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Time at speed of light

  1. Jun 10, 2014 #1
    So, if I travel in a ship at light speed in a line from earth to space for 10 minutes. Than turn around and travel back at light speed for 10 minutes. To earth time, I would be gone 20 minutes... But everywhere I read people are saying that earth will be in the future by the time I come back. But I only raveled 20 minutes since light goes so fast. So why would earth be in the future? I understand that on the spaceship time stops. So in reality I should not of aged those 20 minutes while the people on earth have aged 20 minutes.. So how come when i research. Everyone saying that thousands of years would have went by.. But in reality light moves fast. And if time stopped. How does light go back and forth so fast if time is really stopping? Seems like light it self would stop if time stops right? Because it light is still moving that fast and can come back to earth in 20 minutes and observe the same earth as when the light left. Why does it change if a human is on board observing?
  2. jcsd
  3. Jun 10, 2014 #2


    Staff: Mentor

    You can't travel at light speed. No object with nonzero rest mass can travel at light speed. The best you can do is to travel close to light speed.

    20 minutes, by Earth clocks, after you set out *is* "the future", relative to the time you set out.

    No, it doesn't, because you and the ship can't travel at light speed. You can make your elapsed time on the ship very small by traveling very close to light speed, but you can't make it zero.

    Where did the thousands of years come from? The only time you've mentioned so far is 20 minutes. If that meant 20 minutes by Earth clocks, then that's 20 minutes by Earth clocks.

    If, OTOH, you meant 20 minutes according to your clock on the spaceship, then yes, when you return to Earth, thousands of years could have elapsed by Earth clocks, if you were traveling close enough to the speed of light.

    Time does not "stop" for light. The concept of "elapsed time" is not well-defined for light.

    Oh, and I see this is your first post, so welcome to PF!
    Last edited: Jun 11, 2014
  4. Jun 11, 2014 #3
    I know it's not possible guys.
    Let's say .0001 to the speed of light okay.
    So if I go and come back after 20 minutes of earth time.. Than when I returned, I would come back to 20 minutes after the time I left and that's all right? Not any more years or anything crazy?
  5. Jun 11, 2014 #4
    I say that because I seen a post where someone said if you were to skype in a spaceship with someone on earth. (Speed of light) they would see you moving in like a fast forward looking motion. And in would observe them moving in "slow mo" but I would have to be gone for 1 hour in my relative time in the ship. But if I counted to one hour in my head instead of looking at my watch than returned after that hour. Would I still come back in future or would I come back to "one hour later earth time"
  6. Jun 11, 2014 #5
    Thanks for the replies. I'm on the physics app for my phone so I couldn't tell just one person answered all those questions. When we are talking short intervals like 10 , 20 minutes on earth. (Watching someone moving at light) then that isn't enough time for time to change drastically to the person leaving and coming back right?

    Maybe this is a easier way to ask this question.
    How long would someone need to travel at light, according to time in the ship.. To have a big effect on time change when they come back to the planet?
    Sorry for so many posts, I'm all over the place. I didn't know how to let my thoughts out all at once, I made a profile and spewed out my confusion.
  7. Jun 11, 2014 #6
    How far away are you going anyway? That matters you know. The closest star is 4.243 light years away so if you went there and back at close too the speed of light then more then 8.486 years have passed on earth. How much time would have passed for you depends on how fast you where going.
  8. Jun 11, 2014 #7

    Exactly, so let's say I went there and back at basically speed of light. Would I come back to a 8 year in the future earth? Or since I was traveling 8 years in the ship at light speed. Would I come back to a earth way farther into the future?
  9. Jun 11, 2014 #8


    Staff: Mentor

    Since you specified 20 minutes of Earth time, then 20 minutes will have elapsed on Earth when you return. How could it be anything different, since you explicitly said 20 minutes of *Earth* time?

    Of course, since you are traveling at 0.9999 times the speed of light, then by your clock on the ship, only 17 seconds will elapse during the trip. So when you get back, you will have experienced only 17 seconds, but clocks on Earth will have advanced by 20 minutes.

    How did I calculate that? The key number is usually called ##\gamma##, and the equation for it is:

    \gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}

    This ##\gamma## is the ratio of Earth time to elapsed time aboard your ship, if you travel out and come back at a constant speed ##v##. (We are ignoring any effects due to the fact that you have to accelerate to start the trip, then again when you turn around, and finally when you stop to land back on Earth.) For ##v = 0.9999 c##, the above equation gives ##\gamma \approx 70.7##.

    More on Wikipedia here (Wikipedia calls ##\gamma## the "Lorentz factor"):


    The ##\gamma## factor tells you that. For example, if you wanted to know how much time would elapse on the ship if 1000 years had elapsed on Earth clocks when you got back, just divide 1000 by ##\gamma## for your speed. For the same speed as above, this gives ##1000 / 70.7## or about 14.1 years.
  10. Jun 11, 2014 #9
    Why would you come back way farther in the future? I don't really get what your issue is. I'm not a physicist, or particular knowledgeable about this subject, but certain things about relativity seem pretty easy to understand. One, you can't go the speed of light. Two, the closer you get to speed of light the slower time goes. Three, time on earth would be unaffected by your trip. So if you where going 99.9999999 percent of the speed of light to that star and back then somewhere between 8 and 9 years would have passed on earth, and very little time would have passed for you.
    Last edited: Jun 11, 2014
  11. Jun 11, 2014 #10
    It's quite simple, if you traveled back and forth close to light speed for 20 minutes "Earth's time", as far as the Earth is concerned you were gone for 20 minutes. What you probably want to know is that according to your own clock you would have been gone less than 20 minutes. How much less than 20 minutes depends on how close to light speed you traveled. Here are a few we examples:

    @ 86.6% of c (light speed) your clock would say the trip was approximately 10 minutes

    @ 99% c, less than 3 minutes

    @ 99.99% c, less than .3 minutes or less than 54 seconds

    That should help you get the general idea.
  12. Jun 11, 2014 #11

    Peter you are the man for all those answers. That's exactly what I was looking for, an equation to figure out all that stuff lol
  13. Jun 11, 2014 #12
    And so if 20 minutes pass on earth witch means 17 seconds about in light time.. How can 17 seconds pass when in reality that light has been gone for 20 minutes. I know I would only feel 17 seconds being gone. But technically I would have really been gone 20 minutes not 17 seconds. Does this make light actually younger than it really is? Let's say a light from a star took 1 million light years to reach us, that light is 1million standard years old. But to be light since it's been moving so fast, it thinks it's only for example 100 years old correct?
  14. Jun 11, 2014 #13
    It does. Thank you very much.
  15. Jun 11, 2014 #14


    Staff: Mentor

    I would recommend that you stop thinking of it as "light time". It's "spaceship time". The concept of "elapsed time" does not make sense for light. Wherever I'm responding to a question you ask about "light time" below, I'm actually responding to the question with "light time" changed to "spaceship time".

    "In reality" there is no unique "time" that you have been gone. Time is relative. I know your intuition says that there should be one "real" time, and that the "spaceship time" is somehow "not real"; but that intuition is wrong, and you need to unlearn it if you want to understand relativity.

    No, of course not. "How young" the spaceship is is just how much time has elapsed on the spaceship's clock. *That* is not relative; all observers will agree that 17 seconds elapsed on the spaceship's clock between its departure and return.
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