# double-slit experiment showing faster than light information transfer?

In the figure shown below, a laser sends a continuous stream of a large (almost infinite) number of photons per second at a double slit, and the photons are then are displayed on a screen.

The upper slit is A and the lower slit is B. C is a point on the screen exactly opposite B. The distance between the slits and the screen is adjusted such that at C, there will be a bright pattern if the A slot was closed (ie clump pattern) and dark band (from an interference pattern) if both A and B are open. A, B, and C form a right triangle.

A and B are initially open and we see an interference pattern on the screen therefore C is dark. At a specific time we now close the slit at A. The very next photon (which is just in front of the slits) then will have its wave function collapse instantaneously and will only go through B. This photon will take some time to arrive at C. The time it takes is the distance from B to C divided by the speed of light. But since the BC distance is shorter than the AC distance, the information that the slit at A was closed arrives faster than light speed which should normally take distance AC divided by the speed of light. How am I wrong here?

• It is not clear if some kind of signal that does not allow to send infomation travels faster than light, like in the bohm model , but it looks like this experiment could give some answers to what you are asking: en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
– user65081
Jan 20, 2019 at 0:31
• You may also like this paper It is as if Anton Zeilinger would rather not go there, but I think it a pretty conclusive observation of non locality. Jan 21, 2019 at 2:16