Starblade Darksquall wrote in message
om...
"Old Man" wrote in message
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Starblade Darksquall wrote in message
om...
There seems to be a very big split between those who beleive that
gravity is a force between two bodies mediated by the graviton 2-boson
and can be unified with the other forces, and those who beleive that
gravity is the bending of timespace, and that freefall can be taken to
be a proper reference frame. But there is a test we can do which will
distinguish between them.

If gravity is a force that can be mediated by graviton particles, then
in a non-free falling reference frame, Newton's laws would be
preserved. This means anything that can be effected by gravity also
has a gravitational effect on the first object. Since we all know that
light is effected by gravity, then analogously it ought to have a
gravitational field, so that Newton's laws are then correct in a
regular inertial reference frame.

However, if gravity is the bending of timespace, and is a natural
state of motion like inertia, then Newton's laws would only be
conserved in a state of free-fall. Newton's laws would be preserved in
a reference frame of free fall. Since light falls with a constant
acceleration, then it would not be required to exert a gravitational
force on anything in order to preserve Newton's laws, and therefore it
would not have a gravitational field of any sort.

All we have to do, then, is determine whether light exerts a
gravitational force on things around it. Then we could determine which
of the models is correct and which is incorrect.

In the center of momentum reference frame, which for light
necessarily involves two or more photons, total light energy
gravitates according to M = E / c^2.


Uh, sir? All matter, and energy, is accelerated at exactly the same
rate. Therefore, calculations such as M = E / c^2 are meaningless.
Especially since M = E / c^2 doesn't represent the true mass of the
photon, which is exactly ZERO!

That's right. Rest mass of a single photon is zero, and a single
photon doesn't have a rest frame. However, observe that two
photons, with identical energy, E, but with appositely directed
momentum, E / c = p1 = -p2, have total rest energy of 2*E wherein
the gravitational mass of the two photon system is M = 2*E / c^2.
In the center of momentum system, total energy gravitates. If
the two photons are travling in the same direction, such that
p1 = p2, then the total rest energy is zero because, in this case,
it is impossible to define a center of momentum frame.
[Old Man]

For a single photon in the gravitational field of a spherically
symmetric mass distribution, the photon's effective potential
is proportional to [1 / r^3] rather than the [1 / r] proportionality
of the Newtonian potential for a massive particle. Unlike
massive particles, the photon follows a "null geodesic". Old
Man interprets this to mean that a single photon does not
produce a gravitational field. [Old Man]


While I have no idea about what that 1/r^3 vs 1/r stuff means, if what
you're saying is true (that a single photon does not produce a
gravitational field) then that means GR holds.

*Recycles old siggy.*

(...Starblade Riven Darksquall...)