if they felt they could, to the others. In her book it is a treament for worms in children. Could you find this out please? — Preceding unsigned comment added by 137.92.97.212 (talk) 03:18, 18 January 2012 (UTC)

A medical book from the 1840's used "opening medicine" to describe laxatives. (The editor formerly known as Edison). BnBH (talk) 05:13, 19 January 2012 (UTC)

In other words, "that opens the bowels" (OED).--Shantavira|^{feed me} 11:45, 19 January 2012 (UTC)

January 19

superconducters

what is the value of resistance offered by superconducters connected in parallel? (0/0,1/0,0) — Preceding unsigned comment added by Sanoy samuel (talk • contribs) 05:51, 19 January 2012 (UTC)

It will be very small, approaching zero for ideal superconductors. You can solve this analytically by applying L'Hôpital's rule to the parallel resistor equation, with R1 and R2 each approaching zero for ideal superconductors. Nimur (talk) 06:15, 19 January 2012 (UTC)

$\lim _{(R_{1},R_{2})\to (0^{+},0^{+})}{1 \over 1/R_{1}+1/R_{2}}=0.$ -- ToE 06:21, 19 January 2012 (UTC)

Blot (biology)

Was the term 'blot' in use for laboratory techniques before the invention of the Southern blot method? --NorwegianBlue ^talk 08:52, 19 January 2012 (UTC)

The OED seems to imply that the use of "blot" as a means of recording proteins onto film doesn't show up until Southern blot. Browsing around results in JSTOR for "blot" in biological journals doesn't seem to bring up any consistent technical usage of the term prior to Southern blot. The place to really look is where the term "Southern blot" comes from. (I know the inventor's name is Southern, but did he call it a "blot" or did others coin the term?) It's not in the original paper by Southern linked to in the article; the only use of "blot" is the sentence: "After the appropriate period, strips are removed from the solution or paraffin oil, blotted between sheets of filter paper and washed, with stirring, for 20 to 30 min in a large volume of the hybridization solvent at the hybridization temperature." The usage of blotting paper seems responsible for the term. I can't help but wonder if it was influenced by the Rorschach inkblot. This isn't meant to be a conclusive answer, just what I was able to come up with. There doesn't seem to be much out there on the history of the Southern blot, unfortunately. It would make for an interesting historical study in the proliferation of laboratory techniques. --Mr.98 (talk) 19:27, 19 January 2012 (UTC)

Nice answer. I'll only add that, publishing his work in 1975, Southern was likely fairly familiar with blotting paper, so that "oil, blotted between sheets" wouldn't need any additional referents. SemanticMantis (talk) 20:05, 19 January 2012 (UTC)

Also, blotter paper is used in thin layer chromatography, which was developed in the 1940s, according to History_of_chromatography#Thin_layer_chromatography. So, if biologists were using TLC in the 1960s, they may well have been using "blot" in a similar context before the Southern blot technique was invented. SemanticMantis (talk) 20:13, 19 January 2012 (UTC)

Thanks a lot, both! I've just received the 1975 paper from my library, which confirms that Edwin Southern didn't use the word 'blot' as a noun at all, only as a verb, and only in the sentence that Mr.98 quotes. I really liked the reference to the Rorschach blot, which predates the Southern blot by half a century! Blotting paper indeed must be the origin of the term. Although blotting paper is used in chromatography, I've never heard the term 'blot' being used to describe a chromatogram. I think the reason is that in Southern/Northern/Western/etc blots, the action occurs somewhere else (gel electrophoresis), and the separated mixture of biological substances in transferred -- blotted --- onto blotting paper. In chromatograpy, however, the separation occurs in the filter paper itself, and there is no blotting step. --NorwegianBlue ^talk 21:45, 19 January 2012 (UTC)

Evolution again...

Well, this time I want to ask about two things I can't relate about genes and evolution.I can't relate the "all or none" nature of genes and how they work, and the smooth, gradual nature of evolution(or even smooth differences between individuals, which can be explained easier by considering the large number of genes...or can it?).I mean in cases like evolutionary arms race for example, it's often said that for example, lions become faster as the zebras become faster.What does this "getting faster" mean in genetic terms.I mean genes don't get "amplified".Does each little level of getting faster require a different genetic change?

Also is this right to say that when the environment doesn't change in a long time, some traits in species tend to be exaggerated?(please don't sacrifice any of my questions for the other, this is the last level for me to get a satisfying picture of how evolution works, or maybe one of the last ones!)

Last question: is it true that chromosomes only form during cell division?--Irrational number (talk) 15:56, 19 January 2012 (UTC)

For question 1) it isn't exactly clear that genes (DNA) are the ONLY thing which affects evolution, there are also epigenetic factors, research in this field is somewhat nascent, but there is considerable evidence that some heritable characteristics can be passed outside of nucleic acids. Also, there is not universal agreement among the scientific community that evolution is constant and gradual. There are two competing theories on the rate of evolution (and, as always, reality probably lies somewhere between them). The kind of evolution you are talking about is called phyletic gradualism, while the competing theory, which says that evolution occurs in fits and starts, is called punctuated equilibrium. Both ideas are probably at work in evolution. The reason lions and zebras both get faster has to do with natural selection: in a very broad sense lions that are too slow to eat zebras starve to death and thus don't get to have slow babies to pass their slow genes onto, while zebras that are too slow to avoid lions don't get to have slow babies to pass their slow genes onto. For your last question, yes, the actual "chromosome" structure you recognize in the pictures only forms during certain phases in the cells lifetime. At many times, the individual chromosomes aren't visually identifiable, see Chromatin which has some nice descriptions on the various forms it takes. The chromasome structure most people will recognize seems to happen during the metaphase part of the cell's life cycle. --Jayron32 16:40, 19 January 2012 (UTC)

On the very first part of your question regarding the 'all-or-nothing' 'present-or-not' nature of genes, you may want to have a look at our article on gene expression. Getting from a particular DNA sequence to a visible trait is a multistep process that can be tweaked all the way along by a pretty huge number of factors. There are some very complex mechanisms that regulate the effects of single genes. (If you think about it, your own body is a powerful demonstration of this. Trillions of cells with the same DNA, yet some of those cells are germ-hunting macrophages that can engulf and eat bacteria; some are two-meter-long neurons that can electrically link your brain to your toes with a single cell; some are hair-making trichocytes that spin out nothing but fibers all day long. If you were to separate out all these different cell types and look at them in a petri dish, you'd have no idea they all came from the same organism—yet their genes are identical.) TenOfAllTrades(talk) 16:59, 19 January 2012 (UTC)

hm... I had the very question that "why are the cells in my body different when they are genetically identical" here in refdesk...--Irrational number (talk) 17:12, 19 January 2012 (UTC)

(edit conflict)x2 Well we obviously can't enumerate which genes (and proteins, etc.) specifically make modern lions and zebras faster than their ancestors, but the overall genetic change meant they are phenotypically expressed to make the animals faster. It doesn't necessarily mean it has to be the same gene again and again, as genes aren't usually that discrete (e.g. there is no "fast gene"), but sometimes it does.

The evolutionary arms race is best explained by the Red Queen's Hypothesis. The name comes from Red Queen's race in Lewis Carroll's Through the Looking-Glass, from a conversation between Alice and the Red Queen:

"Well, in our country," said Alice, still panting a little, "you'd generally get to somewhere else — if you run very fast for a long time, as we've been doing."

"A slow sort of country!" said the Queen. "Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"

Imagine that species (or even different sexes in a species) are constantly in a coevolutionary arms race to outwit the other. As soon as one overtakes the other in weapons/skill/reproductive potential, the other must adapt or die. A classic example would be the extreme toxicity of the rough-skinned newt (Taricha granulosa). Why is it so poisonous when only a tenth of its poison could already kill a full-grown man? The answer lies in its predator - the common garter snake (Thamnophis sirtalis). The snakes continuously adapt to the toxins of the newts so they can eat them. In turn, newts continue to become more poisonous so they can avoid being eaten. Snakes who have lesser resistance to tetrodotoxin die and thus only the snakes with the strongest resistances pass their genes to the next generation. Newts with less lethal poison get eaten, so only the more poisonous pass on their genes to the next generation. Each drives the other's adaptations, and in turn, evolution, until you end up with newts with extreme amounts of poison and snakes with extraordinary resistance to that poison due to the positive feedback. To an outsider species not part of the coevolution, the adaptations of the two seem too much; but to both species, they are simply maintaining the status quo. The number of newts getting eaten and the number of snakes dying is probably the same as when the two first began their predator-prey relationship (hence "running to stay in the same place").

And yes, the rate of evolution is still unclear but it may be both gradual and slow over long periods of time, or rapid in short bursts.

As for the second question, not necessarily. Isolation can induce allopatric speciation usually through genetic drift, and different environments induce different evolutionary pressures. The longer an organism is in a certain environment, the degree into which it has evolved to fit that environment is usually greater. Not always though, some environments can exert enough different evolutionary pressure to keep organisms always "on their toes" so to speak, so it could readily adapt to new environments. Some have limited effect such that the organisms begin to "forget" their former "malleability". Instead, their evolution is focused on exploiting the environment they are in as best as they possibly can; with the disadvantage that if the environment were to abruptly change or if they find themselves in another environment, they would be more vulnerable (e.g. whales have adapted so much to life on the sea that beaching them will mean death).

And your last question is a bit vague. If you meant the formation of genetic material, see mitosis and meiosis. Chromosomes are DNA. They divide and reorganize during these phases, but they do not form out of nothing. If you meant the visible clumped chromosomes (chromatin), refer to Jayron's answer above.-- Obsidi♠n Soul 17:40, 19 January 2012 (UTC)

Genetically engineering healthy foods

So far genetic engineering has been done to benefit farmers, but not consumers (except perhaps by lowering price). For example, crops have been engineered to be either disease or parasite resistant, or to better tolerate pesticides. Now my questions:

1) Is anyone working on genetically engineering foods to actually be healthier (or perhaps to make healthy foods more palatable) ?

2) For a specific example, could chicken eggs be engineered to have more good cholesterol and less bad cholesterol ? StuRat (talk) 18:27, 19 January 2012 (UTC)

1) See golden rice. Also, I would not say that GMO crops that increase yield "benefit farmers, but not consumers". Independent of price, genetic engineering, as well as "traditional" techniques of hybrid crops and artificial selection, have dramatically increased worldwide food production over the past ~50 years, effectively increasing the carrying capacity of Earth. SemanticMantis (talk) 19:20, 19 January 2012 (UTC)

The infamous Flavr Savr was clearly intended to be appealing to consumers even if it failed. In addition to what SM has said, note that disease and parasite resistance, and in some cases even better tolerance of pesticides may benefit consumers in ways besides yield. For example by reducing the amount of pesticides that have to be used, or allowing the use of potentially less dangerous pesticides (in the case of pesticide tolerance). (In a more roundabout fashion, they may in some cases reduce environmental damage which is likely to be a benefit to consumers living near the farmers and perhaps even some further away. And remember with the carrying capacity/yield thing, it not only means you can support more people at a lower price, but for a given number of people use less land.) I expect the controversy of GMOs means that there's less incentive for more obvious benefits to consumers. If your product is healthier or more palatable, you're likely going to need to label it as such. And when you label it, some people will start to ask why and will quickly find out it's from a GMO. Nil Einne (talk) 05:07, 20 January 2012 (UTC)

But the problem is people's perceptions of GMO foods. If there was something demonstrably healthier, like the egg I mentioned, perhaps this would overcome people's reluctance to embrace the "unknown". Specifically, something "improved" that actually shows up on the nutrition label. StuRat (talk) 05:14, 20 January 2012 (UTC)

That's a nice theory. When you're spending millions of dollars developing, marketing etc a GMO however, you have to live in the real world where your theory could easily fall flat on its face, particularly in Europe. Nil Einne (talk) 15:54, 20 January 2012 (UTC)

That's my point exactly, GMO foods without any obvious consumer benefit have fallen flat in Europe, so it's time they try something else. StuRat (talk) 16:02, 20 January 2012 (UTC)

2) From the looks of it, egg cholesterol can be modified simply with drugs or diet, which is probably cheaper. The fact we haven't been able to breed low-cholesterol chickens also implies that so far we have no way knowing which genes are responible for egg cholesterol levels. It does seem at least theoretically possible - chickens produce more cholesterol than their embryoes need, so GM chickens would still be able to breed. Smurrayinchester 09:54, 20 January 2012 (UTC)

Benefit to farmers, but not consumers? Norman Borlaug is often credited with saving over a billion people worldwide from starvation (he used multiple techniques, not just GE food). Von Restorff (talk) 12:32, 20 January 2012 (UTC)

To clarify, I mean obvious benefits to those consumers who can freely choose between GMO and non-GMO foods. This doesn't apply to those in third world nations, who will eat any food they can get, even if it glows. To some extent, it doesn't apply in the US, where GMO foods are not labelled as such. However, some consumers will still find out which brands use GMOs and avoid them. Europe is probably the biggest market where consumer perception of the value of GMOs is critical. StuRat (talk) 16:06, 20 January 2012 (UTC)

• Point of clarification / mincing words: GMO crops are often designed to be herbicide resistant, e.g. Roundup Ready soybeans, or to be pest resistant, e.g. Bt corn. Pest resistant crops may allow for lower or less use of pesticides. In contrast, pesticide tolerance is not a normal goal of GMO crop design. Commonly used pesticides, even the most noxious (e.g. Sevin), don't harm the conventional crops they are applied to. SemanticMantis (talk) 15:35, 20 January 2012 (UTC)

See pesticide. Herbicides are a class of pesticides. You're correct herbicides are by and large the only pesticides for which tolerance is engineered in plants (for obvious reasons). Nil Einne (talk) 15:44, 20 January 2012 (UTC)

Duly noted, thanks. Now I'll start correcting my weed science associates when the get sloppy with terminology ;) SemanticMantis (talk) 16:16, 20 January 2012 (UTC)

Genetically_modified_tomato#Improved_nutrition lists a couple of examples. SmartSE (talk) 17:23, 20 January 2012 (UTC)

Why is it easier to make an electric car than it is to make an electric airplane?

^Topic ScienceApe (talk) 20:28, 19 January 2012 (UTC)

Weight of the batteries. AndyTheGrump (talk) 20:32, 19 January 2012 (UTC)

... Rather, energy density of the batteries. Avgas contains some 100x more energy per kilogram than a lithium ion battery. Nimur (talk) 20:40, 19 January 2012 (UTC)

Yup, strictly speaking, you're right. Plus, using Avgas, the plane gets lighter as you use it up - so you need less power, which makes it even more efficient. AndyTheGrump (talk) 21:28, 19 January 2012 (UTC)

Note that it's not being electric that's a problem for airplanes, it's storing the electricity. Solar-powered electric motor airplanes do have some potential, in the form of an unmanned, light-weight, observation platform. StuRat (talk) 04:16, 20 January 2012 (UTC)

Induced gamma emission may be of tangential interest: among the potential applications is powering electrical airplanes. 157.193.175.207 (talk) 09:11, 20 January 2012 (UTC)

And to state the obvious, but that completes the answer to the questions: The car has all 4 wheels continuously touching the ground which supports it, so the weight is less of an issue (although it is still a bit of an issue).--Lgriot (talk) 09:13, 20 January 2012 (UTC)

There are a couple of battery electric aircraft in development, such as the Sonex ESA and an electric version of the Pipistrel Panthera. The problems encountered are a very short range and aircraft endurance (by modern standards). With a normal engine (and a typical fuel system) these aircraft would far outpace themselves in terms of performance. The electric Panthera is intended to have a range of 215nm (400km), per the company website, and sacrifices two of the four seats to accommodate the batteries. The all gasoline version is intended to do 1,000nm for comparison. I personally think that battery powered aircraft technology will creep into the market and become somewhat popular, if for no other reason than with prices often at $5 a gallon or more for AvGas in the United States (and therefore probably much more elsewhere), many of us simply cannot afford to fly a lot of the gasoline powered aircraft very much anymore. For local sport-type flying, I suspect that the electric aircraft would cut down on the costs tremendously (after initial purchase). Falconus^{p t c} 20:04, 20 January 2012 (UTC)

Photosynthesis Poster

Hello. Where can I buy a poster of photosynthesis appropriate for university? I prefer a supplier that can deliver to Canada. Thanks in advance. --Mayfare (talk) 20:55, 19 January 2012 (UTC)

My first reaction was Fisher Scientific, fishersci.com - where I found this photosynthesis poster. You'll have to decide if it's appropriately detailed; there are a few other items - lab kits, CD ROMs, and so on. Nimur (talk) 21:07, 19 January 2012 (UTC)

As a supplement, there are nice graphics in our article on Photosynthesis as well (more in Commons:Category:Photosynthesis). You can print them freely (and even sell them), with the only requirement being that you provide attribution to the artist(s).-- Obsidi♠n Soul 21:38, 19 January 2012 (UTC)

Several of those charts are not in English. If you need assistance translating or modifying them, you can post a request for translation here on the reference desk, or on WP:TRANSLATE. Nimur (talk) 01:36, 20 January 2012 (UTC)

Forging a ring from fountain pens

I'm into forging a ring from iridium-platinum (or iridium-osmium) tips of fountain pens (according to some sites, 30-40 tips for one ring is sufficient). The idea is to melt those tips in a form and then cool the alloy down. Is there a relatively simple and cheap device to create the appropriate melting temperature? I'm also concerned about the proper way of cooling, thanks. --46.204.99.30 (talk) 22:46, 19 January 2012 (UTC)

Not being funny but I think you'd be better off finding some other to do. The time taken to achieves this, would make it more economical to use other sources of platinum. The pen tips have only enough plating to ensure that one has a non corrosive tip which will allow ink to flow smoothly to the paper. Your sources may be misleading you. --Aspro (talk) 23:14, 19 January 2012 (UTC)

Iridium, platinum, and osmium all melt quite a bit hotter than any steel you've ever worked. Are you a skilled metallurgist? Do you have a furnace? High temperature crucible? Oxy torch? No, there is no way to do this on the cheap-and-easy. This article seems to be a pretty good overview. Nimur (talk) 03:26, 20 January 2012 (UTC)

In which U.S. states are capybaras legal?

In which U.S. states are capybaras legal as pets? I'm hoping for a link that will show all states instead of having to go to each state individually. Thanks, 72Dino (talk) 23:53, 19 January 2012 (UTC)

In urban areas of the Southern U.S. (like Houston and New Orleans especially) you can find nutria living in drainage ditches and sewers and the like. Nutria are very similar to capybara, so perhaps if you hang out with some of the homeless in New Orleans, one will just come along and decide to be your pet. No need to import yet another invasive giant rodent into the U.S. One is enough, thanks. --Jayron32 01:50, 20 January 2012 (UTC)

However I'm guessing if you're Catholic, you can't eat your pet nutria during lent if the urge ever arises. Nil Einne (talk) 05:13, 20 January 2012 (UTC)

This page has a summary on laws about exotic animals in each state. None of them specifically mention capybaras.

Many states ban or place severe restrictions on "inherently dangerous animals" (Virginia), "potentially dangerous animals" (Connecticut), etc, laws which are sufficiently flexible that they might apply to capybaras. However, many of these laws have grandfathering provisions, so if you had a capybara for long enough you could keep it.

Realistically if an animal is sufficiently unusual, a state may never have considered if it should be allowed. Most states have a State Veterinarian who will answer questions on this sort of thing. --Colapeninsula (talk) 12:43, 20 January 2012 (UTC)

crazy

No medical advice. See talk page.
The following discussion has been closed. Please do not modify it.
how do i know that im not crazy? I cant remember where i learned it but i know crazy people doesnt know their condition, so what assurance do i have that everything i see and does is normal? — Preceding unsigned comment added by Arah18 (talk • contribs) 23:57, 19 January 2012 (UTC) Anosognosia (an unawareness of one's condition) is common with some mental illnesses, but certainly not all of them. The way to find out if you yourself have a mental illness is to ask a psychiatrist. Red Act (talk) 00:08, 20 January 2012 (UTC) If I show you a proof that you are not crazy, would it help? How could you know that you are judging the validity of the proof correctly? Looie496 (talk) 00:54, 20 January 2012 (UTC) OP here, Thats exactly whats bugging me. What if that psychiatrist is just a creation of my craziness. MahAdik usap 01:31, 20 January 2012 (UTC) Are you able to function reasonably well in society? ←Baseball Bugs What's up, Doc? carrots→ 02:11, 20 January 2012 (UTC) "Craziness" is a qualitative label, mental illnesses are conditions while fitting into society is something different altogether. Many people hold the opinion that those with mental conditions simply need a bit more adaptation to fit into places such as work settings. [3] ~AH1 (discuss!) 03:11, 20 January 2012 (UTC) Depends on the mental condition, I would attest. I've known people with mental conditions that basically allowed them to do absolutely nothing — not even really survive — without heavy duty medication. I'm sure there are a lot of mental conditions which are just on the spectrum of "functional" but there are lots that are quite clearly physiological dysfunction, and I don't have a lot of truck with folks who would deny this fairly obvious fact for ideological reasons. There are experiments one can do to confirm that a tiny chemical change can make a huge difference in one's perceptions of reality. --Mr.98 (talk) 03:16, 20 January 2012 (UTC) Assuming this is meant philosophically and not psychiatrically, this is not too different from the famous question asked by Descartes: How do you know you are not dreaming? Or put another way, how do you know you're not just a brain in a vat? What if life is nothing but a simulation? There isn't any great answer to this, to be honest. If you assume that your ability to distinguish reality from fantasy is totally inhibited, then anything's up for grabs. Descartes thought he thought, therefore he was, but even that's not as iron-clad as he made it out to be. In the end, I fear, you just have to do your best, and try to be aware of your own limitations. But there's zero guarantee that you'll be assessing reality correctly. Alas. But this is not medical advice, and should you suspect that you indeed are suffering from mental illness, seeking out a professional is a good idea, even if you fear that professional only exists in your head. It's better than nothing. --Mr.98 (talk) 03:16, 20 January 2012 (UTC) This is a fantastic question! I think people who are really delusional often get very upset when someone calls them crazy, they are 100% certain they are NOT crazy and take great offense at the suggestion. If instead you have not 100% ruled out the possibility you might be a bit nuts and are willing to genuinely question this with honest introspection, in an on going "sanity checking" process, I think that is actually a decent sign that you aren't totally nuts. In the end, I think it is probably impossible to know for certain, solipsism is a related article. Vespine (talk) 03:35, 20 January 2012 (UTC)

January 20

odd rabbit behavior

Where I live on the Kenai Peninsula we have snowshoe hares running around. As is indicated on the article, they undergo population spikes every so often, and one has been ongoing here for the last two years. Something I have observed is that at this time of year they are quite often found on the edges of roads, and they have this annoying/.stupid habit of running out in front of moving cars in the dark. I have seen them sit on snow berms on the side of the road and wait until a car got close, so that it's lights were illuminating the area right in front of the hare, and then they run out. So my question is, why would they wait like that? It seems completely counter-intuitive that theirt instinct would tell tham to wait until a threat was close before running directly in front of it. — Preceding unsigned comment added by Beeblebrox (talk • contribs) 00:58, 20 January 2012 (UTC)

Our Spotlighting article (while a pretty crappy one) describes some of this behaviour, telling us that "...many animals (e.g. foxes and rabbits) often remain to continually stare at the light and do not appear to see the light as a threat as they normally would view a human". HiLo48 (talk) 02:03, 20 January 2012 (UTC)

Interesting question, and I don't think there's a definitive answer. A few possibilities to consider:

• Your observations may have some bias, i.e. you aren't as likely to notice if the rabbits run away from the road when the car approaches.
• Fight or flight type instincts don't have to be perfect to be beneficial on average. For example, if the rabbits bolt in a random direction when threatened, this still may save more lives on average, compared to doing nothing. So it's not that surprising that the behavior stays around, even when it sometimes ends up killing the rabbit.
• As for the timing: the classic natural predator of the hare is the lynx. Footage such as this [4] shows how the hare gets away by being better at cornering, not by being faster in a straight run. In this case, the hare is better off not bolting until the last second.
• In many modern environments, cars may well kill more hares than lynxes. This could be providing selective pressure, and perhaps future generations will be slightly better at avoiding cars.
• Lastly, peak population density puts all kinds of pressures on the critters, they are competing intensely for good forage sites, etc. This is bound to make them act a little funny. SemanticMantis (talk) 02:21, 20 January 2012 (UTC)

• The rabbits are probably thinking the car is a predator, and applying their evasion method for those predators. Since those predators aren't as fast as cars, the rabbits can dart across their path, get the predator to turn sharply and tumble over, and thus get away. Obviously, this doesn't work well with cars (although some idiot will occasionally crash their car swerving to avoid them). StuRat (talk) 04:05, 20 January 2012 (UTC)

There is a theory here in Australia that Kangaroos, Deer, Water Buffalo, etc run out in front of vehicles at night because they are trying to escape from a scary/confusing situation, so they run into the only areas they can see clearly in, ie the area lit up by your headlights! I've certainly found that if I turn my headlights off they don't run out in front of me. — Preceding unsigned comment added by 124.191.177.248 (talk) 08:01, 20 January 2012 (UTC)

And of course one more thing to add is that hares have been selected to avoid lynx for millions of years, by lynx killing those who didn't behave in a logical lynx avoidance pattern, whereas cars have only been around in any significant number for 60 years. Evolution doesn't work that fast! A new hare behaviour may occur at some point cause by selective pressure, but by the time it occurs, we may already be in the process of replacing cars with another transport mechanism, which may or may not kill hare in a different manner. --Lgriot (talk) 09:32, 20 January 2012 (UTC)

Blood dye

A few nights ago, I had a dream in which it became fashionable to dye one's blood green. This gave the skin a greenish tinge. Trendy people carried around syringes of green food coloring which they administered to themselves like insulin shots. I awoke with many questions. In principle, would it do harm to inject food coloring into the blood? Or is it inert and harmless? How much would it take to dye all the blood in one's body? Would a pocket-sized syringe be sufficient? Assuming you're pale in complexion, would your outward appearance be dramatically greener? How green could the blood get? (In my dream it acquired a lurid, absinthe-like appearance.) How long before the effect wore off? LANTZY^TALK 04:01, 20 January 2012 (UTC)

Not sure if it's harmful or not when injected, so don't try it. You probably could change blood color dramatically, but green would be difficult, being the opposite of red. I suspect green coloring would make the blood closer to black. You could probably manage a color closer to red, like orange or purple, provided you could find a safe dye. And yes, changing your blood color would dramatically change your apparent skin color, if you are pale to begin with. StuRat (talk) 04:10, 20 January 2012 (UTC)

If you want to change your skin colouration, overenthusiastic consumption of carrots (or other carotene containing vegetables) can turn you distinctly orange. Not recommended though, as there can be side effects. As for other colours, gold salts are apparently quite effective in turning people shades of grey. Irreversably.... AndyTheGrump (talk) 04:21, 20 January 2012 (UTC)

Read Argyria if you want to be blue-ish gray. Can someone find a picture for Stan Jones (politician)? Why green blood, and not blue? Von Restorff (talk) 12:05, 20 January 2012 (UTC)

Insulin shots are administered under the skin into fatty tissue, not into veins or arteries. Edison (talk) 05:45, 20 January 2012 (UTC)

Eating beetroot turns your urine pink (well it does with me), so presumably the red colouring must be in your blood to get to your kidneys. I wonder if it makes the blood look redder? No side effects apart from bright red faeces! — Preceding unsigned comment added by 124.191.177.248 (talk) 08:06, 20 January 2012 (UTC)

If you want to change the colour of your faeces, I recommend several pints of Guinness for a noticeable darkening, or spinach for a greenish tint. Then again if such things matter to you, I'd recommend expounding your interests elsewhere... AndyTheGrump (talk) 08:19, 20 January 2012 (UTC)

Methemoglobinemia results in blue or brown blood, and can visibly affect people's appearance: see the section on "Carriers" (sadly it doesn't have photos, but there's one here if you scroll down past the X-Men character). --Colapeninsula (talk) 12:52, 20 January 2012 (UTC)

That photo is of Paul Karason. He used to look like this, but later he looked like this. His girlfriend didn't seem to mind. Von Restorff (talk) 13:01, 20 January 2012 (UTC)

He isn't that way because of his genes. He's that way because he has Argyria — he essentially overdosed on collodial silver because he believed quack stories about its antibacterial powers. Totally different causes — the blog is wrong to list him that way. That blog in general is not what I would call a reliable source. --Mr.98 (talk) 13:54, 20 January 2012 (UTC)

Farming fish

Are there any fish that are bred and harvested specifically for consumption? My understanding is that fish are mostly just caught in the wild, and they are left to replenish their numbers on their own without external forces to ensure their numbers are high. Is this correct? ScienceApe (talk) 11:44, 20 January 2012 (UTC)

Some fish are bred and harvested specifically for consumption (e.g. salmon, trout, sea bass, carp, tilapia, and catfish). See fish farming. Today more than 40% of the fish consumed is farm raised. Von Restorff (talk) 11:46, 20 January 2012 (UTC)

Farmed fish is a very big industry, especially within salmon production. In fact, almost all salmon that is offered to consumers is farmed. See the article on Fish farming. DI (talk) 11:52, 20 January 2012 (UTC)

Huge industry - also including shrimp, sturgeon, and many other fish. Lovely exhibit at EPCOT if you ever get there. Alligators do count as seafood? Collect (talk) 13:16, 20 January 2012 (UTC)

In the United States, most fish markets have signs which say which ones are wild and farmed near the price markers. My recollection is that most of them I've seen are farmed. The proportions no doubt vary by country, but "aquaculture has been growing rapidly worldwide, and in 2009, farmed fish and shellfish surpassed wild-caught stocks as the major source of seafood worldwide." So I'd say your understanding of this is probably not correct. --Mr.98 (talk) 13:18, 20 January 2012 (UTC)

It should also be noted that many farmed fish are better in terms of environment and sustainability. The top wild-caught fishes, especially cod and tuna, are apex predators in their environment, and so have a very low "convertability", i.e. they need a lot of food and territory to survive. These large commercial fish are essentially the lions of the fish world, so are impractical to farm; they also take a long time to grow to economical size for catching, so they take some time to rebound from overfishing. The reason you don't raise lions for food is that lions eat grass eaters. So, you have to first farm a whole lot of grass eaters (like, say, cows and goats) and then feed the grass eaters to the lions. It is much more economical and sustainable to just eat the grass eaters yourself. It's the same with fish: the wild fish we catch and eat themselves eat the small-to-medium sized fish similar to what we now farm. Now, fish farms are not perfect, and some have their own environmental problems, but aquaculture is likely the best route we have for sustainable fish consumption. I've seen shows that make the case that tilapia is one of the best options, even better than salmon, in terms of sustainability. --Jayron32 17:07, 20 January 2012 (UTC)

But there is some tuna farming. See Tuna#Aquaculture. 75.41.110.200 (talk) 22:06, 20 January 2012 (UTC)

solar panel construction

How do I wire (solder) my solar cells together to build a solar panel??? Please do not direct me to the buy a plan site I got ripped off already so I am broke.... I have 300 6 by 6 inch crystaline or monocrystaline cells but I dont know how to put them in a panel and wire connect them correctly.......Thanks Joe Green in Beaumont Texas.... — Preceding unsigned comment added by 24.242.111.205 (talk) 13:14, 20 January 2012 (UTC)

If you type "build a solar panel" into the search box at the website www.google.com and select the first result, it is this website which has the plans to build a solar panel from 6-inch monocrystaline cells. Those plans calls for 36 cells per panel, so you should be able to construct several. --Jayron32 15:23, 20 January 2012 (UTC)

Do the solar cells already have wire leads? That makes things much simpler. I have rarely been successful soldering leads to the tinned area on a solar cell. Edison (talk) 16:11, 20 January 2012 (UTC)

What do you plan to power with the final unit? It will make a difference - whether you want to connect them in parallel or in series; whether you need load balance circuitry or active power control circuitry, and so forth. Nimur (talk) 19:53, 20 January 2012 (UTC)

drug test

does tramadol show up the same as hydrocodine/oxycodine in a drug test — Preceding unsigned comment added by 71.51.57.30 (talk) 15:54, 20 January 2012 (UTC)

We cannot tell you, we cannot give medical advice. Von Restorff (talk) 16:53, 20 January 2012 (UTC)

I dont believe this is a medical advice Q so I will answer. Both Hydrocodone and Oxycodone cand be destinguished from other opiates of which Tramadol is one.--89.243.132.82 (talk) 17:24, 20 January 2012 (UTC)

Is 'cand' a typo of 'can' or 'cant' (a misspell of can't)? — Preceding unsigned comment added by 80.58.205.34 (talk) 18:26, 20 January 2012 (UTC)

Its 'can'. My error.--89.243.132.82 (talk) 19:09, 20 January 2012 (UTC)

Ear wax

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis or prognosis, or treatment recommendations. For such advice, please see a qualified professional. If you don't believe this is such a request, please explain what you meant to ask, either here or on the talk page discussion (if a link has been provided). --Jayron32 20:49, 20 January 2012 (UTC)

Efficiency Test for Evolutionart Algorithms

I was working on evolutionary algorithms, particularly a variation of DE that we have developed. As with most updates, it gives better results but with higher computing time, so i want to find out how good it really is quantitatively, like with some measures. For example the likes of Amdahl's Law or those in the wiki page on Algorithmic Efficiency that deal with cases were one or more of the desired characteristics are getting better at the expense of others. Basically i need some kind of measure depending on the final error and the computing time. Any suggestions?--tathagata 19:11, 20 January 2012 (UTC) — Preceding unsigned comment added by Nonstop funstop (talk • contribs)

Polyneuropathy and GABA

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis or prognosis, or treatment recommendations. For such advice, please see a qualified professional. If you don't believe this is such a request, please explain what you meant to ask, either here or on the talk page discussion (if a link has been provided). --Jayron32 20:47, 20 January 2012 (UTC)

Atomic explosion in your hands

You have 2 half-spheres of pure plutonium-239 in your hands. Each half-sphere has a sub-critical mass - 9 kg. You join the half-spheres. Now you have a sphere of pure plutonium-239 with a super-critical mass.

Should it explode?

P.S. Anyway, don't try this at home. --Zhitelew (talk) 21:18, 20 January 2012 (UTC)

No. Read the second paragraph for your answer. KägeTorä - (影虎) (TALK) 21:24, 20 January 2012 (UTC)

They have a mass just above critical (in this case nuclear reaction becomes subcritical again within a few seconds). How about a half-spheres with sub-critical masses (each)? --Zhitelew (talk) 21:34, 20 January 2012 (UTC)

No, they were subcritical. Read the 'Incidents' section of that article to see. KägeTorä - (影虎) (TALK) 21:38, 20 January 2012 (UTC)

I mean joining the half-spheres:) Something like Gun-type fission weapon (but with the low speed). Also, small explosion is ok, I don't require a full explosion of the mass --Zhitelew (talk) 21:47, 20 January 2012 (UTC)

At the *very* best, you'll get a fizzle. But really you'll just die of radiation sickness. You simply can't whack the spheres together fast enough to get a decent explosion. Not even Thin Man was fast enough, which is why they abandoned it. See Predetonation. --Sean 21:49, 20 January 2012 (UTC)

Well, the OP did specify pure Pu-239. It's possible you could use pure Pu-239 in a gun-type device and get a substantial yield. But pure Pu-239 is hard to manufacture. The plutonium they had during WWII was not pure; it had lots of Pu-240 in it, which is why they couldn't use it in the Thin Man. --Mr.98 (talk) 21:56, 20 January 2012 (UTC)

If it's just a critical mass, you probably see nothing, but the rate of reaction rate is increasing. Eventually the thing will heat up and melt. If it's supercritical, the bits may have a small explosion and separate. But not a full nuclear explosion. Your hands are not adequate for holding together an exploding core enough for the reaction to build to that level. The cores in nuclear weapons need to be held together for a small amount of time for the reaction to really propagate like they do in a nuclear weapon.

Even rather simple nuclear weapons need a little more "help" than what you're describing to get even kiloton ranges of explosives: confinement, a tamper, a neutron initiator, etc. What you're describing is essentially an unshielded fast-neutron reactor, not a bomb. --Mr.98 (talk) 21:56, 20 January 2012 (UTC)

Escape velocity of major star

Could there exist a star with about 10 × 10³² kg in mass, therefore having an escape velocity of about 300,000 km/s (more than the speed of light?) If it could exist, would that mean that light could not escape that star? And hypothetically, what would that star look like? 64.229.180.189 (talk) 22:05, 20 January 2012 (UTC)