Re: Dr Martin Gottschall's Gravity Series
From: Uncle Al (UncleAl0_at_hate.spam.net)
Date: 11/30/04
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Date: Tue, 30 Nov 2004 15:19:25 -0800
Eric Erpelding wrote:
>
> Dr Martin Gottschall has written a four part series on Gravity. It is
> at:
>
> http://www.acufos.asn.au/gravity/gravity.html
>
> The series, entitled "Reflections on Gravity," describes his theory
> that gravity is electromagnetic in origin.
[snip]
Gottschall is an empirical ass.
Class. Quantum Grav. 17 4125 (2000)
It is possible to obtain exactly the Maxwell equations from the
Einstein field equation, but there is a long list of caveats which
inevitably breaks the analogy with electromagnetism.
The source of monopole radiation is a changing monopole moment for a
charge q or for a mass m. Since charge and mass are conserved, there
can be neither monopole electromagnetic radiation nor monopole
gravitational radiation.
The source of dipole radiation is a changing dipole moment.
(Punctiliously, you need a second time derivative of the dipole
moment.) For a pair of charges
d = qr + q'r'
and there's nothing special about the derivatives. For a pair of
masses, the gravitational dipole moment is
d = mr + m'r'
and its time derivative is
mv + m'v' = p + p'
By conservation of momentum the second time derivative of the
gravitational dipole moment is zero, and you can go to a center of
momentum frame and set the first derivative to zero as well. There is
no gravitational "electric dipole" radiation.
Consider the analog of "magnetic dipole" radiation. The gravitational
equivalent of the magnetic dipole moment for a pair of charges is
M = mv x r + m'v' x r'
("x" is the cross product, "mv" is the "mass current")
But M is the total angular momentum, which is also conserved. There
is no gravitational "magnetic dipole" radiation.
The next moment up is quadrupole, with no relevant conservation laws,
so gravitational quadrupole radiation is permitted. You can use this
argument to advocate that gravity must be a tensorial (spin-2)
interaction. Electromagnetism is mediated by spin-1 photons.
-- Uncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/qz.pdf
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