Wikipedia:Reference desk/Archives/Science/2014 March 23

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March 23

The Universe

1. The universe had a beginning as a tiny point which expanded in the Big Bang. This would seem to suggest to me that there the universe has a size, and therefor an edge. But the Universe article states that the "size of the Universe is unknown; it may be infinite." How could it have expanded from a dot the size of an atom to infinite?
2. If there is an edge to the universe, what would happen if you were in a spaceship traveling toward it?

46.7.249.24 (talk) 00:05, 23 March 2014 (UTC)

If it is infinite, then it would have always been infinite, that dot would have been the bit that expanded to the visible universe we now know. If you were in a space ship then the universe would include you and the spaceship too, so you would not be past the edge. To be beyond the edge, there should be no light or radiation of any form, so no cosmic microwave background, or any cosmic rays or gravitational waves. You would not be able to detect gravity from the part beyond the horizon. Graeme Bartlett (talk) 00:55, 23 March 2014 (UTC)

• No, the universe has no edge, just like you don't fall off the edge of the finite surface of the earth, just in higher dimensions. The Big Bang is posited to have begun with a singularity, not at a point. Our article shouldn't be making silly claims like that the universe may be infinite. That would mean no matter how big it actually is, it's actually bigger. μηδείς (talk) 00:57, 23 March 2014 (UTC)

What are you talking about, Medeis? It wouldn't mean any such thing. Infinite geometric structures may or may not correspond to physical reality, but there's nothing wrong with them logically; they have a perfectly coherent mathematical description. --Trovatore (talk) 05:21, 23 March 2014 (UTC)

Our reference desk editors shouldn't be making claims like that the universe definitely can not be infinite. 88.112.50.121 (talk) 01:38, 23 March 2014 (UTC)

And here I thought there were an infinite number of IP 54 troll addresses available? Perhaps You've not heard of Einstein? The Possibility of a “Finite” and Yet “Unbounded” Universe μηδείς (talk) 03:11, 23 March 2014 (UTC)

88.x is right: you shouldn't claim that the universe definitely can't be infinite. It may be infinite for all we know. He/she didn't object to the claim that it may be finite and unbounded, since for all we know it may be that too. -- BenRG (talk) 05:16, 23 March 2014 (UTC)

The possibility of a finite and unbounded universe does not imply that the universe is actually finite, or actually unbounded. There's simply no observational evidence to suggest that the universe is finite (or that it isn't), and no theoretical reason that it should be one way or the other. --Bowlhover (talk) 05:17, 23 March 2014 (UTC)

You folks are just being silly. If you can coherently define a non-self-contradictory "infinite" model of the universe, feel free both to present it explicitly and give what observational evidence that you have that supports the model. When you actually pay attention to what infinite means ("has no actual total size, stands in no definable mathematical relation to its parts") you find your claims are outside the realms of evidence, science, reason and logic. μηδείς (talk) 19:14, 23 March 2014 (UTC)

Medeis, sorry, that's not what "infinite" means, not at all. Take a baby example, the real line; that is, the set of all real numbers, considered as a geometric object. Do you claim that it "has no actual total size" or "stands in no definable mathematical relation to its parts"? Or do you claim it's finite? --Trovatore (talk) 22:22, 23 March 2014 (UTC)

Medeis is an ultrafinitist. Count Iblis (talk) 15:08, 25 March 2014 (UTC)

I sort of doubt that. Ultrafinitism is a specific form of reaction against the Cantor/Dedekind/Weierstrass/et-alia revolution. I see no indication that Medeis is taking any of those figures into account in the first place. --Trovatore (talk) 15:47, 25 March 2014 (UTC)

Numbers are abstractions, not physical objects. Numbers can in the imagination be extended potentially infinitely, but there is no actually existing infinite number of anything. Potentiality and actuality. That's pretty much Philosophy 101, given it's been know since Aristotle. μηδείς (talk) 06:12, 24 March 2014 (UTC)

On this point, Aristotle was flat wrong. So were a lot of other very smart people for a very long time. It wasn't until Georg Cantor that the notion of the actual infinite really started to come into focus. You really can't cite pre-Cantor thinkers on this point; basically everyone was wrong. --Trovatore (talk) 06:23, 24 March 2014 (UTC)

Out of curiosity, do you also reject out of hand a universe that's endless in time? Because there's not much of a distinction between time and space in general relativity. If it makes you philosophically happier, you can imagine "the machine that runs the universe" not only simulating later times at later meta-times, but also simulating an ever-widening area of the infinite universe, so that although the amount that's being simulated at a given meta-time is finite, any part of the spatially and temporally infinite universe will meta-eventually be simulated. This is like the trick for simulating all Turing machines in parallel on a single Turing machine: first do one step of machine #1, then one step of #1 and #2, then one step of #1, #2 and #3... -- BenRG (talk) 21:10, 23 March 2014 (UTC)

I don't accept a priori answers to anything except the law of noncontradiction. My understanding is there will be a big crunch or a heat death either of which will be a singularity beyond which we can't measure. But the easy answer is that time exists within the universe, not the universe within time. Historico-epistemologically, the answer is that savages have all sorts of beliefs, like the earth being flat and having edge, to spontaneous generation, and animal species being unchanging.

Saying the universe is infinite, meaning I can't imagine hitting a wall, is just as conceptually juvenile. It is entirely possible, using the right math, to describe a finite self-bounded universe, just like the surface of a sphere is finite and self-bounded, without need of a wall or edge or infinite horizon. I'll gladly admit that I can't really imagine a beginning of time or an end to it. But I don't then claim omniscience and say I know it is infinite. μηδείς (talk) 06:12, 24 March 2014 (UTC)

Well, no one here has claimed to know that space is infinite either, or if so, it's escaped my notice. On the other hand, you seem to claim to know that it's finite. How do you know that, exactly?

You said in an edit summary, there's no burden to disprove a self-contradictory statement; it does that for you. Perhaps, but surely there is a burden at least to say what the alleged contradiction is, if it's not evident to others. So far, I have not seen any clear explanation of what you think is self-contradictory. The closest you've come seems to involve using private definitions of "infinite" that don't correspond to what anyone else means by the word. --Trovatore (talk) 06:57, 24 March 2014 (UTC)

Maybe this is where the Friedmann–Lemaître–Robertson–Walker metric comes in. InedibleHulk (talk) 01:04, 23 March 2014 (UTC)

Even if we ignore the metric expansion of space, you still could never get to the edge of the universe, as it's expanding at the speed of light, and you'd have to go faster than that to catch up, which is impossible. However, with the model of multiple big bangs each spawning their own "universe", it's possible another universe's edge could move past us. Depending on the age of that universe, the edge might be very dilute, so we might not even notice it passing. StuRat (talk) 03:14, 23 March 2014 (UTC)

Erm, this seems to contain or risk inducing misconceptions. There is no "universe's edge", however you look at it, only what is called a cosmological horizon, which is another concept entirely. Unfortunately, "universe" is sometimes used to mean "observable universe", which may lead to this misconception. —Quondum 04:26, 23 March 2014 (UTC)

The homogeneous and isotropic dust of galaxies that we find ourselves in could be finite in size and surrounded by something else, presumably whatever provided the initial conditions for inflation or whatever produced our cosmos. That environment would be hostile enough to destroy anything you threw into it, like throwing something into the Sun except more so.

It's not logically impossible that space could literally just end. You are made of fields, and those fields hitting the edge of space would be something like ocean waves hitting a beach. The result would depend on the details of physics at the boundary, but you probably wouldn't survive as an organism. Recently it has been argued (controversially) that space actually does end at a black hole event horizon, so there is precedent for this.

It's also possible for space to wrap around so that it's finite in size and has no boundary, as Medeis said, but there's no evidence that it does.

Finally, it is theoretically possible, I think, for inflation from a small starting region to generate a literally infinite cosmos of galaxies. The picture on the right shows a cosmological model, the Milne universe, which on the one hand expands at the speed of light from a starting point, but on the other hand is spatially infinite at all times after the big bang from a cosmological standpoint, because the cosmological space at a given cosmological time is a hyperbolic surface like the "surface of last scattering" in the image, not a horizontal surface. There's a similar way of fitting an infinite, exponentially expanding flat space (like the future of our universe according to ΛCDM cosmology) into a region of de Sitter space that only expands at the speed of light. If you're willing to accept the possibility of a temporally infinite universe, there's no reason not to accept that it might be spatially infinite also. -- BenRG (talk) 05:16, 23 March 2014 (UTC)

Here's what I think is the "simple" answer to the original question. There was never a time at which the entire universe was a single point, because the so called "moment of the Big Bang" never happened at all.
Note that this doesn't mean the Big Bang never happened. There's ample evidence that the Big Bang theory is correct. But the theory per se really never talks about that time t=0 itself. It'll tell you what we think happened 1 second after the Big Bang, or 1/100 of a second, or 10⁻³⁵ seconds, but not 0 seconds. There simply was never such a time at all.
This will make more sense if you take the logarithm of the time coordinate. Think of t=0.01s as being as long before t=0.1s as t=0.1s is before t=1s. Then you see that the time t=0 is moved back infinitely far (log t=−∞). In this sense, the Big Bang is consistent with the proposition that the Universe has "always existed"; it's only our time coordinate that is limited to values greater than 0. --Trovatore (talk) 05:34, 23 March 2014 (UTC)

I tried this argument with my computer science lecturer relating to the halting problem, by suitably scaling the computation speed, you can solve the problem in a finite amount of time – after all, every 'tick' of the algorithm still takes a finite, nonzero time. No prizes for guessing the response. Roger Penrose takes the opposite tack to yours, suggesting that one can go beyond "infinity" (the big bang into the past, and the infinite expansion into the future, across an innumerable number of big bangs). One should take care of artefacts introduced by the choice of coordinates. —Quondum 06:26, 23 March 2014 (UTC)

Well, actually I don't know what the response was. Given a computer that can run at unbounded speed how much are you selling those for, btw? your answer was quite right; you can indeed perform that supertask in finite physical time, but not in a finite number of steps. If I has been your lecturer, I would have pointed out that your objection was correct, but not directly relevant to the material being discussed. Then I would have called you up after class and mentioned that there are models of hypercomputation where such things are considered. Is that what he did?

My point here is mostly that, sure, given a universe that's currently infinite, you have a weird problem if you try to explain the topology of a manifold-like-thingie that is just a single point at one time slice, but then infinite at every other time slice. But that's not really a problem, because the time slice where it's just a single point is not something that we have any coherent description of in any other way either, and indeed there's no reason to think that it corresponds to any actual event in the past. --Trovatore (talk) 06:45, 23 March 2014 (UTC)

I should have guessed that models have tried to go beyond the Church–Turing thesis. I had hoped to spark a bit of a debate with the lecturer (I raised the point after class; she was cute), but only got a dismissive response. This preceded nearly all the dates on references in the articles you linked. The manifold concept is inadequate for explaining a singularity, and we know we're at sea at Planck scales, so for now we can simply acknowledge the lack of coherency. —Quondum 19:17, 23 March 2014 (UTC)

As my fellow ref-deskers have failed to condense this subject into terms that proceed from your question to the understanding of nature and geometry of space-time necessary to understand our best hypotheses on this subject (which, to be fair, is no easy task), and given that by the end I tend to be the most verbose party of all, I've decided in this case to simply link you to Shape of the universe and Observable universe and provide a couple of the briefest quotes from good ol' Carl Sagan, whose gift for reducing these concepts--which are non-intuitive to usual human perception of the physical world, visual and otherwise-- to something digestible was unparalleled (consequently, if you are neophyte to cosmology with a deep interest, you could do a lot worse than to pick up his book Cosmos as a primer):

"[Prior to the Big Bang] all of the matter and energy now in the universe was concentrated at extremely high density...perhaps into a mathematical point with no dimensions at all. It was not that all matter and energy were squeezed into a minor corner of the present universe; rather, the entire universe, matter, energy, and the space they fill, occupied a very small volume. There was not much room for events to happen in.

"In that titanic cosmic explostion, the universe began an expansion that has never ceased. It is misleading to describe the expansion of the universe as a kind of distending bubble viewed from the outside. By defintion, nothing we can ever know about was outside it. It is better to think of it from the inside, perhaps with grid lines--imagined to adhere to the moving fabric of space--expanding uniformly in all directions."

Since the general point of your question comes to how the universe can be finite but unbounded (a concept others have touched upon above) you might find the article General relativity to be of use, as it addresses the basics of space-time curvature, though I think it might be a bit technical, starting from the place of your question. Although better understood in mathematical terms, just try as best you can (or anyone can) to visualize the most common model -- a universe with curved space, such that you can travel indefinitely in it and yet never hit an edge. We don't know for certain what the shape of such curvature would be, so the old suggestion that you could fly off and never change course and eventually end up back where you started is dubiously correct, but a good way to try to get a handle on the concept anyway. Snow (talk) 10:49, 23 March 2014 (UTC)

I have nothing against these quotes but I don't see how they help with the original question. Finite and unbounded models of the universe were historically popular for philosophical reasons, but in modern cosmology they're not theoretically well motivated. If there are finitely many galaxies, they're probably surrounded by something else, not wrapped around. -- BenRG (talk) 21:10, 23 March 2014 (UTC)

According to quantum mechanics, the amplitude of making some particular observation is given as the path integral over all fields and space-time configurations of exp(i S) where S is the action containing both the space-time degrees of freedom and the matter fields. The most important contribution to the path integral comes from the one which minimizes the action, and that then yields the classical equations of motion (The Einstein equations coupled to the equations of motion for the matter fields). Then since you can only ever make a finite number of local observations and you would need to integrate out everything that you don't observe in the path integral formulation, an infinite unverse should be physically equivalent to a finite one where all possible local configurations occur with the correct amplitudes. Count Iblis (talk) 14:50, 23 March 2014 (UTC)

Did you make up this argument or paraphrase it from somewhere? First, I'm not sure how you're going to apply the path integral to observations of the whole universe. What are the ingoing and outgoing states? Second, are there no measurements happening anywhere else in the universe? If you want to allow the possibility that the universe is a show put on for our benefit, you don't need quantum mechanics. You can just say the whole universe is a light year in radius and at the edge are particle-generating machines that mock the appearance of the rest of it. That's a lot of machinery but still much cheaper than making all of those galaxies. -- BenRG (talk) 21:10, 23 March 2014 (UTC)

These are some half baked ideas I have yet to work out in detail. The latter possibility you mention should contribute to the path integral but it will presumably only make a negligible contribution. Count Iblis (talk) 17:23, 24 March 2014 (UTC)

Coherent radar

Various articles use this term in describing particular equipment, but I don't see it defined or its uses or benefits listed. See Journal article about photonics-based coherent radar Jim.henderson (talk) 01:59, 23 March 2014 (UTC)

Wave radar has some information: "Coherent radars measure Doppler-modulation as well as amplitude modulation, while non-coherent radars only measure amplitude modulation. Consequently, a non-coherent radar echo contains less information about the sea surface properties." — A basic understanding of wave phase coherence would be helpful; but a simple analogy would be a laser which utilizes phase-coherent light.  —71.20.250.51 (talk) 04:11, 23 March 2014 (UTC)

You may be interested in the use of the term as it applies to radio demodulation, as it is the same concept. Coherent demodulation allows more detail and information to be extracted from a signal than noncoherent demodulation does, and hence allows transmitted waveforms to be used that require far less power to be used for the same effective received signal. Noncoherent demodulation was used originally, for example, with amplitude modulation receivers, which requires the transmission of a strong carrier for the same demodulated signal quality compared with say double sideband or single sideband that requires coherent demodulation. The advantage of noncoherent demodulation is that the receiver circuitry can be considerably simpler, something that is less of a consideration with modern electronics and techniques. —Quondum 04:18, 23 March 2014 (UTC)

Thanks; that clarifies it somewhat. I should have mentioned that my question was inspired by a news report that tried to hook a radar improvement to the great buzz about Flight MH370. Surely that connection does not warrant attention in a Wikipedia article, but I hope editors who understand the technical issues will add appropriate links among those technical articles and maybe a paragraph or two of explanation based on the above. Jim.henderson (talk) 13:29, 23 March 2014 (UTC)

Coherent detection of a signal buried in Additive white Gaussian noise obtains +3 dB better Signal-to-noise ratio than incoherent (envelope) detection. 84.209.89.214 (talk) 15:47, 23 March 2014 (UTC)

Yup I failed to mention that even for waveforms designed for noncoherent detection, coherent detection does a better job in this sense. —Quondum 19:27, 23 March 2014 (UTC)

The news story sounds like someone riding on a news event to sell their equipment. It's a technology for doing the same job better by replacing the radar electronics. I'm no expert, but I'd be surprised if any modern radar (even the simple Doppler that the cops used for measuring road speeds) is not coherent. —Quondum 19:27, 23 March 2014 (UTC)

...alas, no! There are incoherent scatter radars like the AMISR [1], at the Poker Flat Rocket Range. They are rare, and they are used for special purposes - typically for studying the chemistry and physics of the mesosphere and ionosphere. What makes them "incoherent" is that the radio return doesn't bounce off any specific object: these RADARS detect incoherent returns from the entire ensemble of ions and molecules in the upper atmosphere. Nimur (talk) 15:15, 24 March 2014 (UTC)

Since this "incoherent" applies to the scattering, not the radar system, I see no cause for lamentation. The radar system itself will no doubt still use coherent detection. —Quondum 03:43, 26 March 2014 (UTC)

Here's a tutorial from Haystack Observatory's Millstone Hill ISR: Incoherent Scatter Radar Tutorial. Here's more about AMISR's system description. The phase of the individual radiating elements - AEUs - in the AMISR are controllable (it can be considered a phased array). Nonetheless, an entire community of experts call these "incoherent scatter RADARs" - in contrast to coherent scattering RADARs. I think it's semantics; it's the same machine, whether the adjective "coherent" describes the machine itself, or the physics that the machine relies on. Nimur (talk) 02:38, 27 March 2014 (UTC)

How much greater is gravity on the dark side (away from earth) of the moon than on the light side (facing earth)?

How much greater is gravity on the dark side (away from earth) of the moon than on the light side (facing earth)?68.36.148.100 (talk) 05:06, 23 March 2014 (UTC)

I'm guessing that you think the Earth's and Moon's gravity will add together on the far side, while the Earth's gravity will be subtracted from the Moon's on the near side. This would be true if the Moon was stationary with respect to Earth, but, since it orbits, the apparent centrifugal force exactly cancels this effect, although there may be slight tidal effects left over. Also, "dark side" and "light side" aren't good ways to describe the near and far sides of the Moon, as which side is dark or light changes, while the side facing the Earth doesn't (except for some wobble). StuRat (talk) 05:19, 23 March 2014 (UTC)

If the moon affects the gravity on earth why doesn't the earth affect gravity on the moon?? eg: quote from Moon article:"The tides on Earth are mostly generated by the gradient in intensity of the Moon's gravitational pull from one side of Earth to the other, the tidal forces."165.212.189.187 (talk) 17:39, 26 March 2014 (UTC)

Does the moon truly "wobble" or is it just that we can see more than 50 percent of it simply because of its elliptical orbit? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:21, 23 March 2014 (UTC)

If one side always faced the Earth, with no wobble, why would an elliptical orbit show us more than half of it ? (I can see why observations from different points on Earth would see slightly different views, including due to the same point on Earth rotating each day, but that's a different issue.) StuRat (talk) 05:28, 23 March 2014 (UTC)

I figured out the answer to my own Q here. The Moon rotates at a constant speed, so this means it would only have one side exactly facing the center if it was also revolving around the Earth at a constant speed, which would require a circular orbit. StuRat (talk) 22:13, 23 March 2014 (UTC)

Does it literally "face the earth", or does it face a focus point of its elliptical orbit? If so, that might allow us to see beyond the conventional edges a bit compared with when its truly facing us, at apogee and perigee. I'm just not so sure the orbit is sufficiently off-circular to explain the percentage we can see. Does it literally rock back and forth (i.e. wobble or oscillate) regardless of where it happens to be in its orbit? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:49, 23 March 2014 (UTC)

To put it another way, is it wobbling with respect to itself, or only with respect to the view from the earth? Possibly oscillating as the last vestiges of its presumed, ancient rotation before the earth started slowing down its rotation? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:52, 23 March 2014 (UTC)

Moon wobble (aha, we haven article on it!) is with respect to an Earthbound observer. The article gives a better explanation than I could, so I'll simply leave the link. Someguy1221 (talk) 05:58, 23 March 2014 (UTC)

Perfect. Thank you! ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:58, 23 March 2014 (UTC)

Aye. Thanks, too. Got me humming "Good Librations". The Funky Bunch version, not Beach Boys. InedibleHulk (talk) 20:25, 23 March 2014 (UTC)

If anything gets you humming Marky Mark over Brian Wilson, you have problems I am not allowed to recommend fixes for, per Wikipedia's medical disclaimer. But you still got problems. --Jayron32 22:08, 23 March 2014 (UTC)

I didn't exactly choose it. Just popped into my head. And I wasn't thanking him for the tune, just the info. Bandwise, Beach Boys win. Song-for-song, this one's catchier. But yes, I'm nuts. InedibleHulk (talk) 22:27, 23 March 2014 (UTC)

I diagnose an ear worm, and recommend using mental floss to remove it. StuRat (talk) 22:43, 23 March 2014 (UTC)

Thanks, but it's too late. That article beaned me, and now I can't not confuse my worm with palinacousis. My lobes keep telling me "Come on" and "feel it". I think I'll need more than floss for this. InedibleHulk (talk) 22:55, 23 March 2014 (UTC)

Or get some Brain Bleach from That Other Wiki(tm)... - ¡Ouch! (^{hurt me} / _{more pain}) 11:14, 26 March 2014 (UTC)

The question is about the Dark side of the Moon and you're humming that? ¡Pobre! Wnt (talk) 23:30, 23 March 2014 (UTC)

Great as they are, they don't make good humming tunes. Anyway, I've moved on to this Godzilla melody. Learned a lot about space from that game. InedibleHulk (talk) 01:11, 24 March 2014 (UTC)

Isn't the relevant focus of the ellipse always on the line joining the two bodies? —Tamfang (talk) 00:23, 24 March 2014 (UTC)

I think so; if one mass is negligible compared to the other, one can treat the center of gravity of the heavy body as one focus. On the still far from equal Earth–Moon pair (80:1 IIRC) the common center of gravity, which should be the location of a focus, is still well inside Earth.

The dark side is facing away from the Sun, and that's not always the same part of the Moon. The side facing away from Earth should be called the back (or the far side) of the moon. The terms where "dark" means "unseen" often cause confusion. Another thing I cannot find the original source for... :(

The center of the Moon should experience no net gravity, in the sense that if there were a small cavity, a body inside the cavity wouldn't be pulled towards any wall. For a BIG cavity, (small) bodies inside would be pulled either towards the side facing Earth or to the opposite side, depending on which is closer. - ¡Ouch! (^{hurt me} / _{more pain}) 15:29, 24 March 2014 (UTC)

There is no gravity inside a central cavity in a spherically symmetric object (assuming no external influences), regardless of the size of the cavity. It is a consequence of the inverse-square law. Hence, the only influence felt would be Earth's tidal force, which would have the effect ¡Ouch! describes (as well as an apparent repulsion from the "equator" between the near and far points). Also, on the Moon's surface, effective gravity would probably lower at the near and far points of the moon, and higher elsewhere. —Quondum 03:31, 26 March 2014 (UTC)

Regarding gravity inside a hollow sphere, see Shell theorem. --Jayron32 13:56, 26 March 2014 (UTC)

Alcohol flush reaction

Why is it that the alcoholic flush reaction common in asian people seems to be more noticeable after 1 or 2 drinks but fades quickly, even if they continue to drink, after an hour or so? 82.132.244.11 (talk) 09:37, 23 March 2014 (UTC)

Does our article on the subject address your question? μηδείς (talk) 10:18, 23 March 2014 (UTC)

Another problem related to thermal imaging

I think every weapons amateur like me knows about thermal camouflage like "Nakidka" , or BAE′s ADAPTIV , I think such camouflage has brought an end to the thermal IR cameras forever - of course in the case of a war between the world greatest armies , not in the case of a war between NATO and Taliban who will not have any thermal camouflage forever also - , so I think we will return to night vision again . Sorry for annoying you about these confusing topics but I like it , and I hate forums . Tank Designer (talk) 11:09, 23 March 2014 (UTC)

No need to apologize; that's what we're here for. However, what exactly is your question? You should know by now we're not here to discuss what you or we think will happen. For that you do need a forum. Rojomoke (talk) 11:28, 23 March 2014 (UTC)

Just because thermal camo exists doesn't necessarily mean it will be widely used. Some possible reasons not to:

1) The expense. Perhaps that money could be better spent elsewhere.

2) The inconvenience/discomfort.

3) Keeping the heat in could cause them to get heat stroke.

4) Presumably they have to keep their entire face covered, which could limit their view.

5) As per the previous discussion, thermal camo would work best when the background temp was close to body temperature. If the soldier is much hotter than the background, he will be a lot harder to hide.

6) If the thermal camo uses a coolant like liquid nitrogen to absorb the heat, it will need to be exchanged for more coolant frequently.

So, I can imagine their use being restricted to commandos and such.

UPDATE: I see this technology is not used on infantry but only vehicles and structures. However, some of the items I listed still apply, like the expense, and infrared cameras are still of value to detect infantry, if nothing else. Also, tank crews seem to often have the hatch open when not in combat, for ventilation, to get a better view, etc., allowing them to be spotted by spy planes. StuRat (talk) 12:59, 23 March 2014 (UTC)

(edit conflict)::The problem with this stuff is the expense of fitting all your vehicles with it. As far as I can see, nobody has actually started the process yet and Adaptiv seems to be still in development. Then there are other IR signatures that can't be hidden, such as exhaust plumes. The M-1 Abrams has a particularly "massive IR signature" although there have been recent upgrades which are supposed to suppress a lot of it; diesel engines have a visible exhaust plume too. Alansplodge (talk) 13:08, 23 March 2014 (UTC)

Thank you so much for your efforts , you ended my worries by your valuable information . Tank Designer (talk) 15:14, 23 March 2014 (UTC) Thanks are not enough , God bless you .Tank Designer (talk) 15:21, 23 March 2014 (UTC) — Preceding unsigned comment added by Tank Designer (talk • contribs) 15:20, 23 March 2014 (UTC)

We have articles about Nakidka and Adaptiv (ADAPTIV is a redlink - if that's a common variant, please make the redirect). I don't know anything about it, but the photos don't look 100% impressive. Question: couldn't you camouflage against IR and night vision simply by dyeing fabric with camouflage patterned India ink (carbon), since aromatic carbon seems to absorb/emit every possible frequency the same way in a manner that depends only on the number of molecular layers and the fine-structure constant? Wnt (talk) 17:27, 23 March 2014 (UTC)

I'm not clever enough to say, but one suspects that if it were that simple, somebody would have already thought of it. Alansplodge (talk) 21:28, 23 March 2014 (UTC)

Yeah, I suppose in the far IR (heat glow) this is kind of like camouflaging a light bulb by wiping off obscuring material. It might confuse that it's a light bulb, but... On the other hand, if you backed your fabric with a space blanket... Wnt (talk) 23:26, 23 March 2014 (UTC)

It doesn't seem thermodynamically possible to hide something for very long. If your human or vehicle is converting an internal energy source into something else - there will be heat generated as a by-product. The heat absolutely has to go somewhere or else the internal temperature will climb. For humans, that would be quickly fatal - and even unmanned vehicles would eventually get hot enough to melt something important. So the heat has to go somewhere. Using a blanket to absorb the IR light will only cause the blanket to heat up and become visible.

Probably the best solution has to be to convert the heat into something else. You can have a reservoir of something cold (liquid nitrogen, for example) and warm it up - but you'll eventually run out of cold places to store your waste heat - and then you're back to being visible again.

Just like visual camouflage, you're not making the object transparent - so it'll still block the background that's behind it - which may yet make it detectable.

I suspect that you could use some of these techniques to hide a warm object for a while - but eventually, it's either going to become visible in IR - or it's going to cook whatever is inside. But modern IR cameras are really amazingly sensitive - and it's hard to imagine that you'd be able to fool one of those. However, from a military perspective, most of the world's combatants won't have access to the very best cameras - so there is certainly some mileage in using thermal camo. It's not going to obsolete IR cameras in the longer term though because the fancy electronics and whatever that you need to push the sensitivities way up will slowly get cheaper and more available - and I very much doubt that the camo technologies can keep up with it, simply because they are fighting the inevitability of the laws of thermodynamics.

SteveBaker (talk) 14:59, 24 March 2014 (UTC)

You forgot emissivity. You can have a surface that has high emissivity at certain wavelengths and low emissivity at others. So, you can choose a surface treatment/cover that has high emissivity at wavelengths that are already present in the environment. This is really the same as colour camoflage cothing but extended into selective infrared wavelengths. Metals and most man made materials have moderately high emissivity at heat wavelengths. But air has very low, near zero, emissivity. So what you can do is use a heat pump and heat exchanger to heat air which you blow through. Because of air's very low emissivity, you can exhaust air at a much higher temperature than you need to keep metal surfaces in order to have the same detectability. 124.178.107.238 (talk) 16:07, 24 March 2014 (UTC)

Convection and evaporation are alternatives to emission. A person could avoid all light emissions and still stay cold with a breeze over sweaty skin. Wnt (talk) 19:05, 24 March 2014 (UTC)

I support SteveBaker′s opinion , but I respect all the contributions . Tank Designer (talk) 21:07, 25 March 2014 (UTC).

Nucleosynthesis

A version of the periodic table indicating the origins – including reference desk/archives/science/2014 march 23 – of the elements. All elements above 103 (lawrencium) are also man-made and are not included.

Hi all,

Does anyone have a definitive list of the origins of the elements?

After I created this graphic:

https://commons.wikimedia.org/wiki/File:Nucleosynthesis_periodic_table.svg

based on http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryN.html and http://www.daviddarling.info/encyclopedia/N/nucleosynthesis.html , I found that it appears to conflict statements e.g. "Big Bang nucleosynthesis produced no elements heavier than beryllium..." in https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis#Heavy_elements and "In addition to the above light elements, tritium and isotopes of aluminium, carbon (carbon-14), chlorine, iodine and neon are formed within solar system materials through cosmic ray spallation..." in https://en.wikipedia.org/wiki/Cosmic_ray_spallation .

Who's right?

Thanks, cmɢʟee⎆τaʟκ 17:54, 23 March 2014 (UTC)

• Ah, this is the subject of perhaps the most wonderful non-fiction book I have ever read, P. A. Cox's The Elements; Their Origin, Abundance, and Distribution. It is available used for the cost of shipping, and probablt downloadable at places like scribd for free. μηδείς (talk) 19:06, 23 March 2014 (UTC)

I'd take the .info source above with a grain of salt; it looks quite spammy.

I'd take the Meteorite source with a bucket of salt. Excerpt (from Neutron star, same page):

"This intensifies the magnetic field of the star to around 1012[sic!] times that of the Earth."

If a "scientific" page can't even get its exponents right, I don't trust its claims too much either.

I wonder if large stars can synthesize that many elements heavier than iron without a supernova, and there must be some amounts of lithium from the Big Bang (which is supported by our BBN article), too, and I wouldn't count any elements as purely man-made. The issue is probably that they were synthesized in supernovae but decayed quite quickly. There is even a kind of supernova afterglow that's caused by radioactive decay. - ¡Ouch! (^{hurt me} / _{more pain}) 06:59, 26 March 2014 (UTC)

Oops, I got it wrong; the BBN article mentions not only lithium but also beryllium, and trace amounts of carbon (the latter being so low that it is completely lost in the "noise" of stellar nucleosynthesis, which assembles substantial amounts of carbon, so the BBN carbon cannot be detected).

The r-process is restricted to a supernova, but the s-process, which occurs in asymptotic red giants, "accounts for approximately half of the isotopes of the elements heavier than iron". so the elements marked as "large stars" are probably accurate. - ¡Ouch! (^{hurt me} / _{more pain}) 06:52, 27 March 2014 (UTC)

agent orange

when to kill foliage how far will the contamination travel? — Preceding unsigned comment added by 47.16.152.38 (talk) 19:31, 23 March 2014 (UTC)

Right down into the roots of the plants it touches, and then it remains in the soil, possibly being washed to other areas. The main danger is if the dead foliage is burned, producing dioxin, but the early production process of "Agent Orange" introduced a very nasty dioxin: 2,3,7,8-Tetrachlorodibenzodioxin. This caused illness even without burning any foliage, and contaminated soils for many years. I used to use these chemicals (2,4,5-T and 2,4-D, sold as "SBK" in the UK, hopefully without any 2,3,7,8-Tetrachlorodibenzodioxin), in small quantities, of course, and I still have some, but now seldom use it. The 2,4-D is usually broken down fairly quickly by soil bacteria, so is still considered relatively safe in some jurisdictions. Dbfirs 22:20, 23 March 2014 (UTC)

Bizarre question regarding McDonalds French Fries

Hey- Whenever I cook oven fries at home, whether they are thick or thin they always let off steam when they are fully cooked. However when I was eating McDonalds French Fries, I could not see any steam at all. These fries were piping hot and just removed from the fryer. Why was there no steam coming from the french fries? I have had this 'query' before in previous McDonalds restaurants. Thanks guys! --TįsILIi Achooo (talk) 19:59, 23 March 2014 (UTC)

I suspect that this could be because the atmosphere at MacD's is already warm and full of moisture so the steam escaping from the French fries doesn't condense into visible mist, but that's really only a guess. Alansplodge (talk) 21:23, 23 March 2014 (UTC)

While warmer air does indeed hold more moisture before it becomes visible water droplets, moist air holds less additional moisture. You want hot, dry air to absorb the most moisture without seeing any steam. StuRat (talk) 22:06, 23 March 2014 (UTC)

"Everything you've always wanted to know about McD's French Fries, but were afraid to ask", can be found in this excerpt from The Atlantic Monthly of: Schlosser, Eric ; Fast Food Nation (Houghton-Mifflin, 2001) — (I haven't read it; don't know if it answers your question) — My guess is that the crust, being extra-crispy, doesn't allow steam to escape.  —71.20.250.51 (talk) 21:41, 23 March 2014 (UTC)

• McD fries are quite thin, so they have much more fried skin outside relative to steamy boiled potato inside. See square-cube law. I suspect their cooking standards also emphasize crispiness through preparation. μηδείς (talk) 01:44, 24 March 2014 (UTC)

• I don't think the speculations above are correct. The actual answer, I believe, is that McDonald's fries, like many restaurant fries, are actually fried twice. First they are fried for a long time at a relatively low oil temperature, which removes a lot of water. Then they are frozen. The pre-frying allows the frozen fries to be cooked quickly and then to stay crisp for a relatively long time after cooking. If your fries come out steaming, then they probably become limp very quickly, even if they initially come out crisp -- that's a result of the high moisture remaining inside. Looie496 (talk) 14:30, 24 March 2014 (UTC)

That's the preparation standards I mentioned. Fries are usually cooed at least twice before serving, and the ones you buy to cook at home have already been cooked. μηδείς (talk) 16:55, 24 March 2014 (UTC)

Do they employ doves to coo at them ? :-) StuRat (talk) 17:09, 24 March 2014 (UTC)

Hehehehehemheheh. μηδείς (talk) 19:09, 24 March 2014 (UTC)

Does space occupy a train carriage as a train carriage occupies space?

Bit hesitant to ask this question, but a friend made me reel when we were watching a train pull up to the station by saying something like, 'Here it comes, bearing the space of its first voyage within it.' Is this in any way true? Does spacetime, defined by the walls of the interior, length by width by height, move along inside the train, or does it move through the train as the train moves through space? Makes me wonder about the qualities of a material that isn't permeable to space...

Thanks for your indulgence...

Adambrowne666 (talk) 22:39, 23 March 2014 (UTC)

Well, when the train first accelerates, it presumably moves out of the space it was in. While it is moving steadily, it is at rest, so the same space it was in. But since it is accelerating against gravity (gravity = acceleration), the space is actually moving down all the time... and accumulating at the bottom of Earth's gravity well? Help, is there a physicist in the house? :) (My guess is that space isn't supposed to be a physical substance, only a geometry, and it's supposed to be the same from all frames of reference and in all locations) Wnt (talk) 23:24, 23 March 2014 (UTC)

get ready to really reel: even "at rest" the train is whipping around the earths surface at 500 mph! 68.36.148.100 (talk) 23:36, 23 March 2014 (UTC)

ha, yes, good point - reeling now - and yes, thought it was likely a category error on my part; i was thinking of space as a thing, or something, whereas it isn't, or something. Adambrowne666 (talk) 23:50, 23 March 2014 (UTC)

Asking if space is moving is kind of like asking time is aging. Is a specific day (say, 12 March 1834) moving in time? Of course it's not. The train is simply moving through space, but if you want, you can think of it as stationary, thus defining a coordinate system centered on the train which moves with respect to the coordinate system of the station. In this sense, space is "moving", though this is misleading, as no observable change (beyond the train's movement) is taking place. Choosing to describe the train as stationary and the train as moving makes no difference in physical reality. If we think of space as a substance, then yes, we could say that the train is still "bearing the space of its first voyage" (just as it's still bearing the first wad of gum to be stuck under one of its seats), but space is most emphatically not a substance, and so that sentence is I'm afraid meaningless. -Anagogist (talk) 14:30, 24 March 2014 (UTC)

@Adambrowne666: here's an approach to your question on another tack: sure, you can think of the car as carrying space along with it. Why not? There are no preferred frames of reference in Newtonian mechanics, or in special relativity. Unless you still believe in Luminiferous_aether, the space encapsulated by the car is ontologically synonymous with the boundaries the car defines in some coordinate system. So this is just a long winded way of saying that the space can be defined with reference to the car, and if you want to think of the car as carrying around that space, that will lead to no problems or non-physical results that I know of. You're just saying that e.g. the top left corner of the car is the same point in space, regardless of where the car may be -- and this is trivially true, with respect to the car's frame of reference. This is really more a question of philosophy than physics though. See also Special_relativity#Lack_of_an_absolute_reference_frame, Preferred_frame, Moving_frame, and Philosophy_of_space_and_time. SemanticMantis (talk) 15:39, 24 March 2014 (UTC)

but it's definitely NOT the same space wrt the CMB. 68.36.148.100 (talk) 21:54, 24 March 2014 (UTC)

• The OP's question isn't so off base in consideration of frame dragging around a dense spinning object, which is sometimes referred to as "dragging space". The conceptual extreme of this is a Tipler cylinder. However, our articles carefully avoid that way of putting it, saying that it "warps spacetime in such a way"... which is a bit different. Wnt (talk) 15:45, 25 March 2014 (UTC)

Thanks, all, for the illuminating and well-written answers, and thanks, too, for the ping, Mantis. So - in my naivety - is there any way that a certain space can be defined aside from coordinates? Is there ever something -- aside, I suppose, from deformations come of gravity - that can distinguish one chunk of space from another? I realise the answer is very likely no.... Adambrowne666 (talk) 23:46, 25 March 2014 (UTC)