[Dean's World] Dave Price: Time Travel? Really? Surely Not

[notify at powerblogs.com]

Posted by Dave Price:
Time Travel?  Really?  Surely Not
http://www.deanesmay.com/posts/1164084293.shtml


   This is interesting. In a modern twist on the quantum slit experiment,
   a University of Washington physicist has [1]proposed an experiment
   that quantum mechanics says should should work, but would
   constitute... well, time travel, by 50ms. The scientists involved,
   including Jon Cramer, who proposed the experimnt, all seem to agree it
   should not and probably will not work -- but no one can explain why.

     In that final phase, one of the entangled photons will be sent
     through a slit screen to a detector that will register it as either
     a particle or a wave -- because, again, the photon can be either.
     The other photon will be sent toward two 10-kilometer (6.2-mile)
     spools of fiber optic cables before emerging to hit a movable
     detector, he said.
     Adjusting the position of the detector that captures the second
     photon (the one sent through the cables) determines whether it is
     detected as a particle or a wave.
     ...
     Because these two photons are entangled, the act of detecting the
     second as either a wave or a particle should simultaneously force
     the other photon to also change into either a wave or a particle.
     But that would have to happen to the first photon before it hits
     its detector -- which it will hit 50 microseconds before the second
     photon is detected.

   My guess is that the first photon will act as the signaller to the
   second, which wouldn't normally happen but only implies that the
   particle "knows" its partner has been slowed down, rather than
   requiring information to move backwards in time from the second to the
   first.
   Of course, if it does work the other way, that makes things really
   interesting, because as you may have already figured out this effect
   could in theory (and perhaps even in practice) be used to violate
   causality, the classic time-travel paradox. All you need is a switch
   faster than 50ms to take the information from the fast photon and
   apply it to reverse the decision to measure or not measure the state
   of the second. It's just like going back in time and killing your
   grandfather before your father was born (if the time between now and
   when your father was conceived was less than 50ms).
   And then the Universe would end. Or something.

References

   1. http://seattlepi.nwsource.com/local/292378_timeguy15.html