I was reading the book The outer limits of reason by Nosin S. Yanofsky and I stumbled upon this experiment. There's a wall with two holes (S1 and S2) with a polarization filter on the right of the holes and a light source on the left of the hole. One of the polarization filters is horizontal and the other is vertical. After them, there's at distance d, and then a diagonal (45 degrees) polarization filter.

This whole experiment gives the same result as if there were no polarization filters. The 45 degrees filter can be moved. This has the (mind-blowing for me) effect of making the photons go back in superposition if the filter is moved in. This, says the author, is the proof that the photon knows (sorry for my layman terms but I'm no expert at all) that the 45 degrees polarization filter will be there before it is moved in. The author says that this is basically the end of free will, because the photon will know before you even do it that you are going to put the polarization filter there.

I reasoned about this by myself for some time and I have some questions. If I attach a computer with a robotic arm that moves the 45 degrees filter in and out depending on the random generation of a binary number (0 or 1), I must conclude that the photon knows the random number that will be generated, and that's ok. But what if the random number is generated from the observation of the superposition of another particle (I know that this can be done)? How will the photon behave? There are these 2 possibilities from what I understand.

  1. The photon knows the superposition before it's measured, which goes against the principle that "there's no property until you measure it" and therefore quantum mechanics is wrong (I believe this to be unlikely)
  2. I misunderstood something (very likely) and I'd like for someone to explain me what I didn't understand.

experiment setup


2 Answers 2


The trouble with interpreting the experiment as "the photon knows what will happen" is that it introduces the notion of time in a paradoxical context, from which one can derive paradoxes just like you did in your thought experiment.

The situation has indeed a non-local flavor that, if I wanted to give the photon some metaphorical cognition, I would frame as "the photon has a non-local awareness of the whole set-up". More soberly, we could just say "the photon behaves consistently with the set-up, and it is impossible to describe its evolution in a local way", which is just another way to say that there is no trajectory in QM.

So, yes indeed, attempting to describe a quantum superposition in classical terms leads to paradoxes. Nothing new here - welcome to quantum mechanics.

And one of its amazing aspects is that, although QM seems non-local, causality is somehow always preserved (you can see non-causal correlations, but have no way to abuse them in any practical way). When people explore this issue in detail, they do not conclude that causality is dead in the water, only that we cannot describe what is happening in a classical way - there has to be a quantum-specific causal structure, so to say; see Quantum Mischief Rewrites the Laws of Cause and Effect for an introduction.

So whatever paradox you may consider, it will never unambiguously lead to the conclusion that something is broken within physics itself. It is only our classical intuition that is badly shaken, again and again.

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    $\begingroup$ Thank you for your answer. But I'm still confused about what would happen if the experiment was modified as I said, do you have some suggestions about that? $\endgroup$ Commented Jul 3, 2022 at 19:45
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    $\begingroup$ Well you propose a specific way to get a random value deciding when to install the filter - as such this does not change anything to the base experiment, which is not concerned by that decision process. Including the decision process and having it of a quantum nature just complexify the overall entanglement by superposing yet another system to the base experiment. This is no problem in QM, it just makes the overall thing even harder to grok. I do not believe it would demonstrate any inconsistency in the principles of QM. $\endgroup$ Commented Jul 3, 2022 at 21:09
  • $\begingroup$ Thank you very much $\endgroup$ Commented Jul 3, 2022 at 22:26
  • $\begingroup$ @EmanueleUngaro Richard Feynman used the path integral to explain why photons travel/prefer certain paths, these paths are of higher probability. In the DSE there are no photons in the dark bands, only the bright bands have photons. There are many forces acting on the excited electron before it even releases the photon, the EM field is very dynamic. These forces influence the photon path ... thus the photon knows nothing but the EM field knows everything! $\endgroup$ Commented Jul 4, 2022 at 10:58
  • $\begingroup$ @PhysicsDave So it also knows what the random number generated from the observation of a particle in super position is? $\endgroup$ Commented Jul 13, 2022 at 21:54

The photons know nothing and it has nothing to do with free will or observations. There is a simple and physical explanation as to what is happening. (1) S1 and S2 produce coherent light sources, which are required to create an interference pattern. (2) When you introduce the vertical and horizontal filters, the two sources become 180 degrees out of phase and are no longer coherent. this disrupts the interference pattern. (3) when you introduce the large diagonal filter, the two incoherent sources become coherent again. There is no mystery here.

  • $\begingroup$ Could the down voter at least at explain their disagreement or confusion? $\endgroup$ Commented Jul 4, 2022 at 18:11

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