On Fri, Jan 17, 2020 at 5:03 PM Alan Grayson <[email protected]> wrote:
>> Yes, you can use that to represent a curved path in 4D (one of time 3 of >> space) Minkowski Space where Special Relativity lives, but as you say that >> doesn't really get to the fundamental issue because Minkowski Space is flat >> and Special Relativity says nothing about gravity, for that you need >> General Relativity and GR doesn't live in Minkowski Space. >> In General Relativity curved Spacetime is what gravity is, and in GR if >> there is any curvature in the Spacetime of the universe, and we know there >> is because we know that gravity exists, then, unless vacuum energy also >> exists and is fine tuned to one very precise value, the universe can not be >> stable, it must be either expanding or contracting. There are thermodynamic >> reasons to think it can't be contracting so it must be expanding. >> And that is why no physicist would say that Carroll's statement "*the >> manifestation of spacetime curvature is simply the fact that space is >> expanding*" was controversial . >> > > > *The question is, what does he mean? Is space expanding BECAUSE of > curvature? If so it's expanding because of gravity, since you wrote that > gravity and curvature are equivalent. But since gravity is attractive (as > far as we know), how could it be responsible for expansion (as > distinguished from contraction)? AG * > If the universe consisted of a cloud of particles that were not moving with respect to each other the gravitational attraction between the particles would indeed cause the universe to contract, but the particles ARE moving with respect to each other, so what will happen? It depends on how they are moving, but General Relativity can tell you one thing, unless you invoke a very fine tuned vacuum energy (aka the Cosmological Constant) that cloud of particles will NOT remain the same size, it will either expand or contract. We learn from observation that it's expanding which is consistent with thermodynamic reasoning. John K Clark -- 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 [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAJPayv30T_ONgMgUfYVc0ZHJtdUK0-y_WditR7JYmScnb6evyA%40mail.gmail.com.
