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Is time dialation really *time* dialation?

  1. Feb 7, 2005 #1
    I was reading some interesting articles at wikipedia on black holes and time dialation:


    I remember this subject interested me greatly when I was in high school physics. I'm pretty rusty now, so bear with me. The "facts" I present may be warped and demented. Fair warning.

    Reading this information, and going on what little I remember, time dialation is the apparent slowing down of time of an object as far as an observer is concerned. One of the wikipedia articles goes as far as mentioning that this slowing down of time would make it possible to time travel. I'm not so certain I believe whether this phenomenon is *actually* an effect of time.

    To illustrate my point, consider this hypothetical situation:
    Your friend is in a car, driving at 20m/s. He throws a ball out the window, in the opposite direction that he's driving. Let's say he throws the ball hard enough to give it a 30m/s velocity in the opposite direction that he's driving.

    You are watching your friend from behind the car as he drives away from you. When he's driving at 20 m/s and throws the ball out the window at 30, you see the ball move 10 m/s towards you. If he accelerates to 30 m/s and throws the ball in the opposite direction with the same velocity as before, you see the ball fall to the ground, never approaching you. From the driver's perspective, of course, as he's in a moving vehicle, the balls are all moving away from him.

    What if you apply that same principle to light? The car becomes an object travelling towards a black hole. The ball becomes a photon. You are still you.

    As the object accelerates, the photons that reflect from it, or are thrown from it, head in your direction at increasingly slower speeds. As these photos reach you, you see an image of the object. But as the photons are reaching you so slowly, and continue to take longer and longer, the image you see appears to be decellerating.

    You perceive decelleration, but the object is actually doing the opposite.

    The object flying towards the black hole's event horizon will appear to take forever to get there. What's really going on, is the *photons* are taking "forever" to get to you, but the object is long gone.

    I'm just wondering if this "time dialation" is simply perceived, but does not really exist, and so prohibits the exploitation of time dialation to travel into the future. I would tend to think so... am I off base?
  2. jcsd
  3. Feb 7, 2005 #2
    This might just be me but the idea you are suggesting is not affected much by time. Objects become more 'squashed', to an observer, in the direction they are travelling. I don't think the speeds are great enough to suggest a real change in time at all.

    So long as the photon is reflected far enough away from the black hole then it should travel at a constant speed towards you and so would not appear to deaccelerate.

    The Bob (2004 ©)
    Last edited: Feb 7, 2005
  4. Feb 7, 2005 #3

    I'm not speaking strictly in terms of a single photon, nor of the squashing perspective, but of the effect of time dialation specifically and speculating about what it really is/ what causes it.

    A single photon may not accelerate/decelerate. But consider the fact that in real life, you are not observing a single photon... you are observing many of them in successive "waves".

    If a constant stream of photons is leaving this accelerating object and heading in your direction, and if we believe that the photons will travel more slowly as the object accelerates (given the example I posted), the photon density in the stream heading in your direction will decrease as the object accelerates.

    Er, in other words... The faster the object travels, the longer it will take photons to reach you, both due to larger distances involved, but also due to the initial velocities on the photons. If you only consider a single photon, this means nothing. If you consider that your eyes are pecieving a constant stream, then it makes sense - as the object accelerates, its image takes longer to reach you. It accelerates, but appears to decellerate.
  5. Feb 7, 2005 #4


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    Without having to read any further, this is where you made a fatal error and a major fault in your understanding of Special Relativity.

    The speed of light is a CONSTANT in ALL reference frame. It doesn't matter where it originates, how it originates. All observers observes the SAME speed of light, no matter how they are moving relative to one another. This is the MOST fundamental postulate of SR, and the one that results in the time dilation. Time dilation is a consequences of this postulate. You cannot fully understand this unless you first understand and learn all the postulates of SR. Only then will you realize that your velocity addition or subtraction (what is now known as galilean transformation) will not work at speed close to c, which requires the lorentzian transformation.

    Take note that if time dilation isn't "real", your GPS system will go horribly wrong and our lives would have been in danger.

  6. Feb 8, 2005 #5


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    one more thing to add to what others have said is that, in reality, because of what we learned regarding special relativity, velocities do not add linearly as you have done. i think, if you have a car traveling at [itex] v_1 [/itex] and someone throws a ball in the same direction from the car at a velocity [itex] v_2 [/itex], that the velocity measured by an observer on the ground is

    [tex] \frac{v_1 + v_2}{1 + \frac{v_1 v_2}{c^2}} [/tex]

    the velocities will not be the same as simply adding them [itex] v_1 + v_2 [/itex].

    r b-j
  7. Feb 9, 2005 #6
    Agreed in order to understand how things dilate at higher velocities it is quite necesary to understand the lorenz transforms for objects moving at speeds close to light. They actually happen at lower speeds its just so minor that we ignore it.
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