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The standard test for whether two things are really entangled with one another in the spooky-action-at-a-distance sense of the EPR picture is to see whether measurements of the states of the two particles violate one of the Bell inequalities, meaning that the correlation between the states is stronger than can be explained by any local hidden variable ...

The original goal of the EPR paper was to show that quantum mechanics is incomplete. Hence, that extra variables have to be added to complete it, contrary to what Cedric claims. The goal of EPR is to show that either nature is non-local (and thus in conflict with SR) either quantum mechanics is incomplete. Since Einstein was not ready to abandon locality and ...

In quantum mechanics, two observables that cannot be simultaneously determined are said to be non-commuting. This means that if you write down the commutation relation for them, it turns out to be non-zero. A commutation relation for any two operators $A$ and $B$ is just the following $$[A, B] = AB - BA$$ If they commute, it's equal to zero. For ...

I have to think David will agree, on reflection, that his answer has failed to capture the essence of entanglement. Any stream of particles, if not specially prepared, will measure +h/2 or -h/2 at detector A; they will do with respect to the x axis, or the y axis, or any axis. Exactly the same is true at detector B. How can this very ordinary circumstance ...

In precise terms, the Heisenberg uncertainty relation states that the product of the expected uncertainties in position and in momentum of the same object is bounded away from zero. Your entanglement example at the end of your edit does not fit this, as you measure only once, hence have no means to evaluate expectations. You may claim to know something but ...

In the original EPR gedanken experiment, they assumed two particles that have perfect correlations in position, i.e., they are described by a delta function. That does not pose a problem for a thought experiment but cannot be performed in a lab because such a state cannot be normalized and is therefore unphysical. However, in quantum optics, many ...

The answer to this question comes from the very definition of measurement. In plain words, by definition, when you measure the state of a system you interact with that system and therefore perturb its state. This is known and is very well documented. Take the double-slit experiment for example: Your setup is an electron beam that you fire towards a wall ...

Jherico, I see that you are keen in finding answers to your questions, or putting your views across for a debate, and this is really good. This is what science is all about. I think your questions deserve attention and proper debate. Here is an effort from my side to help dilute some of the misunderstanding through the comments section of this forum. ...

The majority opinion is that Einstein was wrong. However, I see some problems with the standard quantum mechanics (SQM) approach that you outlined. SQM contains two major parts: unitary evolution (described, e.g., by the Dirac equation) and the measurement theory (e.g., collapse, or the projection postulate, which, loosely speaking, states that, after ...

The wave formulation has in its seed the uncertainty relation. Let me be precise what is meant by the wave formulation: the amplitude over space points will give information about localization on space, while amplitude over momenta will give information about localization in momentum space. But for a function, the amplitude over momenta is nothing else but ...

The answer to the question depends a bit on what is meant by "mediated". A composite quantum system composed of two or more quantum subsystems can be in a quantum state in which the subsystems are entangled from the beginning, i.e., from the initial state. If the composite system evolves without any interaction among the subsystems, then the form and degree ...

I think you answered your question. "According to EPR experiments measurements of the entangled states are at odds with SR": if you mean that we cannot consider that the result of a measurement made on one entangled particle "propagates" to another one because this propagation would violate SR principles, you have to rules out a interaction in the sense of ...

Pretty sure that EPR does not state entanglement is at odds with SR or if it does it is incorrect. The point of the EPR paper was that the consequences of entanglement were so strange they could not be real. Experimental evidence however supports entanglement and has never shown any violation of SR.

There are no problems with second detector measurements. They occur as they would without first detector. The "problem" is that if you check them with the knowledge of the results of first detector, you might notice that measurements on both ends are correlated. If you measured x components in both, you definitely got opposite results. While if you check x ...

An experiment very close to the proposed experiment has been done by Zeilingers grad-student Dopfler in 1998. She used a down-conversion crystal to produce pairs of entangled photons in a quantum-eraser type of experiment involving a dual-slit. One of the members went through the dual-slit and was detected by a detector A which is scanning the space behind ...

If we want the position and the momentum to be well-defined at each moment of time, the particle has to be classical. We inherited these notions from classical mechanics, where they apply successfully. Also they apply at macroscopic level. So, it is a natural question to ask if we can keep their good behavior in QM. Frankly, there is nothing to stop us to do ...

There is no experiment in which genuine information could be sent faster than light and there is no contradiction between this fact and quantum mechanics – as built by the Copenhagen school. Quite on the contrary, the proper, Copenhagen-like interpretation of quantum mechanics is needed for a description of known experiments that is compatible with special ...

The process used in this kind of source is the spontaneous parametric down conversion (SPDC, see, e.g. Wikipedia for details). It is a nonlinear optical process in which from a photon with angular frequency $\omega_0$ you get two photons with frequencies $\omega_1$, $\omega_2 = \omega_0-\omega_1$. These photons are then phase matched and have correlated ...

if can help Open timelike curves violate Heisenberg's uncertainty principle http://arxiv.org/pdf/1206.5485v1.pdf ...and show that the Heisenberg uncertainty principle between canonical variables, such as position and momentum, can be violated in the presence of interaction-free CTCs.... Foundations of Physics, March 2012, Volume 42, Issue 3, pp 341-361 ...

You are asking if a more complete theory might show that HUP is wrong and that position and momentum do exist simultaneously. But a more complete theory has to explain all the observations that QM already explains, and those observations already show that position and momentum cannot have definite values simultaneously. This is known because when particles ...

Conclusion 1. Definitely not a causal interaction - see Alain Aspect delayed choice experiments 2. Rather the amplitudes of causally separated particles are merely remain correlated due to past common origin events 3. See Smerlak and Rovelli at http://arxiv.org/abs/quant-ph/0604064 . See also Rovelli at http://arxiv.org/abs/quant-ph/9609002 for a coherent ...

There is no effect of the one measurement event upon the other. It is not until the results from both measurements are brought together for comparison, and accumulated statistically, that it gets interesting. The only interactions relevant to the entanglement are at the source, when the singlet spin system falls apart into two spin one particles (or ...

At the risk of confusing you even more: the value "that would be if the measurement was not made" simply does not exist. Take a your favourite simple quantum system, e.g., the spins of two electrons. The values of the various components and combinations of their spin do not exist before you decide which observables you will measure. It is simply impossible ...

Unfortunately I just joined so it seems my low reputation doesn't allow me to simply reply in a comment. I should also say this answer is a reply to your answer. I'm somewhat confused by your first point, and I think that it may be claiming something that is false, though I might just be misinterpreting it; namely the claim that QM can be replaced by a ...

I answer my own question only because the original answer provided doesn't directly answer it... although it lead me to the correct interpretations. So to address my points... We can absolutely replace QM with a deterministic theory and get the same predictions, we don't actually have to stop before we turn our wave functions into probabilities either as ...

The only way to make Heisenberg's principle irrelevant is to measure the speed and the position (to make it simple) of a fundamental particle. In other words, you would have to observe a particle, without having it collide with a photon or reacting to a magnetic force, or without interacting with it. There might be an other way, which would be to find a ...

It's not a "paradox". Einstein was troubled about the objective reality of complimentary variables. Before EPR, it was thought it's not possible to measure complimentarity variables simultaneously. He argued that some property has an objective value if without in any way disturbing it, we can know what it is with certainty. If we measure the z spin of an ...

In a certain way, to see the paradox in the EPR experiment you have to have absorbed quantum mechanics into your blood. Otherwise one might not see the surprise in the result. I think that a much better paradox, for someone just learning the theory, is the GHZ experiment. You begin with three photons in a linear superposition of two pure states. In the ...

No. The more or less formal definition of interaction between two systems is that you have a system 1 with Hilbert space $\cal H_1$ and a system 2 with Hilbert space $\cal H_2$. If system 1 were in isolation from other systems, it would have the Hamiltonian $H_1$ to govern its time evolution. Likewise for system 2 and $H_2$. When the systems are ...