I had three semesters of QM in grad school and I still have trouble with it.
Well, the OP author is arguably the greatest living physicist, so it's not surprising that 3 semesters isn't enough :)
The Trouble with Quantum Mechanics
21–30 of 126 posts
Re: The Trouble with Quantum Mechanics
#22Earlier quoted context omitted.
>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'…
There is the silicone oil droplet phenomena which is a classical version of the pilot wave, and it does reproduce some quantum behavior, including the double slit. That a pilot wave has been discovered (granted at macro scale) does make one reconsider. https://youtu.be/WIyTZDHuarQ
It does not, really. The macroscopic realisation is not particularly surprising (although it is quite awesome and original). If you put a ball floating on top of a wave you will observe the predictions from a mathematical model of that system, which is exactly what the pilot wave theory is, and there is nothing surprising here. Moreover, the macroscopic model simulates something which by definition is an unobservable construction in the quantum model. It does not simulate any inherently quantum behavior (classical waves is a thing we already knew exists).
Re: The Trouble with Quantum Mechanics
#23Earlier quoted context omitted.
>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'…
There is the silicone oil droplet phenomena which is a classical version of the pilot wave, and it does reproduce some quantum behavior, including the double slit. That a pilot wave has been discovered (granted at macro scale) does make one reconsider. https://youtu.be/WIyTZDHuarQ
As far as I know there is no analog of entanglement in these experiments. If someone has heard or read something in that regard, I'd be interested in a reference!
Re: The Trouble with Quantum Mechanics
#24I know that Eliezer Yudkowsky is very controversial and some of his statements cannot be taken seriously, but I really liked his sequence of articles on QM. It made it a little bit more accessible to me, who knows very little about it. I cannot judge how accurate it is; but from googling around, it doesn't seem he does any strong errors. Also he makes some very strong statements in the end which I found preposterous…
One of the issues with engaging with Yudkowsky's posts is how verbose his arguments are. Fortunately, for something as well formalized as quantum mechanics, there's a succinct counter-example: A formalism of quantum mechanics that makes perfect sense with neither the idea of wavefunction collapse nor some abundance of ontologically dubious and extravagant entities (alternate universes you can never observe). That formalism is relational quantum mechanics.
Accessible overview: https://plato.stanford.edu/entries/qm-relational/ In particular, this overview addresses the fundamental difference between Everett's theory and Rovelli's theory.
Original paper: https://arxiv.org/abs/quant-ph/9609002
A fluffy paper by the same author about QFT and its relational nature: https://arxiv.org/abs/hep-th/9910131
In short: The way you can evade the difficulties MWI addresses completely by changing one premise of the problem: that physical systems have states independent of their observers. In relational quantum mechanics, there is no universal wavefunction because there is no external observer of the universe. Call it the zero worlds interpretation. How parsimonious!
There is an element of Yudkowsky's sequence that deals with relative configuration spaces, but this is a subtly different concept and is really just a discussion from a clever guy unequipped with the right concepts about the difference between, say, affine spaces and vector spaces or torsors and groups. The idea that there is a configuration space independent of the observer is the assumption he misses.
In general I find Yudkowsky's extreme certainty in his own arguments to range from amusing to obnoxious. It's not hard to find places where uncertainty creeps in. It doesn't come from mistakes in his reasoning because what he considers he is usually meticulous about considering, but from what he doesn't consider.
Re: The Trouble with Quantum Mechanics
#25What 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…
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 both of them at the same time, but we can not choose the value (it is random) hence there is no communication and no "weird peeking into the black hole".
Re: The Trouble with Quantum Mechanics
#26What 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…
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?
Re: The Trouble with Quantum Mechanics
#27Earlier 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…
Then how is it different than just splitting a coin, throwing the other half far away and then looking whether you have heads or tails? There is never any spooky action at a distance.
Re: The Trouble with Quantum Mechanics
#28What 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…
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.
An electron is a lepton, it isn't made up of quarks. What does it mean to rip it apart?
Re: The Trouble with Quantum Mechanics
#29Earlier quoted context omitted.
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…
Then how is it different than just splitting a coin, throwing the other half far away and then looking whether you have heads or tails? There is never any spooky action at a distance.
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_theorem
TLDR:
Think of it this way: We've got two players, Alice and Bob, and they're playing the following game. Alice flips a fair coin; then, based on the result, she can either raise her hand or not. Bob flips another fair coin; then, based on the result, he can either raise his hand or not. What both players want is that exactly one of them should raise their hand, if and only if both coins landed heads. If that condition is satisfied then they win the game; if it isn't then they lose.
They can win 75% of the time if they just never raise their hands. Using a shared entangle state they can "cheat" and win 85.3% of the time by using a specific protocol, because they rely on some new form of correlation. But they still can not use this correlation to send messages (see the no communication theorem). Naively (this naive intuition does break!), you can imagine them having two slightly correlated coins - sure, after Alice flips hers she knows Bob's result, but she did not decide the result of her coin so she can not use it to send information to Bob.
Re: The Trouble with Quantum Mechanics
#30Earlier quoted context omitted.
There is the silicone oil droplet phenomena which is a classical version of the pilot wave, and it does reproduce some quantum behavior, including the double slit. That a pilot wave has been discovered (granted at macro scale) does make one reconsider. https://youtu.be/WIyTZDHuarQ
It does not, really. The macroscopic realisation is not particularly surprising (although it is quite awesome and original). If you put a ball floating on top of a wave you will observe the predictions from a mathematical model of that system, which is exactly what the pilot wave theory is, and there is nothing surprising here. Moreover, the macroscopic model simulates something which by definition is an unobservable…
Nobody tells that walkers are simulate all quantum behavior and doing that correctly. However, they helps to understand some of quantum puzzles. For example, droplets have spin. Can you predict behavior of the classical droplet spin in compare to the puzzling quantum spin?