Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time dilation- V relative to what?

  1. Mar 20, 2007 #1
    Time dilation-- V relative to what?

    I'm new to the concept of relativity, and only know what I've read in the few books I have or information that has been exchanged by my peers.

    I understand that as you approach c, you experience less time. I feel that I finally understand the "experiencing less time" part. I also know that velocity is relative. Anyone anywhere can declare themselves at rest (even if they are experiencing a net force, is this correct?).

    Now that I've learned both of these concepts, they don't seem to make much sense together. Something experiences less time when it has a velocity close to c relative to what? What if two people, A and B, were both wearing watches; they are in deep space. A is traveling at .9c and B is stationary. Obviously, A's watch is going to be slow compared to B's. Is this the only way to prove that A was, in fact, the one at a higher velocity? But why can't he say that he was at rest, and therefore believe that B's watch MUST be the one experiencing less time, only to be proved wrong when they met again?
  2. jcsd
  3. Mar 20, 2007 #2


    User Avatar

    Staff: Mentor

    Velocity is always measured relative to another object. If two objects are moving wrt each other, it is impossible to show which is moving and which is stationary, so either can reasonably assume they are the stationary one. But bringing them together requires breaking this symetry - one object must accelerate.
  4. Mar 20, 2007 #3
    I'm sorry to say thins but I think you've still haven't got it.

    Nope. When a non-zero net force is acting on you, you're in a non-inertial frame. The (Galilean) principle of relativity states that all INERTIAL frames are equivalent.
  5. Mar 20, 2007 #4
    As a direct answer to the thread question - Anything you like.

    But I shall elaborate.

    No, you experience exactly the same time. The point is that everybody else sees you experiencing less time. The point of relativity is that these two apparently inconsistent observations aren't inconsistent at all.
  6. Mar 20, 2007 #5
    So with the responses I've gotten, I feel like I have less of an understanding than what I came here with. :P

    So you mean that one of them will be experiencing less time, but they'll never know because they will never be able to be re-united? If my understanding of your statement is correct, then that can easily be bypassed by telling each of them to send a radio signal when their watch hits a specific time. Both of their watches will not be in sync with each other-- one of them will send the signal before the other. However, they are both declaring themselves at rest... so one of them must be wrong?

    So reading this, I think my understanding of relativity is bent a little. From what I understand, if I'm traveling at c, then the time I experience less time compared to everyone else (I've read that particles traveling near c have a longer lifespan, which goes along with my understanding, along with the twins paradox). Am I correct up to this point? I don't understand what you mean by "No, you experience exactly the same time. The point is that everybody else sees you experiencing less time." Could you please explain further?

    Thank you for the help, everyone. It's much appreciated. :)
  7. Mar 21, 2007 #6


    User Avatar
    Gold Member

    Simply put, if you used a telescope to peek at a clock in a spaceship moving at a constant velocity relative to your spacesip - their clock would appear to be running slower than yours. The crew of the other spaceship, peeking at your clock would see it running more slowly than theirs.

    This is not a contradiction although it appears so.

    Statements like 'as you approach c' do not mean anything unless you specify a frame of reference in which the velocity measured.

    There's no such thing as an absolute frame of reference so we can choose who is 'stationary' more or less arbitrarily.

    Some physicists just remember the maxim 'moving clocks run slower'.
  8. Mar 22, 2007 #7
    Thank you very much for your explanation!!!! So I have been wrong the whole time, and I think I understand it now.

    So both parties, traveling at a difference in velocities of .9c, will both see the other going in slow-motion?
  9. Mar 22, 2007 #8

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Let's see if I can restore your state of confusion. :biggrin:

    If each observer watches the other observer's watch with a telescope, each sees the other watch running at a rate different then their watch by a numerial factor equal to the Doppler shift. This numerical factor is very different from the amount of time dilation.

    Now I'll throw another spanner into the works. Time dilation, appropriately considered, occurs regardless or whether the two observers are moving away from each other, or towards each other. This is not true of what is seen in the telescopes.

    If someone is moving away from you, then, as you say, the image of his watch in your telescope moves more slowly than your watch; however, if the other observer is moving towards you, then, in your telescope, you see his watch moving faster than your watch.
    Last edited: Mar 22, 2007
  10. Mar 22, 2007 #9
    Haha, you cannot! At least not with what you said. :P

    I actually understand the Doppler effect. Blah, frequency of light, blah, c is constant, blah, if you're moving away from the light source you intercept the wave as a slower frequency. :D
  11. Mar 23, 2007 #10
    SR says if two spaceships S and S' pass each other with relative velocity v, two clocks separated by a distance d in S will measure a single clock in S' to have logged less time as it transits the distance d than the two clocks in S

    This is reciprocal - the situation is symmetrical since either spaceship can consider itself to be at rest. This is therefore an apparent or observed phenomena - it says nothing about how fast the clocks are running - in fact all clocks run at the same speed in their own spaceship so long as they are not accelerating. To measure an actual permanent age difference, some asymmetry is required. Einstein first introduced what he called a "peculiar result" in his 1905 paper. First he synchronized two clocks that were at rest but separated - then accelerated one to a velocity v and headed it toward the other clock. He states that when the moved clock reaches the other clock it will read less than the clock remaining at rest - here the asymmetry is found in the fact that only one clock has been accelerated - there are no age difference problems in SR that do not involve acceleration in some form or another. Acceleration destroys the symmetry.
  12. Mar 24, 2007 #11
    So time passage variation due to v elocity is an illusion?
  13. Mar 24, 2007 #12


    User Avatar
    Science Advisor

    No, it's very real- If persons A and B are moving at a very large velocity relative to the other, then each will see the others clocks (and heart beat and hair growing, etc.) as slow compared to his own. but it is NOT an "illusion".
  14. Mar 24, 2007 #13


    User Avatar

    Staff: Mentor

    That's normal - this is a strange concept when you first hear about it.
    I don't know why you would say that - you can re-unite people in spaceships if you choose to. One of them has to fire their rockets, which then breaks the symetry and determines who's clock shows less elapsed time when they meet.

    If two ships from different galaxies are flying toward each other and they've never met before, either one can be considered stationary. But if one fires its rockets to mee the other, the one who fired his rockets has changed his state from moving to not moving, breaking the symetry.
    Answered with the telescope analogy by others.
    Yes. Suppose you were in a spaceship flying in circles around someone to avoid the issue of changing distance. Every time you complete a circle, you send a signal of what time it is to the guy you are flying around. He compares it with his watch and finds that you think it takes less time to complete the circle than he does. This is what GPS satellites do.
    That just means that if you close your blinds and never communicate with the ouside world, you can fire your engines as long as you wish and you will never notice any difference in the way time and other issues in physics work inside your spacecraft.
    Last edited: Mar 24, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook