Wikipedia:Reference desk/Archives/Science/2012 May 1

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May 1

Entangled Qubits as Polarized Light

I'm trying to find a way to visualize entangled states, as might be used in a quantum computer. The various articles I can find, and my physics textbook, make vague references to polarized light as a simple model for a qubit. I can understand the one-qubit case, since the components of the electric wave in two orthogonal axes for a coherent beam of light or a single photon will consist of two phasors, represented as two complex numbers, where overall amplitude and phase can be ignored. What's left is the two dimensions of polarization, representing angle and eccentricity. Gates are passing through eg a fluid that rotates polarization, and measurement is passing through a polarizing filter. So a two-qubit register should be two beams of light, or perhaps a beam of light with two colors. I haven't found anything that specifies which it is. Where I get lost is understanding how these bits can become entangled, and where the process can go from there. Black Carrot (talk) 00:30, 1 May 2012 (UTC)

Size of the cat's eye nebula

Our article says that the core is 0.2 ly. Obviously it doesn't burn like a star, so what defines the core? And how large is the entire nebula? Even a ballpark figure would help me. Thanks, --T H F S W (T · C · E) 05:19, 1 May 2012 (UTC)

The angular diameter is about 5 arcmin, which corresponds to roughly 5 ly at that distance. The planetary nebula itself corresponds to the bright inner core, so that is material that was ejected at once when the star entered its PN phase. The larger fainter nebula is material that was ejected over a longer period before that, when the star was a red giant. --Wrongfilter (talk) 08:35, 1 May 2012 (UTC)

So the core is the part we see in all those classic pictures? --T H F S W (T · C · E) 16:16, 1 May 2012 (UTC)

Air in submerged car

Assuming a normal family car was submerged without it being smashed about, and there was one adult inside, roughly how long could they survive on air trapped in the vehicle?

If they inflated the air bags and punctured them, would that help, and if so, to what extent? --Dweller (talk) 09:36, 1 May 2012 (UTC)

Airbags don't contain air - they contain the output of gas generators, which are essentially little rocket engines. You wouldn't want to be breathing that. -- Finlay McWalterჷTalk 09:42, 1 May 2012 (UTC)

I did wonder! OK, how about the air naturally in the car? --Dweller (talk) 10:37, 1 May 2012 (UTC)

Since cars are far from airtight the air would remain "trapped" for only a short time. It would probably take only a matter of seconds rather than minutes for the cabin to fill up with water as the air escapes through the door "seals" and other gaps. Roger (talk) 10:46, 1 May 2012 (UTC)

Interesting. The engine is the heaviest part of the car, so I guess it would nose-dive, leaving whatever air there is against the back window. In a saloon car with no hatchback, there's no door seals there. Would there be as much as a minute's worth of air, or more? --Dweller (talk) 10:49, 1 May 2012 (UTC)

Chappaquiddick incident#Testimony and cause of death contains some supposition on this subject. -- ToE 11:22, 1 May 2012 (UTC)

Mythbusters did a number of submerged car escape myths (you can google for videos). Based on their videos the engine pulls the car down nose first and it seems to take about 30-40 seconds for the front to fill and 60-75 total seconds for the air to also escape from the back seat. Of course, the precise time will vary with the make of the car, etc. Dragons flight (talk) 11:50, 1 May 2012 (UTC)

Have a look at [1] for a practical demonstration. The accompanying notes are interesting. Bazza (talk) 13:37, 1 May 2012 (UTC)

It's also important to note that you normally do not want to stay as long as possible in the car. Quite in contrary, you should try to open the windows to let water flow inside (so you can open the doors. 188.76.235.0 (talk) 13:49, 1 May 2012 (UTC)

If car does drop to the point where you can't open the door and your power windows are shorted out and you are in shallow water, then waiting for the car to fill with water so you can open the door may be your only option. Hopefully you could use that time to unbuckle, get into position, calm down and take deep breaths, for your break for the surface once the pressure equalizes. StuRat (talk) 18:10, 1 May 2012 (UTC)

Googling for something like cubic meter breathe air suffocate suggests that for a person in rest, one cubic meter of air should last about two hours until it has too little oxygen and too much carbon dioxide. Heavy physical activity can cut that time by a factor of ten; I imagine the stress of being in a submerged car will cause you to breathe heavily. If your car's interior volume is, say, three cubic meters, that would be of the order of a couple of hours. If partially filled with water, adjust accordingly. If your head is barely above surface, in a bubble of a couple of bucketfuls of air, let's call it one minute per bucket tops. 88.114.124.228 (talk) 16:13, 1 May 2012 (UTC)

This formula from an atmospheric sciences course is pretty fascinating. --Sean 20:59, 1 May 2012 (UTC)

And I think once you're underwater, kicking on the window to break it is much less effective. Ever since reading about it somewhere, I make sure that I keep a sturdy metal instrument in the glovebox with which to shatter the window should something like this ever occur. DRosenbach ^{(Talk | Contribs)} 20:15, 4 May 2012 (UTC)

What does this mean? Gravitational waves should penetrate regions of space that electromagnetic waves cannot.

165.212.189.187 (talk) 13:04, 1 May 2012 (UTC)

As far as I know, electromagnetic waves, or radiation, can penetrate any region of space. So, the statement does not make sense to me either. Plasmic Physics (talk) 13:16, 1 May 2012 (UTC)

Well, light can be blocked by solid materials, whereas gravity cannot. That must be what they are referring to. Goodbye Galaxy (talk) 13:35, 1 May 2012 (UTC)

Electromagnetic waves of frequencies up to up to microwaves are dramatically attenutated by magnetic shielding and by faraday cages - see http://en.wikipedia.org/wiki/Faraday_cage. It is quite simple to make a faraday cage to reduce incident radiation to levels too low to measure. The next thing after microwaves is infrared thru to light - which is blocked by all sorts of things including metals. Gravitational waves are predicted to penetrate all these things. Keit58.164.230.165 (talk) 13:46, 1 May 2012 (UTC)

Could it be worded better?165.212.189.187 (talk) 14:12, 1 May 2012 (UTC)

But do faraday cages block ALL radiation?165.212.189.187 (talk) 15:47, 1 May 2012 (UTC)

Since you can see into a Faraday cage, obviously not. Gandalf61 (talk) 15:52, 1 May 2012 (UTC)

The blocking of radiation depends on the scale of the mesh. A tighter mesh blocks more ranges of radiation. OsmanRF34 (talk) 15:59, 1 May 2012 (UTC)

EM radiation is not blocked 100%, it decays at an exponential rate, but never reaches zero completely. Plasmic Physics (talk) 22:08, 1 May 2012 (UTC)

Touche. I was particularly thinking of a solid one to rule out the light issue. Then the word "should" should be removed bc we know it DOES, right?165.212.189.187 (talk) 17:17, 1 May 2012 (UTC)

No, we don't know that yet. Gravitational waves have not been successfully detected. Various theories predict that it could penetrate regions impervious to EM waves, but we don't know that for sure yet. 173.32.168.59 (talk) 17:49, 1 May 2012 (UTC)

Ok I get it. SVT is the best theory so far.165.212.189.187 (talk) 18:06, 1 May 2012 (UTC)

Faraday cages are not perfect. The scale of the mesh is not the only factor. Since the wires or mesh (or even solid sheet) must in practice have electrical resistance, there will be an excess voltage within the walls and the attenuation imperfect. However, by suitable choice in wire/mesh weight, and double screening if necessary, any required attenuation can be achieved. But the attenuation is not an exponential decay. Exponential decay of EM waves occurs in dielectrics, however, and gravity waves are thought not so attenuated. Ratbone120.145.52.89 (talk) 02:10, 2 May 2012 (UTC)

Maybe gravity waves can escape from a black hole but light waves cannot? Although I'm not sure about the precise physics of gravity waves (is anybody?). --Colapeninsula (talk) 09:01, 2 May 2012 (UTC)

The physics of gravitational waves in the classical theory of general relativity is understood very well, although we know this is not the complete story - for that, we would need a theory of quantum gravity. Gravitational waves do not escape from the interior of a black hole, but they are emitted when two black holes (or other massive astronomical objects) orbit one another, or when two black holes collide and merge. BTW, don't confuse gravitational waves with gravity waves. Gandalf61 (talk) 12:02, 2 May 2012 (UTC)

Amputating limbs of quadriplegics

Do quadriplegics need to have their limbs amputated sometimes? Why doesn't the blood clot, if they necessarily won't be moving their limbs? — Preceding unsigned comment added by 188.76.235.0 (talk) 13:22, 1 May 2012 (UTC)

Bones make blood. I think they would want to keep limbs where possible. Zzubnik (talk) 14:33, 1 May 2012 (UTC)

Not sure if that's a valid reason. After all, fewer limbs also mean less blood is needed, so less red-cell generating capacity is OK. Some reasons to keep the limbs might be psychological considerations and the hope that they might one day be cured of their quadriplegic condition, say by use of stem cells to regenerate the nerves. StuRat (talk) 17:59, 1 May 2012 (UTC)

And they do have problems with blood clots, see Tetraplegia which notes that such people have high rates of Deep vein thrombosis. --Jayron32 16:23, 1 May 2012 (UTC)

wasp

A few months ago I noticed a wasp in a cupboard in my house. I don't know how long it had been there, but it looked pretty dead to me. I didn't get rid of it because, well, I couldn't be bothered to and it wasn't affecting me. A few weeks later I go in the cupboard again and it's still there, exactly the same as before, it hasn't moved at all. So I am now certain it is dead. Again I just leave it because I'm really lazy. Today, I noticed a wasp buzzing around the house so I opened a window and let it go. Then I thought.. no way, it couldn't be the wasp in the cupboard, could it? I checked and it's gone. So.. do wasps hibernate or something? I'm assuming it was the same wasp, although I can't be certain. It looked like a normal wasp to me and not a big queen wasp. — Preceding unsigned comment added by Waspgirl03 (talk • contribs) 20:32, 1 May 2012 (UTC)

I doubt if it's the same wasp. It might have been cleaned up by somebody else, blown out of the cabinet when somebody opened the door, or perhaps some other critter dragged it off to eat it. (After it dehydrated, it would be far lighter and more prone to blowing around.) StuRat (talk) 20:43, 1 May 2012 (UTC)

Nobody else is here but me, and I am certain I didn't accidentally move it. I looked around nearby I didn't find it. Also, it wasn't on it's back it was actually hanging onto a cardboard box in a pose which seemed like it could have been alive if I had seen it outside and moving Waspgirl03 (talk) 21:16, 1 May 2012 (UTC)

Social insects will routinely "bring out their dead" in order to keep their place tidy. Perhaps you saw the pall bearer leaving. --Sean 21:02, 1 May 2012 (UTC)

I don't understand. Bring out the dead wasp from my cupboard to somewhere else? For what purpose? And why did it wait several months to do it? Waspgirl03 (talk) 21:16, 1 May 2012 (UTC)

If they're living up there they would do it as part of general housekeeping. I don't know about the delay. --Sean 14:32, 2 May 2012 (UTC)

According to various web pages wasps do hibernate, usually in warm sheltered places, and at least in the UK tend to emerge somewhere around mid-April -- so all that seems consistent, but it would have to have been a queen. Looie496 (talk) 21:57, 1 May 2012 (UTC)

I'm in the UK, so I guess it must have been a queen then. Do the queens look very similar to normal wasps? I assumed they would be a lot bigger Waspgirl03 (talk) 22:01, 1 May 2012 (UTC)

They're _noticeably_ bigger, but not a _lot_ bigger. This site has a picture of a (very dead) worker and (equally dead) queen next to each other for comparison. See Vespula vulgaris for our article on the beasties, incidentally. Tevildo (talk) 22:21, 1 May 2012 (UTC)

I figure I should know the answer to this kind of question, since my user name is vespine, (it's not because I'm an expert, I just like wasps, and the word vespine) .. So after reading the wasp article, I think I found the answer: In most species of social wasp the young queens mate in the vicinity of their home nest and do not travel like their male counterparts do. The young queens will then leave the colony to hibernate for the winter once the other worker wasps and founder queen have started to die off. So a YOUNG wasp might not even be fully grown yet, making it probably indistinguishable from any other wasp. Vespine (talk) 02:22, 2 May 2012 (UTC)

I am inclined to go for 'the same wasp' idea. If it was hanging vertically then this would indicate to a strong degree that it had deliberately placed itself there. Queens hibernate ready to emerge the following spring to start a new colony. Hibernation in many creatures can look very much like the animal or insect is dead by dint of the fact that the body processes slow down and emulate a near death state. The fact that you saw a wasp and the dissappearance of the 'body' seems a moderately high coincidence. Thank you for leaving the sleeping wasp in peace they are useful creatures who have a place in every gardener's heart. Richard Avery (talk) 07:39, 2 May 2012 (UTC)

Natural selection needs selection for it to work. That wasp will go and breed an army with preferences for house interiors. You should have just killed it. SkyMachine ⁽⁺⁺⁾ 10:32, 2 May 2012 (UTC)

Hmm, nice attitude. Since wasps have probably been hibernating in houses since they have existed and they are hibernating for 99.9% of the time they are in the house they would not appear to be a significant threat to the normal house occupier. The idea that this will produce future generations of house-hibernating queen wasps, well, welcome to the present! Richard Avery (talk) 14:57, 2 May 2012 (UTC)

Well that because people have not been doing their job. We have the power to control these things and select for a world that will not kill us, or even one that will benefit us. How do you think we developed agriculture in the first place. The point is you let the population survive, but you control selection by removing undesirable characteristics present in individuals capable of reproduction. SkyMachine ⁽⁺⁺⁾ 17:01, 2 May 2012 (UTC)

Personally I don't consider a single wasp sleeping in my cupboard over winter to be undesirable, I actually kinda like it. I am happy to have given her a home for that time. Anyway, thanks for the answers everyone. Very interesting stuff Waspgirl03 (talk) 22:08, 2 May 2012 (UTC)

Wow, that's amazingly tolerant. I would kill or eject, if possible, any insect or arachnid or rodent found in my home, as I don't want to share my living space with them. I would leave most of them alone outside, but make an exception for those which can cause injury, like stinging insects and mosquitoes. Those I kill on sight. StuRat (talk) 22:39, 2 May 2012 (UTC)

If Waspgirl were to be stung or bitten by one of those little devils, it might lower that tolerance level a tad. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:22, 3 May 2012 (UTC)

Yea, I had one fly down my shirt and sting me a dozen times. This might explain why they must all die, IMHO. StuRat (talk) 00:25, 3 May 2012 (UTC)

Not all of them, just that one. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:24, 3 May 2012 (UTC)

There is a certain pleasure in ruining mosquito breeding spots in the spring. Somehow the bites just aren't quite as annoying when you know you've gotten back at them already. Wnt (talk) 18:49, 3 May 2012 (UTC)

Over here (NZ) there are places named after the abundance of mosquitoes, if you go there for at least two hours, you look like you have the measles, occasionally people develop allergies toward so many mosquito bites requiring medical attention. Plasmic Physics (talk) 21:02, 3 May 2012 (UTC)

From the 'deep hibernation' behaviour of the specimen which you have described it is very important to be sure. You should carefully check through you whole house, the kind of wasp that you describe can only be one of a few kinds if it is actually the same wasp. I do believe it is a north american native, and if it has made it's way in a shipping container by sea, or otherwise made it's way to the United kingdom it can do very serious ecological damage. The north United States Government centre for disease control has procedures in place for outbreaks of this kind, however if you are in Asia, or even worse, in the United Kingdom, the situation is dire, and I cannot stress this enough, you need to prepare your household according to this guide FIRST. before you inform the authorities. DO IT NOW. Penyulap ☏ 10:22, 6 May 2012 (UTC)

Can't you at least use small font? Nil Einne (talk) 14:56, 6 May 2012 (UTC)