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I don't understand why your clock slows down

  1. May 10, 2010 #1
    I've managed to grasp general theory of relativity pretty well but I'm stumped on special theory.

    I understand nothing can travel at the speed of light, and that the speed of light is always constant. So the closer you get to catching the speed of light, it still looks like it's beating you by 671 million miles an hour.

    But why does time slow down for you, the closer you get to this speed?

    I just don't see the connection.

    Your clock is slowing down too, just like lights clock. I get that you can't go the speed of light, and light will always speed off from you at the speed of light, but why does your clock slow down? What does that have to do with anything?

  2. jcsd
  3. May 10, 2010 #2


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  4. May 10, 2010 #3

    ugh i get why this would happen for a light clock but what about a regular watch. There is no area that a light signal has to bounce back and forth between. I don't see why a regular clock would tick slower...

    edit: wait I get it, it's all because of the principle of relativity.
  5. May 10, 2010 #4
    ok I almost get it.

    the clock slows down because there needs to be changes in physics for it to make sense that the light wave always advances on at the speed of light.

    But why is that the answer? Time slows down, but I don't see how that explains why the light wave proceeds on at the speed of light. The light wave's clock is also slowed down to nothing, so your clock is just matching that clock.

    How does that explain why the light is always speeding off at the speed of light?
    Last edited: May 10, 2010
  6. May 10, 2010 #5


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    I don't think you've grasped that the slowing down of clocks is a velocity dependent effect that is observed between inertial frames of reference. There is no absolute motion so talking about a clock slowing down because of its velocity is not sensible. Only relative motion can be detected and measured ( provided this uniform motion, i.e. not accelerated motion ).

    Look up 'time-dilation'
  7. May 10, 2010 #6


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    Hm, the point is that there is no universally correct time or whatever - both are dependent on the observer. And it's not this phenomena which "explains" why light travels at a certain speed - it's vice versa.
  8. May 10, 2010 #7
    As far as we can tell, it does appear to be true that the speed of light is the same for all observers. This fact was predicted by Maxwell when he united the electric and magnetic forces into one. From his equations it was possible to calculate that the resulting electromagnetic field travels at a constant speed relative to all observers. It was when Maxwell realised that this constant was the very same speed at which light was known to travel that he guessed that light was, in fact, electromagnetic radiation !
    There is, as yet, no intuitive explanation to why the universe should act like this. Since Maxwell's work, numerous experiments have been performed to test the prediction that electromagnetic radiation travels at the same speed for all observers - and none have failed. Instead of being a prediction from theory, it now became to be used as an assumption to build theories upon. Einstein was so convinced of its truth that he modified Newton's theory of gravity to encompass the constancy of light. Likewise, in the 1940s, Feynman, Tomomaga, Bethe and others incorporated the idea into Quantum Mechanics. The resulting theories, General Relativity and QED, are probably the most accurately tested to date - and they require that the speed of light is constant.
  9. May 10, 2010 #8
    This is the exact phenomena that I imply in regards to achieving a straight line through spacetime. From my understanding the change of course in your position in time or "clock" does not fall out of synchronization relative from light's clock, because the light is the clock. There for the action of change of motion at near the speed of light will result in the object in question being de-synch from time therefore traversing through an alternate path in spacetime.
  10. May 11, 2010 #9
    I think I get it. There's no universally correct time. Time only slows down for the person chasing light. For the person observing it, time doesn't slow down.

    The 2nd person who posted, put up a really helpful link.

    That link says Einstein came up with the idea that time slows down for the one chasing light because something must explain why one person sees the light speeding off at the speed of light and the observer sees the person chasing the light, right on the tail of the light.

    It says there must be a change in how we view physics.

    This is I think why time slows down for the one in motion, because light then can get away at the speed of light. But time goes by normally for the observer, which is why he sees the person chasing light, right on the tail of the light.

    Is that right?
  11. May 11, 2010 #10
    I think you are pretty close to an understanding there. Here is something else to consider. See what you think.

    You are stationary. Your friend is hightailing to the right at 0.9c relative to you chasing after a photon. As you said, from your point of view his clock is running slow and that partly accounts for why he sees the speed of that photon as c from his point of view. Now imagine there is a photon going to the left of you and you are still stationary. From your friend's point of view you are going at -0.9c and chasing after the left going photon (although you were not aware of this!) From your friends point of view your clock is going slower than his clock and that is why you "imagine" the speed of the left going photon relative to you is c. If you followed the above you will realise that travelling at 0.9c is no different to being stationary and is just a point of view.
  12. May 11, 2010 #11
    yeah that makes sense!

    It was just hard for me to make that connection between your clock slowing down and the light escaping. But really it is good someone mentioned the light escaping explains why time slows down if you move close to the speed of light, not the other way around.

    I was also thinking about it in kind of silly terms like: If I am walking to work and time slows down for me as I move toward work. I'm gonna be late to work because time would go by normal for my boss, who needs me at work.

    And I think that is why the light gets away if you try to catch it, because time slows down for you but not the light, and it just zooms ahead.
  13. May 11, 2010 #12
    Almost, given two objects you cannot say that one of them is in motion and the other one is not, they are both in motion with respect to each other. Motion is relative.
  14. May 11, 2010 #13


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    I tried to tell the OP this in my post but I think he's away in his own world.
  15. May 14, 2010 #14
    now i can see what troubles you. I didn't understand first what do want from slowing of light clock.
    Now see, if a man in rocket is traveling at 80-90% of c and he has a light clock in rocket in which a pulse of light is bouncing up and down. for the man in rocket pulse of light is bouncing normally but for you (who is sitting on his roof with a telescope and watching the clock) the pulse isn't seem to bouncing up and down instead it will seem to bouncing diagonally and thus will have to travel a long distance and thus will take more time to complete one oscillation. what i mean to say is that due to this phenomena time will appear to go slowly for you. this is what Einstein wants to say and it is also happen to a normal clock but you have to keeep in mind that time is not absolute and changes for everyone with diffferent speed.
    thanks for putting up this question.
    hope you will understand.
  16. May 14, 2010 #15
    This is a bad example, because such behavior of light is against Maxwell equations.
  17. May 15, 2010 #16
    you understand it wrongly. Listen Tantalos pulse of light is seems to traveling in diagonal way to you but it did bouncing up and down all the time and didn't goes against the Maxwell law. it is relativity you are talking and in it such phenomena occurs and this point is confirmed by a Harward university professor (i didn't remember his name) who shows it in Discovery channel's programme.
  18. May 15, 2010 #17
    Objects with mass will do, but not light. Light if sent in direction perpendicular to the moving spaceship will not move along with the spaceship. The same says QM - momentum of photon is p = hk, where k is a vector in the direction of the moving wave. If the light pulse is very short it will miss the mirror.
  19. May 15, 2010 #18


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    No. If the light strikes the mirror in the rest frame of the mirror and emitter, it will be seen to strike the mirror in all frames. If you draw a space-time diagram of these events it will be clear that the existence of the events ( i.e. the coming together of world lines ) is unchanged by Lorentz transformations.
  20. May 15, 2010 #19
    So let's place in the spaceship 2 lasers pointing at each other in direction perpendicular to the movement of this spaceshishp and emitting light waves in opposite phases. Then the observer on the spaceship will say that the light waves cancelled. But the observer in the rest frame will say no, the lights are 2 diagonals that cross at one point. Can it be possible?
  21. May 16, 2010 #20


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    The phase at the meeting point is not changed by Lorentz transformation. If we had a detector in the middle of the ship we would see the beams arriving in a certain phase. The intertial observers see the same same phase.

    In the pictures I've indicated the peaks in the light wave as red dots.

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