Along the lines of what Larry asks: Obviously the angles are not going to be perfect. The two photons will come in with slightly different angles. So you would think the photons will not be perfectly indistinguishable. Now I wonder if it is the case that if you put your detectors in close, so that they “see” a relatively wide range of angles, then you get the interference and the photons are treated the same; whereas if you put your detectors far away, they might become sensitive to a smaller range of angles, so that they could distinguish the two photons, then the interference might go away?
But in that case, you could make an FTL signaling device by putting one detector at a distance, and moving the other detector from close to far. When close, you get interference and get two photons or none, while when far, you get single photons, a difference detectable by the remote detector. Clearly this can’t happen.
I’m sure Jeff is right and that a fuller investigation of the wave equations would explain exactly what happens here. But it does point out one big problem with this level of description of the quantum world: the absence of a primary role for space and time. We just have events and configurations. How does locality and causality enter into this? Where do speed of light limitations get enforced? The world is fundamentally local, but some ways of expressing QM seem to ignore that seemingly important foundation.
With the setup you describe, the remote detector still has to wait until the photons would be expected to arrive to know whether the other detector had been moved, right? This seems like it would be exactly at-light-speed signaling.
Along the lines of what Larry asks: Obviously the angles are not going to be perfect. The two photons will come in with slightly different angles. So you would think the photons will not be perfectly indistinguishable. Now I wonder if it is the case that if you put your detectors in close, so that they “see” a relatively wide range of angles, then you get the interference and the photons are treated the same; whereas if you put your detectors far away, they might become sensitive to a smaller range of angles, so that they could distinguish the two photons, then the interference might go away?
But in that case, you could make an FTL signaling device by putting one detector at a distance, and moving the other detector from close to far. When close, you get interference and get two photons or none, while when far, you get single photons, a difference detectable by the remote detector. Clearly this can’t happen.
I’m sure Jeff is right and that a fuller investigation of the wave equations would explain exactly what happens here. But it does point out one big problem with this level of description of the quantum world: the absence of a primary role for space and time. We just have events and configurations. How does locality and causality enter into this? Where do speed of light limitations get enforced? The world is fundamentally local, but some ways of expressing QM seem to ignore that seemingly important foundation.
With the setup you describe, the remote detector still has to wait until the photons would be expected to arrive to know whether the other detector had been moved, right? This seems like it would be exactly at-light-speed signaling.