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(Science Daily)   A physicist may have figured out if that damned cat is alive or dead. Or maybe not   (sciencedaily.com) divider line 6
    More: Interesting, Schrodinger, caterpillars, quantum superposition, quantum physics, physical review, quantum states, American Physical Society, University of Arkansas  
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4139 clicks; posted to Geek » on 26 Aug 2013 at 6:02 PM (45 weeks ago)   |  Favorite    |   share:  Share on Twitter share via Email Share on Facebook   more»



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2013-08-27 05:45:33 PM
1 votes:
Does the cat continue to consume half of it's normal food and water per day while in the box until you open it up at which point it eats all or none of it?
2013-08-26 08:31:21 PM
1 votes:

czetie: Felgraf: lifeboat: But I thought I've also read that an entangled particle can be manipulated -- you can change the spin, for example -- and the other particle will follow suit instantly. Does this not imply FTL communication? Could you not devise an apparatus that communicates intentional changes to a particle's state?

This is the part that's wrong. This *IS* how it works in sci-fi.. but, as it turns out, this isn't how it works out in reality.

This is where a lot of the confusion comes from.

What he said, except even more so. Suppose you create two entangled electrons, and send one to Alice and the other to Bob. Because electrons are quantum particles they always have a spin of exactly plus or minus 1 (for appropriate choice of units). If Alice measures her electron along the up/down axis and gets +1 in the "up" direction she can confidently state that Bob will get "down" (or -1) from the same experiment, because they are entangled. So far so good.

However here's the confusing part where quantum mechanics completely departs from any classical analogy. Alice measuring her electron's spin as up does not cause Bob's electron to have spin down in any classical sense. Bob can still measure the spin of his electron along any axis he chooses. If he happens to choose the same up/down axis -- perhaps by prior arrangement with Alice -- he will certainly get down as his result. But if he chooses some other axis, something totally non-classical will happen. He will measure plus or minus 1 along whatever axis he measures on -- because any measurement of an electron's spin always results in plus or minus one. Alice measures Spin Up, but Bob does not measure Spin Down.

Note that this is entirely different from what would happen if we had two classical objects, say two basketballs spinning in opposite directions. If Bob measures the spin of his basketball on some other axis, he will get a number that is less than 1 (and maybe even 0 if he measures perpendicular to the up/dow ...


Aye, the really twisted thing about the experiment (if I recall corectly), is that the only thing it *really* disproved was local hidden variable theory.

Which means, technically, one could still have a 'hidden variable' theory... but it would have to be a *non local* hidden variable, and trying to figure out how that would work makes my head hurt and there is a REASON I am an experimentalist, and not a theorist.
2013-08-26 08:29:34 PM
1 votes:

kahnzo: czetie: Memo to everybody: if your supposed explanation of Schrodinger's thought experiment includes something called a "measurement" (or an "observation"), it's already wrong. And if it includes "wave function collapse", it's doubly wrong.

None of these things has any definition or description in quantum theory, they are just useful approximations for describing what is happening in most practical circumstances. But you can't use them to probe the foundations of QM; that requires you to be precise.

So what's your preference?  Ensemble? Transactional? Many Worlds?  Consistent Histories?
You can't say "You can't do that!" and then lift your skinny fists like tiny antennas to heaven.

The problem doesn't go away because we can do things with more precision.  In fact, violations of Bell's Inequality are quite beautifully astounding, but in no way offer a satisfying resolution to the underlying weirdness.


There's no getting away from the weirdness of QM, regardless of what interpretation you prefer. It just does things that have no classical analogy. If you're looking for a satisfying resolution to the underlying weirdness, well, you will have to get used to disappointment.

Having said that, I think that Many Worlds has the most promise, although it's still incomplete. It doesn't help, though, that most people completely misunderstand what Many Worlds actually says. Sean Carroll explains it far better than I ever could. The neat thing about Many Worlds, properly understood, is that it has the potential to explain both what is happening at a proper fundamental level and why the apparent macro world is so closely approximated by classical physics in which waves have "collapsed". But any interpretation that accords a special place to measurement or observation that is somehow different from "a whole lot of interactions, each of which individually is quantum" is definitely not fundamental (which was precisely Schrodinger's point.)

/In my youth I was partial to de Broglie/Bohm pilot wave theory, and in my heart I still wish it were tenable.
2013-08-26 08:16:49 PM
1 votes:

Felgraf: lifeboat: But I thought I've also read that an entangled particle can be manipulated -- you can change the spin, for example -- and the other particle will follow suit instantly. Does this not imply FTL communication? Could you not devise an apparatus that communicates intentional changes to a particle's state?

This is the part that's wrong. This *IS* how it works in sci-fi.. but, as it turns out, this isn't how it works out in reality.

This is where a lot of the confusion comes from.


What he said, except even more so. Suppose you create two entangled electrons, and send one to Alice and the other to Bob. Because electrons are quantum particles they always have a spin of exactly plus or minus 1 (for appropriate choice of units). If Alice measures her electron along the up/down axis and gets +1 in the "up" direction she can confidently state that Bob will get "down" (or -1) from the same experiment, because they are entangled. So far so good.

However here's the confusing part where quantum mechanics completely departs from any classical analogy. Alice measuring her electron's spin as up does not cause Bob's electron to have spin down in any classical sense. Bob can still measure the spin of his electron along any axis he chooses. If he happens to choose the same up/down axis -- perhaps by prior arrangement with Alice -- he will certainly get down as his result. But if he chooses some other axis, something totally non-classical will happen. He will measure plus or minus 1 along whatever axis he measures on -- because any measurement of an electron's spin always results in plus or minus one. Alice measures Spin Up, but Bob does not measure Spin Down.

Note that this is entirely different from what would happen if we had two classical objects, say two basketballs spinning in opposite directions. If Bob measures the spin of his basketball on some other axis, he will get a number that is less than 1 (and maybe even 0 if he measures perpendicular to the up/down direction). That's how classically spinning objects behave.

Now what makes this really interesting is that when Alice makes her up/down measurement the result will be completely random; but when Bob makes his off-axis measurement, the result is not completely random. Suppose he measures at 45 degrees off the vertical. Because the electrons were entangled, his result is more likely to point in the direction closer to the down direction ("downish", if you will) than it is to point in the upish direction. He might get either, but he's more likely to get downish -- and QM tells you exactly how much more likely. In the jargon, the results are correlated.

Here comes the really brilliant part: John Stewart Bell realized that the amount of correlation predicted by QM was different from any possible "classical" theory (we can define exactly what we mean by "classical" here, but it's too long for this post). This is called Bell's Theorem or sometimes Bell's Inequalities. And he proposed that this would be an effective test of whether QM was correct: If you ran the experiment many times, the number of times you got downish versus upish would tell you whether QM was correct, or whether (as Einstein and others suspected) there was some deeper, purely classical theory for which QM was an incomplete approximation. Years later, Alain Aspect and his team were able to perform the fiendishly difficult experiments and demonstrated that QM is correct. (Technically speaking, you have to give up either "realism" or "locality", or both).
2013-08-26 06:47:37 PM
1 votes:
I'm really tired of hearing about that cat and, frankly, I don't care if it's alive or dead. What I want to know is, where is my FTL internet connection, Mr. Quantum scientist dude.

Oh, and would it kill you guys make a cheap, self-locating TV remote control?  How about entangling my remote control to the coffee table.
2013-08-26 06:39:12 PM
1 votes:

Donnchadha: It's really easy to know if the cat is alive or dead -- you look in the damn box.




THAT IS NOT ALLOWED!!
 
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