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Re: [Phys-L] foundations of physics: Galilean relativity



On 10/05/2015 10:45 AM, Jeffrey Schnick wrote:

If I understand them correctly, John and Moses both think that the
acceleration of the photon is the same as that which a particle at
the original position of the photon would experience meaning that as
the photon gets farther and farther from the planet, the photon's
acceleration remains constant at its original value in its original
direction. That makes no sense to me.

That makes no sense, and it's not what I think.

The gravitational effect on a compound object is not determined
by the position of its center of mass. This should be obvious.
It's got nothing to do with Galilean relativity, special relativity,
general relativity, or anything like that.

Consider the following two cases: In case [1] have a a barbell of
mass 2M. The center of mass is marked by ":" which is very near
the planet P. The gravitational acceleration of this barbell is
very small compared to case [2], where the actual mass (not
just the center-of-mass) is close to the planet.


M.......................:.......................M [1]
P




.......................M:M....................... [2]
P


================

Here's what I think: I think it is not a good use of resources
to compare apples to orangutans. If you want to compare a pair
of photons to an (electron,positron) pair, it would be a better
use of resources to choose a short-enough timescale so that
the photons remain in the vicinity, so that both pairs have
the same geometry, i.e. picture [2] above.

*IF* we are comparing apples to apples, *IF* the geometry
is the same, then everything (including photons) falls at
the same rate, in accordance with the equivalence principle.

Another possibility is to put the photons in a reflective box,
to ensure that they remain in the vicinity for a long time.
However, this is tricky, because when the photons reflect off
the walls of the box they create stress in the box, and stress
itself contributes to the gravitational field. So for some
purposes you can't use an ideal, imaginary box; you have to
account for the physical properties of the box. At this point
you have to drag in the heavy machinery of general relativity.
(Depending on what questions you want to ask, it /might/ be
possible to ignore the details of the box, but not necessarily.)