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OK, here's a question from a non-physicist about the quantum eraser experiment. If my understanding is correct, photons and their entangled counterparts become a particle and create a non-interference pattern when at least one of the photons is detected during it's path through the double-slit and on to the film (destination). Even when an entangled photon does not pass a detector, and passes the slits and hits hits the film before it's partner has hit a detector, it behaves as a particle simply because we will have knowledge of the path that it's entangled partner took.

Supposedly, it is the fact that an observer has 'knowledge' of the entangled partner's path (or direct knowledge of the path, if we had it) that makes the photon behave as a particle.

My question is, how literal are physicists speaking when they talk about knowledge? In other words, the experiment is done by a physicist, who understands what is happening when a photon passes a detector before going through the slit. But what if my stupid brother-in-law, who can't even grasp basic geometry were to do the experiment with no one else around? Would the photons go back to producing an interference patter due to the failure of the experimenter to know what is happening?

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  • $\begingroup$ Photons don't have paths. $\endgroup$
    – CuriousOne
    Commented Jan 11, 2016 at 3:28

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I think that "knowledge" is a very poor word here. In answer to your last question, assuming the experiment was actually performed according to instructions, exactly the same result would be obtained in any experiment. The only relevant part of a physicist's actual knowledge is that such knowledge is generally required to design the experiment (and see why it has to be done) set up the equipment, and operate it properly. However, the objects within the experiment behave totally independently of the physicist's thought processes. You cannot get the result you want by thinking about what you want.

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My impression from the wording of your question is that you don't have a clear picture of what the laboratory experiment is. The motion graphic at http://chesters.org/marvin/eraserPost/ might help.

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    $\begingroup$ That's a nicely done site $\endgroup$
    – Punk_Physicist
    Commented Jan 11, 2016 at 3:22
  • $\begingroup$ Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. $\endgroup$
    – Daniel Griscom
    Commented Jan 11, 2016 at 4:13

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