On Wednesday, April 24, 2019 at 6:46:49 PM UTC-6, Brent wrote: > > > > On 4/24/2019 4:17 PM, [email protected] <javascript:> wrote: > > > > On Wednesday, April 24, 2019 at 5:11:13 PM UTC-6, [email protected] > wrote: >> >> >> >> On Wednesday, April 24, 2019 at 3:34:28 PM UTC-6, Brent wrote: >>> >>> >>> >>> On 4/21/2019 7:35 PM, [email protected] wrote: >>> >>> >>> >>> On Sunday, April 21, 2019 at 8:07:28 PM UTC-6, Brent wrote: >>>> >>>> >>>> >>>> On 4/21/2019 6:31 PM, [email protected] wrote: >>>> >>>> *Here's something odd. At 9:45 in Susskind's Lecture 2 on GR, he says >>>> the metric tensor is a Kronecker delta function. But I could swear that >>>> the >>>> diagonal of -1,1,1,1 represents flat space in SR. AG??* >>>> >>>> >>>> What's odd about that??? Flat space is just special case of curved >>>> space in which the curvature is zero. >>>> >>>> Brent >>>> >>> >>> *Sure, but he seems to be saying that the Kronecker delta is the metric >>> tensor for curved space. Isn't that how you interpret his comment?* >>> >>> >>> No.?? After he goes thru the derivation with delta function in it, then >>> he says it's different for a curve?? space. >>> >>> Brent >>> >> >> *I just reviewed it again. That's not my reading. In any event, it's not >> clear what he means, and using Bruno's suggestion, t' --> it,?? doesn't >> really help either since you end up with the Lorentz metric which is far >> from Euclidean intuition for demonstrating deviations from flatness. >> Further, there are transformations that keep spacetime flat with NON-zero >> off diagonal elements, such as a simple rotation. AG??* >> > > > *Using the Lorentz metric, how is "flat" spacetime defined mathematically? > AG * > > > The general definition is that the Riemann tensor is zero.?? This is > independent of what coordinate system is used.?? > https://en.wikipedia.org/wiki/Riemann_curvature_tensor > > If the Lorentz metric applies globally the space is flat. > > Brent >
Are the double question marks significant in some way, or typos? AG -- 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 post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
