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Why must we travel through something at C?

  1. Jan 6, 2010 #1
    Hi, I'm super new (registered one minute ago) because I tried yahoo answers, but realized no one there cares. So I came here (I've read a few random questions on my trek through the "internets")

    So, I'm just a kid, well I'm a sophomore in high school, but same thing. I have a pretty basic understanding of physics, I've read Hyperspace and I'm halfway through The Elegant Universe, but thats basically it. As a kid who naturally believes (or wants to) in the fantastical, I've (I'm embarassed to say it) googled time machines... and from reading the Elegant Universe I figured if you just stick a guy in a car and go like 99.99% the speed of light, then it's basically like a forwards time machine (well, cheating, but sure.)
    But I want a backwards time machine. And I posed this to my physics teacher (physics honors, you know, heh) and he stared me down and told me that our universe is based on "cause and effect", and that if we made a time machine that goes backwards, it would destroy everything we know about the universe and based all our conclusions on. I'm not quite sure what to make of that... but I feel a little bit beat: all I can figure is that, to go back in time, you have to go negative lightspeed, which I know, is ridiculous. I thought for a second that if you went faster than light, somehow that would work, but that was pretty stupid because then... well, that upsets the whole reasoning (p.s. I don't understand why we CAN'T go faster than C)
    Basically, is it completely hopeless like it seems to be? Or do you have any good arguments I can use to thwart my physics teacher's plans to abolish all my hopes and dreams of backwards time travel? (Even on a theoretical note).
    And... WHY is C the defining object of all? How do we know we go through time at the speed of light? I understand why time goes slower as you approach C, but not why, or how we know, that we rocket through time at lightspeed.

    Any comments would be nice, just please don't make fun of me for not really knowing what I'm talking about here. :D Thanks!
  2. jcsd
  3. Jan 7, 2010 #2
    You might try: http://en.wikipedia.org/wiki/Temporal_paradox

    I only skimmed the article, but it looks like a decent place to start.
  4. Jan 7, 2010 #3
    Questionable statement. And strictly speaking it is not yet known if the backward time travel is possible or not.
  5. Jan 7, 2010 #4
    I've always had a problem with that interpretation of cause and effect, because if the fastest speed one can travel with is the speed of light, and if one could somehow go back in time, the effects of you going back in time would never reach the time you left, as this would mean information would have to be transferred at exponents of light speeds to reach to the future.
  6. Jan 7, 2010 #5
    Because we want to get there in time. The problem with accelerating to c is that mass increases as V increases. (the relativistic effect) To move at the speed of light, mass becomes infinite. That is why photons and electrons with a rest mass of approximately zero are about the only things that can travel at C. Really, your level of understanding is not quite sophisticated enough for you to handle the background. You Physics teach is right. You can't use C to travel backward in time. While you are traveling at c time compresses, but that is not really the same thing as traveling through time.
  7. Jan 7, 2010 #6


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    Science Advisor

    What? A photon has a rest mass of exactly zero and always travels at c whereas an electron has a nonzero rest mass, and can never reach the speed of light.
    To say "while you are travelling at c" is misleading in the first place. There is no reference frame attached to a beam of light. In my opinion, trying to describe what it might be like to travel at c is poppycock. If we're throwing physics out the window in the first place, just say we can travel back in time by clicking our heels together three times and be done with it.
  8. Jan 7, 2010 #7
    Kinetic energy is [tex]E_k=\frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}-mc^2[/tex]

    Notice that Ek goes to infinity as v goes to c. Therefore it would require an infinite ammount of energy just to get TO c, let alone beyond it.

    There are theoretical tachyon particles which DO travel faster than c, but these are not conventional localizable particles.
  9. Jan 7, 2010 #8


    Staff: Mentor

    In General Relativity time travel to the past is theoretically possible (usually requiring negative energy density). If you want to find information about time travel to the past in GR you want to search for "closed timelike curves". Matter (including people) follow a class of paths through spacetime called timelike, and if your path is closed and timelike then you have a GR-style time machine.
  10. Jan 7, 2010 #9
    Time travel would seem to create paradoxical havock for causality or logic, making it hard to imagine what time travel would actually mean. No physics needed to understand that. To see why travelling faster than c (the speed of light in a vacuum) could lead to such paradoxes, you need to understand the basics of special relativity, in particular the relativity of simultaneity (best learnt from actual textbooks such as Taylor & Wheeler's Spacetime Physics). A good starting point is the relativity chapter of Benjamin Crowell's free online textbook Simple Nature: http://www.lightandmatter.com/

    There's a nice example of an FTL paradox in Roger Penrose's The Emperor's New Mind, and a similar one in this thread: https://www.physicsforums.com/showthread.php?t=269480 See Kev's "Log of superluminal radio conversation" in #3 and his diagrams in #10.

    Three approaches to the problem that I've seen are Hawking's "chronological protection conjecture" that the laws of physics always will somehow ensure that time travel is never in practice possible, the Many Worlds Interpretation of quantum mechanics (as used by David Deutsch in The Fabric of Reality to argue for the possibility of pseudo time travel to alternate versions of the past), and--the most fun idea--Novikov's "self-consistency principle" that literal time travel could happen in circumstances that would create a causal loop where the result of something travelling back in time to affect its former self could be the very thing that causes it to travel back in time!

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