On 5/24/2020 7:51 AM, Alan Grayson wrote:
On Saturday, May 23, 2020 at 12:06:33 PM UTC-6, Brent wrote:
On 5/22/2020 11:25 PM, Alan Grayson wrote:
On Friday, May 22, 2020 at 11:03:40 PM UTC-6, Brent wrote:
On 5/22/2020 9:48 PM, Alan Grayson wrote:
On Friday, May 22, 2020 at 9:05:23 PM UTC-6, Brent wrote:
On 5/22/2020 6:26 PM, Alan Grayson wrote:
On Monday, May 18, 2020 at 3:28:40 PM UTC-6, Alan
Grayson wrote:
Suppose the universe is a hyper-sphere, not
expanding, and an observer travels on a closed loop
and returns to his spatial starting point. His
elapsed or proper time will be finite, but what is
his coordinate time at the end of the journey? TIA, AG
It's not a dumb question IMO. If you circumnavigate a
spherical non-expanding universe, what happens to
coordinate time at the end of the journey? Does
something update the time coordinate? Or does it
somehow miraculously(?) remain fixed? TIA, AG
Are you supposing the universe is a 3-sphere? In that
case It's just like going around a circle. The degree
marks on the circle are coordinates, they have no
physical meaning except to label points. So if you walk
around the circle you measure a certain distance (proper
time) but come back to the same point.
Or are you supposing it's a 4-sphere so that all
geodesics are closed time-like curves? I don't know how
that would work. I don't think there's any solution of
that form to Einstein's equations.
Brent
I'm supposing a 4-sphere and (I think) closed time-like
curves. The traveler returns presumably to his starting
position, but is the time coordinate unchanged? AG
I don't think there's any very sensible answer in that case.
Goedel showed there can be solutions with closed time-like
curves if the universe is rotating. But solutions of GR
don't have any dynamic connection to matter and the entropy
of matter. In the same spirit there could be a solution to
quantum field theory that was close around the time like
curve...in which case you'd experience "Groundhog
Day"...including your thoughts.
Brent
What does entropy have to do with this problem? AG
Increasing entropy points the direction of time.
Brent
Let me pose the question another way: Is coordinate time ever updated? AG
In general relativity points are labelled by coordinates which locally
smooth functions that define a Reimannian space. You can change them to
whatever you want whenever you want. They are just labels. I'm not
sure what "update" means in this context. Physical time is measured by
clocks, and they measure proper time along their world lines.
Brent
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