I'm a complete newbie to Quantum Theory, but I want to know more, so I've been watching few YouTube videos (an example below).
https://www.youtube.com/watch?v=ZuvK-od647c
All videos I've watched explain, that when an entangled particle has its spin measured, it will instantaneously communicate its measurement with its entangled partner, so that when it too is measured in the same direction, it will have the opposite spin.
Often these videos explain why these particles must be communicating with each other, rather than containing "hidden information" (Bell's theorem), but do not delve into why it must be instant.
Answer
There are three kinds of possibilities that can explain the COMBINATION of two types of below observations relating to entanglement.
Correlation of two particles of any single entangled pair. Perfect anti correlation (opposite spins when measured in same direction) is a specific case of this, which is most commonly sighted
Statistical correlation between measurement outcomes of numerous pairs when measured at any angles. 1.) can also be considered a special case of 2.)
In order to explain above combination of observations, three kinds of possibilities are there. These are just possibilities, no one knows how actually the correlations form.
1) There is an active link between the two particles of entangled pair that is able to signal at FTL, thus measurement of one particle influences the state of other and then it is easy to expect the observed statistical correlation. This possibility is the one you are talking about and it is also most commonly assumed by most public when they try to explain/understand the correlations.
By signal, here I mean the means by which the entanglement is supposedly collapsed on measurement of first particle. This signal can not be used/detected by any observer for any/information/communication purpose. It just a speculation on how the entanglement may be collapsing. see no-communication theorem on wiki.
2) Reality/Locality is not as we understand it. Meaning, two particles, being spatially far off can still be considered at same location in quantum sense. This allows measurement of one particle to influence that of the other without violating light speed and then it is easy to expect the observed statistical correlation. This appears to be the line taken by most main stream scientists.
3) There is a yet to be discovered classical mechanism that forms the statistical correlations over duration of the experiment. Local Hidden Variables alone is not sufficient to support such mechanism and that has been proven by Bell's inequality and supported by experimental outcomes.
In order for such a mechanism to be at work, it has to involve "Local Hidden Variables" PLUS some kind of global memory/accumulation/balancing/synchronization. Global means that the the natural environment in the vicinity of the experiment accumulates information about creation and measurement outcomes of previous pairs and steers the creation and measurement outcomes of subsequent pairs in such a way that it balances out per quantum mechanics, over large number of pairs.
I have not seen enough literature which would convince that this possibility (3) has completely, critically and honestly been ruled out. Most literature mentions Bell's inequality that only disproves local hidden variables.
Some people also speculate many worlds, which in my opinion is even more weird.
Some people also try to explain it in terms of random probability just like toss of coin eventually turns out ~50/50 percent heads and tails. This explanation becomes weak when you try to explain both kinds of observations listed in the beginning of this answer. You can not explain perfect anticorrelation as random probability. For some states, perfect anticorrelation is guaranteed. There are no guarantees in randomness.
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