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Question About Time and the Speed of Light

  1. Jul 27, 2012 #1
    I recently watched a documentary on Albert Einstein that said the inspiration (or epiphany I guess) for the slowing of time as speed increases occured to him while he was on a bus. It said that one of Einstein's day dreams, was that he would imagine himself riding on a beam of light. Well, while he was on the bus, he was having this day dream, and he was watching a clock tower out the back window, leading him to theorize that because he was going the spped of light, the light from the clock tower wouldn't be able to reach him, so the image of the clock would remain the same as when he started going light speed, thus stopping time. I was confused by this. Yes, the image of the clock would remain the same, so if you were to read it, it would never change. My question is, how is that actual time dilation and not just the image staying stationary (like taking batteries out of a clock- time would still pass, the clock just wouldn't show it)? Sorry this may sound stupid, because I don't know much about the subject- I'm still in high school; but I just don't understand how time is linked to light. Does this have something to do with the increase in mass that occurs at that speed, therefore causing a greater impact in the fabric of spacetime with greater gravitational forces etc.? I know that supermassive objects like blackholes create that event horizon that bends space and time but I have no idea if that's in any way relevent haha. Again, sorry if this all sounds like nonsense- because maybe it is- but I don't understand this concept very well.
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  3. Jul 27, 2012 #2


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    That epiphany is just an idea. It lead him to a more well-thought model. Don't take it as correct.

    Read the FAQs; they should help you out.

    https://www.physicsforums.com/forumdisplay.php?f=210 [Broken]
    Last edited by a moderator: May 6, 2017
  4. Jul 27, 2012 #3


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    When you watch a receding clock, you're not seeing just time dilation nor are you seeing just the image being slowed down because of the increase in the light transit time. It's a combination of both.

    You can be sure that this story could not have been an inspiration for Einstein because he would also have considered what the image of an approaching clock tower would look like out the front window where it would look sped up, correct? The clock isn't going to change its tick rate just because you pass it, would it? Of course not.

    But according to the Principle of Relativity (not to be confused with the Theory of Special Relativity), someone in the clock tower looking at a clock on the bus will see it sped up by the same amount on approach that the traveler see the clock in the tower sped up and both see the other clock slowed down by the same amount after the bus passes the clock tower. In order to explain this, you have to conclude that clocks must be affected by motion but it's never clear exactly how to assign that effect. This was known even before Einstein but since it didn't occur to prior scientist that the propagation of light could be relative nor that time could be relative, they offered an explanation that was based on an absolute frame for the propagation of light and for the flow of time.

    It was quite a challenge for anyone to think in a new way and that's what Einstein did although I don't know what his inspiration was.
    It isn't directly related to mass or gravity, that came later in General Relativity. I suggest you look up Relativistic Doppler in wikipedia to understand what you see of clocks that are traveling at high speeds.
  5. Jul 27, 2012 #4
    That isn't why time dilation occurs. It isn't just something that observer sees, moving observers actually measure less time in between two events. This is because the speed of light is the same value for all observers, regardless of their state of motion, as long as they're in an inertial frame of reference (constant velocity). In order to preserve a constant speed of light, an observer will note that observers who are moving with respect to him are recording less time, and have been length contracted (they're shorter along their direction of motion).

    As I've said, the key is that observers agree on the speed of light in a vacuum. For a qualitative description, remember that speed is distance divided by time. Next, you need to understand what a frame of reference is - it's essentially a coordinate system in which an observer considers himself to be at rest, and everything else to be moving around him. So, moving at 100 m/s (relative to the earth), you are justified to say that in your FoR, you're at rest, and everything else is moving by you at 100 m/s. So, two inertial reference frames will measure different speeds for objects in classical mechanics. However, it turns out that light travels at the same speed for ALL inertial reference frames.

    Naturally, since an observer in motion measures some object as traveling a different amount of distance than an observe in a different FoR, they must disagree on the speed. However, this can't be the case for light, since everyone agrees on it's speed. Since speed is distance over time, these inertial reference frames must therefore disagree on the elapsed time.

    That's the absolute basic idea. You should look into some resources, if you're interested in the topic.

    EDIT: Ah, ghwellsjr beat me to it.
  6. Jul 27, 2012 #5
    Thank you! That was the perfect way of explaining it I think. I'd never really considered that in those reference frames, everything can be relative, but light would stay constant, meaning time would have to adjust. That definitely answered my question.
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