[ExI] FTL question

[Anders Sandberg]

On 2013-07-31 01:50, Dan wrote:
> Anyone here care to answer? I'll post your reply back to A2. Thanks!

Short answer: AFAIK, yes, the drive does allow causality violations.

Middle answer: As the saying goes, "FTL, causality, relativity - choose 
two". Any method that allows you to get outside your own lightcone in a 
asymptotically flat spacetime (wormholes, warp drives, teleportation) 
allows sending messages into the past due to the non-Lorenz invariance 
of event ordering outside lighcones.

Long answer: Everett, Allen E. (15 June 1996). "Warp drive and 
causality". Physical Review D 53 (12): 7365–7368 shows that you can get 
closed timelike loops in the Alcubierre spacetime, and that allows you 
to send information back in time, messing with normal notions of causality.

You can still save some of it by arguing for the Novikov 
self-consistency principle ( 
http://en.wikipedia.org/wiki/Novikov_self-consistency_principle ): 
inconsistent states have zero probability amplitude. But as Scott 
Aaronson and others have proven, in such worlds you still get awesome 
computation power from CTCs and there P=NP. So many would say they are 
weird enough to look noncausal.

There is also the problem that the basic drive spacetime doesn't contain 
a creation and endpoint of the drive field. Krasnikov has shown that it 
is generally impossible to whip up such a field and turn it off (S. V. 
Krasnikov, “Hyper-fast Interstellar Travel in General Relativity,” Phys. 
Rev. D 57, 4760 (1998) [arXiv:gr-qc/9511068]) but round-trips (and hence 
CTCs) are possible.


-- 
Anders Sandberg
Future of Humanity Institute
Oxford University