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MWI and FTL Communication Possibilities

  1. Aug 20, 2008 #1
    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?
  2. jcsd
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