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 -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/942bdc36-7cb7-4103-a696-75d2d39cacf8%40googlegroups.com.
