(Henry Spencer) wrote in message ...
In article ,
Rob Dekker wrote:
There is nothing that constantly
travels back and forth between the Earth and the Sun to keep Earth in its
orbit, ...

This is an assumption.
I think the current theory postulates that 'gravitons' are constantly
exchanged, at speed of light.

No, that is precisely *NOT* what the current theory is, which was my point.
Gravitational waves, aka gravitons, convey only *changes* in the field.

Quantum electodynamics and general relativity were weak points in my
studies, and my studies were decades ago so I may be asking nonsense:
Physicists often talk of photons in two forms, actual and virtual,
where actual photons convey changes in position and virtual photons
convey forces for current positions.

This is in analogy to all other forces of nature, where some 'particle' is
associated with the force.

The analogy is false, or at least misleading. Gravity seems to be a
rather different kind of force. Physicists are still struggling to fit it
into the same sort of theory as the other forces.

But hard proof either way is still is not there...

On the contrary. It is easily proved that gravity is *not* the result of
the exchange of particles at the speed of light. If that were so, gravity
would show aberration, which would make planetary orbits unstable.

If I remember my classical mechanics correctly, two body forces are
stable for r^x if and only if x is greater than or equal to -2. The
classical electrostatic and gravitational models are both r^(-2), in
their behavior so both are capable of potentially forming stable
systems, but become unstable for small perturbations from this model.
When full classical electromagnetic theory is introduced the fields
produced by orbiting charged particles cause the classical
electromagnetic system to be unstable. Stability only reappears when
quantum physics is introduced, and then only for the lowest energy
state. Would this gravitational "abberation" have a correspondence to
the magnetic field?

Given that a multibody system coupled using 1/r^2 forces is generally
unstable, but we have proof of the existence of such systems (e.g.,
our solar system, galaxy, etc.) an important question is whether the
instability would be important over the time period during which the
systems have existed, and studies have been sufficient to show that
the result is inconsistent with this interpretation.

This analysi, of course, would be easiest for two body systems
involving large masses and short distances, i.e, an encounter with a
black hole or neutron star. I had the impression that asttrophysicists
considered such systems to be unstable, but I vaguely remember that
tidal forces and mas exchange are important contributors to that
instability.

Light and other forms of electromagnetic radiation *do* show aberration:
a moving observer, looking to the side, sees stars slightly ahead of their
true positions, because a photon which to him seems to come straight in
from the side, in fact had to angle forward a bit to get down the barrel
of his moving telescope. That's a very loose explanation, but a more
rigorous treatment gives the same result. Astronomers noticed this in the
early 18th century.

Planetary orbits are stable only if gravity appears to come from precisely
the true position of the Sun. If gravity traveled at a finite speed and
thus showed aberration, the Earth wouldn't be here.

Does abberation affect virtual photons?