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What happens after you travel at a relativistic speed

  1. Mar 28, 2010 #1
    If a space ship is moving at a relativistic speed past the earth, the people on earth would believe that the time in the spaceship was moving slower. The people on the spaceship would believe the time on the earth was moving slower. So what happens when the spaceship stops? Do both times revert back to normal? Because if the times just go back to the same speed, then the people on earth would still be seeing the people in the spaceship only in the past... and that doesn't make sense. So what actually happens?
  2. jcsd
  3. Mar 28, 2010 #2
  4. Mar 29, 2010 #3
    Okay, you have two frames of reference...one in which a spaceship is traveling at relativistic speed and one which, for arguments sake, we'll consider to be stationary.

    As the spaceship decelerates, the time dilation effect becomes less pronounced until the spaceship is stationary...at which time the two frames of reference are temporally equivalent. The end result is that the people on the spaceship have effectively traveled into the future relative to the people in the stationary frame (meaning that during the spaceship's travels, a greater amount of time has elapsed for the people in the stationary frame of reference). With the spaceship stationary, both sets of observers now view each other in the same frame of reference...there is no "seeing the people in the spaceship only in the past".
    Last edited: Mar 29, 2010
  5. Mar 29, 2010 #4
    At high velocities time and position are not orthogonal as they are approximately at low velocities. It is no longer enough just to talk about an objects time you must also consider its place (position).
  6. Mar 29, 2010 #5
    I'm pretty sure that each reference frame would view the other as having a slower time.

    Ok, let us consider position along with time. How does that explain what happens when the spaceship stops?
  7. Mar 29, 2010 #6
    What happens when the spaceship stops? You ask. Nothing happens. As the spaceship passes close to earth at high speed both observers have approximately the same space-time position. After the ship has moved away for a long time the ship and the earth observer have different space-time positions, dS=x. If the ship stops quickly then the difference in space-time positions is still approximately dS=x. Nothing happens. Both will observe the others clock as progressing at the same rate as their own clock (after stopping). Before stopping both observe the other's clock as progressing slowly relative to their own clock.
  8. Mar 29, 2010 #7
    Ok, so the spacetime positions are the same. How is it possible to view someone else's time as slower and still have both be the same spacetime?
  9. Mar 29, 2010 #8
    Maybe I misunderstood your question. See the first paragraph in "Time dilation and space flight" in the "Time dilation" Wikipedia entry. It describes the effect of a spaceship traveling close to the speed of light and the differences in time that are experienced by the people on the spaceship vs the people on Earth. One of the sentences states "the ship's clock (and according to relativity, any human travelling with it) shows less elapsed time than the clocks of observers on Earth"...which means that the clocks on Earth are running faster than the clocks on the spaceship.

    I'm not sure what you mean by both being the same spacetime. The spaceship traveling close to light speed will be in one frame of reference and Earth will be in another. Although they both physically exist in the same spacetime, the spaceship is encountering greater time dilation effects due to its speed and therefore its frame of reference has changed from that of Earth's. When the spaceship comes to a stop, the spaceship is now in the same frame of reference as Earth (very oversimplified because I'm discounting all other movement as well as the differences in spacetime distortion just due to the mass of the two objects).
  10. Mar 29, 2010 #9
    Well, the Earth clocks are running faster according to people on Earth. But the people on the spaceship see the earth go by at a constant velocity and therefore, the same time dilation phenomena that the people on earth see when watching the ship occurs when the people on the ship are watching the earth. I asked many questions about this long ago and I'm pretty sure that that is the general consensus regarding this topic. Can any other members confirm this for me?

    Well, I meant the same position in spacetime. I was simply asking edpell how the spacetime positions could be the same when (relative to another) the times were different. You (Temporal) seem to have a small misunderstanding of how time dilation works, so I would look that up before attempting to answer this. (Unless I'm wrong about the whole dilation thing, that's always a possibility :) )
  11. Mar 29, 2010 #10
    As I thought...I misunderstood your scenario. The observers in the spaceship can be considered as motionless in their frame of reference as they see the Earth go by which explains why the observers on the spaceship would see the Earth clock as moving more slowly. Thanks for the clarification.

    Please point out what misconceptions you think I may have concerning time dilation so I can learn from this discussion.
    Last edited: Mar 29, 2010
  12. Mar 29, 2010 #11
    What you're describing is just Differential Aging. The leg of the journey, or the fact that the ship stops doesn't matter, as edpell has already said. Once the ship stops, observers on earth and the ship could agree on a clock's tick rate.
  13. Mar 29, 2010 #12


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    No, not in the past. At least, not per se. During their travel in the spaceship they will not have experienced as much time passing as those on Earth. But it doesn't mean they're in the past; it means they're younger.
  14. Mar 30, 2010 #13
    It's also good to point out that time was never "ABnormal"! All of this does make sense in the context of a constant "c", and presumably the ship must go through a braking period to come to a stop... so time is never jumping around, or "snapping back", because it was never deformed to begin with, anymore than spacetime is ALWAYS deformed by energy/momentum.

    When the spaceship stops, 'Earth' and 'Ship' will not agree on the total amount of time passed subjectively, but their "tick rates" will be similar again. If we ignore that, this doesn't make sense as a Relativistic effect, and it seems like some kind of magical "time jump". "What happens after you travel at a relativistic speed?"... the same thing that happened before, accounting for Time Dilation/Differential Aging.

    The issue is the desire for people to hang on to absolute notions in a relative world.
  15. Mar 30, 2010 #14
    Thanks for all of your posts. I still do not understand completely, but your information helps me narrow down my question. So, now I understand that when the spaceship stops, clock's on both the earth and the spaceship will not agree. The question is, how is the amount of time passed on the spaceship and the earth the same? Or is it?
  16. Mar 30, 2010 #15
    It is different, and both earth (A) and spaceship (B) MUST agree on that at the end of the journey. That's why we have the bother of Time Dilation and Differential Aging in the first place. We all have to agree on the laws of physics at the end of the day. That's why a round trip to near lightspeed wouldn't end in a happy homecoming. Your descendants would be REALLY dead and gone (depending on trip velocity and duration of course), and everyone would agree. It's that last concept, that is RELATIVE. The subjective experience is relative, but the absolute result is not. The velocity of light in a given medium is absolute, and time and space and our subjective experiences are defined by that.

    You've really learned a lot in a short period in this thread you know... restores some nonexistent faith on my part.
  17. Mar 30, 2010 #16


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    Different inertial frames define "simultaneity" differently, meaning for two events that happen far apart in space, they disagree on whether they happened at the same time-coordinate or different time-coordinates (see the relativity of simultaneity for more info). Suppose the ship is heading away from Earth at a relative speed of 0.6c, and then when its own clock shows that 8 years have passed, it decelerates so it is at rest relative to the Earth. In the frame where the Earth is at rest, the event of the ship's clock reading 8 years is simultaneous with the event of the Earth's clock reading 10 years, so in this frame the ship's clock has only elapsed 0.8 times the amount the Earth's clock has elapsed. On the other hand, you could consider the frame where the ship was initially at rest while the Earth was moving away from it at 0.6c. Since this is an inertial frame, the ship won't continue to be at rest when it accelerates, instead this frame says that the Earth continues to move at 0.6c and the ship suddenly accelerates so it is moving at 0.6c too (you can think of this frame as the rest frame of an inertial observer who was initially moving alongside the ship but didn't change velocities when the ship did). In this frame, the event of the ship's clock reading 8 years (which is still when it changes velocities) is simultaneous with the event of the Earth clock reading 6.4 years, so in this frame the Earth's clock only elapsed 0.8 times the amount the ship's clock elapsed between the moment the ship left Earth and the moment the ship accelerated. Now, if the ship were to actually turn around and return to Earth, both frames would agree about exactly how much time had elapsed on each clock at the moment they reunited at a single location (see post #36 on this thread for an analysis of such a situation in two different inertial frames), it's only for events at different locations that different frames can disagree about simultaneity.
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