Query about quantum superposition and wave functions

[tim1608]

Hi Everyone

I have four questions about the nature of quantum superposition and wave functions:

1. If a particle is quantumly superpositioned in more than one location then as soon as the slightest evidence of the particle's existence in one of the locations is detected by a "measurement", does this mean that all traces of the particle's existence in any of the other locations must disappear?

2. Is it remotely possible that, under certain circumstances, a quantumly superpositioned particle can simultaneously interact with other particles in multiple locations as if it is simultaneously at all of those locations? (I can understand that one objection to this scenario might be that it violates the conservation of energy. Well, as I see it, this would not be the case if the particle is only devoting a fraction of its energies to each location.)

3. As well as collapsing, can wave functions "uncollapse"?

4. I have a hunch (which might be wrong) that it may be better to think of wave functions as continuously evolving in response to particle interactions as opposed to fully "collapsing" and/or "uncollapsing". Might there be some element of truth in my hunch?

Thank you very much.

 

[.Scott]

1. If a particle is quantumly superpositioned in more than one location then as soon as the slightest evidence of the particle's existence in one of the locations is detected by a "measurement", does this mean that all traces of the particle's existence in any of the other locations must disappear?

To answer the gist of your question, yes - but with two caveats. In general, there would never have been any "trace" of the particle being anywhere before the measurement - so there really isn't any "disappearance". Second, I need to leave the "slightest evidence" part up to real physicists. I believe that you can collect inconclusive "evidence" without fully collapsing the wave function.

2. Is it remotely possible that, under certain circumstances, a quantumly superpositioned particle can simultaneously interact with other particles in multiple locations as if it is simultaneously at all of those locations? (I can understand that one objection to this scenario might be that it violates the conservation of energy. Well, as I see it, this would not be the case if the particle is only devoting a fraction of its energies to each location.)

It's not only possible, it's routine. Perhaps the easiest to understand example would be light passing through a holograph plate. Where a photon will end up is statistically related to a region on the plate where the photon might have been.

3. As well as collapsing, can wave functions "uncollapse"?

Sort of. I think I should leave this question up to a real physicist, but I'll try it anyway. There are delayed erasure experiments which might make you think that the wave function somehow got undone - and I don't believe the "collapse" mechanism is fully understood. I think the problem is that "uncollapse" suggests that "collapse" occurs neatly over a span of time so couldn't the clock be turned backwards - but at its core, I don't think it follows such a neat time line.

4. I have a hunch (which might be wrong) that it may be better to think of wave functions as continuously evolving in response to particle interactions as opposed to fully "collapsing" and/or "uncollapsing". Might there be some element of truth in my hunch?

Yes.

 

[tim1608]

Hi .Scott

Thank you very much for your reply. I relation to Question 2, since I wrote the original post in this thread, I have read about "Bose-Einstein Condensates". Apparently, BECs are single particles in multiple locations simultaneously but they have only ever been created in extremely cold environments. Do you think that there could ever exist a scenario for some sort of room-temperature equivalent of a BEC?

Thank you very much

 

[.Scott]

Do you think that there could ever exist a scenario for some sort of room-temperature equivalent of a BEC?

Perhaps with magnons.
But I wouldn't hold my breath.

 

[WannabeNewton]

Apparently, BECs are single particles in multiple locations simultaneously ...

Where in the world did you read that?

 

[hilbert2]

Perhaps with magnons.
But I wouldn't hold my breath.

Actually, that has been done already: http://www.nature.com/nature/journal/v443/n7110/full/nature05117.html .

 

[tim1608]

Where in the world did you read that?

Jeffrey Satinover said it in the "What the Bleep" movie.

(Please note that my use of Jeffrey Satinover as a source in this post does not indicate agreement with is views on homosexuality. Unlike him, I do not believe that homosexuality is a defect. I do however believe that homophobia is a defect.)

 

[Cthugha]

Jeffrey Satinover said it in the "What the Bleep" movie.

That movie is considered pseudoscience. At best it is misrepresenting science. At worst it is simply nonsense. Please do not try to understand physics based on that fringe movie.

 

[bhobba]

Jeffrey Satinover said it in the "What the Bleep" movie.

That movie is utter junk - forget about it.

At the level you are I suggest the following - the Kindle versions are dirt cheap - but true (well mainly anyway - they do have a few very minor issues no need to go into here):
https://www.amazon.com/dp/B004ULVG9O/?tag=pfamazon01-20
https://www.amazon.com/dp/B008ABSSIW/?tag=pfamazon01-20

After reading those, and you are interested in going further, do another post and myself, and others, will be only too happy to provide further suggestions.

Thanks
Bill

 

[kurros]

.Scott answered the basic questions pretty well, but I wanted to add the following:

3. As well as collapsing, can wave functions "uncollapse"?

4. I have a hunch (which might be wrong) that it may be better to think of wave functions as continuously evolving in response to particle interactions as opposed to fully "collapsing" and/or "uncollapsing". Might there be some element of truth in my hunch?

Yeah these days a more "full" study of what "wavefunction collapse" really is takes place in decoherence theory. https://en.wikipedia.org/wiki/Decoherence

The general idea is that wavefunction collapse is related to the irreversible information loss that occurs as some small, coherent, quantum system interacts with a larger system ("the environment", or the measuring apparatus). I have not studied it in detail myself, but my quantum optics friends assure me that this is the correct way to think about things.

So, then, "uncollapse" is not really possible because there is an entropy barrier to it happening. Like how it is "theoretically" possible for a smashed glass to un-smash itself, but in the real world it is never going to happen.

 

[bhobba]

The general idea is that wavefunction collapse is related to the irreversible information loss that occurs as some small, coherent, quantum system interacts with a larger system ("the environment", or the measuring apparatus). I have not studied it in detail myself, but my quantum optics friends assure me that this is the correct way to think about things.

Your friends are correct.

THE book to get on it is Decoherence and the Quantum to Classical Transition by Schlosshauer:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

Note - decoherence does not solve the measurement problem - but morphs it into something else. It explains (without going into exactly what the terms mean - its tied up with proper and improper mixtures) apparent collapse, but not actual collapse.

If anyone's interested the following gives a good account:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Personally I subscribe to the ignorance ensemble interpretation which solves the issue in a very simple way - I interpret an improper mixture as a proper one. Of course the issue with that is, how exactly does nature accomplish this wonderful feat - ah well - science always has issues - you explain one axiom by reducing it to others - but then they need explaining.

Thanks
Bill

 

[DrClaude]

Jeffrey Satinover said it in the "What the Bleep" movie.

Note that discussing that movie is off-limits for PF.

https://www.physicsforums.com/showpost.php?p=3929007&postcount=2