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Acceleration and Time Dilation

  1. Sep 19, 2010 #1
    If an observer is in a rocket traveling at a constant speed, both the rocket and the observer are in the same frame of reference - right? Anything influenced within the frame of reference will behave the same regardless of speed. The closer a rocket gets to the speed of light, the speed limit within the frame of reference is reduced - I assume... right? Limiting the amount of movement. At 99.999% The speed of light, any movement within the frame of reference would be almost null. However, at speeds at, lets say, 50% the speed of light - most movement - such as a ticking watch, a heart beat... etc. Would be the same as if the speed was less. Time Dilation would occur, blah blah... so on.

    Now, can acceleration be described as moving from one frame of reference to another? The reason I ask this is because I want to know the effects of the local observation of time dilation durring acceleration. If we could accellerate to a reasonable speed close to the speed of light within 5 seconds, would the local observer feel time slow, and litterally see his watch slow down until a constant speed is reached - in which the feeling of time would resume as normal? Of course the crushing forces of accelleration would kill him... haha...

    I hope I'm clear on what I'm saying - not a physist here, just interested in physics. Any insight on the effect of acceleration on time would be great. Thanks
  2. jcsd
  3. Sep 19, 2010 #2
    When you talk about acceleration, you enter the realm of GR, rather than SR. Now I haven't studied GR much, but I would think that since the local observer's nervous system is always in the same reference frame as his watch, he wouldn't feel or see anything abnormal in his watch.
  4. Sep 19, 2010 #3


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    I think you might have a misunderatanding about the nature of uniform ( unaccelerated ) motion. Someone in uniform motion has no velocity. They are at rest in their own frame. Such an observer ( also called 'inertial') may appear to be moving when observed from certain other frames, wrt which they have a relative velocity.

    Regardless of any relative velocities, the inertial observer experiences no relativistic effects. Clocks run as before, heartrates, ageing and all processes are normal.

    When an observer accelerates (with rockets) their velocity wrt to other frames changes, and they themselves feel the acceleration while the motors are running ( as you know), but that is not a relativistic effect.

    Relativistic effects happen only to other observers, looking into your frame.

    Except for this, what you've posted is true. SR can deal with acceleration, as long as the space-time remains flat ( although spatial hyperslices may no longer be Euclidean).
    Last edited: Sep 19, 2010
  5. Sep 19, 2010 #4
    the observer ... is not mivong and has no velocity , hence we cannot apply SR to him ... his watch , will not slow down for him but it will be normal i guess ... but if you'd kept on sending him messages about your time every second then a time dialation would happpen as compared to the observer your time would be slower and you might think that the observer's time is also slower caz according to you he has an acceleration ,(negative here )
  6. Sep 19, 2010 #5
    If the rocket travels at a constant speed it is not accelerating. It confuses me since the title of your topic implies acceleration.

    Perhaps it is best to accurately state the question.
  7. Sep 19, 2010 #6
    The post(after the first paragraph) was about acceleration - though I can see how it might be confusing - I apologize.

    The first paragraph was to show the consistency of movement within a frame of reference with constant velocity. For example, when a car reaches its highway speed and maintains that speed - everyone in the car can behave as if they were standing still. A baseball can be tossed back and forth, a pendulum can dangle straight down, a glass of water will remain level. In acceleration - objects within the accelerating frame resist movement(because they are in there own frame of reference as well) thus moving them against the accelerated direction. If the cars engine was in the back and the front was cut completely off exposing the driver, the passenger, and any loose object to the opening - if the car suddenly stopped the driver, passenger, and objects would remain in motion and move outside the car - and without friction or gravity(or any other outside forces as if they were in space) they would all remain in their frame of reference while the car was removed from the frame. Acceleration can be observed as movement within a general frame of reference(and to an outside observer, movement within movement.) - thus the object moving(accelerating) is leaving the frame of which the observer is in.

    So, my first post obviously doesn't make since (it was late and I was tired)- because since the watch is accelerating along with the observer and the rocket - the time dilation would be almost non existent to the observer. However it would dilate - right? Because the simple act of relative motion dilates time - very little at small speeds - but it does. So the simple act of rising your watch to check it causes the watch to lose time(by several hundred trillion trillionths of a second I'm sure.) - at any speed or acceleration.

    Now, lets take a infinitely long wall of clocks. Imagine that it is covered in clocks that are 10 feet in radius and lined up one after another. If this wall was to accelerated at 60,000,000m/s2 to 99.999% the speed of light while the observer remained still, the wall would reach its max speed in just under 5 seconds. The simple act of relative motion dilates time. Since acceleration to the still observer is basically movement within movement I'm assuming(since I'm more awake and my previous thought seems silly) that the observer would see the time on the wall of clocks rapidly slow down until it reaches the constant speed. However, if the wall was accelerating and then the observer suddenly started to accelerate along with the wall - then the effect of the time slowing and then speeding back up would be observed. Right?

    Just curious... thanks.
  8. Sep 19, 2010 #7


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    There's no time dilation on the comoving watch. Time dilation is only seen from a frame which is moving wrt the watch.

    Be aware that speed and acceleration are completely different things. The only measureable velocities are relative velocities, but acceleration with rockets is local and not relative.
  9. Sep 19, 2010 #8


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    There's more to consider here than you think. The still observer would see the clocks slow, but not all at the same rate, while they are accelerating. This is due to the effects of length contraction and the Relativity of Simultaneity. Once they reach reach a constant speed, they will run at the same rate, but show different times. If the observer were accelerating along with the wall, he would also note that the clocks all ran at different rates during the acceleration. Clocks in the direction of the acceleration would run fast compared to clocks in the opposite direction and the further apart the clocks, the greater the difference in their tick rate.

    It is probably best to avoid looking at situations that involve acceleration until you really understand all the consequences of constant velocity.
  10. Sep 19, 2010 #9
    Thanks Janus
  11. Sep 19, 2010 #10


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    That's actually not true, see this section of the Usenet Physics FAQ.
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