On Wednesday, April 24, 2019 at 8:59:52 PM UTC-5, Brent wrote: > > > > On 4/24/2019 6:36 PM, [email protected] <javascript:> wrote: > > > > On Wednesday, April 24, 2019 at 6:46:49 PM UTC-6, Brent wrote: >> >> >> >> On 4/24/2019 4:17 PM, [email protected] 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 > > They are some glitch in my email program which puts in extra ??? It is > mysteriously inconsistent. > > Brent > > I can't imagine interfacing with Google Groups via anything else (like any email system) but its own web interface.
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