On Wednesday, April 24, 2019 at 6:46:49 PM UTC-6, Brent wrote: > > > > On 4/24/2019 4:17 PM, agrays...@gmail.com <javascript:> wrote: > > > > On Wednesday, April 24, 2019 at 5:11:13 PM UTC-6, agrays...@gmail.com > wrote: >> >> >> >> On Wednesday, April 24, 2019 at 3:34:28 PM UTC-6, Brent wrote: >>> >>> >>> >>> On 4/21/2019 7:35 PM, agrays...@gmail.com wrote: >>> >>> >>> >>> On Sunday, April 21, 2019 at 8:07:28 PM UTC-6, Brent wrote: >>>> >>>> >>>> >>>> On 4/21/2019 6:31 PM, agrays...@gmail.com 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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
