On 12/25/2013 9:15 PM, LizR wrote:

On 26 December 2013 15:56, meekerdb <meeke...@verizon.net <mailto:meeke...@verizon.net>>wrote:## Advertising

On 12/25/2013 2:45 PM, LizR wrote:On 26 December 2013 07:23, Jesse Mazer <laserma...@gmail.com <mailto: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."`

But if you consider yourself to be a worldline then you have no 4-velocity, only a3-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... Inline images 1 And here's a diagram of how it actually is... Inline images 2 ...both are from Chapter 11 of FOR.

`First, I said nothing about a "present moment"; that's Edgar's concept. I referred to the`

`4-velocity. By treating the velocity as a 3-vector, instead of suppressing the`

`0-component, the above diagrams do not show how one's clock runs slower relative to the`

`coordinate frame. When you use some of your 4-velocity to move thru space, there is less`

`of it available to move you through time. So when you say it is the angle between the`

`"vertical axis" and the world line, that's a statement in a specific coordinate system.`

`But one's proper velocity is always 1, independent of coordinates.`

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