Universal expansion, coordinate systems, time and the block universe

[studiot]

This question/discussion proposal was inspired by the discussion in this thread, but is not directly on topic there.

 

http://www.scienceforums.net/topic/102170-an-issue-i-have-with-gr-physics-versus-newtonian-physics/page-2#entry966276

 

It appears to me that accepting universal expansion has-implications-for/constrains acceptable coordinate systems.

 

Consider static points on the classic 2D manifold surface of a balloon.

 

As manifold expansion occurs:

 

1) For xyz coordinate systems all of x and y and z will be different before and after expansion.

 

2) For cylindrical coordinate systems the angle will remain unchanged, but r and z will be different.

 

3) For spherical coordinate systems both angles will remain unaltered but r will change.

 

Only option (3) has two invariable coordinates (latitude and longitude or HA and azimuth) that we can use to keep the points in the 'same place' as the balloon expands.

 

Since the expansion takes place over time my question for discussion is how does the choice of coordinate system affect the 'arrow/reversibility of time' and the block universe?

 

[MigL]

Not sure if I'm understanding cor/rectly...

You seem to be attaching a 'fixed' co-ordinate system ( ie an absolute frame ) to the universe and noticing changes as the universe expands.

 

The co-ordinate system is the math that is 'modelling' the expansion.

It is 'expanding' to model the actual expansion.

If it wasn't, it wouldn't be a good model.

And of course, some models ( ie co-ordinate systems ) will work better than others, depending on the topology.

 

Or have I totally mis-understood

[Strange]

Since the expansion takes place over time my question for discussion is how does the choice of coordinate system affect the 'arrow/reversibility of time' and the block universe?

 

 

All these coordinate systems (and any others) describe the same thing in different ways. They are not describing different things (as shown by the fact that there are consistent rules to transform between them).

 

You can, for example, choose a coordinate system to describe the universe where it doesn't expand but where the speed of light varies over time. That sounds like the sort of thing you are describing.

[Mordred]

For strictly coordinate spatial change without any other considerations its time independant. There is no variable time involved. The choice of coordinates has no affect on time in regards to block.

 

In terms of entropy this does have influence on the arrow of time via entropy