# Re: How the STc principle (special relativity) puts both the arrow of time and a common present moment on a firm physical basis.

```On 12/25/2013 10:53 PM, LizR wrote:
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On 26 December 2013 19:11, meekerdb <meeke...@verizon.net <mailto:meeke...@verizon.net>> wrote:
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On 12/25/2013 9:15 PM, LizR wrote:
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```    On 26 December 2013 15:56, meekerdb <meeke...@verizon.net
<mailto:meeke...@verizon.net>> wrote:

On 12/25/2013 2:45 PM, LizR wrote:
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```        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.
```
```
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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.
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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.
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There are other viewpoints though. QM makes for some interesting questions about time as raised in this speculative paper by a couple of top experimentalists:
```
====================

A few discontents in present-day physics' account of time are pointed out,
and a few novel quantum-mechanical results are described. Based on these, an
outline for a new interpretation of QM is proposed, based on the assumption
that spacetime itself is subject to incessant evolution.

...
One of us (AE) owes this insight to a student's question about SchrÄodinger's
cat.
She argued that, if the box is opened after suąciently many hours, it should be
possible to know whether the cat has been dead or alive during the preceding
hours. If it has been alive, it would soil the box and leave scratches on its
walls,
whereas if it has been dead, it would show signs of decomposition. Here too, the
measurement at the moment of opening the box must select not only the cat's
state at the moment of opening the box but its entire history within the box.

=====================

Brent

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