Time travel, Ask a Physicist, and what's fair game

In my last “Ask a Physicist” column, I all but dared the io9 readers to send me questions about time travel.  And they did.  In droves.  It seems that I’m morally compelled to do next week’s column on time machines.

I’m not exactly shy about writing about time travel (and see the links within the links), but I’ve gotten a little disenchanted with it.   I have always taken the position that the only time travel that possibly makes any sense to discuss are those permitted by general relativity.  Otherwise we’re in the realm of pure science fiction.  The problem is that every GR solution for a closed timelike curve (aka time machine) seems to have a possibly insurmountable physical requirement: exotic matter may not exist, cosmic strings may not exist, our universe has no net sense of rotation, etc.  (If you don’t know about these time machines, don’t worry, I’ll write about them next week).

But here’s the rub: since general relativistic time machine may not be physically possible, discussing them is only incrementally better than talking about TARDISes and DeLoreans and the like.  I’ll put in all the caveats about how most of these things may not be physically possible, of course, but history (and the comments section) teaches me that people will use that as an opportunity not to question the possibility of time travel, but to upgrade their favorite crazy version of time travel to “just as physically viable as wormholes.”

I’ll tell you one thing I won’t do in this column.  Except for the briefest of mentions, I’m not going to talk about particular paradoxes.  Here’s my plan: I’ll mention their existence, and then the comments  will be all about debating them.  That, and Primer.
-Dave

p.s. Since we’re doing time machines one way or another, we should make it a good one.  Which time machine models do I need to discuss under pain of your unending disappointment?

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3 Responses to Time travel, Ask a Physicist, and what's fair game

  1. jin choung says:

    QUESTION:

    are any moments actually “lost” or whether we can re-experience them or not, are they just as persistent and real as the current moment?

    meaning that as a human being, i am irrevocably driven through the time line but according to physics as we know it, is there any indication that “past moments” are as real and in existence just as much as the current moment – even though i will never be able to go back?

    in this case, consciousness would be like the stylus that carves the grooves into a recording phonograph – it’s writing the history so it won’t ever go back but the marks that it made PERSIST and are not “gone” the moment the write head passes over it.

    and is it possible to conceive of a state of existence where someone can see all these moments of time like pages in a book or frames in a film strip?

    jin

  2. Papa says:

    “Time flies like an arrow. Fruit flies like a banana.”
    Groucho Marx

  3. Tyra says:

    If one was to build a time machine why is it that some peoples theory say that it will only work with the time it was built. How can you travel further back then the time machinewas built. What would you use as a communication device to send a message if you were to speak into the past ? Coukd you communicate with sound . How could you calculate the time you would go. How would you turn it off and on.

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