In a different thread, a user stated the following about events preceding or following other events:
However, if the two events are causally connected ("event A causes event B"), the causal order is preserved (i.e. "event A precedes event B") in all frames of reference.
My question is what does "causally connected" really mean? What does "causes" mean? Further, given that we know that we can have instantaneous effects in typical quantum processes (e.g. flip the polarizer, effect on another a reading light years away, even though we cannot transmit useful information with this), does that not constitute "causing" for the purpose of this statement?
Answer
Further, given that we know that we can have instantaneous effects in typical quantum processes (e.g. flip the polarizer, effect on another a reading light years away, even though we cannot transmit useful information with this), does that not constitute "causing" for the purpose of this statement?
Correct, that does not constitute "causing" in this context. For the same reason I wouldn't call this sort of thing an "effect." It's really just a correlation.
In general, causality is just what you'd think it is: if event A causes event B, that means the physical state at (location,time) B has some logical/mathematical dependency on the physical state at (location,time) A, such that what happens at A influences what happens at B. Just how that dependency is expressed depends on which physical theory you're using. In classical mechanics, the dependency exists if a particle is able to travel from A to B. In quantum field theory, the dependency exists if quantum operators at A and B don't commute with each other. And so on. The exact mathematical expressions can get a bit technical if you're not familiar with the theories, but they're all expressing that same underlying idea.
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