Re: Principle of Equivalence

Sat, 11 Jul 2020 20:50:07 -0700 


On 7/11/2020 8:07 PM, Alan Grayson wrote:



On Saturday, July 11, 2020 at 8:41:21 PM UTC-6, Brent wrote:



    On 7/11/2020 4:29 PM, Alan Grayson wrote:



    On Saturday, July 11, 2020 at 5:05:02 PM UTC-6, Brent wrote:



        On 7/11/2020 12:54 AM, Alan Grayson wrote:



        On Tuesday, July 7, 2020 at 10:06:44 PM UTC-6, Alan Grayson
        wrote:



            On Tuesday, July 7, 2020 at 8:50:50 PM UTC-6, Alan
            Grayson wrote:



                On Tuesday, July 7, 2020 at 8:05:28 PM UTC-6, Bruce
                wrote:

                    On Wed, Jul 1, 2020 at 4:18 PM Alan Grayson
                    <[email protected]> wrote:

                        How, exactly, is the Principle of
                        Equivalence used by Einstein to develop GR?
                        TIA, AG



                    This lecture by Sean Carroll should answer all
                    your questions:

                    URL: https://wp.me/p2WMeM-3vl

                    Bruce


                I'll watch it tonight, but I think I've figured it
                out; specifically, the EP implies space-time is
                curved by the presence of mass/energy (and this is
                independent of the need to express the laws of
                physics in a coordinate independent way via tensors). AG


            Here's my reasoning regarding the EP; if an observer is
            in a box subject to uniform acceleration, a beam of
            light starting on the extreme left side (moving
            transverse or perpendicular to the acceleration vector),
            will hit a lower point on the right side, showing that
            uniform acceleration results in curved paths in
            space-time. But if this result is identical to gravity,
            locally, it means that curved paths in space-time are
            produced by, or are equivalent to gravity.



        That makes no sense.  You're saying that because curved paths
        can be produced two different ways then they must always be
        produced the second way.



            BUT gravity is only observed in the presence of
            mass/energy. ERGO, the EP implies mass/energy curves
            space-time. AG



        And that's not even true.  Gravitational waves can propagate
        thru the vacuum.  The Schwarzschild solution is for empty
        space.  De Sitter space is an empty cosmos.


    Light can propagate through empty space, but it can't arise from
    nothing. Same presumably for gravitational waves. AG


    Are you going to say the same thing about matter?  electric charge?

    Brent



I'm not sure what your point is. All I was saying above is that 
mass/energy causes curvature of space-time as per the EP. A 
gravitational wave moving through empty space seems no different in 
principle than light moving through empty space. In both cases there 
is a mathematical solution for the wave motion, with the source of the 
wave left undefined. But every wave motion must has a source. AG



And every source must have an origin?  There is nothing in the equations 
that says there must be a source.


Brent

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