I'm a lay physics enthusiast and I came up with a thought experiment that I can't fully wrap my head around:

Alice and Bob are worried about an impending attack by the dreaded Xenomorphs, so they set up an early warning system.

Bob stays on Earth where he sets up an interferometer, while Alice travels near to the Xenomorph home planet and sets up a monitoring outpost with her own interferometer and a particle detector. Exactly half way on her trip between Earth and the Outpost, Alice deploys an Entangled Particle Pair Generator (EPPG).

The EPPG which sits halfway between Alice & Bob's interferometers two entangled streams of particles (one aimed at Alice's outpost, the other at Earth). Their plan is that when Alice sees the Xenomorph fleet head off to Earth, she will turn on her detector which will 'collapse the wave form' of the particle stream entering her interferometer and destroy the interference pattern. It should also collapse the wave-form of the entangled particle stream at Bob's end and Bob should stop seeing an interference pattern almost immediately.

I know that this won't work. You can't transmit data faster than light and you can't transmit data through entanglement, but when I try to think about what would actually happen, I get confused. From what I understand, entanglement is essentially the same phenomenon as observation, so when Alice detects/measures/observes particle $p$, Bob shouldn't see the waveform of the entangled particle $p'$ "collapse" on his side. From Bob's perspective, Alice just becomes entangled with $p$ and $p'$.

The problem I run into is: If Alice and Bob communicate over a classical channel, how do they reconcile their different measurements? If the EPPG sends a constant number of particles (say a million/second) and maybe a signal that numbers each particle, then Alice could send her detector readings back to Bob eg:

  • particle 1,000,000 was spin up and impacted interferometer at position (0, 123)
  • particle 1,000,001 was spin up and impacted interferometer at position (10, -3)
  • particle 1,000,000 was spin down and impacted interferometer at position (x, y)
  • etc...

How would Bob be able to reconcile Alice's record of a non interference pattern on her interferometer when his own record should show a clear interference patter?


closed as unclear what you're asking by Norbert Schuch, Daniel Griscom, Gert, Ryan Unger, ACuriousMind Dec 31 '15 at 3:10

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    $\begingroup$ Can you be clearer about exactly what entangled state these particles are created in? (It appears you've got a state space that accounts for both spin and position.) If so, it will become easy to compute what happens. If not, then of course there's no way to say exactly what happens, beyond the fact that, as you say up front, the system can't be used to communicate anything you couldn't communicate classically. $\endgroup$ – WillO Dec 30 '15 at 23:52

In both cases, Bob won't see an interference pattern. Even when Alice doesn't measure her half of the EPR pair, he won't see an interference pattern.

As far as what you see locally is concerned, the existence of an entangled copy is equivalent to a measurement having already been performed (with an unknown result). It's only when the two parties compare notes later that things get confusing and you can distinguish entangled quantum spins from classical coin flips and who measured what starts to matter.

More concretely, the problem with your "early warning system" is that it raises the alarm even if there's no attack coming. False positives.

  • $\begingroup$ Nice! Every introduction to EPR should start with your explanation before people get the wrong impression about what EPR does. $\endgroup$ – CuriousOne Dec 31 '15 at 4:21

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