On 12/25/2013 2:45 PM, LizR wrote:
On 26 December 2013 07:23, Jesse Mazer <laserma...@gmail.com <mailto:laserma...@gmail.com>> wrote:

    The notion that everything "travels through spacetime at the speed of 
light" was
    popularized by Brian Greene, but it only works if you choose a rather odd 
    of "speed through spacetime", one which I haven't seen any other physicists 
make use of.

Mainly because it doesn't make sense. Speed is change of position with time, hence "speed in spacetime" equates to the angle a world-line makes relative to some world-line chosen as a basis, e.g. the rest frame of the Hubble flow. Things don't move through space-time, they move through space. They are 4 dimensional objects embedded in space-time.

But when you are "standing still" your time coordinate keeps increasing. Your 4-velocity in your own inertial frame is always (1 0 0 0).


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