MWI and FTL Communication Possibilities

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I know that currently accepted theory is that entanglement cannot - and never will - be used to send FTL communications.

Leaving that minor detail aside, what I am not clear on is why _quantum_ FTL communication would necessarily violate SR if one believes in MWI.

Let's say Cramer's experiment worked. If A sends a message to B using this method, the result is that B experiences what would be - quantum mechanically speaking - a very impropable but nonetheless possible series of observations which he interprets as a message. In fact, what is really happening (in MWI) is that A has moved into a branch in which B has experienced this very unlikely series of observations which look like a message.

And let's say further that A and B are moving relativistically w/r/t one another and therefore when B makes these observations, A hasn't actually sent the message (from B's perspective). But, nonetheless B is in a universe in which the communication-related particles have nonetheless done what they did. So, permit B to send a message to A telling him not to send the message to B. Paradox?

Not necessarily, right? Because B is in a universe/branch in which the message was "received" - aka the particles behaved in such a way as to transmit information - does not mean that A - in B's branch - deciding not to send the message will change what B had already observed.

Put in cheap sci-fi terms, a new timeline has been created.

Put in QM terms, A and B simply find themselves in whatever branch of the wavefunction creeates a consistent causal history. B's sending a message back to A telling him not to send the original message will surely be received in some _other_ branch of A, but because B (after receipt of the message) only exists in branches in which the message was sent and the relevant particles have already decohered, B's actions are not going to affect what B has already observed. If B went and asked A whether he'd received the message, he'd surely say no.

Does this jive with orthodox MWI or am I completely in left field? And if I am right, wouldn't Cramer's experiment - if successful - prove MWI above any other interpretation?


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The splitting of the worlds (assuming the whole thing hangs together pending a derivation of the Born rule) simply does not permit any signalling. Bob's statistics are unaffected by anything any version of Alice does or how many versions of Alice there are simply due to how marginal probabilities work in Quantum Mechanics.

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