Notices
Results 1 to 2 of 2
Like Tree1Likes
  • 1 Post By DrRocket

Thread: Time machine in a medium

  1. #1 Time machine in a medium 
    Junior Member
    Join Date
    Mar 2013
    Posts
    2
    I think it is believed that we can make a time machine if we move faster than the speed of light. Obviously we know light travels at lesser speeds in mediums of high refractive indices. Is that correct or I have misread the concept?
    Reply With Quote  
     

  2. #2  
    Senior Member
    Join Date
    Jan 2013
    Posts
    504
    Quote Originally Posted by Robinol View Post
    I think it is believed that we can make a time machine if we move faster than the speed of light. Obviously we know light travels at lesser speeds in mediums of high refractive indices. Is that correct or I have misread the concept?
    It you naively insert a superluminal speed into the equation for "time dilation" in special relativity you get an imaginary time dilation, not a negative time dilation.

    Photons always travel at c, even in a medium.

    While the phase velocity of light through a medium may be less than c that doesn't help you anyway, as the "speed of light" as the term is used in relativity refers to the speed of light in a vacuum.

    While there appears to be nothing in the general theory of relativity that prohibits closed timelike curves (the criteria for "time travel"), it is generally believed, though not proved, that such are not possible. The impossibility of closed timelike curves is the "chronology protection conjecture" of Steven Hawking -- which is the conjecture that macroscopic closed timelike curves are impossible. Hawking has proved this conjecture under the "weak energy condition", which seems to be reasonable at macroscopic scales, though not at the scales of quantum physics.

    Note that there are exotic solutions of the Einstein field equations that admit closed timelike curves, for instance behind the event horizon of Kerr black holes. So more than general relativity must be involved in any proof of the chronology protection conjecture.

    Closed timelike curve - Wikipedia, the free encyclopedia

    Phys. Rev. D 46, 603 (1992): Chronology protection conjecture

    Is time travel allowed? | plus.maths.org

    [0801.0735] Topological censorship and chronology protection
    Robinol likes this.
    Reply With Quote  
     

Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •