Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dark Matter and Thorne's unlikely time travel

  1. Sep 28, 2010 #1


    User Avatar
    Gold Member

    There are a lot of "IF's" in the following, so this may be more a question of logic than of physics.

    Also, I have two questions revolving around the same themes; they are independent from one another so a response can be to one or both of them.

    (1) In http://www.phy.duke.edu/~hsg/55/related-files/sciam-wormholes-jan-2000.pdf it is pointed out that a space-time "bubble" of the sort needed for science-fiction "warp drive" would require an inordinate amount of negative energy around it. Since dark energy must have negative pressure, then IF dark energy is variable and IF the variations could be localized, could dark energy serve the same purpose as the negative energy in this construction?

    (2) First the background, then the question:

    (a) The background: In 1988 Kip Thorne proposed a method to attempt to make a time machine out of a wormhole, supposing that a wormhole could be made traversable and relatively stable. (The account is in his popular book "Black Holes & Time Warps: Einstein's Outrageous Legacy".) He points out that the quantum fluctuations would recycle themselves and thus the positive energy inside would snowball until, most likely, the wormhole would be destroyed, saving us from time-travel paradoxes. This version of Hawking's "Chronology Protection" has a quantum phenomenon saving the problem of General Relativity.

    (b) The question, in two parts:

    (i) Is the following reasoning correct? [For my own comfort, I use the language of foundational mathematics and formulate my question in terms of axioms.] Most of the axioms for General Relativity are presently independent of the axioms of Quantum Mechanics, so that one could conceive of a universe in which the axioms of General Relativity remained the same while many of the present axioms of Quantum Mechanics were absent or negated. (OK, this may change when a theory of Quantum Gravity arrives, but until that time....). In this universe the savior "Infinite Quantum Fluctuations" then may be absent, and Thorne's time machine would be possible, leading to the contradictions of time-travel, leading to the conclusion that the axioms of General Relativity must be faulty, or that there must be a saving mechanism in General Relativity itself to forbid the kind of twisting of spacetime in Thorne's mechanism.

    (ii) If this is correct, is there such a saving mechanism in General Relativity which is independent of the mechanism of the snowballing quantum fluctuations?
    Last edited by a moderator: Apr 25, 2017
  2. jcsd
  3. Sep 28, 2010 #2


    User Avatar
    Science Advisor

    Why should time travel lead to any "contradictions"? The assumption usually made by Thorne and other physicists who analyze the possibility of time travel in GR is that something like the Novikov self-consistency principle would apply, so however physically implausible time travel may be it needn't lead to any logical paradoxes. The fact that some theory allows time travel is not enough reason to rule it out a priori, although of course physicists have a number of reasons to suspect it won't be allowed in a final theory of quantum gravity.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook