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

Space-time contraction question

  1. Aug 1, 2006 #1
    We know from experiment that time (a measure between two events) slows as we approach speed of light, but does the meter stick (a measure between two entities) then shorten as we approach speed of light ? If yes, since time stops at speed of light, then does meter stick disappear at speed of light ?
  2. jcsd
  3. Aug 1, 2006 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Time and space are relative terms that depend on the relative motion between the measuring frame and the rest frame of the object being measured.

    The length of a meter stick moving at relativistic speed relative to you would appear shorter than 1 m. But your meter stick would also appear shorter to the relativistically moving observer. The effect is similar for time - time in the moving frame appearing to move slower.

    Time does not stop at the speed of light. The speed of light cannot be reached. Light always moves away from the moving observer at the speed of light. A meter stick will never appear to have 0 length in any inertial frame (mind you, receiving a light signal from the moving meter stick in order to measure it could be difficult so it may seem to disappear). For things that travel at the speed of light (e.g. photons), time and space has no meaning. Time and space has meaning only in relation to matter and space.

  4. Aug 3, 2006 #3
    Thank you, but do not these two statements form a contradiction... "The speed of light cannot be reached" & "For things that travel at the speed of light (e.g. photons), time and space has no meaning" :confused: Also, if photons are not within "space-time" where are they ? Your answer defines photons as being "things" that exist so they must exist somewhere, but where if not within "space-time" ?
  5. Aug 3, 2006 #4
    length and time

    i think that the relativistic effects you mention depend on the way in which you measure length, clock reading and time interval. i would avoid to say that clocks are slowing down.
  6. Aug 3, 2006 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    The situation is this:

    Massive particles and physical objects cannot move at the speed of light. They must always move slower than 'c'.

    Photons and other massless particles must move at exactly 'c'.

    People often try to think about what a photon would experience as if it had human qualities. This is a mistake.

    I would say that the issue here is that one is "anthropormorphizing" the photon -one is treating it as if it were a human being, and it's not.

    See for instance


    Of course, part of Einstein's genius was to see that these questions didn't lead anywhere.

    In an abstract sense, one can construct coordinate systems in which the trajectory of a photon is stationary.

    These are called "null coordinates" and the transformation rules are quite simple. Using geometrized units in which c=1, the transformations to null coordinates are just:

    u = x-t
    v = x+t

    These coordinates are perfectly valid mathematically, and are even used in General Relativity. They describe a coordinate system in whch a photon moving along the x axis is represented by a single number (u, or v, depending on which way the photon is going).

    Combined with standard 'y' and 'z' coordinates, one can construct a complete 3-d coordinate system out of these null coordinates.

    This abstract mathematical description is probably as close as one can come to ascribing a "point of view" to a photon. Note that this coordinate system does not have any such thing as a "time" coordinate - instead, one has two null coordinates (one for photons moving in the +x direction, another for photons moving in the -x direction), and two spatial coordinates.
  7. Jan 12, 2009 #6
    ok,i have a doubt on Alcubierre warp drive. According to this theory if space-time wrap can be created by bending it and establishing a connection through worm hole then interstellar travel is possible more than light speed. what will be the time taken by the bended space from point A to point B to become normal if the gap is about 5 Light year?
  8. Jan 12, 2009 #7


    User Avatar
    Gold Member

    Wormholes do not allow us to travel at faster than light speeds; they allow us to shorten distances. At no point is the craft moving faster than light.
  9. Jan 12, 2009 #8
    Keep in mind such time dilation and length contraction is relative to the other observer's frame...not yours. You see those effects relative to another frame of refernce, and from that reference you are observed the identical way; but in your local frame everything remains the same. Unless you are entering a black hole.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook