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When particles are generated or interact in such a way that they cannot act independently, what connects them, especially over large distances... What causes a particle that's lightyears away to theoretically change instantaneously, for example, to state of down spin as soon as it's relative particle is in a state of up spin?

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closed as unclear what you're asking by tparker, John Rennie, Yashas, Jon Custer, Bill N Mar 16 '17 at 15:07

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  • $\begingroup$ A single (entangled) state describes the system of two particles. If you make an observation all of whose eigenstates are unentangled, then the state collapses into one of those eigenstates, ending the entanglement. So the answer to "what causes the instantaneous change?" is "a well chosen observation". Of course there's more to be said about what's really going on with wave function collapse, but that's no different for entangled states than for unentangled states, and no different for two-particle systems than for one-particle systems. $\endgroup$ – WillO Mar 16 '17 at 5:45
  • $\begingroup$ @WillO but what collapses, what is the entaglement? $\endgroup$ – A-Aron Campbell Mar 16 '17 at 5:48
  • $\begingroup$ "Entanglement" means the condition of being entangled. Two particles with state spaces $H_1$ and $H_2$ are said to be entangled if their joint state (in $H_1\otimes H_2$) is not of the form $h_1\otimes h_2$. $\endgroup$ – WillO Mar 16 '17 at 5:50
  • $\begingroup$ @WillO so am I just really over thinking this? $\endgroup$ – A-Aron Campbell Mar 16 '17 at 5:52
  • $\begingroup$ I'm afraid I don't have enough information to know whether you're overthinking or underthinking. :) The above is all there is to be said. $\endgroup$ – WillO Mar 16 '17 at 5:54
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"Cause" isn't (can't be) the right word. The measurement outcomes are $100\%$ correlated, but that doesn't mean one caused the other (no more than the spring blooming of flowers in New York State followed several weeks later by the monsoon season in India means that the first caused the second). For entangled particles separated by several light-years, not causal is even more evident: call the two measurement events $m_1$ and $m_2$, and then one observer may see $m_1$ occurring before $m_2$ while a second observer (in a different inertial frame) may see $m_2$ occurring before $m_1$. So there's no unambiguous way to even say which one came first, the $m_1=\mbox{chicken}$ or the $m_2=\mbox{egg}$. So how can you say one "caused" the other when you can't even say one occurred before the other?

The only thing you can unambiguously say is that the single preparation event that prepared both particles in a single entangled state occurred before both subsequent measurements (it's in both their past light cones). And it's that preparation event which "caused" the subsequent measurement correlation (just like it's the Sun/seasons that caused both the blooming flowers and the monsoon season). That explanation may not seem $100\%$ intuitively satisfying, i.e., it may seem like there's still something left unexplained, but that must necessarily be a problem with our intuition (developed by observing countless classical phenomena).

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