Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time problem

  1. Nov 3, 2004 #1
    I have a unclear problem about time travel:
    As you all know,when a spaceship move pass earth with the speed nearly c, the observer will regconise that the watch on the spaceship is moving slower than his own clock.
    However, to the astronaunt who is on the spaceship, what he saw is the Earth is moving away from the Spaceship,therefore he has to experinence that the clock on the Earth has to be slower than his own.However, that's not correct according to the twin paradox because it'll be impossible to say which one is older.
    I have an explanation that using the Doppler effect, you will have different time for the astronaunt and the obsever.Suggest that at time t the spaceship sends a light signal to the earth and it bounces back to the space ship after it travelled a distance x.To the obsever,he has travelled a distance y.Because c is a constant,therefore,x/t1(t1 is the time for the spaceship to cover that distance) has to be the same as y/t2(t2 is the time for the Earth to cover that distance).Assume that x>>y because the speed of the space ship is closer to C, i will have t1>>t2.That is wrong but i can't correct it myself.
    Please give your comment
  2. jcsd
  3. Nov 3, 2004 #2
    It's hard to follow your example that you propose after this statement, but I have to point out that your summary of the situation is correct...an observer on earth will contest that the rocket's clock is ticking slower than his own, and an observer in the rocket will contest that earth's clock is ticking slower than his own. This IS in fact correct, but you say that it's not.
  4. Nov 4, 2004 #3
    My problem is if each observer experiences other's clock slowing down so how come time travel can be true.At least in theory because their clocks are both slowing down therefore, how can one tells who is older after the astronaunt finishes his journey and return home.Evereybody knows that he will be younger than his brother,but if his brother clock actually slowing down in his observation, it means that his brother who is younger,not him.
  5. Nov 4, 2004 #4


    User Avatar
    Science Advisor

    As long as the "astronaut" continues traveling at a constant speed relative to the "stay at home", yes, each observes the other as aging more slowly than himself. That's not a "paradox" nor even a problem- they are never in the same "frame of reference".

    In order that they be able to compare their ages in the same frame, we must have that "the astronaunt finishes his journey and return home" he must accelerate and so the "standard formulas" (that assume constant relative speed) no longer apply. Roughly speaking, it is the astronaut, who undergoes the acceleration, that will age slower.
  6. Nov 4, 2004 #5
    But in practice, we know that the twin paradox has been checked by using the accelerator to build up particles speed and we can observe some particles which life is very short becomes longer enough for us to notice and make some records.If we are in the same frame as the short life particle, how can we observe that phenomena.Obviously, we can observe the particles life become longer, but in thier observation,will our lives become longer too?
  7. Nov 4, 2004 #6
    Because we are not in the same reference frame as the particle. Relative to us, the particle undergoes a rapid acceleration that we don't experience. This places us outside its reference frame.
  8. Nov 5, 2004 #7
    So as you say that can i understand that time travel is impossible not only because we can't achieve the speed of light but also because the diferrence between each other's time frame?
  9. Nov 5, 2004 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What do you mean by "time travel"? If you get in a rocket ship and move away from Earth very fast and then turn around and come back, you will have aged less than the rest of us when you land. Actually, even if you just go to the bathroom while your friends stay on the couch watching TV, you will have aged less than them when you get back. I wouldn't call this time travel. It's just time dilation.
  10. Nov 8, 2004 #9
    Thank you i see what you mean.
  11. Nov 9, 2004 #10
    Make sure to keep asking questions if you don't understand something, AVNguyen. Coo? Coo.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook