3 ms·

If A and B are entangled, and you observe B, would there be any way of knowing B was observed from observing A? If yes, that's binary communication. If no, the

• cortic 4y ago
  correction; If no, then how do we know A changed at all (when we observe B)?
  • l33tman 4y ago
    You don't, until you meet up and compare notes.
    • cortic 4y ago
      Okay, so how do we know that A wasn't like that all along? edit; Been trying to read up on this, i think i get it; Its a random outcome whether B has been looked at or not, but when you check them later, they are opposite pairs randomly distributed.
      • l33tman 4y ago
        Yes exactly. Nothing "changes" at the other end (although, there has been arguments regarding the Wigner's Friend thought experiments about this... see Guerin 2021) Just be careful when reading up on this about a subtlety here - as long as both A and B measure the same property, say "spin along Y axis", a measurement they will always both measure the opposite of, the experiment will seem just like the "unopened envelopes" popular description of entanglement which is not really magical. It's annoying that this particular description keeps being made.. The interesting things happen when you start mixing up the measurements so A and B measure different, but correlated, properties. In the context of Bell's tests, they will not measure in the exact same basis (like vertical polarization), one of them will rotate the basis slightly, say 30 or 60 degrees. And the resulting correlations between A and B in this scenario can't be explained by hidden variables ("unopened envelopes describing the result of all possible measurements") but is predicted correctly by quantum mechanics.