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The Trouble with Quantum Mechanics

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Re: The Trouble with Quantum Mechanics

#31

Post by ebcode »

thisrod wrote: post #20
EGreg wrote: post #9

What 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…

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?

Re: The Trouble with Quantum Mechanics

#32

Post by krastanov »

lisivka wrote: post #30
krastanov wrote: post #22

Earlier quoted context omitted.

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?

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 contraptions the pilot wave (or the wave function) of a single particle, but the nice intuitive demonstrations fail when you try to scale it up to more particles (or anything that would be exhibiting the interesting, nontrivial quantum behavior).

Re: The Trouble with Quantum Mechanics

#33

Post by krastanov »

ebcode wrote: post #31
thisrod wrote: post #20

Earlier 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?

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 sending messages to the past.

[1]: https://en.wikipedia.org/wiki/Relativity_of_simultaneity

Re: The Trouble with Quantum Mechanics

#34

Post by jayajay »

EGreg wrote: post #9

What 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…

Pilot wave theory only postpones the question of "what is a wave function and how does it collapse?", it doesn't actually say anything new. I think Wigner's Friend is really no fundamentally different than Schrodinger's Cat.

Re: The Trouble with Quantum Mechanics

#35

Post by gizmo686 »

ebcode wrote: post #31
thisrod wrote: post #20

Earlier 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?

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,z), then we can compute precisely where and when Bob observed the event occurring (t',x',y',z'). This conversion is known as the Lorentz Transformation [1], and can be derived mathematically from the assumption that the speed of light is constant regardless of reference frame.

Where this gets weird is the case where Alice and Bob observe 2 events: X and Y. In this case, it is possible that they will disagree about which event happened first. Once you accept this, it should become clear why causality requires some speed limit. You can do the math based on the Lorentz Transform and confirm that this limit is the speed of light. Intuitively, this is a direct consequence of the fact that we defined the Lorentz Transform to make the speed of light constant.

In case the need for a speed limit is not obvious, let as pretend that it does not exist. Suppose that, from Alice's perspective, X happened before Y. Further, suppose that Carol happened to be in a spaceship that passed by X at the instant it occurred, moving at a velocity that would take her by Y at the instant it occurred. [2] From the perspective of Bob, Carol would have traveled backwards, going from Y to X.

We can make this situation even worse by considering Carol's perspective. Recall that we defined Carol as starting at X and traveling to Y. In the same way, we can define Dave as starting at Y and traveling to X (recall that, if we were Bob, we would be convinced that Y happened first, so with a fast enough ship, Dave and make it in time). In this situation, both Carol and Dave exist at Y, so Carol can give Dave a copy of her diary of the trip. However, after Dave makes the trip to X, he will meet Carol again, so he can give her a copy of her diary of the trip she is about to take.

[0] This means that if I am on Earth, and you are in a space-ship moving at 99% the speed of light (from my perspective), and we both measure the speed of light, we will arrive at the same answer.

[1] https://en.wikipedia.org/wiki/Lorentz_transformation

[2] This is possible only because there is no limit to how fast Carol's Spaceship can travel.

Re: The Trouble with Quantum Mechanics

#36

Post by naasking »

tps5 wrote: post #6
demonshalo wrote: post #4

Earlier 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…

't Hooft's approach is the only superdeterministic theory I've heard of, so it's worthwhile for the novelty alone.

I've never been pursuaded by arguments that science is impossible in a superdeterministic world though. The fact that all current observations are consistent with a superdeterministic world and yet we still came up with a viable theory already proves that science is possible in such a world.

Re: The Trouble with Quantum Mechanics

#37

Post by EGreg »

krastanov wrote: post #29
EGreg wrote: post #27

Earlier quoted context omitted.

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_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?

Re: The Trouble with Quantum Mechanics

#38

Post by mathoff »

wolfgke wrote: post #3

Relevant for those who are interested in the topic: Two rather unusual interpretations if quantum mechanics: - De Brogle-Bohm theory: https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory - Gerard 't Hooft - The Cellular Automaton Interpretation of Quantum Mechanics: https://arxiv.org/abs/1405.1548 (see also https://en.wikipedia.org/wiki/Superdeterminism )

In philosophy, this is what we call four dimensionalism: https://en.wikipedia.org/wiki/Four-dimensionalism https://plato.stanford.edu/entries/temporal-parts/

Re: The Trouble with Quantum Mechanics

#40

Post by baytrailcat »

ebcode wrote: post #31
thisrod wrote: post #20

Earlier 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?

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 using which he measures speed of light. You are sitting at home postulating about his experiment. Consider he is moving at velocity v, so is his experiment equipment. How can he arrive the same result (3*10^8 m/s) for speed of light? You conclude that only way is that whatever clock he is measuring the experiment with is running slower than yours (velocity=distance/time, he is measuring less distance, so time must be less too). Now imagine he is going at the speed of light. (The actual equations breakdown at this point, but you can imagine he is infinitesimally close to speed of light). How is he arriving at c? Only way is time is not passing for him (according to you). Now imagine he is going faster than speed of light. At this point, the equipment he is sending photons with is traveling faster than photons itself! How does he arrive at c? His clock must be running backwards! And keep in mind, your friend in all these cases is happily measuring speed of light as c without any of these conundrums you have. And now you can publish a paper claiming that your friend travelled back in time! Did he really do that in a "Back to the future" kind of way? Perhaps not.
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