wrote:

One could not go anywhere, I'm afraid. For example:

1. Gravity in general relativity is purely attractive. Electromagnetic
interactions are observably both attractive and repulsive.
2. Gravity in general relativity is a spin 2 interaction. This leads
to very specific properties in, for example, the angular dependence
of scattering. Electromagnetic interactions at the atomic scale
are observably spin 1 interactions.
3. Gravity in general relativity couples to mass quadrupole moments and
higher, and has no dipole coupling. Electromagnetic interactions at
the atomic scale observably couple to dipole moments.
4. Gravity in general relativity produces gravitational waves, which,
among other things, have distinct polarizations that differ by 45
degree (not 90 degree) rotations; couple to all forms of energy,
including electrically neutral matter; and are radiated in a distinctive
quadrupole pattern. Electromagnetic interactions at the atomic scale
produce photons, which observably have none of these characteristics.
5. Gravity in general relativity couples to binding energy. Electromagnetic
interactions at the atomic scale observably couple only to electric charge
and current.

Gravitational and electromagnetic interactions are *extremely* different;
electromagnetism is very, very different from "strong gravity."


What a refreshing pleasure to read a succinct, well-reasoned,
informative scientific argument.

I admit having vacillated between periods when I was impressed with
similarities between GR and EM, and periods when I have been impressed
with what appear to be very fundamental differences between GR and EM.

Your list of differences makes a very good case for inherent
differences. Still, I maintain an openness toward a deep, and as yet
undiscovered, connection between GR and EM for the following reasons
(my list).

1. In the 1920s Kaluza and Klein showed how you could take GR, recast
it in a 5-d form, and pull *both* rabbits (GR and EM) out of the same
hat. Although their efforts were not entirely successful, what was
achieved was impressive enough to make Einstein return 4 separate times
to serious attempts at a 5-d unified theory. Is it possible that the
differences between GR and EM are more "superficial" than we realize,
and that some unified field theory might one day reveal an intimate
relationship between them? Or is the situation like Pauli's quip: 'What
God has cast apart, let no man try to put together'? I can accept
either answer - as long as it is reasonably definitive and comes
directly from *nature*.

2. Both EM and GR are 1/r^2 interactions. GR has gravitoelectric and
gravitomagnetic phenomena that are remarkably analogous to EM
phenomena. There are also the previously discussed intriguing analogies
between hadrons and black holes.

3. When I first learned that G(n-1)/G(n) ~ 10^38, I found it
fascinating that this is also approximately the ratio of the strengths
of the EM/GR interactions (~ 10^38). If that is a coincidence, then
nature has been both subtle *and* malicious.

4. I confess to scientifically unsupported, intuitive doubts about the
spin-2 issue, but I am willing to be persuaded by *empirical* evidence.

Bottom line: I agree with you that GR and EM give many indications of
being fundamentally different interactions, and that accurate models of
nature currently require both. However, I maintain that GR and EM are
of equal strength and play equally important roles within atoms.

Robert L. Oldershaw