On Friday, January 17, 2020 at 5:08:14 PM UTC-6, John Clark wrote: > > On Fri, Jan 17, 2020 at 5:03 PM Alan Grayson <[email protected] > <javascript:>> 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 >
Sometimes a picture works best. Below is a diagram that represents how space can be flat in a curved spacetime that expands space. LC [image: vsl.gif] -- 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/aa944b86-c18c-4394-970b-875ab42e56f1%40googlegroups.com.
