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Re: Gravity and light question

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Tom Roberts wrote:

> On 7/18/16 7/18/16 - 11:31 PM, Thomas 'PointedEars' Lahn wrote:
>> The Einstein field equations are:
>>   R_µν − 1∕2 R g_µν + Λ g_µν = 8 π G∕c⁴ T_µν
>> The shape of spacetime on the left-hand side is determined by the stress–
>> energy tensor T_µν on the right-hand side *and vice-versa* (its
>> componenent T₀₀ is the energy density which by the mass–energy
>> equivalence is equivalent
>> to the mass density in the trivial case).  [This is succintly summarized
>> in John Archibald Wheeler’s “Spacetime tells matter how to move; matter
>> tells
>> spacetime how to curve.”  So there *is* a difference between light and
>> massive objects: light, being massless, does _not_ affect spacetime
>> considerably.]
> 
> Your last claim is wrong. Just LOOK at the equation you wrote above -- the
> curvature (LHS) is related to the energy-momentum tensor T. Light clearly
> has nonzero T, and thus affects the curvature.

Straw man.  I said “does _not_ affect spacetime *considerably*” (i.e., 
compared to massive objects).  I did not say “does _not_ affect spacetime 
*at all*”.
 
> Beware of sound bites -- they invariably omit some details.

I beg your pardon?
 
> It's just that in practice it's very difficult to have a large amount of
> energy-momentum in light; it's much easier for massive objects, as mass
> has an enormous amount of energy and dmomentum (compared to other types).

AISB.

>> The curvature of spacetime, then, is found in R_µν, the Ricci curvature
>> tensor, and R, the scalar curvature.  Obviously, R_µν − 1∕2 R g_µν ≠ G ∕
>> c².
> 
> Hmmm. It is quite common to define the Einstein curvature tensor G with
> components:
>  
> G_µν = R_µν − 1∕2 R g_µν

I know.  Irrelevant.  Had I meant the Einstein tensor, I would have written 
indices (to indicate covariance and/or contravariance).

> (There's no 1/c².)

Of course not.  That the curvature of spacetime were defined by “G / c^2” 
was “Odd Bodkin”’s claim, which was thus disproved.

I value your professional insight, but you should read postings more 
carefully, so as not to leave the impression that you are posting
follow-ups only for the sake of posting.

-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

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Re: Gravity and light question Odd Bodkin <bodkinodd@gmail.com> - 2016-07-18 16:17 -0500
  Re: Gravity and light question Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-07-19 06:31 +0200
    Re: Gravity and light question Odd Bodkin <bodkinodd@gmail.com> - 2016-07-22 07:13 -0500
      Re: Gravity and light question Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-07-23 15:52 +0200
    Re: Gravity and light question Tom Roberts <tjroberts137@sbcglobal.net> - 2016-07-22 11:19 -0500
      Re: Gravity and light question Ben Wagner <benwa@ubuntusite.org> - 2016-07-22 18:40 +0000
        Re: Gravity and light question Tom Roberts <tjroberts137@sbcglobal.net> - 2016-07-22 22:43 -0500
          Re: Gravity and light question Poutnik <poutnik4nntp@gmail.com> - 2016-07-23 07:23 +0200
            Re: Gravity and light question Poutnik <poutnik4nntp@gmail.com> - 2016-07-23 08:15 +0200
      Re: Gravity and light question Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-07-23 15:38 +0200

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