On 26 December 2013 19:11, meekerdb <meeke...@verizon.net> wrote: > On 12/25/2013 9:15 PM, LizR wrote: > > 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. > > > Hey, it's not something I made up. Check Weinberg's "Gravitation and > Cosmology". He uses the 4-velocity frequently, e.g. in Ch9 eqn 9.8.1 thru > 9.8.6 he writes the T^00 component of the stress energy tensor as > rho*U^0U^0, where U^0 is the time-like component of the 4-velocity of a > perfect fluid. Robert Wald does much the same in "General Relativity". Or > look at page 50 of Misner, Thorne, and Wheeler where they write,"More > fundamental than the components of a vector is the vector itself. It is a > geometric object with a meaning independent of all coordinates. Thus a > particle has a world line, P(tau), and a 4-velocity U=dP/dtau, that have > nothing to do with any coordinates." >
OK, Brent, my apologies if I have misread you. But you are supporting a view that doesn't make sense in terms of SR - nothing is actually moving through spacetime, and giving (apparent) support to the notion that it is isn't going to help. I don't have most of those books you mention, but I do have "Gravitation" (which my other half got for his 18th birthday in 1973) open to page 51, box 2.1 - "Farewell to "ict" - and have just had my mind suitably boggled by reading about 4-velocities. Please note, everyone (I'm sure Brent knows this already) that these are NOT velocities *through* space-time, they are handy vectors for working out what is going on at a point along an object's world-line. The object doesn't move through space-time, it exists at various points in space-time which joined together make a 4 dimensional object known as a world line. One can draw vectors at points along this world line and use them to work out its "4-velocity", which I assume is a quantity useful for working out how its clock goes in relation to other objects, and/or how the various Lorentz transformations work - or something along these (world) lines - but this does *not* mean that things are moving through space-time or that there is a common present moment, or that the past doesn't exist, or any of the other things Mr Owen has claimed. I think Brent, who knows all this stuff backwards and sideways, is just toying with us .... naughty Mr Meeker. -- 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.
