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What does time dilation look like?

  1. Dec 28, 2006 #1
    Imagine a highly advanced spaceship that can go at 99.99 percent the speed of light. Assume that it takes a year to accelerate to get to that speed, and another year to slow back down. It has a crew that, on the voyages between stars, enjoys playing various games as the ship's computer takes over all other duties. Some crew members practice archery, and some play laser tag. Their ship is currently in orbit around our sun, and they are all playing their favorite pasttimes, waiting for an assignment. They get one: the order is to travel to a far away star system. They set out for it. After accelerating to their maximum velocity of 99.99 percent light speed, they all decide to relax and play.

    Will anything be different? Will the archers find it impossible to fire arrows in the direction they are accelerating, because of the energy necessary to make anything go even very slightly faster than their current speed? Will the laser tag players see light go much slower than it did when they weren't moving?

    Thanks for any and all help given.
  2. jcsd
  3. Dec 28, 2006 #2
    i'm not sure, but i have almost sure that no, it happens because, for example, an arrow, if fired from the person, the speed of the arrow is x(the speed the man trow it) in relation to the person, not to the referencial in cause.

    you need ALOT energy to trow an arrow from the same place you launche the spaceship and put it at velocity "c". inside the spaceship, you are already at a velocity "y=99.99%c" so, when you trow the arrow, it velocity is "x" in relation to the person, and 99.99% in relation to the referencial. So, the energy needed is the same...as if you were in where you're now...

    however, in the case of the laser, no: the laser will pass on him at the same speed "c", that is 1 of things that Einstein said when made his relativity teory: the light, no matters the referencial, travels everytime at speed of light...if the person is at 99.99%c, then he see the laser at "c"...

    however, i'm an aprendice...so, problaby, hardcore will come soon explainig it right...:biggrin:

    i put this so i can learn too if i'm wrong...:tongue:

    regards, Littlepig
  4. Dec 28, 2006 #3


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    No, the crew will not experience anything different inside the spaceship. In fact, the only way they would be able to tell that they were moving would be if they were to look out the window.

    You might try looking at the Pirelli award winner multimedia presentation


    which makes much the same point. It also covers some other very basic points of relativity at a very elementary level. The most useful aspect is probably the interactive quiz program that goes along with the multi-media presentation.

    For your answer about what time dilation would look/feel like, I think the multimedia explanation above makes it reasonably clear - go on to the point in the adventure where Al says "Still one bounce per second...."
    Last edited: Dec 28, 2006
  5. Dec 28, 2006 #4
    That is a bit confusing since your wrote earlier:

    That implies that all relax and play activity happened after the vessel stopped accelerating.

    One of the postulates of the special theory of relativity is that all laws of physics are the same for unaccelerated (inertial) motion, so that obviously includes all the above mentioned play activity since the vessel is no longer accelerating.

    Actually it really does not matter if the vessel moves at some X miles per hour relative to Earth. It could very well move some Y miles per hour relative to some other planet in some other star system. There really is no such thing as the absolute speed of something. You can even say that the vessel is standing still and the earth is moving at X or that other planet is moving at Y, it would not make any difference.

    One more thing, even after the vessel accelerated to 99.99 percent of light speed, the speed of light to those in the vessel is still the same as it was when they were orbiting the Earth.
    Last edited: Dec 29, 2006
  6. Dec 28, 2006 #5
    Well - something is different - the occupants of the spaceship will not have as much time to finish a game as they had on earth - for example, if the game of life takes 80 years for a person on earth, and this time interval starts at the moment the acceleration phase ends and terminates at the moment the deceleration phase begins, the occupants of the spaceship will not be able to complete the game since the local spaceship clock will only log a few seconds of time during the 80 years measured by earth clocks during the inertial interval
  7. Dec 29, 2006 #6
    I am confused by this statement, since it seems to me that no such scenario is discussed here.

    Note that at the moment the vessel stops accelerating it maintains its relative speed. In other words it does not decelerate just because it stops accelerating.
  8. Dec 29, 2006 #7

    Chris Hillman

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    As Jennifer pointed out, it seems that you started asking about one scenario and switched without warning to asking about a second scenario.

    Pervect already gave some pointers regarding the first scenario (the "coasting rocketship".) In the second scenario, it seems that you are asking about something like the constant magnitude of acceleration scenario which is treated in "The Twin Paradox" at http://www.math.ucr.edu/home/baez/RelWWW/group.html.
    (Here, the direction of acceleration vector reverses at the halfway point of the journey, but the magnitude is constant throughout.) Once you have figured out what this magitude is, you can get a rough idea even with Newtonian intuition, if you can imagine what happens inside the cabin of a bus which is accelerating with constant magnitude acceleration. The reasons you offered are not correct, but you should expect Coriolis type effects when you toss a frisbee, etc. If you look up "Rindler observer", this should help you to understand what to expect inside the cabin of your relativistic rocket in a constant acceleration scenario.
    Last edited: Dec 29, 2006
  9. Dec 29, 2006 #8


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    yes - but according to their time (proper time) they'll only live for 80 years.
  10. Jan 26, 2007 #9
    Indeed, something is different! And correctly so, as predicted by the theory of relativity. Two clocks do not necessarily record the same elapsed time between two events.
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