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# Quantum entanglement does not provide for faster than light communication. Or FTL travel.

There's a quantum no-communication theorem which shows that if two particles are in an entangled quantum state, you can't communicate information from an observer at one particle to an observer at another particle, simply by measuring the state of one particle.

It LOOKS like quantum entanglement can affect something at a distance, instantaneously.

Here's an example:  Two particles A and B are created in such a way that A and B have zero total spin, but it's unknown what spin A or B have individually.

So the quantum state looks something like:

(A has spin up AND B has spin down)  OR (A has spin down AND B has spin up).

Then the particles fly apart.

Then the spin of particle A is measured, in the up/down direction.  This measurement either shows that A has spin up, or A has spin down.

Because the total spin is 0, the observer at A now knows that B has the opposite spin from A.

And if B's spin is now measured in the up/down direction, B will have the opposite spin from A.

But this does NOT mean that information was transmitted from A to B.

Indeed, if the measurement events at A and B are such that a signal faster than light would be required to transmit from one measurement event to another, the time order of those measurement events depends on the observer!

One observer might see the measurement at A happening first.  Another observer, moving at a different velocity, might see the measurement event at B happening first.

So there isn't even a causal relation where one measurement causes the other to have a certain result.

All we can say is that there's a logical connection between the two measurements, such that the measurement on A has the opposite result from the measurement on B.

The speed of light is still the universal speed limit for transmitting information.

And people, because people have information too :)  A Star Trek transporter which sends people faster than light, is impossible.

If any faster than light communication is proven, that would mean that quantum mechanics is wrong, or that general relativity is different from special relativity in a way that allows faster than light communication in some instances.

I did thought-experiments, attempting to arrange for faster than light communication by measuring particle A, and I can see how this doesn't happen.

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### Replies to This Discussion

One intuitive explanation given early on was that it's like one astronaut A having the top or bottom half of a page from a book and another astronaut B having the other half— without anyone looking to see which part either has.

Then they separate at a long distance. However once astronaut A has looked at the half page he has, he knows whether it's the top half or the bottom half and hence he also knows which half astronaut B must have without exchanging any information whatever.

That was a neat explanation, but I believe recent experiments may obviate it as an intuitive understanding.

Yes, and the other thing to notice is that if particle A's spin gets measured, that conveys no information to an observer at particle B.
The observer at particle B might know ahead of time when particle A's spin will be measured, but they can only find out the result of the measurement in the ordinary way, via walkie-talkie or whatever - a signal transmitted at light speed or less.
Or, the observer at B can find out the spin of particle A by measuring particle B's spin. But they could have done that even if particle A's spin weren't measured.

So measuring particle A's spin doesn't result in any new information available faster than the speed of light at particle B.

recent experiments may obviate it as an intuitive understanding.

How's that?

I don't understand the experiments, but they involve what is called "entanglement swapping" and result in entanglement between two particles which have not arisen from the same source as usually is the case. The particles may never even have existed at the same time. It seems to be getting spookier with each experiment.

Perhaps you can explain it to us.

Yes, they entangled photons that never coexisted.

And in another experiment, photons were entangled AFTER they had stopped existing.

I don't see any suggestion that this requires a change in quantum mechanics.

But it seems to suggest that spacetime isn't fundamental at the quantum level.

I don't see any suggestion that this requires a change in quantum mechanics.

Perhaps not, but it does signal that something very interesting is going on.

Thanks for the information Luara.  I knew almost nothing about entangled particles.

I read once that the basic reality on a quantum level may be information, not particles and spacetime.

These entanglement experiments suggest something like that.

There's a hypothesis that the universe IS mathematics.  And perhaps the universe being constructed of information fits in with that hypothesis.

In response to the "why is there something rather than nothing" question, maybe "there's something" because we are a very complicated kind of mathematical structure that is self-aware.

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