Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 10/04/2009 14:50:30
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Mr Scientist (and SC) made me wonder about entanglement. Lets say we have two entangled particles, one is made to be placed in a gravity well. We do not observe them while doing so, but we do it afterwards. Now those particles will have not only traveled in distance but also in time differently when we finally compare them. Will the entanglement be there? And we need to do it with something 'ticking' not photons I think, otherwise it would maybe be possible to test with a optical black hole.
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I'm not sure the scenario you describe would be possible. Entangled photons can become disentangled. All of the entangled particles I've heard about are photons; the entanglement relates to the polarization of the photons. I suspect that entangled photons share the same em fields. If this suspicion is the reality, entanglement would be broken by altering the phase of one of the photons.
This Wiki article (http://en.wikipedia.org/wiki/Quantum_entanglement) does not limit the property of entanglement to photons. But I don't remember any experimental evidence of entanglement other than that of photons.
Quantum entanglement is a possible property of a quantum mechanical state of a system of two or more objects in which the quantum states of the constituting objects are linked together so that one object can no longer be adequately described without full mention of its counterpart — even though the individual objects may be spatially separated. This interconnection leads to non-classical correlations between observable physical properties of remote systems, often referred to as nonlocal correlations. For example, quantum mechanics holds that states such as spin are indeterminate until such time as some physical intervention is made to measure the spin of the object in question. It is equally likely that any given particle will be observed to be spin-up as that it will be spin-down. Measuring any number of particles will result in an unpredictable series of measures that will tend more and more closely to half up and half down. However, if this experiment is done with entangled particles the results are quite different. When two members of an entangled pair are measured, one will always be spin-up and the other will be spin-down.[citation needed] The distance between the two particles is irrelevant. Theories involving 'hidden variables' have been proposed in order to explain this result; these hidden variables account for the spin of each particle, and are determined when the entangled pair is created. It may appear then that the hidden variables must be in communication no matter how far apart the particles are, that the hidden variable describing one particle must be able to change instantly when the other is measured. If the hidden variables stop interacting when they are far apart, the statistics of multiple measurements must obey an inequality (called Bell's inequality), which is, however, violated — both by quantum mechanical theory and in experiments.
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You should in theory be able to entangle any particles. Photons are preferred because they're easy to produce, and don't interact with things as readily as a lot of other particles. We're also very good at sending photons places (fiber optics). I know people have worked on entangling electrons, for example: http://www.newscientist.com/article/dn3449-triple-electron-entanglement-boosts-quantum-computing.html
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It has been awhile, but I remember now reading about other experiments where electrons are entangled. But in the case of electrons, it is their spin state. If one entangled pair is spin-up the other will always be spin down. I always thought of the spin state of an electron and the polarization of a photon as different things.
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Entanglement is a case where people tend to think of things in an overly simplified way. It's basically the concept that particles have interacted in some way such that to accurately describe the system, you need to write a quantum description of the system as a whole, and can't separate out the individual particles. This could be as simple as entangling photons so that their spin measurements have to be correlated, but it isn't the only case by far.
In general, when the particles interact with their surroundings, they end up becoming entangled with these surroundings. In theory, if you could write the full quantum state describing the particles and their surroundings, you would have all the information on the system. In practice, the surroundings are usually too complex to account for in a quantum mechanical treatment, and the entanglement seems to vanish (as it's "eaten up" by the surroundings).
As to your particular question, you would probably need a good quantum theory of gravity in order to write the full entangled state of the system, but its probably safe to say that in practice it's going to be waaaaaaaaaaaay too complicated.
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but its probably safe to say that in practice it's going to be waaaaaaaaaaaay too complicated.
I believe this is the reason physicists settle for the probability argument instead of searching out possible evidence for the deterministic cause........................Ethos
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I don't think that time dilation would be a factor in the entanglement. The two entangled particles might be different ages, but 'now' is identical for both of them.
Well, this actually raises the issue of time ticks again. If 'now' is a zero-length point there is no problem as zero will be the same size regardless of time-dilation. If 'now' must have a non-zero length though, this length will differ between the two reference frames occupied by the entangled particles and if the simultaneous resolution of the entangled state takes a non-zero period of time, one particle must start it's resolution before, and subsequently complete its resolution after, the other particle.
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Mr Scientist (and SC) made me wonder about entanglement. Lets say we have two entangled particles, one is made to be placed in a gravity well. We do not observe them while doing so, but we do it afterwards. Now those particles will have not only traveled in distance but also in time differently when we finally compare them. Will the entanglement be there? And we need to do it with something 'ticking' not photons I think, otherwise it would maybe be possible to test with a optical black hole.
particles move in space only and not in time
time is a run of clocks that measure their motion in atemporal space