The forum discussion centers on the concept of universe expansion and its implications for spacetime. Participants explore whether the expansion of space implies an expansion of time, referencing Einstein's theory of general relativity and the Big Bang model. Key points include the understanding that the universe does not have a center and that the expansion of space is not synonymous with the expansion of time. The discussion concludes that while space expands, time remains a constant measure, and the concept of time expanding is mathematically redundant.
PREREQUISITESAstronomers, physicists, and students of cosmology who seek to deepen their understanding of the universe's expansion and the relationship between space and time.
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: