🚀 go-pugleaf

matmcinn@nuscc.nus.sg (Mcinnes B T (Dr)) writes:

>[The scene: Aristotle is accidentally translated in time to the late
>20th century, while working on the Physics. He asks a Modern Physicist
>to help him to catch up with things.]

[After the Modern Physicist convinces Aristotle that inertia is
not due to interaction with an "inerton"... Aristotle studies
gravity, and greatly admires Einstein's extension of Newton.]

>A: Well, Einstein has explained gravity by transferring it from the
>domain of the Second Law to that of the First. A free particle moves
>along a "straight line", where this expression is taken to mean a
>geodesic in the appropriate spacetime. Thus we can say, eg, that the
>Earth moves around the Sun, not, as Newton mistakenly thought, because
>the Sun and the Earth interact, but rather because they DO NOT interact!
>If they interacted, the motion would not be geodesic. Magnificent!
>MP: No! No! You have it all wrong! The Earth moves around the Sun
>because there is some interaction [let us call it "gravity"] mediated by
>some hitherto undetected particle which I hereby dub the "graviton",
>with a spin that may readily be calculated........
>Moral: It takes more than an Einstein to abolish the fantasy of the
>"gravitational interaction".

Very funny, but I think you're being a bit harsh on the Modern
Physicist here.  Nobody expects quantum gravity to be a theory
in which all gravitational interactions can be calculated via,
say, Feynman diagrams involving the exchange of virtual gravitons.
What is hoped is just that the dynamics of geometry can be
made into a quantum theory.  *Real* gravitons would be quite
present in such a case: gravitational radiation, waves in
the metric, would occur only in discrete quanta.

Even if it were possible to turn quantum gravity into something
accessible via perturbation theory, one must realize that
physicists are being rather metaphorical when they speak of
virtual particles caroming off each other to mediate forces.
The pictures are largely a computational and visualization aid
for what is really the interplay of fields, which happen
to consist of quantum operators.

Granted that things under the influence of gravity travel
along geodesics, you can't deny that the metric affects where
they end up going.  In that sense there is an interaction
with the "gravitational field," defined as fluctuations in
the geometry.

And if Aristotle said to the Modern Physicist that the 
amount of rest mass belonging to the particle was due
to an "inerton," the Modern Physicist would probably
agree with him!
 
-- 
Matt McIrvin   mcirvin@husc.harvard.edu   Long live short .sigs