Is This What Quantum Mechanics Looks Like?

Veritasium video - physical realization of pilot wave theories

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Is This What Quantum Mechanics Looks Like?

#1  Postby minininja » Nov 09, 2016 8:53 pm

This may not be how quantum mechanics actually works, but it's a really cool physical representation of it that matches quantum effects, including the double slit experiment.



Science is awesome. :mrgreen:
[Disclaimer - if this is comes across like I think I know what I'm talking about, I want to make it clear that I don't. I'm just trying to get my thoughts down]
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Re: Is This What Quantum Mechanics Looks Like?

#2  Postby CdesignProponentsist » Nov 10, 2016 12:32 am

I've been a fan of the pilot wave interpretation ever since hearing of it. It seems to match the behaviors all too closely to be entirely coincidence.
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Re: Is This What Quantum Mechanics Looks Like?

#3  Postby newolder » Nov 10, 2016 9:50 am

Some of the problems associated with the de Broglie/Bohm “pilot wave” idea include the inability to treat spin correctly (for a component of spin in, say, the z direction would break the rotational symmetry of pilot waves) and the fact that the pilot waves must act superluminally and therefore contradict special relativity in order to yield an interference pattern.

Our old friend and blogger Lubos Motl has posted a few times on issues with “pilot wave” mechanics, e.g. Bohmian mechanics is incompatible with loop corrections and Droplets and pilot waves vs quantum mechanics The second is a bit of a rant but does contain enough of the mathematics to show the differences between classical objects guided by pilot waves and the Schrödinger wave function.

So even if you forget about the completely different interpretations of the wave function and the shape of droplets, there is a difference (well, many differences, but I chose this one) at the purely mathematical level. The equations governing the evolution of the wave function must be exactly linear and there can't be any debate about it because it's a matter of consistency. The equations governing the evolution of the shape of droplets are almost certainly nonlinear because there is no general constraint that would ban the nonlinearity, and they are therefore 99.999...% likely to occur. You may find situations and approximations in which the nonlinearities are small or the nonlinear equations emulate some linear ones for other reasons, but fundamentally they are very different.


ETA From May of this year, someone is really getting his goat up now...: Anti-quantum zealots.... :lol:
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Re: Is This What Quantum Mechanics Looks Like?

#4  Postby CdesignProponentsist » Nov 11, 2016 5:06 am

newolder wrote:Some of the problems associated with the de Broglie/Bohm “pilot wave” idea include the inability to treat spin correctly (for a component of spin in, say, the z direction would break the rotational symmetry of pilot waves) and the fact that the pilot waves must act superluminally and therefore contradict special relativity in order to yield an interference pattern.


I'm pretty sure I've heard somewhere that they don't in fact need to act superluminally. I could be wrong. If I find it I'll link it.
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Re: Is This What Quantum Mechanics Looks Like?

#5  Postby newolder » Nov 11, 2016 9:46 am

The alternative (to superluminal) Bohmian view would be trajectories like this (from wiki):
Image
To which Motl adds:
Bohmian trajectories never cross the z=0 plane – the horizontal line in the middle of the picture, in this convention. A particle in the upper half stays in the upper half, and the same holds for the lower half. There's clearly no sense in which this correlation exists in quantum mechanics – or the real world – and this unphysical feature of Bohmian mechanics is also exploited by ESSW to show that the Bohmian paths are "surreal".
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Re: Is This What Quantum Mechanics Looks Like?

#6  Postby DavidMcC » Dec 17, 2016 1:00 pm

CdesignProponentsist wrote:
newolder wrote:Some of the problems associated with the de Broglie/Bohm “pilot wave” idea include the inability to treat spin correctly (for a component of spin in, say, the z direction would break the rotational symmetry of pilot waves) and the fact that the pilot waves must act superluminally and therefore contradict special relativity in order to yield an interference pattern.


I'm pretty sure I've heard somewhere that they don't in fact need to act superluminally. I could be wrong. If I find it I'll link it.

They certainly don't. A standing wave, as its name implies, doesn't travel at all, because it is the superpositioon of waves travelling in both directions. The wavefunction must have already reached all positions in the set-up for the standing wave to exist.
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Re: Is This What Quantum Mechanics Looks Like?

#7  Postby igorfrankensteen » Dec 17, 2016 1:28 pm

CdesignProponentsist wrote:I've been a fan of the pilot wave interpretation ever since hearing of it. It seems to match the behaviors all too closely to be entirely coincidence.


I assume this is because waves behave like waves behave like waves. The dissimilarities with quanta, would be be likewise because waveparticles aren't caused through constant input of several sources of energy from outside, as the oil droplets are.

It is a very convenient illustration of wave behavior.

Ps, I enjoyed that just as the narrator says "the oil droplets almost never recombine," two of them do.
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Re: Is This What Quantum Mechanics Looks Like?

#8  Postby DavidMcC » Dec 17, 2016 1:35 pm

igorfrankensteen wrote:
CdesignProponentsist wrote:I've been a fan of the pilot wave interpretation ever since hearing of it. It seems to match the behaviors all too closely to be entirely coincidence.


I assume this is because waves behave like waves behave like waves. The dissimilarities with quanta, would be be likewise because waveparticles aren't caused through constant input of several sources of energy from outside, as the oil droplets are.

It is a very convenient illustration of wave behavior.

Ps, I enjoyed that just as the narrator says "the oil droplets almost never recombine," two of them do.

So you sat through the entirr video? That's impressive! 8-)
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Re: Is This What Quantum Mechanics Looks Like?

#9  Postby PensivePenny » Dec 17, 2016 1:46 pm

Minininja, thank you for posting this. It was a great presentation that helped a quantum mechanic illiterati like myself. I really like the guy in the video too. What a great science explainer!
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Re: Is This What Quantum Mechanics Looks Like?

#10  Postby DavidMcC » Dec 18, 2016 1:47 pm

newolder wrote:The alternative (to superluminal) Bohmian view would be trajectories like this (from wiki):
Image
To which Motl adds:
Bohmian trajectories never cross the z=0 plane – the horizontal line in the middle of the picture, in this convention. A particle in the upper half stays in the upper half, and the same holds for the lower half. There's clearly no sense in which this correlation exists in quantum mechanics – or the real world – and this unphysical feature of Bohmian mechanics is also exploited by ESSW to show that the Bohmian paths are "surreal".

Can yuou supply the wiki reference, newolder? I can't find the page with that dubious graphic on it, only one with the correct version of the 2-slit interference of waves: Double slit experiment

Image

EDIT: AFAIAC, this could still be seen as a pilot wave model, just the correct version of it.
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Re: Is This What Quantum Mechanics Looks Like?

#11  Postby newolder » Dec 18, 2016 1:56 pm

DavidMcC wrote:...
Can yuou supply the wiki reference, newolder? ...

Lobos Motl used the image on his blog entry Anti quantum zealots .... The wiki reference is the image's url: https ://upload.wikimedia.org/wikipedia/commons/thumb/0/02/Doppelspalt.svg/400px-Doppelspalt.svg.png

Further to the topic question, the top answer at physics stack exchange is also relevant.
...
Bohmian mechanics, which is the most mature form of the de Broglie pilot wave theory does explain measurement through the mechanism of hidden variables (i.e. by saying that there is state in a quantum system which is hidden from us). However, it is also known that Bohmian mechanics needs to be nonlocal to make the hidden variable explanation work. Roughly this means that it implies faster-than-light signalling, which in turn makes it very hard to make sense of causality: in a universe where faster than light signalling can be done, effects can come before their causes. So my belief is that most physicists would say that Bohmian mechanics is not a good explanation.
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Re: Is This What Quantum Mechanics Looks Like?

#12  Postby DavidMcC » Dec 18, 2016 2:17 pm

Ah! I see it's on WikiMedia. I was looking on WikiPedia! My bad. :oops:
I certainly agree that the WikMedia graphic is wrong, but does that have anything to do with Bohm?
EDIT: Also, the FTL implication does not seem to make sense. Does that also come from Bohm himself, or was he being set up?
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Re: Is This What Quantum Mechanics Looks Like?

#13  Postby newolder » Dec 18, 2016 2:34 pm

Bohmian mechanics implies superluminal communication (physics stack exchange).

Adapted pilot wave theory that does not require superluminal communication results in the "surreal" trajectories as shown in the image. (Motl)

Neither fits with current physics.
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Re: Is This What Quantum Mechanics Looks Like?

#14  Postby DavidMcC » Dec 18, 2016 3:23 pm

newolder wrote:Bohmian mechanics implies superluminal communication (physics stack exchange).

Adapted pilot wave theory that does not require superluminal communication results in the "surreal" trajectories as shown in the image. (Motl)

...

.... As has already been said. However, I am not sure that any of that is Bohm's own idea, or whether it is "fake" Bohmian theory, put up either due to misunderstanding or a deliberate attempt to discredit his ideas.
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Re: Is This What Quantum Mechanics Looks Like?

#15  Postby PensivePenny » Dec 18, 2016 3:24 pm

I know this question is a little off topic, but can someone direct a quantum mechanics illiterati to something that can explain waves in a vacuum, specifically with regard to the double slit experiment? What medium is the wave being created in, if there is a single particle? Medium is how I see waves, though I can comprehend forces like magnetism, gravity etc causing waves too. What is the wave in the double slit experiment comprised of?

A link or search term would be appreciated.
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Re: Is This What Quantum Mechanics Looks Like?

#16  Postby DavidMcC » Dec 18, 2016 3:34 pm

PensivePenny wrote:I know this question is a little off topic, but can someone direct a quantum mechanics illiterati to something that can explain waves in a vacuum, specifically with regard to the double slit experiment? What medium is the wave being created in, if there is a single particle? Medium is how I see waves, though I can comprehend forces like magnetism, gravity etc causing waves too. What is the wave in the double slit experiment comprised of?

A link or search term would be appreciated.

EM waves can exist with or without material filling the locations where the waves are. Ie, even if it is a vacuum. Indeed, some media (such as electrical conductors) prevent waves, by reflecting them back at the conducting surface.
Try a search on "electromagnetic wave propagation" (I haven't, but it should work!).
EDIT: the wave in the double slit experiment is basically the same as in any other set-up involving EM waves. The exact spatial dependence of the wave depends, of course, on the details of the setup (including the source of the EM waves as well as what objects they encounter).
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Re: Is This What Quantum Mechanics Looks Like?

#17  Postby DavidMcC » Dec 18, 2016 3:44 pm

Having tried that search. I reckon the Wikipedia URL it came up with is as good aas any:
https://en.wikipedia.org/wiki/Electromagnetic_radiation
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Re: Is This What Quantum Mechanics Looks Like?

#18  Postby PensivePenny » Dec 18, 2016 3:46 pm

Great, thanks David :thumbup:
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Re: Is This What Quantum Mechanics Looks Like?

#19  Postby newolder » Dec 18, 2016 3:53 pm

DavidMcC wrote:
newolder wrote:Bohmian mechanics implies superluminal communication (physics stack exchange).

Adapted pilot wave theory that does not require superluminal communication results in the "surreal" trajectories as shown in the image. (Motl)

...

.... As has already been said. However, I am not sure that any of that is Bohm's own idea, or whether it is "fake" Bohmian theory, put up either due to misunderstanding or a deliberate attempt to discredit his ideas.

The first chapter of a book on Bohmian mechanics is available in the arxiv.

Also, some problems with the theory are noted here.
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Re: Is This What Quantum Mechanics Looks Like?

#20  Postby DavidMcC » Dec 18, 2016 4:02 pm

newolder wrote:...

Also, some problems with the theory are noted here.

That isn't entirely fair, newolder, because AFAIK, there is NO theory that fully combines GR with QT, not even QFT.
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