If I understand "popular quantum mechanics" right, there is no faster-than-light communication via two entangled particles, because changing one of the particles (with the purpose to transfer a signal to the other one) breaks the entanglement.

My question: Why then Einstein opposed to quantum mechanics claiming that entanglement makes possible faster-than-light communication? Was the fact that entanglment breaks when one of the particles is "forced" to be changed unknown in the time of Einstein?

Or what do I misunderstand?

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    $\begingroup$ There are several ways to answer your question, which is also historic, focusing on Einstein. The by far best of these ways is to read the orignal paper ( Einstein, Podolsky, Rosen Phys. Rev. 47 (1935)). The less optimal ways are to ponder quotes like "God doesn't play dice" (which I believe is derived from a private letter of Einstein to Bohr). For the actual physics, the question has been asked many times, just take your pick from the list on the right. $\endgroup$ – Adomas Baliuka May 22 '17 at 20:08
  • $\begingroup$ Even if the entanglement breaks, the state of the second particle changes instantaneously. Thus there must be "something" that propagates faster than light. However it was demonstrated (I believe that after Einstein's remarks), that whatever is communicated, we can not use it to transfer information, so relativity is safe. $\endgroup$ – user126422 May 22 '17 at 21:19

Einstein claimed that "spooky action at a distance" broke relativity by allowing faster-than-light (FTL) communication. If two entangled particles (e.g electrons created as a pair) were separated by a large distance, and a measurement was made on one (like spin), the other particle would simultaneously and instantaneously also have its wavefunction collapse.

Imagine one electron was measured here on earth to be spin-down. This would mean that the other electron, possibly light-years away, would be forced into a spin-up configuration instantaneously. This seems to allow for FTL communication, since there is no delay between the collapses.

The popular resolution is that when we make the measurement on Earth, we cannot choose whether or not the electron is forced into a spin-down or a spin-up configuration; it is random. In addition, there is no way to tell if the other electron has already been measured or not. When we make our 'initial' measurement of spin-down, we don't know if this is due to some alien race already measuring their electron to be spin-up . Therefore, absolutely no information can possibly be transmitted in this fashion.

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It is not the information that we put in as a human. It is the information that the state of one particle was defined and that information must be transported superluminar to the other particle.

Atleast that's my understanding.

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    $\begingroup$ I have no idea what your answer means and I'd be very surprised if the OP does. $\endgroup$ – Adomas Baliuka May 22 '17 at 20:04
  • $\begingroup$ In the classical view information needs to be transmitted. The OP is right that the measurement "deletes" the entanglement, so no further information can be transmitted (e.g. use it to send binary code). But still there is the information of the measurement itself that needs to be transmitted and that may be what Einstein meant (if ever). $\endgroup$ – Leeen gold May 22 '17 at 20:47

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