Entangled particles react instantly across distance, is the universe non-local?

[Max_42]

Two particles that become entangled share a correlated quantum state such that measuring one instantly determines certain properties of the other, no matter how far apart the two particles happen to be, a light year or more if you like. Einstein famously called this spooky action at a distance, and was deeply uncomfortable with what it seemed to directly imply about the fundamental structure of physical reality itself.

The naive reading of entanglement suggests some kind of instantaneous influence traveling between the two particles faster than the speed of light, which would appear to directly violate special relativity's strict speed limit on any and all causal influence. Physicists have worked hard over the decades to clarify that entanglement specifically does not allow for faster than light communication of usable information, since the actual measurement outcomes on each individual side remain genuinely random until the two separate results are later compared, so no meaningful message can actually be sent this way in practice.

But the correlations themselves are still real, are still experimentally measured with extraordinary precision, and are still stronger than any classical, purely local theory can fully account for or explain, a fact definitively established by Bell's theorem and the long, careful, increasingly rigorous experimental tradition that has directly tested and confirmed it over many decades since. Something about the underlying structure of reality really does appear to be non-local in some genuine, physically meaningful sense, even if that specific non-locality can never be directly harnessed to send an actual usable message faster than light.

What this does to our ordinary, everyday notion of cause and effect is genuinely subtle and easy to state either too strongly or too weakly depending on how carefully you phrase it. It does not mean one measurement event straightforwardly causes the other in the standard, familiar sense of one clearly preceding the other and directly bringing it about through some identifiable physical mechanism. It does mean that the correlation between the two separated measurement outcomes cannot be fully explained away by any purely local story, one where each particle simply carried its own pre-determined, independent local properties the entire time.

Some physicists conclude from this that locality itself, the deeply intuitive idea that distant events genuinely require an intervening physical mechanism to meaningfully influence each other, simply is not a fundamental feature of physical reality at the deepest level, however natural and intuitive it continues to feel to us at the ordinary macroscopic scale we actually inhabit and evolved within. Others hold out for some deeper, currently unknown theory that could eventually restore locality in some more subtle, currently unanticipated form we simply have not thought of yet. Either way, entanglement is one of the very few places in modern physics where the actual data itself has directly and forcibly overturned a deeply held prior philosophical intuition, rather than merely refining or extending it.