Wikipedia:Reference desk/Archives/Science/2011 July 10

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July 10

to balance the current

respected

let us take take two ac current wires.The fist one is being converted from DC to Ac and the second one direct ac. Here what i want to know is "is there any device that can connect the both wires by which the output which we are getting is appropriate and the ac current which we converted should be used and if the power from ac is low then it should be balanced with the direct ac for output".Mightyteja (talk) 06:16, 10 July 2011 (UTC)

Slightly reformatted the question to make it easier to read. Richard Avery (talk) 06:35, 10 July 2011 (UTC)

The sources should never be directly connected, of course, because the "mains" source could easily destroy the DC to AC converter (is it a switch-mode power supply or an old-fashioned transformer and rectifier?) Devices are available that will automatically switch off the "mains" AC and will switch on the local source, but they work by monitoring voltage and triggering a conventional switch. There is a short delay between the failure of one supply and the starting of the other. An Uninterruptable power supply works by always feeding power from the converter, and by constantly topping up the backup battery from the "mains" source. I suppose it is possible to design some circuitry that "balances" the loads, but I've never heard of it being marketed, and there would be safety considerations that would need to be built in to the design. Dbfirs 07:17, 10 July 2011 (UTC)

A skilled electrical engineer or electronics designer could certainly design a circuit which had alternating current with a DC offset present on a wire. I can't think of instances where this has been done with generating stations or substations with respect to the power they send out, or with power supplied to a house. It seems more common in signals than in power. The two types of electricity cold be combined at one end of a circuit and separated at the other end. AC and DC power could be made to flow in the same direction or in opposite directions. I wish there were an easy way to draw circuit diagrams here. Inside many amplifiers, the output of the transistor or tube amplifier is exactly that, a combination of direct current and an audio or video signal, which can be considered (by Fourier analysis) as the combination of several AC sinusoids of various frequencies, amplitudes, and phase angles. This combined signal can certainly be decomposed into AC and DC components. A transformer or capacitor could block the DC and pass the AC, for instance. An inductor could oppose (block) the AC but allow the DC to pass. A tuned circuit could block AC of one narrow band of frequencies, or allow passage of AC of one narrow band of frequencies. An output transformer in an audio amplifier or radio receiver sends only the AC to the loudspeaker, for example. Some types of microphones combine a DC power supply with energy sent up to the microphone from the amp input, on a pair of conductors which also sends the audio signal down to the amplifier from the microphone. Some TV antenna amplifiers similarly carry DC power up to the amplifier mounted on the antenna and carry the TV signal down on the same conductors. Trouble comes quickly to such a scheme when there is more than one neutral or ground connection in the system, or when the DC voltage is applied directly across a transformer winding, which has AC reactance but little DC resistance to limit current flow, or when AC is applied directly across a capacitor which opposes DC flow but little reactance to AC flow. Randomly connecting AC and DC power supplies together when one understands little about electricity is very likely to blow fuses or circuit breakers, start fires, damage equipment, or cause personal injury. Edison (talk) 17:41, 10 July 2011 (UTC)

I see that Edison interprets the question slightly differently from my reading, but we agree in the advice given. Dbfirs 19:20, 10 July 2011 (UTC)

"The fist one is being converted from DC to Ac" was the part that confused me the most. How is a fist either DC or AC? "Fist" has traditionally been a term for a style or pattern of telegraphy sending which is recognizable. I recall an instance in a utility substation where a battery ground was detected on the (ungrounded) DC station battery, and it turned out to be an inadvertent connection between an AC circuit (with grounded neutral) and the DC power supply. Even with that goof, everything was working ok. If there had been a second (unintentional) ground, a fuse would have blown or there would at least have been some smoke. I love the smell of burning insulation in the morning. Edison (talk) 00:34, 11 July 2011 (UTC)

I believe what the OP needs is a grid-tie inverter (GTI). He can throw away his DC to AC converter and feed the DC directly into the GTI. His 'direct AC source', whatever it is (grid or local generator) goes to the grid connection of the GTI. The AC source is then reinforced by power from the DC input. GTIs are designed for feeding power from PV installations into the national grid, but presumably they can be used for local, non-grid installations too. --Heron (talk) 18:31, 11 July 2011 (UTC)

Yes, that might work. It would need to have the cut-out circuitry disabled, and possibly some extra control circuitry added to keep it off when the "mains" is working normally and then switch it on when the "mains" voltage drops. A mains isolator would also be required to disconnect the mains supply in the event of a significant drop in voltage to comply with local regulations and to prevent the output going to feed power to other users free of charge. The cost would be high. Dbfirs 11:33, 12 July 2011 (UTC)

Memory

In which form memory store in brain or what is the form of encoding in brain ? — Preceding unsigned comment added by Just institutions (talk • contribs) 07:23, 10 July 2011 (UTC)

It's in the form of the connections between neurons, and perhaps some logic within each neuron, such as "if you get a signal from any of these 3 neurons, but not any of these 5, then send a signal on to this neuron". StuRat (talk) 08:19, 10 July 2011 (UTC)

Memory storage and recall are enormously complex processes that are still not fully understood. See our articles on neuroanatomy of memory, encoding, storage and recall for an overview. Gandalf61 (talk) 10:59, 10 July 2011 (UTC)

Here's a recent article [1] from El Reg about Project SpiNNaker, which will investigate the question using quite a large number of ARM chips.  Card Zero  (talk) 11:48, 10 July 2011 (UTC)

Just a note that there are competing theories. For example, Sir Roger Penrose believes that memory may be encoded on a quantum level through the superposition of proteins in the microtubules of the neurons of the brain. And Rupert Sheldrake's morphic field model holds that the brain is more like a radio receiver which picks up consciousness from the collective unconscious. -- SmashTheState (talk) 14:34, 10 July 2011 (UTC)

The vast majority of neuroscientists believe that the main way that the brain stores memory is by strengthening synaptic connections between brain cells. Our articles on Hebbian theory and Long-term potentiation give more details, although they are written at a technical level that may be difficult for a beginner to understand. Looie496 (talk) 17:32, 10 July 2011 (UTC)

The logic you're looking for is called summation (neurophysiology). There are different neurotransmitters, and depending on the synapse, a neurotransmitter might cause an inhibitory postsynaptic potential or an excitatory postsynaptic potential; these add up in various ways, can interfere with the propagation of each other down the dendrite to the neuron ... and long term potentiation (e.g. via protein kinase C) then customizes this further on a neuron by neuron basis. Bear in mind that if you see a photomicrograph of a dendrite, it is a tremendously long cellular projection which is covered in synapses. It's all quite complicated, naturally... Wnt (talk) 18:12, 10 July 2011 (UTC)

Were you responding to me or to the OP? I'm afraid I don't get the relevance of what you wrote in either case. Looie496 (talk) 18:31, 10 July 2011 (UTC)

"perhaps some logic within each neuron, such as "if you get a signal from any of these 3 neurons, but not any of these 5, then send a signal on to this neuron" I think this is part of memory; without such a complex network, long-term potentiation would not be affecting something complicated enough to matter. Wnt (talk) 05:34, 14 July 2011 (UTC)

Questions about Antimatter?

Hi, I have a few questions about antimatter. I tried reading most of it to try to understand it, but for the life of me I can't. I just have a few questions that if answered might help me understand better.

1. Does antimatter exist on a different dimensional plane?
2. Does antimatter only take shape in the form of a particle? Can it take the form of an object?
3. If #1 and #2 are correct, are there antimatter versions of ourselves?
4. Is it tangible in any way?

Any help would be appreciated! 64.229.153.237 (talk) 18:06, 10 July 2011 (UTC)

Antimatter acts pretty much just like matter, so far as anyone can show. Of course, a complex and fine-tuned system like a human is an exacting test, but so far as I know no one has disproved that you couldn't zap someone with a magic wand and turn him into a perfectly viable antimatter-person by simply flipping every matter particle for the same thing in antimatter. Till he touched something (like the air) and blew up, that is.

Different dimensional plane? Not sure how to define. I remember reading speculations that some part of the universe, now separated from us by further than light can travel since the beginning of time, might have antimatter galaxies. Would that count? Wnt (talk) 18:16, 10 July 2011 (UTC)

(edit conflict) "No" and "objects are nothing more than certain combinations of particles" are the first two answers. The difference between a particle and its antiparticle is that the fundamental charges are opposite. Antihydrogen is trapped in our earthly dimensions using our earthly-dimensioned tools and physics. DMacks (talk) 18:20, 10 July 2011 (UTC)

(edit conflict) Antimatter behaves exactly like matter does, excepting that certain fundemental properties (like electric charge) take the exact opposite value. Otherwise, it is expected to behave exactly the same as matter does. If you are interested, you should read antimatter, where a lot of things are explained. --Jayron32 18:23, 10 July 2011 (UTC)

Well, take a look at CPT symmetry and it particular the section on violations. So maybe not exactly the same, depending on how that comes out. Not that I expect any such thing would be big enough to notice at the level of what is green or sweet-smelling or crumbly. --Trovatore (talk) 19:02, 10 July 2011 (UTC)

Right: Antimatter atoms might have substantially different chemistry than we're used to, but until we get much better at preserving un-ionized antimatter for study, we won't be able to tell. 99.24.223.58 (talk) 20:42, 10 July 2011 (UTC)

In answer to the questions: (1) No. (2) Theoretically every particle has a corresponding antiparticle, though there are a few cases, such as the photon, where a particle is its own antiparticle. (3) No, although theoretically there probably could be. (4) Yes, as tangible as ordinary matter -- however if you bring ordinary matter and antimatter into contact, you get a huge explosion. One more point that is perhaps worth noting is that from a certain point of view, an antimatter particle can be thought of as an ordinary particle going backward through time. (That's how they are represented in Feynman diagrams.) Looie496 (talk) 18:37, 10 July 2011 (UTC)

Although you can think of antimatter that way, it might not be very helpful for someone who doesn't understand Feynman diagrams well to try to do so. 99.24.223.58 (talk) 20:44, 10 July 2011 (UTC)

Quoting myself from a previous thread: "The particle-antiparticle duality exists because of CPT symmetry. The CPT symmetry applied twice gives you back what you started with (it's a fancy kind of mirror reflection). Therefore, for any given starting state, there are two possibilities: CPT leaves the state unchanged (it's a CPT-invariant state) or CPT switches back and forth between two states that are duals of each other. In the second case, there's a convention of naming one state by 'anti-' prefixed to the name of the other state—for example, electron and antielectron. This is not a universal convention. 'Positron' is another name for 'antielectron', and the W⁺ and W⁻ are never called anti-W⁻ or anti-W⁺, even though they're CPT duals of each other. Furthermore, it's essentially arbitrary which of the two states gets the 'anti' prefix. There's no physical 'antiness' property possessed by the 'anti' particles.

"CPT symmetry includes time reversal (the T), so one could say that the dual states have opposite time directionality. But this is like saying that they have opposite space directionality (the P). It doesn't really mean anything. Particles have no intrinsic time direction to start with, so there's nothing to reverse. And there's nothing 'anti' about the particles with 'anti' in their name, so you can't single them out as the ones going backward in time." -- BenRG (talk) 21:59, 10 July 2011 (UTC)

Zero-point energy?

I recently read an interesting article regarding a practical experiment demonstrating the effects of some peculiar quantum physics. Virtual particle pairs are constantly being created and annihilating themselves all around us. Generally they're only "real" in potentia, and expressed as a probability. However, if one takes two mirrors and places them very close together, the Casimir effect will draw the two mirrors together.

Now the interesting part.

At relativistic speeds, one of the photon pairs created by the annihilation of the virtual particle can escape -- becoming essentially what we regard as "real." As I understand it, this is zero-point energy. You're extracting photons from virtual particles, from the potential energy of space/time itself. You are creating energy from nothing. The article states that they've actually managed to create the technology necessary to take this from s imple thought experiment into practice, and have observed photons being created from nothing. Does this violate the First Law? Or does the First Law indicate that we will never be able to increase the efficiency of the machines necessary to create the photons to the point where more energy is being created than consumed? Or does this mean we have now discovered the potential to create infinite amounts of energy, the proverbial free lunch? -- SmashTheState (talk) 18:38, 10 July 2011 (UTC)

It doesn't seem like a violation to me, since you aren't creating energy, but just changing it from one form ("the potential energy of space/time itself") to a more familiar form. It's also not an infinite supply, just far larger than we could ever use. Now, just because it doesn't violate any fundamental laws of physics doesn't necessarily mean it's practical. For comparison, nuclear fusion is absolutely possible as an energy source, but, despite decades of research, we've yet to make it practical. StuRat (talk) 18:50, 10 July 2011 (UTC)

No, anyone who claims to extract energy from the vacuum is deluded or conning you. That's not because there's no energy to extract. It's because if you do extract energy from the "vacuum", you end up in a different, lower vacuum state, one in which the laws of physics are different and we can't exist. The fact that we're still here means that didn't happen. -- BenRG (talk) 22:22, 10 July 2011 (UTC)

You must have misunderstood something.Energy is not being created out of nothing. The 1st law of thermodynamics still applies. Dauto (talk) 20:34, 10 July 2011 (UTC)

It's a myth that the Casimir effect shows the reality of vacuum fluctuations, albeit a myth that a surprising number of professional physicists seem to believe. See "The Casimir Effect and the Quantum Vacuum" by Robert Jaffe. The article that you linked is nonsense. The "phenomenon" these people are investigating is nothing more mysterious than what happens in a light bulb. -- BenRG (talk) 22:22, 10 July 2011 (UTC)

I'm a little suspicious of anyone who makes truth claims regarding scientific matters. Science is the philosophy of empiricism. Empiricism, by definition, can't reference truth, since truth is a metaphysical property. Rationalism and revelation can make truth claims; empiricism can't. I'm not saying your skepticism is unwarranted, but given that you admit this "myth" is accepted by professional physicists, I'm not sure why I should accept your view over theirs. Can you explain why you're utterly certain that the experiment being referenced doesn't demonstrate what it's claiming to demonstrate? To argue it can't because your first principles won't allow it is, as I understand the theory of scientific method, bad science. -- SmashTheState (talk) 23:46, 10 July 2011 (UTC)

I don't know that much about the issue being discussed here. But 'Extraordinary claims require extraordinary evidence' is generally an important part of science. The first law of thermodynamics is very well accepted and supported. This doesn't mean it's impossible to be wrong, clearly that's unscientific. But this does mean you're going to need a lot of evidence before anyone is going to believe you've proven it wrong. If someone claims it's disproven, the most likely possibilities are either they've misunderstood the first law, they've misunderstood their results or their results are just plain wrong. That isn't bad science. (And there are so many of these 'free energy' ideas you really have to be doubly sceptical of anyone claiming to have disproven the first law.)

About the 'professional physicists' thing, I don't know what BenRG is referring to but I would note it's generally a bad idea to think someone knows what they're talking about just because they're in a related field. And related field is an important point. Physics is a wide area of science and the fact someone is physicist doesn't mean they are really that informed on one specific area of physics. This of course applies to most areas of science. A marine biologist (which itself is still a fairly wide field) will probably know more about the biology of fungi then a layperson but I wouldn't call them an expert. If you want to know about fungi affecting apples you'd ask a mycologist of some kind or a plant pathologist.

Most people will be careful when talking about a field they're not that familiar with but that may have been BenRG's point. Some physicists may make comments about the Casimir effect when asked, while emphasising it's not really in their area of expertise, and they'll be wrong. In other words, the more important point is not whether BenRG know what they're talking about but whether the person writing the ref does and whether the physicists who believe other things about the Casimir effect do.

Nil Einne (talk) 00:54, 11 July 2011 (UTC)

Butterfly effect in astronomy

I've often wondered about long-term astronomy predictions (or the reverse, such as determining when eclipses occurred thousands of years ago). Over such time periods, it seems that a close-passing comet or other object is likely to have perturbed the orbits of the Earth and Moon. So, can we tell that this hasn't happened (say from a lack of "wobble"), tell that it has and figure this in, or are we just assuming that any such perturbations are insignificant to the calculations ? StuRat (talk) 19:49, 10 July 2011 (UTC)

Possible perturbations are taken into consideration to estimate how long before the butterfly effect takes over but thousands of years is too short a period to worry about that. It would take millions of years before planetary chaos takes over. Read Formation and evolution of the Solar System#Long-term stability. Dauto (talk) 20:41, 10 July 2011 (UTC)

Thanks, but why couldn't there be a long-period comet that passed near Earth, say 10,000 years ago, shifting the orbit of the Moon significantly ? StuRat (talk) 17:49, 11 July 2011 (UTC)

Highly unlikely that it's mass would be large enough to have an effect. Dauto (talk) 21:43, 11 July 2011 (UTC)

I was under the impression that there could be Pluto-sized objects way out there. StuRat (talk) 00:33, 12 July 2011 (UTC)

Yes, there are likely hundreds of them. What you don't seem to grasp is the vastness of space and how short a time frame a few thousand years actually is. When you put those two things together, the chances that a large enough event happens in such a small time scale is utterly negligible. Dauto (talk) 15:40, 12 July 2011 (UTC)

Skinning beef

I have searched but cannot find a description of the modern method of skinning beef in a slaughter house. Can anyone direct me to an article on this subject, or tell me how it's done? Thanks, wsc — Preceding unsigned comment added by 66.74.14.40 (talk) 19:53, 10 July 2011 (UTC)

See the section including diagrams 28-33 of [2]. Not for the faint of stomach. 99.24.223.58 (talk) 20:51, 10 July 2011 (UTC)

I witnessed this process at a commercial beef processing facility in Eastern Pennsylvania. What I saw was consistent with the description in the above reference, specifically the 'vertical method' with partial automation. ike9898 (talk) 16:06, 11 July 2011 (UTC)

Waves spreading out

Why does a wave, like a water wave, spread out after passing through a thin slit? I'm somewhat familiar with Huygens' Principle, but I would like a more physical and intuitive picture of why it happens. How does the section of water entering into the slit know to spread, when it wouldn't have spread if the barriers hadn't been there? What do the barriers do to communicate their existence to the entering wave? 74.15.136.219 (talk) 20:00, 10 July 2011 (UTC)

Not sure I understand, a wave does spread out without a slit, just like with one. As to why, think of the wave as a lump of water. Gravity would naturally cause it to spread out in all directions. StuRat (talk) 20:13, 10 July 2011 (UTC)

No. Diffraction causes a wave that has passed through a slit to spread out differently than one that has not. --Srleffler (talk) 17:22, 12 July 2011 (UTC)

It's called diffraction. That article may help. Let us know if you still have questions after reading it and we'll be happy to help. --Tango (talk) 20:15, 10 July 2011 (UTC)

Waves do spread into all directions even when barriers are absent. That IS the meaning of Huygens' Principle. Dauto (talk) 20:44, 10 July 2011 (UTC)

See above. Passing through a slit does change how a wave spreads out. --Srleffler (talk) 17:22, 12 July 2011 (UTC)

Maybe this pic, http://tinypic.com/r/11i0d5h/7, will show what I mean. Before the section of water enters the slit, it moves without spreading. So why would it start spreading after leaving the slit? Am I mistaken in thinking that (before the wave hits the slit) if the section of water that would ordinarily not pass through the slit were to magically disappear, the remaining water would continue unperturbed, without spreading? 74.15.136.219 (talk) 21:31, 10 July 2011 (UTC)

I believe you are mistaken. There's nothing special happening at the slit, aside from blocking the rest of the wave. Before the slit, the wave *does* spread out. It's just that the bit of wave on the right spreading out to the left interferes with the bit of wave on the left spreading out to the right, resulting in it looking like no spreading has occurred. (You can potentially visualize this by looking at the second image in Huygens–Fresnel principle, and then mentally adjusting the size of the slit from a point out to being infinite.) -- 140.142.20.229 (talk) 21:41, 10 July 2011 (UTC)

You are the one that is mistaken. Diffraction does cause a change in how a wave spreads. You've got the right mechanism though: the wave before the slit does not spread (assuming it is a plane wave) due to interference between different parts of the wave. When some of those parts are blocked by the slit, the portion that is left begins to spread out.--Srleffler (talk) 17:25, 12 July 2011 (UTC)

Also good pointing out that the water is not moving forward in the picture you've shown. It is moving back and forth, up and down, or a combination of both. Dauto (talk) 22:32, 10 July 2011 (UTC)

I'm trying to see this more physically. When a small portion of water in the wave pulls upward on the water in front of it, are you saying that it creates a circular wave? So that Huygens' principle is not just a mathematical trick, but something physical?74.15.136.219 (talk) 02:41, 11 July 2011 (UTC)

Yes, it's a physical principle. Any disturbance in the water propagates in all directions. Why wouldn't it? I don't know what you mean by a mathematical trick. Dauto (talk) 05:09, 11 July 2011 (UTC)

Thanks a lot. Final question: why isn't there a backwards wave? — Preceding unsigned comment added by 74.15.136.219 (talk) 05:50, 11 July 2011 (UTC)

There are, but all the little waves coming from different points interfere negatively. If you put a patch of material with different properties somewhere in there, the interference will be imperfect and some wave will come back as a partially reflected wave. Dauto (talk) 08:42, 11 July 2011 (UTC)

Some thoughts:

1) Waves that originate as a point source will spread out circularly on a planar medium.

2) Waves that originate as a line source will spread out linearly in a planar medium.

3) Waves passing through a small slit are slowed at the edges, so spread more like from a point source from there. StuRat (talk) 17:44, 11 July 2011 (UTC)

Re (3): a change in velocity is not required.--Srleffler (talk) 17:27, 12 July 2011 (UTC)

Meaningful speculation on existence of anything possible?

Can science meaningfully speculate on why anything (i.e., the universe) exists? 76.27.175.80 (talk) 21:29, 10 July 2011 (UTC)

There probably isn't any way to formulate a testable hypothesis concerning the reason for the existence of the universe, but untestable speculation abounds, e.g., at Big Bang#Speculative physics beyond Big Bang theory regarding why the universe came in to existence. 99.24.223.58 (talk) 21:54, 10 July 2011 (UTC)

Science is a means to describe the universe based on the idea of the testable hypothesis, which is not to say that the entirety of the human experience consists solely of testable hypotheses. Many experiences have facets which subject parts of themselves to testable hypothesis, for example one can study the details of a religious experience via science; what parts of the brain are active during religious experiences, what sorts of genetic markers make people more likely to have religious experiences, etc. It can look at why some sorts of art is better received by certain people, what sorts of music is (in general) thought to be pleasant to listen to, etc. Science can study all of that. But the knowledge behind all of these things is not a substitute for the experience itself. Large parts of the human existence, while explainable by science, are still real parts of the human existence, and cannot be substituted by science. Understanding how acoustics and music theory and sound waves work doesn't make The Dark Side of the Moon a more or less enjoyable musical work to listen to; people will still be moved by a painting like The Scream even if they do (or even if the do not) understand the chemical composition of paint. And people can still have a meaningful relationship with their God even if they understand what physics says that the Big Bang looks like, or how evolution works. Don't mistake the understanding of the details of creation for a substitute for a relationship with the Creator (if indeed you are inclined to believe that sort of thing. I make no pretense that you are required to, just that you are allowed to). The two concepts can exist perfectly well in a normal, healthy, intelligent person. --Jayron32 22:09, 10 July 2011 (UTC)

Its something studied by philosophers rather than scientists. See Ontology, Anthropic Principle, Dysteleology and http://spot.colorado.edu/~heathwoo/phil3600/parfit.pdf 92.28.240.112 (talk) 17:52, 13 July 2011 (UTC)

The Universe exists because there could not have been anything to prevent it. μηδείς (talk) 03:17, 14 July 2011 (UTC)

Car steering

Does a car follow exactly the same curved path backwards as it did when going forwards if the steering wheel follow the same positions (in reverse order)? If it doesn't, is it possible to "park a car" in some defined restricted space, so that it can't be unparked? -- SGBailey (talk) 21:32, 10 July 2011 (UTC)

I'm pretty sure the general answer is "yes it does so no it isn't," unless for example you were trying to park on an inclined oil slick, in which case you might be able to slide in to a position from which you could not drive out. 99.24.223.58 (talk) 21:58, 10 July 2011 (UTC)

Despite the logic of the correct reply above, I usually find it easier to "unpark" than to park in a tight space, but I think this is because I usually drive out forwards. On the rare occasions when I try to reverse out of a tight space, I find that to be harder than it was to drive in forwards. Dbfirs 22:24, 10 July 2011 (UTC)

Also a properly parked car has lower entropy than an un-parked car which means it takes more effort to find the proper maneuver to park it than to find some maneuver to un-park it. Dauto (talk) 22:37, 10 July 2011 (UTC)

Ah, yes, I like the entropy argument! I think that's the main factor, with my forward/reverse being a secondary factor. Dbfirs 06:42, 11 July 2011 (UTC)

In general – and in the absence of significant losses of traction and sliding – the car's path is fully reversible; usually the problem arises because the driver's intuition isn't quite as well developed for driving backwards, not because the car has become irreversibly trapped. (Sometimes, of course, a car may become genuinely trapped if someone else has moved the surrounding cars while you were in the shop.) That said, if the no-sliding condition is not met, one can get into an inescapable spot. See, for instance, this video of extreme parallel parking.

Incidentally, your question reminds me of the Douglas Adams novel Dirk Gently's Holistic Detective Agency, a subplot of which involved the protagonist's attempts to manoeuvre a large sofa up and around a cramped stairwell. The sofa became stuck, apparently irreversibly, and remained in place for much of the novel. Complex simulations of the stairwell were employed in an attempt to compute a path by which the offending furniture could be liberated. TenOfAllTrades(talk) 14:30, 11 July 2011 (UTC)

As an aside, note that the Moving_sofa_problem is still unsolved, even in the simpler 2D case. So Adams' take is actually somewhat accurate ;) SemanticMantis (talk) 16:31, 11 July 2011 (UTC)

DEET insect repellent dissolving things

I'm aware that DEET insect repellent can dissolve various materials. I want to know whether it's possible to rub it on skin, wait for a period of time and then make contact with vulnerable materials? Or is it constantly able to dissolve those things for as long as it's present on the skin? --2.216.135.118 (talk) 21:33, 10 July 2011 (UTC)

I am guessing it's not the actual DEET that's doing the dissolving, but rather the fact it uses alcohol as a carrier. Once dried onto the skin it should be fine, but I'd test first because I know it can stain things too. HominidMachinae (talk) 21:56, 10 July 2011 (UTC)

I agree. Any solvent absorbed into skin loses potency because of the porosity and absorption of skin. 99.24.223.58 (talk) 22:01, 10 July 2011 (UTC)

Oh yes, it's the actual DEET that does the dissolving. Absolutely it is. Regarding skin, my experience is that as long as it looks shiny/wet it is dangerous, but once it has absorbed into the skin it isn't any longer. Looie496 (talk) 22:03, 10 July 2011 (UTC)

Yes, our article on DEET says: DEET is an effective solvent, and may dissolve some plastics, rayon, spandex, other synthetic fabrics, leather, and painted or varnished surfaces including nailpolish. and I expect that the worst effects will disappear once it dries, but remember that the skin produces natural oils and these will act to transfer tiny amounts of DEET to anything touched under some circumstances. Dbfirs 22:21, 10 July 2011 (UTC)

DEET is a liquid at room temperature. Until not so many years ago, you could readily purchase bottles of 99% DEET in camping supply stores and the like. Nowadays the available formulations top out around 30% or so; cost and health considerations led to these more dilute products. (Lower concentrations may also be kinder to your plastic and synthetic products, and don't smell quite so...pungent.) In any case, once you get up to 20-30% DEET, higher concentrations aren't any more effective at repelling insects; they just last longer before needing to be reapplied. 30% is good for 3-6 hours, whereas the old 99% stuff could be effective for 12 hours—if you didn't sweat too much. TenOfAllTrades(talk) 04:09, 11 July 2011 (UTC)

DEET causes issues with some garment materials. Have worked in outdoor apparel and the DEET tests challenged some polymers.TCO (reviews needed) 04:15, 11 July 2011 (UTC)

Destroys acrylic fingernails! Bielle (talk) 04:25, 11 July 2011 (UTC)

First, note that military bug repellent contains much more DEET than what you can buy at the store. I used it when I was in the Marines. Once, I had DEET on when I went to the beach. I carried a radio with me. When I got to the beach, I put the radio down and I noticed that my DEET-laden fingers melted the plastic handle. I still have the radio and there are clear indentions in the handle for each of my fingers. After that, my friends and I experimented on other things. That nearly pure DEET melts a lot of stuff - even on your skin. A good trick was touching styrofoam and melting it with very little pressure. -- kainaw™ 02:14, 12 July 2011 (UTC)

I miss the days when you could buy 100% DEET in civilian stores. When I was a kid it was all we used, and it was the only absolutely effective bug spray I've ever used HominidMachinae (talk) 07:29, 12 July 2011 (UTC)

I have a bottle of 80% here purchased from a civilian store. I've permanently etched my fingerprint into my camera's shutter button with DEET accidentally. JJ Harrison (talk) 07:52, 12 July 2011 (UTC)

Brief flash of light in the night sky

Last night (10/07/2011) around 10.30pm (GMT +8), there was a flash of light in the sky. It was shaped like a ball. It just appeared out of nowhere and vanished a few seconds later after travelling what seemed to be 2 inches from our view. What could this flash/ball of light be?202.152.86.5 (talk) 23:11, 10 July 2011 (UTC) Was it a shooting star? Was it a comet (though I don't recall a tail)?

I'm not sure what you mean by 'about 2 inches'? A meteor might be a possibility - comets don't suddenly appear and disappear. AndyTheGrump (talk) 23:21, 10 July 2011 (UTC)

The OP geolocates to Brunei. The description is not very clear -- a meteor seems like the most likely possibility, but apparently there was a major military air show very recently, so some sort of military aircraft might also be possible. Looie496 (talk) 23:27, 10 July 2011 (UTC)

The stated timezone (GMT+8) looks like Brunei as well. OP, can you be a little more specific about what you say you saw? If the "2 inches" bit of a "flash of light … like a ball … 2 inches from our view" is literal may this have been Ball lightning? Tonywalton ^Talk 23:53, 10 July 2011 (UTC)

I agree that a meteor (also known as a shooting star) is the most likely explanation. It could have been a satellite flare. It could have been an aircraft, although I think 10:30pm is quite a long time after sunset in Brunei so you wouldn't have any reflected sunlight (shortly after sunset, the sun can still be above the horizon from the point of view of an aircraft, so the aircraft can reflect sunlight to the ground). As others have said "2 inches" is a little unclear in this context. How many times the diameter of the moon was it, would you say? --Tango (talk) 23:57, 10 July 2011 (UTC)

Maybe ball lightning? --Jayron32 00:28, 11 July 2011 (UTC)

If the sky was clear I'd say it was almost certainly a meteor. I'm not a fan of "ball lightning" as an explanation for anything as there really isn't any very good evidence for that phenomenon, I think it's a "last resort" type explanation.. It's likely it could have been just a random meteor, but there's a chance it might have been associated with a known meteor shower. For example the July Pegasids are active right now in the northern hemisphere in the region of Pegasus. If you remember where it was in the sky, get a program like "stellarium" and you should be able to work out what constellations are in that part of the sky, you might be able to match it with a known meteor shower. Vespine (talk) 04:13, 11 July 2011 (UTC)

Imagine this, viewed from the rear, in sunset colors. ::yes, just make sure you are deaf or it will be ovsious that it is a plane. Googlemeister (talk) 20:52, 11 July 2011 (UTC)

Could it be a plane from that air show, high enough up so it reflects the sunlight, going supersonic and creating a water vapor halo that catches the sunset ? If the plane was black it might not reflect much sunlight itself. StuRat (talk) 17:33, 11 July 2011 (UTC)

It's hard to say what it was. At 10:30pm local time, a plane probably wouldn't be able to get high enough to catch any sunlight. Apparently it was not an Iridium flare, though: no Iridium flares above that location in the past 48 hours. WikiDao ☯ 17:56, 11 July 2011 (UTC)

If not sunlight, perhaps a searchlight was pointed at it. It would seem rather odd to do an air show at night without some form of lighting, after all. StuRat (talk) 18:56, 11 July 2011 (UTC)

The airshow was apparently held on July 9th, the day before.[3] I've only seen airshows held in the daytime. WikiDao ☯ 19:08, 11 July 2011 (UTC)

What part of this does NOT fit "meteor"? I think that's by far the simplest explanation and seems to fit the given details perfectly. Barring any reason to discount that hypothesis why are people still trying to shoe horn other explanations? Vespine (talk) 02:38, 12 July 2011 (UTC)

Because wild speculation is what the Ref Desk is all about! :) -RunningOnBrains^(talk) 09:10, 12 July 2011 (UTC)

I'd expect that to have made the local news, since they described it as a "ball", not a "point", so it's pretty big. Also, meteors aren't very common outside of meteor showers. StuRat (talk) 03:59, 12 July 2011 (UTC)

In all seriousness though, there is no reason to believe it was anything but a bolide (bright meteor, or shooting star as the original poster puts it); they present in all sorts of colors, shapes, and durations. We are asked to identify an object which

• Was a flash of light in the sky
• Appeared suddenly
• Moved across the sky
• Disappeared

All of which easily fit the MO of a meteor. Just because very bright meteors don't happen every day doesn't mean it isn't the most likely explanation. I've seen two in my lifetime, and I'm not exactly outside staring at the night sky all the time (or very often at all); neither of these was during a meteor shower. -RunningOnBrains^(talk) 09:10, 12 July 2011 (UTC) Meteors outside of meteor showers are very common. Under good conditions, if you are actually looking for them, you could easily see several an hour on a typical night [4]. Since we have ruled out an Iridum flare (there are some other satellites that can cause flares, but not many), I think we can be 99% certain it was a meteor. --Tango (talk) 12:09, 12 July 2011 (UTC)

looking for a picture of fluorine reacting

Can anyone help here...or point me to more places to ask for help?

I want a video (or perhaps a photo) of fluorine gas reacting with something. There is nothing PD (or even that I can think of as an easy donation). Looking for something like in the videos below. Will go into a Featured Article Candidate. Either someone to make it for us or who has it.

http://www.youtube.com/watch?v=mG6EG_igTGw

-or-

http://www.youtube.com/watch?v=V1FsO5zaf6M

TCO (reviews needed) 23:50, 10 July 2011 (UTC

See Fluorine

as linked to from the The Periodic Table of Videos.

μηδείς (talk) 03:23, 14 July 2011 (UTC)