On 26 December 2013 15:56, meekerdb <[email protected]> wrote: > On 12/25/2013 2:45 PM, LizR wrote: > > On 26 December 2013 07:23, Jesse Mazer <[email protected]> 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. > > > 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). >
If you insist on using this "velocity through space-time view", yes. But if you consider yourself to be a worldline then you have no 4-velocity, only a 3-velocity, which is measured as the angle your worldline makes to the vertical axis (modulo the usual caveats about there being no preferred reference frames). Here is a diagram of how time isn't... [image: Inline images 1] And here's a diagram of how it actually is... [image: Inline images 2] ...both are from Chapter 11 of FOR. -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
