On 1/16/2020 3:17 PM, Alan Grayson wrote:
On Thursday, January 16, 2020 at 2:02:22 PM UTC-7, John Clark wrote:
On Thu, Jan 16, 2020 at 1:25 PM Alan Grayson <[email protected]
<javascript:>> wrote:
>> as Sean Carroll says "/the manifestation of spacetime
curvature is simply the fact that space is expanding/".
*> If curvature is caused by energy and momentum in
space-time, Carroll's statement doesn't make sense. AG*
Has it ever occured to you that if a professor of physics at one
of the best universities in the world makes a statement about
physics that makes no sense to you it might not be because he is
talking nonsense but because you don't understand something? It
makes no sense to you because you assume spatial curvature and
spacetime curvature mean the same thing. But you're wrong, they
don't. And they don't because the time dimension behaves in a
fundamentally different way than any of the 3 spatial dimensions
do; more specifically if you want to use Pythagoras theorem in
space*TIME *(not to be confused with space) to calculate a
distance in space*TIME* then you have to stick in a minus sign
that Euclid and Pythagoras knew nothing about.
I f
John K Clark
*You're referring, of course, to the Lorentz metric with the minus
sign. Yeah, I've heard of it -- used in pseudo Riemannian manifolds,
and without it those space-time causal cones wouldn't make sense. But
I read again and again that "curvature" of space-time is caused by the
presence of mass/energy, so I find Carroll's comment puzzling. He
seems to be saying that expansion is caused by curvature, when it's
generally thought to be caused by dark energy. *
"Dark energy" is a place-holder name for whatever is responsible for the
accelerating expansion of the universe. So it could be curvature (i.e.
just an intrinsic constant of spacetime) or it could be a quantum field
that would have a corresponding particle that we could look for. And
what would be puzzling about Carroll having a different opinion than
what is "generally thought" when it is a completely unsettled empirical
question on which experts may be expected to have different theories.
In fact there isn't any "generally thought" consensus, and science
doesn't go by consensus anyway. What puzzles me is that you spend so
much time writing to these email lists when your level of understanding
would be improved a lot more by reading a book, e.g. David Mahon's
"Relativity Demystified" or Robert Wald's "General Relativity" or
"Gravity" by Jim Hartle or even Vic's "Comprehensible Cosmos".
Brent
*Also, as you know, I am not an enthusiast (to put it mildly) of the
Many Worlds theory of Everett, but Carroll is. So I don't pray at the
feet of physicists, even those from prestigious universities. AG*
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