This is a really good description of the measurement problem. It takes all the sides of the debate seriously, which is hard for most physicists to do. If you think about quantum mechanics all day every day, you're almost bound to settle on one interpretation as your working model, and it becomes hard to take the others seriously.
The Trouble with Quantum Mechanics
41–50 of 126 posts
Re: The Trouble with Quantum Mechanics
#42Earlier quoted context omitted.
This is a pretty great question with a fairly subtle answer. It is not much more than just flipping a coin, indeed! See section "4. Relativistic Causality" of http://www.scottaaronson.com/democritus/lec11.html for the best explanation I know of. This entire book "Quantum Computing Since Democritus" is absolutely great if you want to understand these topics. See also https://en.wikipedia.org/wiki/No-communication_theo…
But here you have two independent coins. My question is, how is entanglement fundamentally different than having two sides of the SAME coin but not looking until later?
Let's forget how this looks like two correlated coins and focus on your question.
Yes, if Alice and Bob just measure the spin of their corresponding electron, indeed it just looks like half of a precut coin. But if they want to win the game they will do something more complicated. (side note: To use the typical physicist terminology, this precut coin is a hidden variable - something set inside of the electron even before its measurement.)
But Alice and Bob can do something more interesting, something that permits them to win the game we described more often than 75% of the time. This involves Alice measuring the spin of the electron in a projection different than the one in which Bob is measuring (this is part of the "specific protocol" that I mentioned in the previous post). So while Alice is checking whether the result is h=Heads or t=Tails, Bob is checking whether the result is h+t or h-t. Now the two electrons seize to behave like the two parts of the same coin (and this is what permits them to win the game more often than 75% of the time). Explaining how exactly they use this protocol would take too much space, but you can look it up in the link to the book I provided.
P.S. However, as explained in the previous post, this does not permit them to send messages to each other.
P.P.S. Again, Section 4 of the linked page explains this in details!
Re: The Trouble with Quantum Mechanics
#43Earlier quoted context omitted.
What I personally do not like about pilot wave theory is that in the many body versions of the theory that I've seen, you still need a wave function that depends on the positions of all the particles. That is, the "pilot wave", unlike physical fields like electric or magnetic fields, is not a function of location in spacetime, nor is it attached to a particle, but to all particles. This means in a certain sense the a…
Still better than world-splitting at every instant. Isn't it?
Yes, I would agree with that!
Re: The Trouble with Quantum Mechanics
#44What is wrong with the Pilot Wave Theory intepretation? I'm ok with FTL information transfer, and so Bell's inequalities are satisfied. And anyway the other interpretations DO NOT rule out FTL information transfer. For example if one entangled electron flies into a black hole then we would be able to know its spin by measuring the other one even if light from it won't reach us. Also there is this: https://en.m.wikipe…
>I'm ok with FTL information transfer Which will lead to the conclusion that Special Relativity (which was extensively validated) is either wrong or incomplete. I have seen a revival of Pilot-wave theory here on HN, but the real conclusion (a real particle pushed around by the enigmatic pilot wave or quantum potential or whatever) is as bizarre or as satisfying as saying particle is in a superposition. Also, I haven'…
> Which will lead to the conclusion that Special Relativity (which was extensively validated) is either wrong or incomplete.
Not necessarily. The simplest relativistic pilot wave theory requires only a preferred foliation of spacetime.
This is seemingly unappealing to most physicists, but recent work has shown that the wave function itself contains just such a structure, so the needed foliation is present in every interpretation of QM.
Re: The Trouble with Quantum Mechanics
#45Earlier quoted context omitted.
Do you understand the arguments that the ability to send a signal faster than light allows you to send that signal backwards in time? I've never met anyone who who understood special relativity but was OK with that.
How so? Why should faster-than-light travel imply a reversal of the arrow of time? Can you explain the reasoning?
No, sorry, I can't explain the theory of relativity in one paragraph. It takes a book. Spacetime Physics, by Taylor and Wheeler, explains it very well with only high school level maths.
Re: The Trouble with Quantum Mechanics
#46Weinberg mentions the quote by Eugene Wigner regarding the importance of consciousness in the section on instrumentalists. However, I see it is being fundamental to realist's interpretation also. I subscribe the the realist approach, which I view as meaning nothing special happens when a human makes an observation - it is just quantum mechanics as usual. This has interesting implications. The key is that the observer is not outside the system, he is a part of it. His wave function becomes entangled with the system when the measurement is made. But what makes this difficult to analyze is that we don't know what it means for the observer to say "I see the result was *". What physical process allows the observer to be aware of the measurement?
We can make a simplifying and unsatisfying assumption - the observer has a register in his head which records the result of the measurement. Here, it is easy to imagine this register is part of the wave function just like the object being measured. As mentioned above, the wave function for this register is entangled with the object being measured so the measurement and register value are correlated. In our model this corresponds to the person independently thinking each of the possible results, which some people call multiple universes.
But this simple register is not what goes on inside the brain. Not knowing this makes it difficult to accurately model the measurement process.
EDIT: for clarity
Re: The Trouble with Quantum Mechanics
#47Earlier quoted context omitted.
Wouldn't the entanglement just break? The preassure would rip the electron apart to begin with. I'm not sure if that erases entanglement already or if the quarks needed to be annihilated. Either way, are the known laws of physics applicable inside a black hole at all? I don't think so.
The entanglement does not break. The entire black hole becomes entangled with the electron left outside (which is less exciting than it sounds). Sure, plenty of thing break if we start talking about unknowns, like the black hole evaporation for instance, but today's physics has a pretty good idea what happens if we just drop one of the electrons in the black hole and measure the other one. We just learn the spin of b…
> if one entangled electron flies into a black hole then we would be able to know its spin by measuring the other one even if light from it won't reach us
All the arguments here mention what happens under SR but wouldn't GR be more appropriate? If an electron flies into a black hole, from the frame of reference of the observer doesn't it appear to get closer and closer to the black hole event horizon over time, but never actually enter? It only enters a black hole from its own frame of reference, doesn't it? So the outside observer would never see it enter the black hole, and light from the electron would always reach us.
Or is my understanding of General Relativity effects near a black hole event horizon wrong?
Re: The Trouble with Quantum Mechanics
#48Earlier quoted context omitted.
How so? Why should faster-than-light travel imply a reversal of the arrow of time? Can you explain the reasoning?
This is an intuitive way of thinking about this without diving into the math. Here are the postulates: speed of light (c) is same for everyone. And there is no special reference frame; your physics is as good as the physics of your friend moving away from you in a space-ship in a constant velocity v. Now consider that your friend is actually moving in a spaceship away from you. He has an experimental setup with him u…
I did used to ponder over this sort of mystery, but I realized the actual answer to this lied at the particle level as explained in QED by Feynman: A positron can be treated as an electron traveling backwards in time. When it collides with an electron, the two create a photon. Should you ever come close to the speed of light, one way to "exceed" it would require a collision with your anti-matter self, resulting in (self-obliteration and...) photons from everyone's else's point of view. Of course, assuming my understanding is correct, it's also possible that you transform into your anti-matter self... bizarre. Going forward again would require some similar interaction, but I seem to recall formulating a thought experiment where this violates causality, making either the whole idea impossible even from a Relativistic standpoint or forcing some denial of causality, which would make other dependencies on causality (such as science itself) unreliable.
Re: The Trouble with Quantum Mechanics
#49Weinberg mentions the quote by Eugene Wigner regarding the importance of consciousness in the section on instrumentalists. However, I see it is being fundamental to realist's interpretation also. I subscribe the the realist approach, which I view as meaning nothing special happens when a human makes an observation - it is just quantum mechanics as usual. This has interesting implications. The key is that the observer…
In the realist interpretation, even without people, you wind up in entangled states after measurement of a spin (where by "measurement" I mean: allow a giant classical system to interact with a spin and tilt a pointer on some gauge to / wind up with some register holding + or -). It's perfectly possible to formulate the realist ideas without any consciousness or insistence that the measurements register in a brain or observer.
Re: The Trouble with Quantum Mechanics
#50Earlier quoted context omitted.
the Automaton interpretation sounds like something I would enjoy reading. But considering that I do not have the time this week nor enough knowledge in the subject, could you perhaps provide me with a quick summary as to what it is about?
My understanding is this: Hooft is an "instrumentalist" as described in this article. He does not believe that quantum mechanics describes "what's really going on" and is interested in classical models that explain why QM makes such good predictions. "Superdeterminism" is a very interesting perspective that Hooft feels could resolve this dilemma. Others feel that superdeterminism is not falsifiable, so there's no poi…
The physics stack exchange has basically two populations: people learning physics and asking introductory questions, and experts gathering to discuss detailed questions. It's really an example of a great egalitarian online community.