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The Trouble with Quantum Mechanics

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Re: The Trouble with Quantum Mechanics

#71

Post by gizmo686 »

lisivka wrote: post #68
gizmo686 wrote: post #35

Earlier quoted context omitted.

The idea behind relativity is that the speed of light is constant in all reference frames. [0]. In order for this to work, we "warp" spacetime depending on the velocity of the observer. More concretely, imagine Alice and Bob are moving away from each other at 50% the speed of light. They both observe event some event, X, occur and note the location in space-time. If Alice observes that X occurs in the location (t,x,y…

> From the perspective of Bob, Carol would have traveled backwards, going from Y to X. So what? It's Bobs problem. FTL electrons, in medium where speed of light is much less than c, are traveling exactly as you described.

Suppose Carol has a clock. At X it reads 0, and at Y it reads 100 (100 units in the future). From Bob's perspective, Carol's clock would also read 0 at X and 100 at Y despite the fact that she was at Y first.

In other words, as Carol travels 'forward' in Bob's time, her clock runs backwards. Or, from Carol's perspective, Bob's clock would be running backwards.

This isn't actually a problem in relativity, but is the definition of time travel.

Re: The Trouble with Quantum Mechanics

#72

Post by tim333 »

n4r9 wrote: post #65
lisper wrote: post #59

This article presents a very inaccurate view of the realist approach. The universe does not "split" when you make a measurement. The measurement problem is a solved problem. The solution is that measurement and entanglement are the same physical phenomenon. Measurement is just entanglement extended to a macroscopic system through a process called "decoherence". The net result is that, when you do the math, you recove…

> The measurement problem is a solved problem. I think this is overselling it slightly. Yes, Everettian style interpretations have helped to shed insight into the reality of the quantum state. Yes, decoherence has helped us to understand physical systems and their interactions with the environment. But if the measurement problem was solved there wouldn't continue to be a swathe of literature on the measurement proble…

Indeed it's not that solved. A big problem is as Weinberg puts it

>Several attempts following the realist approach have come close to deducing rules like the Born rule that we know work well experimentally, but I think without final success.

which is kind of the heart of things.

Re: The Trouble with Quantum Mechanics

#73

Post by lisivka »

gizmo686 wrote: post #71
lisivka wrote: post #68

Earlier quoted context omitted.

> From the perspective of Bob, Carol would have traveled backwards, going from Y to X. So what? It's Bobs problem. FTL electrons, in medium where speed of light is much less than c, are traveling exactly as you described.

Suppose Carol has a clock. At X it reads 0, and at Y it reads 100 (100 units in the future). From Bob's perspective, Carol's clock would also read 0 at X and 100 at Y despite the fact that she was at Y first. In other words, as Carol travels 'forward' in Bob's time, her clock runs backwards. Or, from Carol's perspective, Bob's clock would be running backwards. This isn't actually a problem in relativity, but is the d…

No, it's not a time travel. Bob will see that Carol travels backward and Carol will see that Bob travel backward, like we see EM emission from FTL electron in reverse order. FTL electron is not moving back in time, nor Bob or Carol.

Re: The Trouble with Quantum Mechanics

#74

Post by gpsx »

evanb wrote: post #49
gpsx wrote: post #46

Weinberg 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…

Yes, I agree you can formulate the realist ideas, because the observer is nothing special at all. And I also think this is how the world works. But from an experimental verification point of view I think you really need a model for what the brain does to say you understand how a measurement is made. What is a theory without verification?

Granted his quote used the word formulate, but I interpret the comment as referring to not just writing some rules but also having some justification for them.

Re: The Trouble with Quantum Mechanics

#75

Post by krastanov »

lisivka wrote: post #67
krastanov wrote: post #33

Earlier quoted context omitted.

This wiki page [1] explains why whether two events happen at the same time depends on your reference frame. If you can send signals faster than light then you can send a signal to an even far away and cancel that event. In another reference frame the sending of the signal would have happened after the far-way event has already happened. Hence, due to the "relativity of simultaneity" FTL communication is equivalent to…

By sending a message in two opposite directions, we can achieve 2c speed of information transfer. How that is possible?

No, we can not, but the reason might seem subtle. S sends the same information in opposite directions to A and B. So A learns what B learns, but A can not use that to tell something to B, that S did not already know. A can not influence B, rather it is S that influenced both A and B.

Re: The Trouble with Quantum Mechanics

#76

Post by krastanov »

EGreg wrote: post #69
posterboy wrote: post #61

Earlier quoted context omitted.

Thanks, I wanted to add that, too, but wasn't sure about it. I still am not, because this opens another can of worms. Can a black hole then not grow if from our POV nothing ever enters?

I thought that from the POV of the object falling into the black hole, time stretches. From our point of view it can very well be sucked in.

It is the other way around. From the POV of the object falling they just fall in. Look up Preskill's work for explanation of how it works when entanglement comes into play.

Re: The Trouble with Quantum Mechanics

#77

Post by krastanov »

lisivka wrote: post #62
krastanov wrote: post #32

Earlier quoted context omitted.

But you can do the same with classical setups that mimic some effects from the typical quantum mechanical formulations. Those classical experiments are indeed amusing and interesting, but they do not illuminate the "quantum puzzles", no matter whether they are modeled after pilot wave theory or after quantum mechanics. And very importantly, those amusing demonstrations do not scale! Sure, you can mimic with classical…

So, your prediction for walker droplet spin is that walkers, in kind of Stern and Gerlach experiment, will behave like classical magnets, not like quantum particles, right?

No, they would behave like a ball floating on top of a wave and given that there are waves involved there will also be interference patterns. There is nothing quantum here. Sure, in one particular way it looks like a quantum particle (to the extent of a cargo cult), but in all the important ways it does not (entanglement, computational power, generalisation to multiple particles).

Re: The Trouble with Quantum Mechanics

#78

Post by gizmo686 »

lisivka wrote: post #73
gizmo686 wrote: post #71

Earlier quoted context omitted.

Suppose Carol has a clock. At X it reads 0, and at Y it reads 100 (100 units in the future). From Bob's perspective, Carol's clock would also read 0 at X and 100 at Y despite the fact that she was at Y first. In other words, as Carol travels 'forward' in Bob's time, her clock runs backwards. Or, from Carol's perspective, Bob's clock would be running backwards. This isn't actually a problem in relativity, but is the d…

No, it's not a time travel. Bob will see that Carol travels backward and Carol will see that Bob travel backward, like we see EM emission from FTL electron in reverse order. FTL electron is not moving back in time, nor Bob or Carol.

The notion of "backwards" in time is a bit fuzzy once you get into relativity, so the three person example I gave may not clearly show it.

However, in the four person example, Dave is able to hand Carol her diary of the trip that she is about to make. If that is not time travel, I do not know what is.

Re: The Trouble with Quantum Mechanics

#79

Post by lisper »

opaque wrote: post #66
lisper wrote: post #59

This article presents a very inaccurate view of the realist approach. The universe does not "split" when you make a measurement. The measurement problem is a solved problem. The solution is that measurement and entanglement are the same physical phenomenon. Measurement is just entanglement extended to a macroscopic system through a process called "decoherence". The net result is that, when you do the math, you recove…

Decoherence is an extremely useful and elegant mechanism for understanding quantum systems, but unfortunately it doesn't solve the measurement problem. At least not to the satisfaction of most practising physicists. The entanglement and trace operation does produce classical probabilities in the observed sub-system. However, it requires you to make a pretty arbitrary division between the observed system and the wider…

> Why is everything not just entangled with everything else up and up the chain until the entire universe is in superposition?

It is.

I'm going hazard a guess that your next question is going to be: why do I not perceive myself to be in a superposition of states? And the answer is that you are not what you think you are. You think you are a human being, a classical physical object made of atoms, but you aren't. This is a very good approximation to the truth, but it is not the truth. The truth is that you (the thing engaged in this conversation) are a software process running on a human brain. You are a classical computing process, i.e. a process that can be emulated by a classical computing model like a Turing machine. The reason for this is that the kinds of things you do necessarily involves copying information (e.g. the process of reading this comment involves copying information from your computer into your brain) and quantum information cannot be copied. Only classical information can be copied. Your conscious awareness of the existence of physical processes is an emergent phenomenon of accumulating memories, i.e. copying information. Because of this you cannot become consciously aware of your quantum nature, and because of that you cannot demonstrate the quantum nature of any system (to yourself) unless it is isolated from you. You could in principle demonstrate the quantum nature of the rest of the universe if you could somehow isolate yourself from it, but that presents insurmountable practical difficulties.

> The problem of where the quantum world ends and the classical world begins is still unanswered.

Because the question tacitly makes the false assumption that there is a hard boundary between the two. There isn't.

Re: The Trouble with Quantum Mechanics

#80

Post by lisper »

tim333 wrote: post #72
n4r9 wrote: post #65

Earlier quoted context omitted.

> The measurement problem is a solved problem. I think this is overselling it slightly. Yes, Everettian style interpretations have helped to shed insight into the reality of the quantum state. Yes, decoherence has helped us to understand physical systems and their interactions with the environment. But if the measurement problem was solved there wouldn't continue to be a swathe of literature on the measurement proble…

Indeed it's not that solved. A big problem is as Weinberg puts it >Several attempts following the realist approach have come close to deducing rules like the Born rule that we know work well experimentally, but I think without final success. which is kind of the heart of things.

Just because Steven Weinberg is unaware of or does not accept the solution does not mean that the problem is not in fact solved.
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