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Time Travel?

  1. Apr 9, 2006 #1
    Hey all,

    If you guys have some free time, perhaps you could take a look at this link.

    http://www.rebelscience.org/Crackpots/notorious.htm

    I'm pretty sure he's not correct, but I can't place the exact point in his thinking where it goes 'wrong', so to say. It just seems like he's saying that A=B, so C must equal A too. :p

    I'd appreciate any ideas about this page.
     
  2. jcsd
  3. Apr 9, 2006 #2

    ZapperZ

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    Is there a particular reason why you are interested in a website like this? Do you not find it rather dubious that all this person can do is play "quote the physicists"? I mean, this is not the bible where what a person says is gospel.

    Zz.
     
  4. Apr 9, 2006 #3
    Zapperz,

    Honestly, I was just looking up reasons by physicists on why we can seemingly travel through space, but not time, even though it is a joint 'spacetime'. I happened to find this website, and wanted to get an 'expert' opinion on why it's wrong. (I am almost positive it IS wrong, but I'm not experienced enough to know where to counter his argument. I'd like to know where his thinking is off, and thereby, improve my own knowledge of the subject.)
     
  5. Apr 9, 2006 #4

    russ_watters

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    It is such gibberish that it is tough to give a debunking of it. But to answer the non-question in your first sentence there - we can and do travel through time. If we didn't, we wouldn't age, and I wouldn't need to get up for work tomorrow...

    If you want more, you'll need to ask a specific question.
     
  6. Apr 9, 2006 #5
    Understood Russ. (Thanks for the words on his site...they seemed to be quite...confusing. Maybe that's why I couldn't figure out where the logic was misapplied...because it was all over.)

    Ok, I guess the question I've been looking for is that we seem to travel through space both backwards and forwards....I can go to any particular 'point'. And yet, we inexorably move FORWARD through time. If spacetime is related, why does this seem to be so? *I'm sure this is somewhat a 'newbie' answer, and honestly, I haven't looked up a ton of topics on this, so if you direct me to links I'll be glad to read. I'm sure it's been answered a dozen different places. :)
     
  7. Apr 9, 2006 #6

    pervect

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    I haven't read the original webpage, but if you want a popular science article about time travel by a reasonably reputable physicist, try Cramer.

    Specifically

    http://www.npl.washington.edu/AV/altvw33.html

    and check for other articles from his "alternate view" page.
    http://www.npl.washington.edu/AV/
     
  8. Apr 10, 2006 #7
    Thanks Pervect, I'll take a look. What is the 'default' position from the scientific community on time travel? Theoretically possible? Only into the future, but not the past? Nothing at all?
     
  9. Apr 10, 2006 #8
    If we percieved time backwards from the norm and everything on Earth did, how would we know? For all I know the universe ended in a big crunch and every moment since then has been a passage towards the start of the universe, with stars dying and being born? if this were the case then our future might be an aliens past?:wink:

    As i understand it is there any proof of times direction, the only things I've read about it appear to be very superficial? Or is this a pointless question anyway? Best left in the realms of metaphysics?
     
  10. Apr 10, 2006 #9
    Schrodinger's dog,

    The whole 'how do we view time' question is a bit metaphysical..but I think the "time travel" question isn't. Of course, we don't know exactly how time 'exists', but I was wondering if the position at large of the scientific community includes or excludes time travel (into a past)...no matter the paradoxes that brings up (ie. grandmother paradox).
     
  11. Apr 10, 2006 #10

    selfAdjoint

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    In curved spacetime, as in GR, it is possible for worldlines to exist that never go faster than light, but still move into the past and return to their starting point. These are called Closed Timelike Curves (CTCs). There is a considerable industry of finding solutions of Einstein's equations that have CTCs in them, and just as you say, the theorists who do this are not concerned with the grandfather or other paradoxes. So far, though, all the solutions with CTCs that have been found have unphysical propereties like negative energy in them. Even the people who found them agree that these are not physically possible.

    So the question arises: can solutions be found that are physically possible and still have CTCs in them? Nobody knows and some scientists have made and taken bets on the issue. But fact this seems to be true: it's a purely mathematical issue. Do the equations have such solutions or don't they?
     
  12. Apr 10, 2006 #11

    pervect

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    It's currently the matter of a some debate. There are some theorems that show, roughly speaking, that if GR is true that it requires negative energy to build a finite-sized time machine. ("Negative energy is a rather lose characterization of what's needed, but it gets the idea across. I don't quite remember the exact requirements anymore).

    Look up some of the old thread's here on Mallet's time machine if you're really interested - his time machine proposal is one of those that falls afoul of this theorem.

    Some of the early proposals got around this theorem with infinite time machines - such as Tippler's infinite rotating cylinder, or Gott's infinte cosmic strings. These were the inspiraton for some of the current theoretical work.

    Some suggest that this neagative energy requirement should rule out time machines, others suggest that negative energy may not be impossible, pointing to and quantum effects like the Casimir force. I'm not sure if "dark energy" from cosmology meets the requirements, but I do recall some author noting that black hole evaporation requires that the space-time around a black hole to have, in bulk, the required "negative energy" property. (This may have been in the context of wormholes, rather than time machines, though).

    Some people suggest that time travel is impossible for different reasons than the problem with negative energy. Hawking's Chronlogy Protection Postulate suggests that any attempt to create a time machine (with wormholes, for instance) will self-destruct due to infinite quantum vacuum fluctuations.

    The effect of quantum vacuum fluctuations can only currently be guessed at, as we don't have a quantum theory of gravity. People have taken a shot at estimating their value anyway (quauntum gravity should make the fluctuations finite but very very large rather than infinite). Results suggest that simple efforts to build a time machine probably will self destruct ("simple" being highly relative), but there is quite a bit of guesswork involved at this point, from what I can tell. I don't really have much of a handle on the issues of quantum gravity, so I can't even follow the guesswork.
     
    Last edited: Apr 10, 2006
  13. Apr 10, 2006 #12

    JesseM

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    My understanding was that negative energy is not generally seen as unphysical, and in fact the energy density between the plates in the Casimir effect would be negative. But this doesn't mean most physicists think time travel is likely, since there are plausible suggestions for how "chronology protection" could be upheld once quantum effects are taken into account, like the conjecture that vacuum fluctuations would build up to infinity and shut down a wormhole at the exact moment that the mouths moved into a position that would allow them to create CTCs. There'd be no way to be sure without a theory of quantum gravity, though.
     
  14. Apr 11, 2006 #13

    pervect

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    Thinking, I recall now one name on the "pro" side of the existence of negative energy of the sort needed to form wormholes - Matt Visser, the author of "Lorentzian Wormholes". For instance, take this quote.

    http://www.physics.wustl.edu/~visser/research.html

    I don't recall any specific names of physicists with an "anti" position, but I do suspect that there are some :-).
     
  15. Apr 14, 2006 #14
    Out of curiosity.

    If an object vibrates at the speed of light at .01 Angstrom omni directionally, will the object experience the same time dilation as an object travelling linearly at the speed of light?:bugeye:
     
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