Relativistic quantum mechanics and causality

[wawenspop]

You forget that measurements take time and are only approximately described by the highly idealized, instantaneous and complete measurements that figure in typical discussions of the foundations of quantum mechanics. The collapse is an idealization, too.

We could be talking about a photo that traveled billions of years in superposition 'spread' out over billions of square miles and at one location it is decohered, so what about the symmetrically same location a billion years away where it did not decohere? Seems there is a 'knowledge' at all locations where the photon 'could be'.

 

[A. Neumaier]

We could be talking about a photo that traveled billions of years in superposition 'spread' out over billions of square miles and at one location it is decohered, so what about the symmetrically same location a billion years away where it did not decohere? Seems there is a 'knowledge' at all locations where the photon 'could be'.

A wave knows its shape locally, so there is no problem. A local measurement at a delocalized state cannot collapse the whole state but only the part within the projection to the local area.

This is enough to make your questions absurd and unanswerable.

 

[Hawkwind]

Exactly. The propagators never get outside the lightcone, neither if you simulate
the propagation on a lattice, nor with the exact analytical solutions.
...

Are you sure ?
Afaik, the propagator of a massless particle has a pole on the lightcone, but summing up the contributions due to internal lines in a diagram (by integration over 4-momenta), there are also contributions from propagators connecting events with timelike or even spacelike distance.
Afaik, causality in quantum field theories is ensured by the additional demand, that the commutators/anticommutators of bosonic/fermionic field operators have to vanish at spacelike distances. This makes sure that events with a spacelike distance cannot influence each other.
Regards,
Hawkwind