On 26 December 2013 07:23, Jesse Mazer <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 definition 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.

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