Dr Joe, I believe that many of us might like some direction as to when to use "space" and when to use "spacetime". Also, in string theory, is there a similar unity/difference between stringspace (11 dimensions) and "string time"?
In a similar vein, starting from matter in spacetime, due to expansion, (inflation) there comes a 'point' when ST expansion exceeds c, and presumably leaves the matter behind. Have I got this right? If ST later slows down again below c (after inflation?) how (if) does it reconnect with matter?
Hi Cat, good to hear from you! The main difference between space and spacetime is….time! “Space” is where everything is located. You can define positions with coordinates. There are three dimensions, x, y, and z (or left/right, forward/back, up/down). “Spacetime” consists of the three spatial dimensions, plus time, so four dimensions in total.
I am not a string-theory expert, so go easy on me! String theory describes space/spacetime and how the universe works. I think it is not analogous, then, to space and spacetime. I would say that one does not have string space and string time.
Our intuition, experience, and language all make analogizing and understanding universal expansion extremely difficult. (I’ve been teaching this for almost 30 years in general education/introductory astronomy courses and it is one of the most difficult concepts for the instructor to describe and for students to understand!) Part of the problem is that universal expansion was discovered by noting that galaxies appeared to be all moving away from us, with those at larger distances moving faster than those more nearby galaxies. We want to think It’s the galaxies themselves rushing into the distances. Instead, and it’s a subtle distinction, it’s not the galaxies moving away from us, it's the space in between expanding. Moreover, the expansion velocity is not the same everywhere: Nearby space expands slowly, and more distant space expands more rapidly.
There can indeed be a situation where two very distant galaxies are “moving apart” at velocities greater than the speed of light. This is not a violation of the cosmic speed limit because the galaxies themselves are not moving (which is what Einstein is concerned with: Individual objects moving through space from one point to another).