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Help With Dilation

  1. Apr 21, 2004 #1
    Hey all, Im not really a physicist or even a science type guy (just a lowly lawyer in training).

    Anyways, I was reading up on time travel and relativity and I get the gist of it however, there is one concept I can't seem to get my mind around. Dilation.

    More specifically why does dilation occur? Does anyone know why or is it just something that has been observed with no real explanation? Thx.
  2. jcsd
  3. Apr 21, 2004 #2
    I too had the same problem with understanding the 'Why' when it came to speed/gravity being able to slow the time of a reference frame relative to others. I never really grasped it until I bought Brian Greene's book, "The Elegant Universe."

    Here's what I learned from reading his introduciton to the book. Einstein's theory showed that an object at total rest, not moving in any of the 3 spatial dimensions, is actually travelling forward in the 4th dimension(time) at 300,000 km/sec(the speed of light). But, if that object decides to move in the spatial dimenson 'Up' at 25,000 km/sec it is sharing it's total net speed between 2 dimensions('Up' and 'forward in the 4th dimension') and is now only moving 275,000 km/sec in the 4th dimension causing it to age slower than an object at total rest, because the rate at which time passes for you depends on how fast you are travelling in this 4th dimension.

    Now you may have heard that a photon is timeless, or that it doesn't 'feel' the effect of time. Why? Because it is putting its total net speed of 300,000 km/sec into a spatial dimension, causing it to travel at 0 km/sec in the 4th dimension.

    Hope this helps, it sure shed a new light on the topic for me. (everything in the above analogy takes place inside a vaccuum which is why 300,000 km/sec was used, otherwise you can just replace 300,000 km/sec with the constant 'c')
  4. Apr 21, 2004 #3
    Hi. It follows from Einstein's two postulates, that the speed of light is the same for all inertial observers, and that the laws of physics are the same in all inertial reference frames.

    The maths and all of special relativity comes from working through those postulates.

    But why it occurs is simply a consequence of "that is the way the universe works" and "all the experimental evidence supports the postulates and the conclusions".

    I'm afraid physics is not a science which deals with a reason for the mechanism, it can only attempt to describe the mechanism.
  5. Apr 21, 2004 #4
  6. Apr 21, 2004 #5
    Thanks for the reply, I'll definately look into the "Elegant Universe".
  7. Apr 29, 2004 #6
    Velocity makes matter expand at the molecular level. If you could compare a clock that is at rest to a clock that is traveling at near the speed of light, the high velocity clock will be many times bigger with the same mass and energy so it runs slower.

    All mass is held together by its internal molecular gravity. Velocity is antigravity.
  8. Apr 29, 2004 #7


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    Wrong, Pocketwatch. The time dilation has nothing to do with the physical natur of the clock. It occurs even for massless "light clocks". It is due to the basic nature of relativity (the Lorentz transformations). And there is no aboslute "going fast" because there is no absolute fixed frame of reference. All motion is relative between two observers.
  9. May 2, 2004 #8
    Okay. Let's suppose a man is in a spaceship that is traveling so close to the speed of light that in the time it takes for him to read your above statement, one year passes on earth. Now, we all know that velocity has no effect on the speed of light, and we know that brain synapses occur at near the speed of light, the only way that it would take him a year of earth time to read your statement is for the distance between his brain synapses to increase tremendously. That is the only way it could possibly happen. You must also bear in mind that everything appears perfectly normal to the man in the spaceship. He is not aware of any change in time.

    I believe that gravity is everywhere in the universe, perfectly balanced from every direction. Velocity has an effect on mass moving within that gravity.
  10. May 2, 2004 #9


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    His speed with relation to earth does not affect the physics in his own frame of reference, the spaceship. According to Einstein's first postulate, called Galilean Relativity, every inertial reference frame experiences the same kind of physics within itself as every other one does. So the space traveler would read the paper in the same way, with the same relation between light and brain function, that he would at home. But an astronomer viewing him through a telescope from earth would see that process taking a year. Likewise he would see the astronomer's actions slowed down by the same factor.

    I know this sounds like saying "It's just an optical illusion", but that's wrong. This is basic physics, all the quantum fields and particles have to obey it and it makes our world to be what it is.

    As to mass, you can do the Lorentz transformations in two ways. In one way, used by Einstein and most modern physicists, mass does not change under transformation (relative speed), but energy increases with relative speed according to a particular formula. So in this setup, there is no mass increase. Between Einstein's time and the modern era, a number of scientists used a formalism where mass increases but energy stays constant. Because of the intimate connection, [tex]e = mc^2 [/tex] in the rest frame, you can do it either way. But whatever formalism is used, mass increase is not a basic underlying reality that can be used by itself apart from other dilations.
    Last edited: May 2, 2004
  11. Jun 21, 2004 #10

    This is a nice analogy, to state things extremely simply.

    However, you should remember that it is meaningless to consider that anything could ever be motionless in space. The entire universe is in motion, and nothing can ever be at "total rest" with respect to the entire universe, or to any part of it.
  12. Jun 22, 2004 #11


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    Does all the matter in the universe have a center of gravity? Is it in motion? If so, relative to what?
  13. Jun 22, 2004 #12
    Yes. The center of gravity in the universe is the site of Big Bang. I suspect that it is not meaningful to consider that this is in motion.
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