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Does time really slow down?

  1. Apr 5, 2005 #1
    I know that these sorts of questions might have been asked before and probably answered as well, but I just wanted to make sure of my understanding.

    Accourding to SR Time will slow down accourding to an objects velocity. Now the question is even if you were able to travel @ the speed of light would you feel any slowness of time? Or is you time slowed compared with a non-moving object?

    My current thought is that it will slow compared to a non-moving object (or from another frame of refrence.)
     
  2. jcsd
  3. Apr 5, 2005 #2
    v <> c!!!

    You can never reach the speed of light. But everybody (even you!) has wounded what happends when v = c. Well...time stops, you have no dimentions, and your mass in infinite (v=c, t=0, m=1...~).

    According to SR, you will still exist, but nothing will exist around you because you are at rest and the universe is moveing at c past you. Time does not slow down for you, it only slows down from the reference frame of anything moveing faster than c (thats if v = c for you).

    Now let me ask you this. How do you intend on traveling faster than light :rofl: The only way I have though of is to have negative mass.
     
  4. Apr 5, 2005 #3
    This is a description of what you would look like to a stationary observer. It is not however the mass that is infinite at v = c, it is the momentum (or energy).

    As far as you are concerned, everyone else is traveling at the speed of light and you are standing still, so you do not feel weird at all (even though you are the one "actually" moving). Time goes normally for you, and you could play with a ball that would move according to classical physics (newton's laws).

    The weird effects of special relativity only occur when observers look at eachother with relative motion between them.
     
  5. Apr 5, 2005 #4

    JesseM

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    You can't really talk about what things would look like "as far as you are concerned" if you were moving at the speed of light, since an object moving at the speed of light does not have a valid inertial rest frame of its own. You can talk about what you'd see in the limit as you approach light speed (relative to some external landmark like the galaxy), though.
     
  6. Apr 5, 2005 #5
    In your attempt to be a pedant you have fallen in to your own trap: there is no such think as an object moving at the speed of light so it is pointless to say that such an object "does not have a valid inertial frame of its own". Your statment is somewhat like saying "unicorns do not have a valid inertial reference frame of their own".

    I think it is fairly obvious that when we discuss what things would look like at the speed of light, our discussion is based on the limiting case v goes to c in the equations of relativity.
     
  7. Apr 5, 2005 #6

    JesseM

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    How are you defining "object"? I'd say a photon is just as much an "object" as any other particle.
    OK, but there are some quantities which don't even have well-defined limits as v approaches c, like the velocity of other things which are moving at c in our frame. If you have two particles A and B which are both moving in the same direction at c in your frame, and you want to know how fast A would be moving in B's pseudo-frame, you could either look at the limit as two particles at rest relative to each other approach c in your frame (in which case you'd get the answer that A is at rest in B's pseudo-frame), or you could look at how B will see a particle A moving at c in your frame in the limit as B's velocity approaches c (in which case you'd get the answer that A is moving at c in B's pseudo-frame).
     
  8. Apr 5, 2005 #7
    JesseM said: "....but there are some quantities which don't even have well-defined limits as v approaches c"

    This sort of apparent ambiguity happens all the time in thought experiments involving a singularity. In reality one of the particles would always be going a little faster than the other and the uncertainty vanishes.
     
  9. Apr 5, 2005 #8

    JesseM

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    Not if they are both massless particles like photons (or if you want to stick with classical physics, electromagnetic waves), in which case they'll both move at exactly c in our frame.
     
    Last edited: Apr 5, 2005
  10. Apr 5, 2005 #9
    JesseM,

    "Not if they are both massless particles like photons...."

    Nice try!

    But you were talking about "quantities which don't even have well-defined limits as v approaches c" That doesn't apply to photons.
     
  11. Apr 5, 2005 #10

    JesseM

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    "v approaches c" in the sense of taking a mathematical limit in order to see what things would look like in the pseudo-frame of something moving at exactly c, not in the sense of actually accelerating a particle to velocities closer and closer to c.
     
  12. Apr 8, 2005 #11
    So my understanding was right.

    No matter what V equals to you would still see time go past at the same rate for you.

    But it is slowed from another frame of referanse.
     
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