Quantum Entanglement

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Quantum Entanglement

Postby Xial » Wed May 02, 2007 5:04 pm UTC

Quantum entanglement is when 2 particles are linked so that a change to the physical state of one (spin for example) immediately effects the other. It seems that if this were true then it would be possible to transmit information instantaneously perhaps eventually over great distances.

How does this not violate relativity which states, among other things, that nothing can travel faster than light (including information)?

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Postby cmacis » Wed May 02, 2007 5:24 pm UTC

Not my field, but hasn't there been no experimental evidence that this can happen over long distances? Isn't it the whole point of the search for a theory of quantum gravity that quantum and relativity are mutually exclusive, so if you take something that links both then you get impossible stuff.
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Postby Xial » Wed May 02, 2007 5:55 pm UTC

Well there was a recent experiment which showed that it could occur over 50km. Even though this isn't that far it would still seem to violate the speed of light.

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Postby Yakk » Wed May 02, 2007 6:13 pm UTC

Because no information is transferred. You can't communicate anything over the described link.

I prefer the many-universes interpritation, in which "observation" events are events that rotate the observer orthoginal to the parts of the universe inconsitent with the observation.

This means when you poke at the spin of an electron at location A, you have no effect on the electron at location B -- rather, you change yourself in such a way that you can only see the events at location B consistent with your observation at location A.

Locality. Cha-ching.

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Postby Xial » Wed May 02, 2007 7:38 pm UTC

But why is no information transfered? If you say that up-spin is 1 and down-spin is 0 then quantum entanglement allows you to send bits of information in binary.

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Postby Shadowfish » Wed May 02, 2007 7:58 pm UTC

You can only measure the spin of the electron you own. You can't set the spin of the electron you own.

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Postby skeptical scientist » Wed May 02, 2007 8:50 pm UTC

Before you measure, (in one standard interpretation) the spins are both 50% up, 50% down, and when measuring you set yours to up or down, and the other to up or down, but you can't observe which. So when you measure yours, you don't change the result of the other guys observation in any way he can detect, since he can't tell if he measured his, which was still in a undetermined state, and determined it, or if he measured his after you already measured yours, since all he knows is that he measured up or down, which could have happened in either case, with equal probability. So you can't send any information this way.
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Postby toysbfun » Fri May 04, 2007 11:04 pm UTC

Actually, I believe Einstein said that nothing could be accelerated to superluminal speeds. Scientists have been making pulses of light go faster than 3*10^8 m/s for years. Making information move faster than light would violate causality though, because it would get the information would arrive before it was sent.

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Postby gmalivuk » Sat May 05, 2007 1:10 am UTC

toysbfun wrote:Actually, I believe Einstein said that nothing could be accelerated to superluminal speeds. Scientists have been making pulses of light go faster than 3*10^8 m/s for years. Making information move faster than light would violate causality though, because it would get the information would arrive before it was sent.


Yeah, special relativity just rules out accelerating matter to luminal (or superluminal) speeds. It doesn't rule out tachyons, for instance, which always travel faster than light (and which, due to some of their other properties such as being repelled by gravity, have only been theoretically posited).
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Postby Tchebu » Sat May 05, 2007 1:32 am UTC

Can someone give a conceptual (not going deeply into the math of it) explanation as to how this entanglement occurs? How does it happen in the first place? Sure, get the point, the two particle are linked in such a way that when you measure one, you also determine the state of the other, but... how does that occur in the first place?

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Postby Gelsamel » Sat May 05, 2007 1:36 am UTC

A magical machine that shoots out 2 particles, one with a certain spin, and the other with the opposite? But the initial spin would be random? Something like that.
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Postby Cosmologicon » Sat May 05, 2007 1:56 am UTC

Gelsamel wrote:A magical machine that shoots out 2 particles, one with a certain spin, and the other with the opposite? But the initial spin would be random? Something like that.

That makes it sound like a hidden variable. That is, that makes it sound as if the particles are actually in one spin state or another after they're shot out but before they're measured. That's not how quantum physics describes it. The particles don't choose their spin state until they're measured, at which point they both choose it to be opposite.

Now, you may say, if their choice can't be influenced in any way, how is that any different from a hidden variable? I don't think there's a simple answer to that without a lot of math, but there is a difference. Hidden variables have been tested in the lab and found not to exist; quantum physics was vindicated in this experiment.

I should point out that there are some bizarre hidden-variable theories that are indistinguishable from quantum, but they're non-local. I think most people would consider that even more counterintuitive than entanglement, so there's no way to get around the screwiness of it all.

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Re: Quantum Entanglement

Postby StardustDeath » Sun Jul 05, 2009 5:32 am UTC

Oops, sorry.
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Re: Quantum Entanglement

Postby Mr Jack » Sun Jul 05, 2009 10:20 am UTC

StardustDeath wrote:You have three options before you.

1) If you belive in Einstein's Theory then yes, quantum entanglement would be wrong. Nothing would be able to travel past the speed of light and all things in the universe happen speratelly in their own seperate dimensions (wheather the fist or the tenth) and everything is just a coincidence.

2) If you a a quantum physisist, you belive that in each of the ten dimensions of this universe (or if you are an extreame one, parrel universes) then you would say that each thing that happens in each dimension (or universe) happens to another at the same time. So each dimension (or universe) is irrevelant as they are all the same thing.

Yes, there is a third choice...

3) If you are a string theorist then you are the arch-nemisis of both of the above. You belive that, yes, there are seperate dimensions and universes but not everything that happens directlly happens THE SAME WAY as in another universe and you don't belive that everything is COMPLETELY different. You say there are an infinite number of universes all tangled up withing each other. Each universe conatins another, and another, and another. When something happens in ANY of the dimensions in your universe, the same thing happense in the corresponding dimension IN A DIFFERNT WAY. For instance, say you are in Pre-K and you have just learned that 1+1=2, well, in a corresponding dimension, IN ANOTHER UNIVERSE, a person exactlly like you (except for personality, but we don't want to get into brane theory) has just figured out that 0+2=2. And maybe in another universe another person has figured out that 3-1=2, and so on and so fourth.


None of these theories are wrong or right for that matter. It all depends on what you BELIEVE in.


That's all patent nonsense. Special Relativity fits perfectly with Quantum Mechanics. The difficulties aligning relativity with QM are far subtler, and have nothing to do with causality. String theory doesn't contradict relativity and QM; it attempts to unite them. Your description of other universes is taken straight out of science fiction, not string theory.

And, even disregarding the nonsense that precedes it, that final statement is horrible. Physicists don't "believe" in one theory over another. Physics theories are models. We don't BELIEVE in models, we USE them.

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Re: Quantum Entanglement

Postby avialexander » Sun Jul 05, 2009 12:24 pm UTC

I don't quite get it. I'll use a modification on the dead or alive cat in a box problem to try to explain my understanding of it. So, if you took a cat and a dog, put them inside two boxes without knowing which was which, the chances that one box has a cat or dog is 50/50. So, after separating the cat and dog by, say, 50 kilometers, you have them both in separate observation areas A and B. Then a scientist at obs. area A decides to open one of the boxes, and finds a dog, so therefore, the box at obs. area B goes from 50/50 to 100% cat. Now, putting aside the disturbing images and a cartoon series that come to mind when one thinks of a 50/50 mix, I come to my questions.
1. How can a qubit be in a 50/50 state, is it a mathematical thing, or is it just another way of giving the probability an image?
2. When you open one of the boxes, you find out its contents, and that means you know what animal the the other box contains, but until the other scientists open their box, they have no way of knowing what it contains, unless you call them and tell them. Applying that to qubits, it would seem that no information is transferred, and you cannot transfer information in that way, unless there is a way to set the qubit to an up or down spin state before you measure it. So how are you supposed to transfer information with qubits?

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Re: Quantum Entanglement

Postby dedalus » Sun Jul 05, 2009 1:45 pm UTC

The one bit here that I'm not sure about is whether current theory is stating that measuring the entangled property of one object actually FORCES the change to the other, or simply means that we know the other is the required value. If it's the second, then it's perfectly within the bounds of relativity. The problem is that I remember reading something about it being the first, which makes things difficult. The reasoning behind it is performing the young double-slit experiment upon a stream of entangled photons when their partners are and aren't being measured; I'm not sure what the actual outcome of this is.
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Re: Quantum Entanglement

Postby gmalivuk » Sun Jul 05, 2009 2:14 pm UTC

For the record, the cat and entanglement are different. The cat is about superposition of states and a wave function that collapses when you open the box, and the entanglement is about separate particles whose quantum state is described by a single formula that collapses everywhere at once (i.e. for both particles) when either one of them decoheres.
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Re: Quantum Entanglement

Postby doogly » Sun Jul 05, 2009 3:51 pm UTC

StardustDeath wrote:You have three options before you.

1) If you belive in Einstein's Theory then yes, quantum entanglement would be wrong. Nothing would be able to travel past the speed of light and all things in the universe happen speratelly in their own seperate dimensions (wheather the fist or the tenth) and everything is just a coincidence.

2) If you a a quantum physisist, you belive that in each of the ten dimensions of this universe (or if you are an extreame one, parrel universes) then you would say that each thing that happens in each dimension (or universe) happens to another at the same time. So each dimension (or universe) is irrevelant as they are all the same thing.

Yes, there is a third choice...

3) If you are a string theorist then you are the arch-nemisis of both of the above. You belive that, yes, there are seperate dimensions and universes but not everything that happens directlly happens THE SAME WAY as in another universe and you don't belive that everything is COMPLETELY different. You say there are an infinite number of universes all tangled up withing each other. Each universe conatins another, and another, and another. When something happens in ANY of the dimensions in your universe, the same thing happense in the corresponding dimension IN A DIFFERNT WAY. For instance, say you are in Pre-K and you have just learned that 1+1=2, well, in a corresponding dimension, IN ANOTHER UNIVERSE, a person exactlly like you (except for personality, but we don't want to get into brane theory) has just figured out that 0+2=2. And maybe in another universe another person has figured out that 3-1=2, and so on and so fourth.


None of these theories are wrong or right for that matter. It all depends on what you BELIEVE in.


Mr Jack already pointed it out, but it's worth reiterating that all of the above is nonsense.
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Re:

Postby vrulg » Sun Jul 05, 2009 8:20 pm UTC

Tchebu wrote:Can someone give a conceptual (not going deeply into the math of it) explanation as to how this entanglement occurs? How does it happen in the first place? Sure, get the point, the two particle are linked in such a way that when you measure one, you also determine the state of the other, but... how does that occur in the first place?


It has to do with conservation of angular momentum I believe. You know what the total angular momentum of the 2 particle system has to be, no matter how far you move the particles apart. So, once you measure the spin of one particle, you automatically know what the spin of the other particle has to be if angular momentum is conserved(angular momentum could not be conserved because either the principle is wrong, or, more likely, something changed the angular momentum before you could measure it)

Of course, whether this is how the entanglement occurs, or just a reason to suspect that it should occur I couldn't say.

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Re: Quantum Entanglement

Postby Xyros » Sun Jul 05, 2009 11:04 pm UTC

To what extent does entanglement occur in the universe?
I know two electrons can be entangled, and I'm pretty sure all the quarks inside nucleons are entangled, as well as their gluons.
But a paradox arrives if all particles are entangled, as quantum mechanics is based on random events based on statistics.
But if all particles are entangled, then the all measurements could be found for the whole system.

Someone please tell me what I'm saying is wrong.
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Re: Quantum Entanglement

Postby gmalivuk » Sun Jul 05, 2009 11:18 pm UTC

Particles are entangled in groups, so the fact that the particles inside one proton are entangled with each other doesn't mean they're entangled with those entangled particles inside another, completely different proton.
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Re: Quantum Entanglement

Postby thoughtfully » Mon Jul 06, 2009 12:38 am UTC

I'm pretty sure that particles entangled in the EPR sense that we're discussing here have to be created at the same time. Just being "entangled" inside a larger quantum system isn't enough.

I don't think anyone's mentioned the excellent book Entanglement. It's a very accessible guide to the theory and experimental results of quantum entanglement.

You should read the Wikipedia article, and also this one to see why its different from the idea of having a red and blue ball or something else of that sort. The quantities measured are quantum mechanical probabilities which can be measured in different ways; not a simple "a" or "b" outcome. To detect the entanglement, the experimental results have to be communicated conventionally, which satisfies causality. It is really nontrivial to grok this stuff, so don't worry if it takes you awhile.
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Re: Quantum Entanglement

Postby Rhubarb » Mon Jul 06, 2009 11:11 am UTC

dedalus wrote:The one bit here that I'm not sure about is whether current theory is stating that measuring the entangled property of one object actually FORCES the change to the other, or simply means that we know the other is the required value. If it's the second, then it's perfectly within the bounds of relativity. The problem is that I remember reading something about it being the first, which makes things difficult. The reasoning behind it is performing the young double-slit experiment upon a stream of entangled photons when their partners are and aren't being measured; I'm not sure what the actual outcome of this is.


It's the second one if you get rid of physical realism, which is IMO a nice way of doing it. Then all states are only taken as relative to the observer making the measurement. If you look in terms of who has what information, and you say that a lack of information means the state of a system becomes a quantum superposition, it starts to make some kind of sense. You have to allow macroscopic observers to exist in superpositions too, so that one becomes entangled with your particles or whatever from the point of view of the other observer. Weird...

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Re: Quantum Entanglement

Postby nilkemorya » Mon Jul 06, 2009 5:37 pm UTC

Rhubarb wrote:
dedalus wrote:The one bit here that I'm not sure about is whether current theory is stating that measuring the entangled property of one object actually FORCES the change to the other, or simply means that we know the other is the required value. If it's the second, then it's perfectly within the bounds of relativity. The problem is that I remember reading something about it being the first, which makes things difficult. The reasoning behind it is performing the young double-slit experiment upon a stream of entangled photons when their partners are and aren't being measured; I'm not sure what the actual outcome of this is.


It's the second one if you get rid of physical realism, which is IMO a nice way of doing it. Then all states are only taken as relative to the observer making the measurement. If you look in terms of who has what information, and you say that a lack of information means the state of a system becomes a quantum superposition, it starts to make some kind of sense. You have to allow macroscopic observers to exist in superpositions too, so that one becomes entangled with your particles or whatever from the point of view of the other observer. Weird...


Okay, number one, Quantum Mechanics doesn't "Make Sense" it accurately predicts effects in physical systems we have observed. Number two, two spin-entangled particles will only remain entangled until one of the particles are measured, at which point the other particle is forced to have the opposite spin(the spin was undetermined, not just unknown, before measurement) and entanglement is broken. Number three, this is distinguishable from systems which have hidden variables mathematically, and has been shown to be the case by: http://en.wikipedia.org/wiki/Bell_test_experiments

I don't think some of that had been mentioned yet in this thread, so I thought I'd throw it out there.

By the way, what exactly do you mean by "physical realism"? That's a fairly ambiguous turn of phrase.
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Re: Quantum Entanglement

Postby Yakk » Mon Jul 06, 2009 7:04 pm UTC

http://lmgtfy.com/?q=physical+realism
http://www.newtonphysics.on.ca/HEISENBERG/Chapter4.html
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I've seen it being used to refer to "when we measure how much something weighs, before we measure it, the thing had a weight to be measured". That the thing we measured was "real" in that it had meaning before we measured it.
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Re: Quantum Entanglement

Postby Rhubarb » Mon Jul 06, 2009 8:20 pm UTC

nilkemorya wrote:Okay, number one, Quantum Mechanics doesn't "Make Sense" it accurately predicts effects in physical systems we have observed.
Yeah, well, you know, that's just, like, your opinion, man. Personally I think quantum mechanics does make some kind of sense when you look at the relational interpretation of QM.
nilkemorya wrote:Number two, two spin-entangled particles will only remain entangled until one of the particles are measured, at which point the other particle is forced to have the opposite spin(the spin was undetermined, not just unknown, before measurement) and entanglement is broken.

Yes, but if there is no physical realism, the spin of a particle isn't ever an objective fact of nature, it is taken as relative to an observer. That way, it can be fixed for one observer after they have measured it, and so is the other entangled particle's spin, but from the point of view of another observer both spins could still be in an entangled state- that's because there's nothing transmitted between the entangled particles when they are measured by the first observer.
nilkemorya wrote:Number three, this is distinguishable from systems which have hidden variables mathematically, and has been shown to be the case by: http://en.wikipedia.org/wiki/Bell_test_experiments

I don't think some of that had been mentioned yet in this thread, so I thought I'd throw it out there.

By the way, what exactly do you mean by "physical realism"? That's a fairly ambiguous turn of phrase.

I'm not talking about hidden variables. And maybe I should have mentioned counterfactual definiteness as well.

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Re: Quantum Entanglement

Postby Technical Ben » Mon Jul 06, 2009 9:31 pm UTC

I still don't see how this differs from the example of the cat and a dog in a box.
How are these particles entangled? What shows that they are not already opposites? If you create a particle pair with 50/50 chance of being up/down or down/up how can that be entanglement?

(I still don't get how you can measure states between 1 and 0 (or up and down) but that's a whole different question.)
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Re: Quantum Entanglement

Postby gmalivuk » Mon Jul 06, 2009 11:20 pm UTC

Yakk wrote:I've seen it being used to refer to "when we measure how much something weighs, before we measure it, the thing had a weight to be measured". That the thing we measured was "real" in that it had meaning before we measured it.

In which case I'm pretty sure QM is not realism, since there are observed phenomena which require that certain characteristics simply do not exist in a determinate form before measurement.
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Re: Quantum Entanglement

Postby Rhubarb » Tue Jul 07, 2009 11:19 am UTC

Technical Ben wrote:I still don't see how this differs from the example of the cat and a dog in a box.
How are these particles entangled? What shows that they are not already opposites? If you create a particle pair with 50/50 chance of being up/down or down/up how can that be entanglement?

(I still don't get how you can measure states between 1 and 0 (or up and down) but that's a whole different question.)


Well the answer to the first bit depends on the last thing you said. You can find a state between 0 and 1 by measuring along a different axis, i.e. not up or down, but left or right, say. However, the value of the spin will still be +1 or -1, so you can see with a bit of thought that the possible states after a measurement exist on the surface of a sphere. A result along a different axis can actually be expressed as a superposition of the 0 and 1 states. A point on the equator of the sphere is an equal superposition of the states at the top and bottom of the sphere. The general state is given by [imath]\cos{(\frac{\theta}{2})}|0\rangle+e^{i\phi}\sin{(\frac{\theta}{2})}|1\rangle[/imath] where [imath]\theta[/imath] and [imath]\phi[/imath] are your polar and azimuthal angles.

The probability of getting a particular result depends on how close the state is before the measurement to the ends of the axis that you're measuring along. So if the qubit is at the equator before the measurement it has a 50/50 chance of being found to be in state [imath]|0\rangle[/imath] or [imath]|1\rangle[/imath] after a measurement along the vertical axis.

The way this differs from the cat/dog thing is, here you can measure along a horizontal axis and find a result that is a superposition of [imath]|0\rangle[/imath] and [imath]|1\rangle[/imath] - you can't make a measurement of the cat and dog and find a cat/dog superposition and say that that's a valid result!

Now the crazy thing about entangled qubits is before they are measured each one exists at the centre of the sphere. That means there's a 50/50 chance of getting a result at either end of any axis you measure along. And yet, despite that, when you measure along the same axis, you always get correlated results!

I hope this isn't too abstract...

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Re: Quantum Entanglement

Postby nilkemorya » Tue Jul 07, 2009 6:27 pm UTC

Rhubarb wrote:
Yeah, well, you know, that's just, like, your opinion, man. Personally I think quantum mechanics does make some kind of sense when you look at the relational interpretation of QM.

Yes, but if there is no physical realism, the spin of a particle isn't ever an objective fact of nature, it is taken as relative to an observer. That way, it can be fixed for one observer after they have measured it, and so is the other entangled particle's spin, but from the point of view of another observer both spins could still be in an entangled state- that's because there's nothing transmitted between the entangled particles when they are measured by the first observer.

I'm not talking about hidden variables. And maybe I should have mentioned counterfactual definiteness as well.


Lemme take a step back, "making sense" is usually defined as fitting in with your view of what the universe should act like. Since quantum systems do things which people who only have their experience with the physical universe to work with(they haven't studied quantum for 20 years to get a feel for those systems), wouldn't that make them by definition unintuitive, IE not make any sense?

If the observers go ahead and compare measurements later though, they will find that the two measurements of the separate entangled particles agree with what they would expect given their own measurement, provided that CFD holds. Wouldn't this exact measurement be a way of testing whether you have CFD or locality, and since this has been done, throw locality out the window?

This is of course unless you go with a many worlds style approach, but then you don't even get factual definiteness.
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Re: Quantum Entanglement

Postby Rhubarb » Tue Jul 14, 2009 10:48 am UTC

I haven't forgotten this thread, I'm just trying to think up a good reply. I'm trying to get it straight in my head as well. :P You're right that the two observers had better agree on what results they get, but agreement of that type does not require realism, that is, the states of the entangled systems being measured and the observers themselves do not need to be absolute or objective, the only thing needed is consistency, which QM can actually give. If we live in a consistent universe, I don't think it is possible to determine whether any physical properties of systems are absolute/objective or not, there is no special system that is able to say that, because that system must also obey the rules of quantum mechanics, and so just sees a consistent universe.

You can keep locality if you get rid of realism. In fact I think locality is extremely important to QM, and in fact is partly why we have QM in the first place. (From here (pdf): "A quantum theory is best understood as a theory about the possibilities and impossibilities of information transfer, as opposed to a theory about the mechanics of non-classical waves or particles.") I think also it is the information that a system has (about the rest of the universe) that defines what is 'real' (for that observer!), so that things only become real once the observer in question actually holds the information describing those things. Since different observers in general will hold different information about the universe, realism must be got rid of.

I'm not actually that sure about CFD, I probably shouldn't have mentioned it, I don't really understand it thoroughly. :oops:

There's an excellent article here about RQM (pdf).

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Re: Quantum Entanglement

Postby Technical Ben » Wed Jul 15, 2009 5:49 pm UTC

Still cannot comprehend why they just don't have opposite spins anyway. That's a whole lot simpler than all this Quantum entanglement. (some phrase about a razor springs to mind ;) ) Why is the spin undetermined at 50/50 up or down. Why can it not be "we have made a particle with up" spin for example? The pair have a 50/50 of up/down or down/up instead of each particle having a 50/50 of up/down?
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Re: Quantum Entanglement

Postby thoughtfully » Wed Jul 15, 2009 5:55 pm UTC

Very simply, that isn't what is observed. The Universe is under no obligation to comply with our expectations.

You might find my recent post in another thread topical.
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Re: Quantum Entanglement

Postby Tass » Wed Jul 15, 2009 7:20 pm UTC

Yeah, basically what you suggest is a hidden variable theory. It is not easy to understand why it fails, but if you want to give it a try look up Bells Inequality.

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Re: Quantum Entanglement

Postby ThomasS » Wed Jul 15, 2009 8:00 pm UTC

I remember discussing the oddness of quantum mechanics and the various ways to interpret it in an old thread. I still think that the oddness is best illustrated by entanglement examples, in particular the psychic rat depiction doesn't require anything more than elementary probability to understand.

This example and Bell's inequality only contradict local hidden variable theories. People normally find non-local hidden variable theories to be too spooky to consider. However, spooky isn't a testable criteria, and I'm not convinced that the universe agrees with us on the meaning of non-local.

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Re: Quantum Entanglement

Postby Technical Ben » Wed Jul 15, 2009 11:17 pm UTC

Ok. You never mentioned that they do not have a direct correlation of spin. The Wiki article mentions measuring at inbetween up/down spins, and these not having a direct correlation (as predicted by exactly opposite spins). "In contrast, Bell's theorem places a straight-line limit on the curve that any local hidden variable model (involving identical particles) can follow from correlated to anti-correlated. The QM prediction for entangled particles breaks this limit. For example, when the relative analyzer alignment is 22.5 degrees QM gives 0.71 correlation whereas the straight-line limit (implied by Bell's theorem) is 0.5. From this, one may conclude that the outcome of Quantum measurements on entangled particles cannot be replicated by a model that employs identical particles that have hidden attributes/properties which locally determine the outcome of measurements."

I'm still unclear on what this implies, but it's helped a little. (I'm still in favour of non local hidden variables or the measurement being the interfering factor ;) )
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Re: Quantum Entanglement

Postby BSamuels » Thu Jul 16, 2009 4:09 am UTC

The law (I forget the name) that says nothing goes faster than the speed of light applies to matter with mass. Hence why photons move at the speed of light, they have no mass so they are exempt from the law.

TBH I don't really know, that was just my educated guess.
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Re: Quantum Entanglement

Postby ThomasS » Thu Jul 16, 2009 4:38 am UTC

BSamuels wrote:The law (I forget the name) that says nothing goes faster than the speed of light applies to matter with mass. Hence why photons move at the speed of light, they have no mass so they are exempt from the law.

There is a phenomenon in relativity theory which prevents objects with mass from reaching the speed of light. Basically they seem to become heavier and heavier as they go faster and faster. This law clearly does not apply to photons/light waves, which do not have mass and insist on traveling at the speed of light.

Somewhat separately, special relativity tells us that different observers might see the same events happen in different orders. Somebody holding a meter stick might set up two lights, one at each end, to come on at the same time. From his point of view, this is what happens. However, somebody moving sufficiently fast might see light A turn on before light B and somebody moving in the opposite direction might see light B turn on before light A. The rule is that if you see light A turn on before light B, and if it would require something moving faster than the speed of light to get from the lighting of A to the lighting of B, then somebody else might see B turn on before A. On the other hand if you could make it from the lighting of A to the lighting of B without breaking the speed of light, then A was lit first no matter who is looking.

So if we did have some information (even without mass) traveling at faster than the speed of light, then somebody somewhere would see that information arrive before it departed. This leads to discussions about tachyons, free will and whether god not playing dice means that the future is already written. This is related to quantum entanglement because whether information is transfered with wave function collapse or not, there is still something frustrating about not being able to discuss what causes it and whether it is instantaneous with respect to which observer. This is part of what makes unified field theories hard.

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Re: Quantum Entanglement

Postby thoughtfully » Thu Jul 16, 2009 6:30 am UTC

BSamuels wrote:The law (I forget the name) that says nothing goes faster than the speed of light applies to matter with mass. Hence why photons move at the speed of light, they have no mass so they are exempt from the law.

TBH I don't really know, that was just my educated guess.

The easy (for me) way to picture this is that all bodies travel though spacetime at a fixed speed, c. There are no other speeds, in this four-dimensional point of view. What can change is the projections of the velocity vector onto the space dimensions and onto the time dimension. A massless particle travelling at the speed of light has zero timelike projection, and experiences no passage of time. A body at rest has a zero spacelike projection, and experiences no time dilation.

Speed in the usual sense is the spacelike projection. A massless particle has its full speed, c, as the spacelike projection.
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Re: Quantum Entanglement

Postby PM 2Ring » Thu Jul 16, 2009 10:28 am UTC

ThomasS wrote:This example and Bell's inequality only contradict local hidden variable theories. People normally find non-local hidden variable theories to be too spooky to consider. However, spooky isn't a testable criteria, and I'm not convinced that the universe agrees with us on the meaning of non-local.

True, non-local hidden variable theories seem to be just as spooky as the action at a distance they are trying to explain. OTOH, I heard about a paper recently that invokes a universal fractal probability function to explain quantum weirdness that sounds promising. I should mention that I'm a fan of the transactional interpretation of QM, but I also have a soft spot for Many Worlds (or at least a modified version thereof).

As for Bell's Inequality Theorem, Penrose gives a pretty good explanation in The Emperor's New Mind. And IMHO it's worth reading John Bell's Speakable and Unspeakable in Quantum Mechanics. Some of the articles in this collection are fairly technical, but many are aimed at lay readers.


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