On 26 December 2013 15:56, meekerdb <meeke...@verizon.net> wrote: > On 12/25/2013 2:45 PM, LizR wrote: > > 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. > > > 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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
