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I may be flat-out wrong here, but it seems to me there is a very common misunderstanding of what the double slit experiment results mean, at least by non-physicists, because of the way it is often explained.

The experiment shows that photons behave differently, as particle or waves, depending on how we choose to observe them. The implication is that reality depends on the observer, that somehow the point of view defines the reality. I think this is a widely held misinterpretation of these results (at least by non-physicists), which is almost never clarified when explaining the double slit experiment.

To avoid this confusion I think it is crucial to always clarify that what makes the photon behave as a particle rather than a wave is that it interacted with something else, regardless of being observed or not. Observation requires interaction, so it is true that observation results in particle-like behavior. However, interaction does not require observation. The photon could have interacted with anything else and it would have behaved as a particle, even if no human ever observed it. So it's not really saying that reality depends on the observer, its just that when quantum waves interact with other systems, they behave as particles. When seen this way, it's not nearly as mind-bending.

Am I wrong?

Thanks!

MajorChipHazard
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  • What particle-like behaviour are you referring to in the double slit experiment? – ProfRob Jul 08 '20 at 22:30
  • @RobJeffries If one observes the photons when they hit the detector, you see an interference pattern, meaning they behave as waves as they pass the slits. However if one observes the photon before the slit to determine through which slit the photon goes through, it generates two lines in the detector instead of an interference pattern, meaning it passed the slits as a particle. – MajorChipHazard Jul 08 '20 at 22:41
  • You might find this interesting if you haven't read it. – Charlie Jul 08 '20 at 22:46
  • Oh, I thought you meant the usual example of one photon at a time in the apparatus generating an interference pattern, but if you check, then each photon goes through one of the slits. – ProfRob Jul 08 '20 at 23:02
  • Photons do not “collapse” into particles. They are detected or not (binary outcome). – ZeroTheHero Jul 08 '20 at 23:41
  • There are multiple interpretations of what the double slit experimental results mean. In my opinion, physicists have been confused about the "proper" interpretation of these results for approximately 100 years now. After thinking about it for a number of years, and after reading about the results from the delayed quantum eraser experiments, it is my opinion that all of the experiments are telling physicists that there is a law of conservation of information involved, but this law hasn't been formally codified yet. Next ... – David White Jul 09 '20 at 02:53
  • In other words, if you know which slit a photon went through, no matter how you design your experiment, the results indicate that you knew which slit the photon went through. If you don't know which slit the photon went through, no matter how you design your experiment, the results tell you that you don't know which slit the photon went through. ALL double slit experiments to date have always given those results. – David White Jul 09 '20 at 02:55
  • @DavidWhite Yes, but my point is it is very important to add to your explanation that it's not really dependent of 'what we know', but 'what we could theoretically know' about the photon. If the photon left any evidence at all that it went through one slit (even if we never see that evidence), it will behave as if it went through that slit. This point is critical to understand it. So the fundamental reality is not 'what is', or 'what we know' but 'what is theoretically knowable'. – MajorChipHazard Jul 09 '20 at 04:36
  • For more on how a photon is sort of like a particle and sort of like a wave, see my answer to How can a red light photon be different from a blue light photon? – mmesser314 Jul 09 '20 at 04:37
  • @MajorChipHazard, note that my comments are not based on what we could theoretically know ... they were based on what we have actually measured. – David White Jul 09 '20 at 17:06
  • @DavidWhite In that case, I would argue your interpretation is wrong, it's falling into the misconception I'm trying to clarify. What we know is irrelevant, it does not affect the results. What affects the results is what evidence there exists, even if we never get to see that evidence.

    If the photon were to interact with another photon right before the slits, it would pass the slits as a particle instead of as a wave, even if we never measure it. That's because its interaction with the other photon left evidence of which slit it went through, even if we never measure that evidence.

     – MajorChipHazard Jul 09 '20 at 21:59
  • @MajorChipHazard, the interference pattern exists even when one photon at a time goes through the slits, as has been demonstrated by experiment. – David White Jul 10 '20 at 18:11
  • @MajorChipHazard Photons do not interact with each other, this a misconception from the time of Huygens (400 years ago?). Photons do not interfere, they each act on their own and choose their own path thru the slits to the screen. The path they choose is based on probability (QM), Feynman described this process well with the "path integral" theory (60 years ago) and single photon experiments were conducted 40 years ago ... but they are not popularized because the Huygens version is so prevalent (and works well). Highest probability paths are integer multiples of wavelength per Feynman. – PhysicsDave Jul 10 '20 at 19:15

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Almost no physicists, or scientists for that matter, believe that "reality is somehow linked to the concept of consciousness". That is assuming "there exists an objective reality". Some philosophers doubt this, but in my opinion that just means you claim that the word "reality" has no useful meaning, in which case the whole discussion is pointless.

In a way, the way you explain away your alleged misinterpretation of QM is correct. However, in the modern formulations of the theory (quantum field theory) the basic objects are quantum fields. The concept of a particle in that setting is emergent and approximate, valid only in certain limits (e.g. non-relativistic limit or scattering limit).

On the other hand, in the modern view of non-relativistic quantum mechanics (which is not a universal theory but still useful and valid for a wide range of phenomena), there are particles which have associated wave functions. The idea that "in certain circumstances the same object can be a wave or a particle" is not a claim of physics. It just means that "the most simple physical description of the phenomena involving that one object may use a particle picture or a wave picture depending on circumstances". This does not meen that we do not have a theoretical framework valid in all circumstances. We do! In fact, both quantum field theory, as well as standard non-relativistic quantum mechanics, both do not need to resort to a fundamental distinction between particle and wave behavior.

Adomas Baliuka
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