On 26 December 2013 19:11, meekerdb <[email protected]> wrote: > On 12/25/2013 9:15 PM, LizR wrote: > > 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. > > > 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 [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.
