The discussion revolves around the concept of the universe's expansion, particularly focusing on the implications of this expansion for time and spacetime. Participants explore theoretical aspects, including how distance and time relate in the context of an expanding universe, and whether time itself can be considered to expand alongside space.
Participants express differing views on whether time can be considered to expand alongside space, with no consensus reached on this point. The discussion includes multiple competing perspectives on the nature of spacetime and the implications of the universe's expansion.
Some statements rely on specific definitions of "universe" and "spacetime," which may not be universally agreed upon. The discussion also touches on complex concepts from general relativity and cosmology that may not be fully resolved within the thread.
ok... and maybe the expansion of time has to do with the way the time between test particles following geodesics would increase over space...JesseM said:
But that doesn't make any sense, unless the test particles are tachyons (and I don't think you can define the notion of a geodesic for tachyons)--if you slice spacetime into a series of non-spacelike-sections, you won't have the same collection of test particles in each slice, you'll just get particles randomly appearing and disappearing from one slice to the next. Try to visualize a spacetime filled with worldines, and what happens if you take a series of slices along the time axis vs. what happens if you take a series of slices along a spatial axis, hopefully you can see what I mean.moving finger said:
It makes sense. I think the reason you cannot make sense of it is because you are stuck in thinking of test particles which have worldlines with a unique separation in their "space values" for each "time value".JesseM said:
OK, but the point is that when you take spacelike slices along the time axis, you can say the two events are just the positions of the particles at that moment in time, so you can compare the distance of the two events in one spacelike slice to with the distance of the "corresponding" two events in a later slice (ie the position of the same two particles at the later time). But what correspondence is there between events in one slice taken along a space axis with events in another slice taken along the same space axis?moving finger said:
What events are further apart? Any two pairs of random events in different slices will be further apart, regardless of how you choose them? If in the first slice I choose the events "clock on Earth reads 2005" and "clock on Earth reads 3005", and in another slice I choose the events "clock on Alpha Centauri reads 2005" and "clock on Alpha Centauri reads 2008", does that mean time has shrunk? What if I had instead chosen the events "clock on Alpha Centauri reads 2005" and "clock on Alpha Centauri reads 4005"? Without some correspondence between the events on one slice and another, without some kind of rule saying "if you pick events A and B in your first slice, you must always pick events A' and B' in the second one", then your choice of which two events in each slice to pick will be totally arbitrary, so there will be no well-defined procedure to decide whether "time is expanding" or not. So, your argument still makes no sense to me.moving finger said: