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).
>

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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.
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