What does quantum entanglement mean for causality?
Due to entanglement, there can be spacelike separated measurements such that there exists a reference frame > where it looks like measurement A precedes and has a causal influence on the outcomes of measurement B, and > also a reference frame where it looks like measurement B precedes and has a causal influence on the outcomes of measurement A.
“Causality” is already a somewhat fraught notion in fundamental physics irrespective of quantum mechanics; it’s not clear that one needs to have some sort of notion of causality in order to do physics, nor that the universe necessarily obeys some underlying causal law. To the extent that quantum mechanics breaks our common-sense notions of causality, it’s only in this very particular sense (where it seems like Alice measuring first “causes” Bob’s measurement to take a certain value, or vice versa), and since neither party can use a measurement scheme like this to send information, the breakage doesn’t invite paradoxes or any sort of other weirdness.
Outside of philosophical musings about causality (which, to be clear, I think are perfectly valid and interesting) it suffices to say that entangled systems exhibit correlations without a common cause, and leave it at that.
“Causality” is already a somewhat fraught notion in fundamental physics irrespective of quantum mechanics; it’s not clear that one needs to have some sort of notion of causality in order to do physics, nor that the universe necessarily obeys some underlying causal law. To the extent that quantum mechanics breaks our common-sense notions of causality, it’s only in this very particular sense (where it seems like Alice measuring first “causes” Bob’s measurement to take a certain value, or vice versa), and since neither party can use a measurement scheme like this to send information, the breakage doesn’t invite paradoxes or any sort of other weirdness.
Outside of philosophical musings about causality (which, to be clear, I think are perfectly valid and interesting) it suffices to say that entangled systems exhibit correlations without a common cause, and leave it at that.
If you’re interested in a recent technical discussion of some of these ideas, I recommend the following paper: https://arxiv.org/pdf/2208.02721.pdf