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Apr
13
revised Quantum non-locality with commuting measurements?
added 369 characters in body
Apr
13
revised Quantum non-locality with commuting measurements?
added 369 characters in body
Apr
13
answered Quantum non-locality with commuting measurements?
Apr
13
comment Shouldn't local realism imply the superdeterminism?
If one assumes that you have no free will in the sense that you could have chosen which questions you ask - or which quantities you measure in the experiment - then it follows that you had no free will to write the particular question you wrote, either, and other users have no reason to try to help you because you are just a mechanical computer pressing the keys on a keyboard in a certain way. Also, if we have no free will, you shouldn't be bothered by the fact that I and others consider superdeterminism to be the ultimate pseudoscience - in your view, we don't have a choice, either. ;-)
Apr
13
awarded  Good Answer
Apr
13
comment Demonstrating quantum non-locality by two consecutive measurements
This is nothing else than the uncertainty principle. One may find the circular polarization of a photon, or its x-vs-y linear polarization, but not both. So if the first one is measured, the result of the second is modified, so if both measurements are made after one another, the results of the second one will show different results than if the first measurement hadn't taken place.
Apr
13
comment Demonstrating quantum non-locality by two consecutive measurements
The entangled state is capable of producing "very many correlations" for many possible future "pairs of measurement", e.g. for two circular polarization measurements, or two x-vs-y linear polarization measurement, or a similar measurement with respect to some axes rotated by an angle such as 45 degrees etc. But it's important that in a single repetition of the initial state, one can't experimentally prove the correlations for all these possible experiments "simultaneously". One can't measure the photons without changing them. One can't measure circular and linear polarization.
Apr
13
comment Demonstrating quantum non-locality by two consecutive measurements
To say what @CuriousOne did in other words, in quantum mechanics, a (strong, full-fledged, standard) measurement always affects the measured system, by bringing it to the eigenstate of the just-measured operator corresponding to the just-measured eigenvalue. It means that the information about the relative phases is destroyed, and this has consequences e.g. that the interference pattern disappears in double slit experiment after the which-way information is measured; or the circular-circular correlations of 2 EPR photons disappears when at least 1 linear polarization is measured before that.
Apr
10
awarded  Guru
Apr
10
comment Why is the speed of light defined as 299792458 m/s?
One meter is just defined in such a way that the speed of light in SI is totally accurately known, 299 792 458. Now, with this well-defined meter (assuming that you are OK with the definition of 1 second and you can reconstruct it), one may measure distances. The question is how accurately we may measure distances in terms of 1 meter as it is currently defined. The accuracy is amazingly good because when we switched to the new definition, the accuracy of $c$ was already good - and we could measure $c$ in terms of the old meter - so the accuracy hasn't "dropped".
Apr
10
comment Why is the speed of light defined as 299792458 m/s?
David, one meter is a unit, a purely human convention. Be sure that the speed of light, which is a fact about Nature, doesn't "depend" on the choice of one meter or anything man-made in general. Even without humans, there would be light and it would move by a speed. Nature doesn't need humans or their inventions and conventions for that. It's just complete nonsense that there is something inaccurate about the definition of 1 meter or the speed of light we know.
Apr
10
comment Why is quantum entanglement considered to be an active link between particles?
There is nothing strange or nonlocal happening in quantum field theory at all and one may explicitly, rigorously, and precisely prove from the actual quantum formalism. To be sure that nothing strange or nonlocal takes place QFT means to understand QFT fully without defects. It doesn't involve being any "radical positivist" or any other bizarre philosophical statement. These are just totally sharp physics questions in QFT that have nothing whatever to do with philosophy as long as one remains a scientist.
Apr
10
comment Why is quantum entanglement considered to be an active link between particles?
Dear Jerry, the wave function isn't "localized to places" in any simple way. The wave function is the description of the whole knowledge. It is not a function of space, like fields are functions of space. For 2 particles, it is a function of both vectors $r_1$ and $r_2$. So it's nonsense to say that "the wave function changes everywhere". What's relevant is whether the observables change somewhere. And locality of QFT guarantees that they won't. Moreover, I only used the word "realized" after "outcome", not "probability", so you're just distorting everything I wrote.
Apr
10
comment Commutation relations in QFT and the principle of locality
... so the commutator of the fields has to be zero. So the equal-time commutator of fundamental fields that may be parts of operators that are measured must be a combination of delta functions and their derivatives, something that only has the support in the infinitesimal neighborhood of $x=y$. Everything else would violate locality i.e. relativistic causality, and that would be in conflict with special relativity and/or internal consistency. The equal-time commutator may also be viewed as a special case of the general commutators that are encoded in Green's functions/correlators.
Apr
10
comment Commutation relations in QFT and the principle of locality
Dear @GennaroTedesco - I downvoted your answer (despite the work and nice layout) mostly because of the bold face word "only" - and because of your later comment that causality has nothing to do with the equal time commutators. Of course they have everything to do with each other. One may derive the delta-function equal-time commutator from the canonical quantization. But that doesn't mean that it's the only way to prove that the commutator is zero almost everywhere. Instead, another proof is indeed on locality/causality: at equal times, $x\neq y$ are always spacelike-separated, so...
Apr
5
comment A tutorial explanation of decoherence?
The Wikipedia article may still be incomprehensible to average high school students or adults with the same physics and mathematics background - one needs some college-like training related to sciences or mathematics. But it's questionable whether this is a problem. I don't think that people outside the "college-like STEM community" should care about the meaning of decoherence. It only creates chaos when people who obviously don't know much about physics in any sense are led to use words such as "decoherence".
Apr
5
comment A tutorial explanation of decoherence?
Things generally become easier to understand to people who have studied things, who have a talent for things, or who are just smarter, but that doesn't mean that it's "not kosher" to express opinions about which texts are technical and which texts are not. So if your attempt was to bully me and declare the rating of "technical vs not technical" to be political incorrect, I have to assure you that you have failed and I will never stop using these important adjectives.
Apr
5
comment A tutorial explanation of decoherence?
Dear @Calmarius, the Wikipedia article on decoherence has almost 1,000 edits from very many different editors, low number of equations, and especially simple enough equations that only need some basic undergraduate linear algebra, and everyone with the understanding of the basic formalism of QM may get this article. At least approximately, it makes sense to divide texts to very technical and not too technical and while there are very technical texts on Wikipedia, the article on decoherence isn't one of them.
Apr
4
comment Few questions regarding String-Net theory and the Standard Model
I think that this question hasn't been replied to because the source of many statements is fuzzy and it's all confusing. Why shouldn't composite fermions ever be singlets? The neutron is basically a singlet, especially if the baryon number is ultimately not conserved. A theory that prevents one from creating arbitrary bound states almost certainly disagrees with the observations - and with locality. So why should one spend too much time with the question what the theory actually is? If it makes these strange prdictions, it's probably not too interesting.
Apr
2
awarded  Good Answer