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Traveling at the speed of light and time travel?

  1. Sep 14, 2010 #1
    I understand that modern day Physics allows for travel into the future, but not the past. I understand that time slows down the closer and closer you get to the speed of light, so that you never exceed that limit. But what if you meet it? I know because of the laws of Physics, you couldn't meet it, only 99.99% of it, but what if you reach 100% the speed of light?

    I understand this violates the currently accepted laws, but would time completely stop for the observer traveling at 100% the speed of light? Because any slight movement of the hand, or head or any ligament would then exceed the limit...

    Again, correct me if any of my understanding is wrong, I'm relatively new to Physics and I really enjoy it.
  2. jcsd
  3. Sep 14, 2010 #2
    Time is relative. It only slows down relative to people moving slower than you. You would perceive time to go on as normal.

    It doesn't really make sense to ask such a question. "What would the law of physics be like if there were no laws of physics?" My point is that you cannot use physics to analyze a situation in which the laws of physics do not hold. You would need a whole new theory to describe it!

    That's not to say that you shouldn't ask the question, just that there is no answer that people can give you given our current knowledge of physical laws. Indeed, it is questions like these that tend to inspire new ideas in physics.
    Last edited: Sep 14, 2010
  4. Sep 14, 2010 #3


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    Your understanding of basic Special Relativity is wrong. When you move, your time doesn't slow down. For example, do you see your time slowing down? After all, according to the creatures on Alpha Centauri, you are moving. Do you see any time dilation effects?

    If A moves relative to B, B will see A's time "slowing down". But it works the other way around as well. Since there's nothing special about B, one can also be in A's frame, and A can see that B is moving. A will see B's time slowing down. But both A and B sees no difference in their own respective time.

    This is one of the most common mistakes of anyone who misunderstood Special Relativity.

  5. Sep 14, 2010 #4
    What you should first understand is that if I see you travelling at 99.99% the speed of light, and you emit light in your forward direction, that light is travelling at the speed of light relative to you, and it is also travelling at the speed of light relative to me.

    i.e. no matter how fast you go, light going in the same direction as you is always travelling at 300 000 m/s relative to you, whether you emit it or I do.
  6. Sep 14, 2010 #5
    Taking into consideration what you said and as well as what ZZ said, if A is moving relative to B, and B emits light in a forward direction, will the light appear to slow down to A?
  7. Sep 14, 2010 #6
    Nope, the speed of light is constant from every frame of reference. They would observe the frequency of the light to be different though, due to the doppler effect and also time dilation/space contraction.
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