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B Time only moves forward, how is it possible to travel to the past?

  1. Oct 15, 2016 #1
    1.if we assume BIG BANG as frame of reference for time then how could one go to past ?
    2.if i travel the time and go to past at the time of BIG BANG, at what place i will be?
    3.why the speed of light is same for everything(moving or stationary)?
    3.and what will be the apparent speed of light beam for another beam of light or particle moving at speed of light?
  2. jcsd
  3. Oct 15, 2016 #2


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    1. We have no evidence that it is possible to travel in time. Also, it doesn't make sense to refer to the big bang as a frame of reference.

    2. We have no evidence of the existence of anything before the big bang. So even if you could travel in time it's not clear that there is such a destination.

    3. It just is. We only justify it by saying we can predict stuff perfectly if we assume that, and we get all sorts of problems if we don't

    3 (again) There is no such perspective. It is self-contradictory to attempt to describe it because light would have to be stationary and moving at 3x108m/s at the same time.
    Last edited: Oct 15, 2016
  4. Oct 15, 2016 #3

    Mister T

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    Time, as we understand it, began at the big bang. So you're asking what would something that doesn't exist be like if it did exist. There's no way to answer that.

    If you're wondering where the big bang occurred, the answer is right where you are now. And right where everything is now. The universe didn't expand into an already existing space. Space itself expanded. And continues to do so.

    It's difficult to answer "why" questions because every answer is just met with another "why". One way to look at it is like this. The laws of physics, as they exist now, are stated in such a way that a state of rest is equivalent to a state of uniform motion. With no way to distinguish experimentally between those two states, the speed of light has to be the same to all observers. Thus the laws of physics treat those two states as being equivalent.

    Experiments overwhelmingly confirm that these laws of physics work.

    The laws of physics that I mentioned above put a strict upper limit on the speed of an observer. Observers cannot reach the speed of light. So again, you're asking what would something that doesn't exist be like if it did exist. You might find an answer to such questions somewhere, but not within the body of knowledge known as physics.
  5. Oct 15, 2016 #4
    If something didnt exist before big bang then in which thing the universe is expanding and which things made big bang to happen? Also according to Mister T there was nothing before big bang so what made energy to convert into mass?
  6. Oct 15, 2016 #5


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    The universe doesn't need to be expanding in to anything. You can write a complete description of a 4d object where each 3d "slice" is bigger than the one on one side of it and smaller than the one on the other side without referencing anything outside - it's called the Friedmann-Robertson-Walker (FRW - sometimes Lemaitre gets a look in and you see FLRW) solution to Einstein's equations.

    It isn't clear that anything caused the big bang - our notions of causality are all based on our experience inside the universe. There's no particular reason why they should apply outside the universe, even assuming that there's anywhere outside the universe to talk about.

    Essentially, it isn't clear that your questions are meaningful. And even if they are, we don't yet have the knowledge to answer them.
    Last edited: Oct 15, 2016
  7. Oct 15, 2016 #6


    Staff: Mentor

    And on that note, this thread is closed.
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