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

Is this a suitable explanation of relativity?

  1. Jan 29, 2008 #1
    this isn't my explanation, but I was just wondering what others thought:

    "Einstein's special theory of relativity showed that an object can be of two different sizes for two different people and that they can perceive the time between two events to have different duratoins. His General theory of relativity suggested that space itself is not just a stage for the material world to act on, but is an active participant in the play. Space, for example, gets bent out of shape by any matter that is hanging about in it. The relativity theory talks a lot about "frames of reference." From the point of view of relativity, things standing still with respect to one another are in the same frame of reference. Things moving relative to one another have different frames of reference. Distorting a frame of reference is equivalent to distorting space itself, and it is this distortion, by the agency of gravity, that the general theory predicts."

    I don't know too much about general relativity, but is the part at the beginning, about special relativity true? I thought if two objects were of different velocities (rather than size) that the duration changes?
  2. jcsd
  3. Jan 29, 2008 #2
    I'm not 100% sure to be honest, but i do think that the velocity, not only the size, has an effect on the duration of whatever the object is doing, such as moving across your line of vision or something. For example, if you're running at the speed of light, meaning your V = C, then gamma = infinity. And gamma is basically the time dilation factor. So while moving at the speed of light, gamma will simply slow things down infinitely, which is why time stops. So i'm not 100% sure, but the velocity of something MUST be included into the equation for determining gamma, or how much the duration of something will be slowed down by. Therefore, yes, different velocities lead to different durations. But the size still could matter too, because i know that in some way the mass of something effects the speed that it can move at, because the more mass of an object, the more energy is required to move it at a high speed. Hope that made sense and actually answered your question.
  4. Jan 29, 2008 #3


    User Avatar
    Science Advisor

    Well, no massive object can ever move at the speed of light in relativity; but the gamma factor is equal to 1/squareroot(1 - v^2/c^2), so you can figure it out for any velocity less than c. So, for example, if a clock is moving at 0.6c relative to me, then in my frame the amount of time between successive ticks will grow by a factor of 1/squareroot(1 - 0.6^2) = 1/squareroot(0.64) = 1/0.8 = 1.25.
    I think the comment about size was not relating to mass, but rather to length--in relativity an object moving to me will shrink in length along its axis of motion in my frame, a phenomenon known as "length contraction" or "Lorentz contraction". Objects shrink by 1/gamma, so in the example above where the object is moving at 0.6c in my frame, its length will shrink by 1/1.25 = 0.8 in my frame.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook