What Do Bell Tests Reveal About Photon Polarization and Reality?

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Discussion Overview

The discussion revolves around the implications of Bell tests on photon polarization and the nature of reality. Participants explore the interpretations of quantum mechanics related to photon behavior during measurements, the concept of superdeterminism, and the ontological questions surrounding the existence of polarization prior to measurement.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory
  • Technical explanation

Main Points Raised

  • Some participants question the assumption that a photon decides its polarization at the moment of detection, suggesting instead that it may have a predetermined polarization that is altered by the measurement process.
  • Others argue that the violation of Bell inequalities in experiments supports the idea that measurement outcomes are not predetermined, as any theory suggesting otherwise would predict compliance with these inequalities.
  • A participant introduces the concept of superdeterminism, suggesting it cannot be ruled out, although they find it a far-fetched notion compared to other interpretations.
  • Some participants propose that the wave function collapse indicates a lack of polarization before measurement, while others suggest that new research might indicate a form of non-local realism that exists prior to measurement.
  • There is a discussion about the implications of weak measurement experiments, with references to Kocsis et al. and their findings aligning with Bohmian mechanics, although the interpretation of these results remains contested.
  • Concerns are raised about the clarity of language used in discussing whether entangled photons possess definite polarizations independent of observation.

Areas of Agreement / Disagreement

Participants express differing views on whether photons have a predetermined polarization before measurement, and the discussion remains unresolved with multiple competing interpretations of the implications of Bell tests.

Contextual Notes

The discussion highlights limitations in current technology to definitively measure certain aspects of photon behavior and the dependence on interpretations of quantum mechanics, which remain a point of contention among participants.

gork
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So I've been reading a lot about Bell tests. I have a question that I can't seem to find an answer to. The tests indicate that a photon decides to be horizontally or vertically polarized at the moment of detection. What are they basing this on? To make such a counter-intuitive assumption seems like it would require a lot of data, but having read z dozen papers on Bell tests by Aspect and others, I don't see it. It seems more likely that the photon has a polarization to begin with but it changes due to the detection method, thus causing an artificial correlation. If this were true, it would mean reality does exist but we erase it when we try to detect it.
 
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gork said:
So I've been reading a lot about Bell tests. I have a question that I can't seem to find an answer to. The tests indicate that a photon decides to be horizontally or vertically polarized at the moment of detection. What are they basing this on?
It follows from the fact that the result of the long series of measurements doesn't satisfy the relevant Bell inequality. Every theory that says that the result of each polarization measurement was already decided before the measurement, predicts that the result of a long series of measurements will satisfy that Bell inequality.

Experiments agree with QM, even when QM says that Bell inequalities will be violated.
 
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But isn't it possible that the variables were decided beforehand and changed by the measurement? The observer would THINK that the measurement decided the variable and that nothing had previously been decided, but the results should be identical if the variables weren't undecided, but were changed by the measurement.
 
What do you mean by "changed by the measurement"? If you just mean that the state after the measurement (if the measurement doesn't destroy the particle) is different from the state before the measurement, that's not relevant here. Either the result (what the measuring device will display when the measurement is over) is determined before either of the particles begins to interact significantly with a measuring device, or it's not. If it is, then the results of a long sequence of measurements will satisfy Bell inequalities.
 
Maybe the OP is referring to a form of superdeterminism?
If so, then no, there is no test that can rule that out.
Although this form of "paranoid" determinism (which basically implies that nature is actively trying to "trick" us) is quite far fetched in my view; even compared to say MW.
 
The CI version of what is happening in a Bell test would say that a photon has no polarization before it is measured, at which point its wave function collapses, which is due to the photon interacting with the detector.

I'm suggesting that the photon has a polarization prior to being measured, but measuring it alters its polarization.

There's of course no way to prove this with our current technology. This is more of an ontological question that could perhaps be measured through weak measurement. Kocsis et al. recently used weak measurement to work out trajectories for single photons in a double-slit experiment, and they exactly followed the predictions of Bohmian mechanics.

QM suggests that photons do not have trajectories until they are measured. I'm suggesting that new research is pointing toward a non-local realism that does exist but we haven't previously been able to measure.
 
gork said:
The CI version of what is happening in a Bell test would say that a photon has no polarization before it is measured, at which point its wave function collapses, which is due to the photon interacting with the detector.

I'm suggesting that the photon has a polarization prior to being measured, but measuring it alters its polarization.
I still don't see if you mean that the result of each measurement is a certainty (unknown, but still determined by this "polarization prior to being measured") before the measurement begins, or that it's not.
 
gork said:
... Kocsis et al. recently used weak measurement to work out trajectories for single photons in a double-slit experiment, and they exactly followed the predictions of Bohmian mechanics.

QM suggests that photons do not have trajectories until they are measured. I'm suggesting that new research is pointing toward a non-local realism that does exist but we haven't previously been able to measure.

Do you have a reference? I couldn't find anything in the archives.

I have to say that your comment about research pointing to non-local realism is highly subject. My own reading is definitely towards the other direction.

Also, you seem to be pretty familiar with the background on Bell tests and yet you use some language which confuses me. When you say an entangled photon has a definite polarization, i.e. is realistic, do you mean to say that is independent of the act of observation? Does it have definite values for any possible measurement angle?
 
  • #10
gork said:
So I've been reading a lot about Bell tests. I have a question that I can't seem to find an answer to. The tests indicate that a photon decides to be horizontally or vertically polarized at the moment of detection. What are they basing this on? To make such a counter-intuitive assumption seems like it would require a lot of data, but having read z dozen papers on Bell tests by Aspect and others, I don't see it. It seems more likely that the photon has a polarization to begin with but it changes due to the detection method, thus causing an artificial correlation. If this were true, it would mean reality does exist but we erase it when we try to detect it.
Interesting proposal.
 
  • #11
gork said:
Kocsis et al. recently used weak measurement to work out trajectories for single photons in a double-slit experiment, and they exactly followed the predictions of Bohmian mechanics.

QM suggests that photons do not have trajectories until they are measured. I'm suggesting that new research is pointing toward a non-local realism that does exist but we haven't previously been able to measure.

I do not think the results point towards any interpretation. As they measured average trajectories, orthodox QM also predicts the same "trajectories" but interprets them differently.

Let me directly quote the paper: "Single-particle trajectories measured in this fashion reproduce those predicted by the Bohm–de Broglie interpretation of quantum mechanics (8), although the reconstruction is in no way dependent on a choice of interpretation."
 

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