Discerning Between QM Interpretations

Discussion in 'Physics & Math' started by RJBeery, Nov 11, 2016.

  1. RJBeery Natural Philosopher Valued Senior Member

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    Yes. I want to send a photon which we consider to be in a mixed state through a polarizer rigged to detect torque. Subjecting a photon to a polarizer is of course a measurement, but the presence of torque implies that a pure state existed prior to such measurement...unless you disagree on this point, perhaps? It's difficult for me to see an argument against it.

    The only way Copenhagen and MWI would plausibly predict torque is if they considered the emitter to be part of the wavefunction, as you mentioned, or if they abandon conservation of momentum. If the former, then the wavefunction never has a physical manifestation because the future act of measurement makes the photon have a pure state at the time of emission which, ironically, contradicts the basic premises of these interpretations.
     
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  3. RJBeery Natural Philosopher Valued Senior Member

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    Something else had been bothering me, Fednis, and I identified it as "timelike entanglement". While timelike entanglement has apparently been discussed as of late, does it officially exist in QM theory? I did find a few links about it but frankly I think any sort of acceptance of timelike entanglement would be, in all respects, identical to interpretations which rely on retrocausality and, as I said, contradictory to the premises of wavefunction collapse. If we accept that the entire Universe can be represented as an enormous quantum system then timelike entanglement would necessitate that we permanently reside in a 4-D Block Universe, devoid of change or options.

    http://phys.org/news/2011-01-physicists-method-timelike-entanglement.html
     
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  5. Fednis48 Registered Senior Member

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    I think I agree that Copenhagen and MWI wouldn't predict a torque, but I don't see why any other interpretation would, either. Talking about the emitter, one would expect a torque because generating a photon produces angular momentum, and total angular momentum must be conserved. But measurement of angular momentum does not inherently change angular momentum, so it need not be accompanied by any torque on the detector. Note that in a quantum superposition, conservation of momentum does not require summing over all the superposed states; total momentum is conserved in each state independently. As a corollary, collapsing the superposition down to just one state does not "change" its momentum in a strict sense.

    Interesting stuff! Honestly, this reminds me of the whole "time crystals" thing that was raising eyebrows a couple years ago, or the "directly measuring wavefunctions" stuff that's still finding publication. Researchers getting too deep into the math and offering some paradigm-shifting explanation for a phenomenon that's actually pretty mundane. In this case, I'd guess they're just performing delayed quantum teleportation. But hey, without really out-there ideas we never would have discovered quantum mechanics at all, so more power to them!
     
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  7. Confused2 Registered Senior Member

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    Aren't quarter wave plates lossless? If lossless then no torque?
     
  8. arfa brane call me arf Valued Senior Member

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    I'm going to make a plug for the informational aspects of interpreting quantum mechanics.
    It actually makes sense to consider entanglement as a computational resource, like memory. Entanglement means particles interacting with each other, with each other's charge, and with each other's angular momentum. So emission of photons is an interaction between the photon and the electron or other particle emitting it. So there's some information "stored" in there.

    But reading this information means losing half of it, apparently. Quantum measurements output exactly half the expected information, like addressing a lossy memory from an informational point of view. Actually I should say classical measurements of quantum information, whatever that is.
    That you can design quantum gates and circuits is a kind of proof of the entanglement as resource interpretation.

    I read somewhere that a program had been written that could "think" quantum-logically, and it churned out some designs for experiments that the designers believed they wouldn't have managed without the software, because we just don't think quantum-logically.

    What the QC angle says is any quantum experiment can be considered as a quantum circuit, with quantum gates. If the quantum world is the real one then so can any experiment, although it may not be "useful" to do.

    BTW torque is something that appears after the quantum interactions, it's a classical measurement so it destroys half the information you want to know about--it's gone once the measurement is made, as if it never existed, and in fact, it didn't.

    Actually, that's a rather eccentric way of saying a wavefunction has no physical significance (you can't measure a wavefunction).
     
    Last edited: Dec 3, 2016
  9. RJBeery Natural Philosopher Valued Senior Member

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    I have a couple thoughts on this. First of all, a quarter wave plate must experience torque. The reason is because we know that circularly polarized light imparts torque when it is absorbed:

    http://advances.sciencemag.org/content/2/9/e1600485.

    If we turn a linearly polarized photon (with presumably zero torque) into a circularly polarized one (with proven torque) then we conclude it came from the quarter wave plate.

    Secondly, I think in terms of analogies and I believe that is useful here. A free-floating experimenter in deep space is holding a basketball. If he wants to spin the ball he will induce a counter-torque on himself; if he merely wants to rotate the ball through some number of radians he will still (temporarily) induce a counter-torque on himself. There are no actions the experimenter can perform on the ball which have no counter-action on himself...

    Now, regarding quantum systems: it is my belief that no measurement is possible on a quantum system which does not disturb the system itself. For example, I've read that Maxwell's Demon might actually work if the demon were able to have knowledge of the configuration of the various particles without disturbing them...it just so happens that such knowledge is impossible. This would mean that all measurements on a quantum system are also actions on that system which would produce counter-actions. Whether we are "rotating the ball" (changing the polarization angle of a photon) or "spinning the ball" (converting it to circular polarization) we should expect a reaction.

    Addtionally, from an Entropy/Information Theory perspective we know that gaining information on a system always has a cost of some sort. (See here: https://en.wikipedia.org/wiki/Entro...and_information_theory#Landauer.27s_principle)
    Put simply, my intuition tells me that we cannot measure a system and certainly can't change a system (as we do with a polarizer) free of charge. This isn't just navel-gazing though, I suggest a way to check whether this phenomenon exists in the paper: set the photon to a known angle of polarization and then check for torque as we put it through a second angle. If such torque is detectable then we can apply this fact in our EPR experiment to draw a distinction between QM interpretations.
     
  10. Dinosaur Rational Skeptic Valued Senior Member

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    It has always interested me that many (including some with serious credentials) advocate/believe the many worlds interpretation.

    It is the easiest to understand, but is silly if you consider the implications. Every possible result of a quantum level process spawns a new universe.

    This implies billions of new universes per second due to events in many small volumes of space. Each new universe spawns billion more in the next second.​

    Those who accept/believe/advocate this interpretation are seduced by its being so easy to understand.

    I wonder how many believers consider the implications.

    BTW: I favor Bohr's interpretation.
     
  11. karenmansker HSIRI Banned

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    Hi RJ! Good discussions here. I like your hypothetical model. I favor D. Bohm's interpretations (re: hidden variables - explicit v. implicit)
     
  12. RJBeery Natural Philosopher Valued Senior Member

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    MWI has many issues that bother me. It's easy to envision (and describe) the world "splitting in two" on a confined quantum system with two equally likely outcomes, but what about outcomes that aren't equally likely? What is the mechanism which would have us find ourselves in the correct worlds such that Born's Rule is exhibited? And we say the "world splits"...does this "split" travel at the speed of light? From which frame? And what about conservation of energy? I would apparently have the ability to create an entire universe by flipping a quantum coin...

    Nothing would give me greater pleasure than to disprove MWI and Copenhagen. That being said I would find it fascinating to disprove the retrocausal interpretations as well. I just need to finish fielding objections and find a Uni lab to do it. Advice is welcome on this, I know nothing of the QM physics lab experimenting..!

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  13. Confused2 Registered Senior Member

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    In the light of the Heisenberg Uncertainty Principle ( https://en.wikipedia.org/wiki/Uncertainty_principle ) I have the feeling that an exact angular momentum leaves a total unknown out there waiting to strike - am I right/wrong and if correct what is the thing waiting to strike? If it strikes does it have any significance to the OP?
     
  14. RJBeery Natural Philosopher Valued Senior Member

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    I've never given much thought to commutation relation for angular momentum. I do have a very vague and half-baked personal theory on the cause for the non-commutation of certain operators in QM (and HUP)...and it's precisely because measurements at this scale affect the system.
     
  15. arfa brane call me arf Valued Senior Member

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    Have a read of the Einstein-Bohr debates, you look like you're trying to cover old ground
     
  16. RJBeery Natural Philosopher Valued Senior Member

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    I'd imagine you are exactly right.
     
  17. RJBeery Natural Philosopher Valued Senior Member

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    I thought of this over a month ago but didn't bother posting it...

    Consider the quarter-wave plate that Confused2 asked about. Light is initially polarized at theta and passes through a quarter-wave plate (QWP) that imparts angular momentum on the light (and compensating momentum on the QWP); a second, connected, QWP is positioned at a distance such that the circular polarization is returned to linearly polarized light, but now at an angle of theta+45 degrees. The "dual QWP" system has now experienced temporary angular momentum followed some time later by a counter-torque which brings it back to having none. This would result in the dual quarter-wave-plate system having been rotated a very small amount. This rotation would persist as long as light continues to shine through the system.

    This would also apply to a traditional linear filter as well but that isn't relevant, the nature of the filter isn't important; what's important is that the rotation occurs if the light has a definite polarization prior to measurement and does not occur otherwise.

    One objection may be that this dual QWP system would rotate regardless of the angle of polarization of the incoming light, and this is true. Let's design a second system, S, which takes light polarized at a specific angle theta and rotates it by 45 degrees clockwise using traditional polarizers. The simplest such system would be a single polarizer set at (theta + 45), and we would expect a "loss" of (1-cos(45)) = 29% of our light. If we add a second, interim, polarizer set to (theta + 22.5) we can reduce our loss of light to (1-(cos(22.5)*cos(22.5)) = 15%. In general, we can use n interim polarizers to reduce this loss of light to (1-(cos(45/n))^n). At the limit we would expect no theoretical loss in intensity...but only if the incoming light had the definite and expected polarization prior to measurement.
     
  18. RJBeery Natural Philosopher Valued Senior Member

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    I was thinking a bit more about this, and there may be a much simpler solution. If we could run the EPR experiment, with photons, many many times we could simply run the photons through a polarizing filter at the expected angle. If no loss of intensity is detected after passing through the filter then we conclude that the photons were in the expected angle prior to measurement.
     

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