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Q & A: quantum entanglement and the relativistic speed limit

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Most recent answer: 06/18/2017
Why doesn't Quantum Entanglement of particles over macroscopic distances violate the Theory of Relativity? Is it a mere consequence of delay in communication of observation over tremendous distances that leads us to the conclusion that instantaneous revelation of state takes place under this principle?
- Manan (age 15)
New Delhi, Delhi, India

The standard summary of the speed limit is "Nothing can travel faster than c." That's a little misleading, because the meaning of "nothing" isn't clear. Relativity runs into big problems if either of these quantities travel faster than c:

1. Anything conserved, such as energy or electric charge. (Faster than c in one frame would mean that the consservation law was broken in other frames.)

2. Information (Faster than c in one frame would mean backwards-in-time in others, leading to all sorts of science-fiction-like paradoxes.)

At first glance it seems that measurement of a quantum-entangled system violates (2), but actually it doesn't. The reason is that the outcome of the measurement is random, so that it transmits no information to the remote entangled object. 

Randomness means that nothing in the vicinity of either measurement determines the outcome. Therefore if in one frame you say "A had up spin, so that made B have down spin" and another frame  someone says "B had down spin, so that made A have up spin" there's no real disagreement. For these purely random quantum events, nothing leads up to determining the outcome, so there's nothing to say whether causation runs A--> B or B-->A. The direction of causation is not a Lorentz invariant for these phenomena!

And that's ok.

Mike W.


(published on 06/18/2017)

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