Re: Ye Old One. The GR Expert.LMAO.

On Tue, 3 Feb 2009 22:08:47 +0000 (UTC),
carlip-nospam@xxxxxxxxxxxxxxxxxxx enriched this group when s/he wrote:

Ye Old One <usenet@xxxxxxxxx> wrote:
On Tue, 3 Feb 2009 01:21:42 +0000 (UTC),
carlip-nospam@xxxxxxxxxxxxxxxxxxx enriched this group when s/he wrote:

[...]
Both photons and massive particles move along geodesics, autoparallel
paths in spacetime. Their paths are different because they have different
initial conditions, but in both cases, the paths are determined by "bent
space" (really "bent spacetime"). There is no qualitative difference.

Yes there is. If the particle has mass then its gravity comes into the
two-body calculation of gravity attraction.

Perhaps the difference here is that I've actually *done* the two-body
calculation, for both massive objects and photons (to second post-
Newtonian order). Whether or not a particle has mass, if it has a
stress-energy tensor, its gravity comes into the two-body calculation.

Gravitation is defined as the natural phenomenon in which objects with
mass attract one another.

In particular, the stress-energy tensor of a photon is not zero, and
it appears on the right-hand side of the Einstein field equations as a
source of gravity.

We know a photon has no
mass for two reasons. First it travels at the speed of light.

Correct -- it has no rest mass . More precisely, its rest mass is limited
by observation to be less than 10^{-18} eV/c^2.

By definition it cannot have mass and travel at the speed of light.

Second it does not have gravity.

It does, in general relativity. See, for example, chapter 30 of Stephani
et al., _Exact Solutions of Einstein's Field Equations_ (2nd edition),
for exact solutions for gravitational fields generated entirely by photons.
Or look up the radiation-dominated FLRW cosmology.

Or look at the famous paper by Aichelburg and Sexl, "On the gravitational
field of a massless particle," Gen. Rel. Grav. 2 (1971) 303 (which has 270
citations listed in the Spires database).

A photon is a massless "particle". It can travel only at the speed of
light. If it had even the tiniest mass it could not travel at the
speed of light. Because it travels at the speed of light it is
timeless. Because it is timeless its interaction with the rest of the
universe is limited. Apart from those photons released at decoupling
all photons are created by matter.

[...]
Note that massive objects also follow geodesics. If you want
to say that a light path is straight, you can, but then you also
have to say that the Earth's path around the Sun is straight.

No, it is not. The Earth is in orbit.

As is a photon passing the Sun. (The photon is in a hyperbolic orbit,
not an elliptical one; but so is a comet.)

No, the photon is following a straight line through curved space.

No. You have a choice -- you can either talk about paths in space or
paths in spacetime.

A photon follows a "straight line" (a geodesic) in spacetime. So does
the Earth.

The Earth follows a path that is defined by all the components
relating to its orbit.

This is General Relativity 10: a *basic* feature of GR is
that all objects moving solely under the influence of gravity move
along geodesics.



The Earth does not move along a straight line in space. But neither
does a photon. This is perhaps General Relativity 101 -- you have to
know how to do a calculation -- but it is a completely unambiguous
result.

The difference is that the Earth has mass, a photon does not.

Look. This is my field. I have close to 100 published papers on general
relativity, with about 3000 citations. I teach graduate level GR almost
every year. I'm on journal editorial boards to deal with papers on GR,
and federal panels to evaluate grant proposals in GR. I've written
papers specifically about the motion of massive bodies (e.g., Phys. Lett.
A267 (2000) 81, gr-qc/9909087) and light (e.g., Class. Quant. Grav. 21
(2004) 3803, gr-qc/0403060) in gravitational fields.

This is starting to feel a bit like arguing with a creationist. I've given
you detailed references and citations (most of which you've snipped);
I've laid out reasonably detailed analysis; I've tried to understand
and clarify misconceptions. You are replying with one-liners, with
assertions with no references, with references to unnamed "physicists
[you] work with." You change the subject (for example, "does a photon
have a gravitational field" has become "does a photon have a rest mass";

Because without mass there is no gravitation.

when I've explained that the source of a gravitational field, in GR and
experimentally, is the whole stress-energy tensor, you clip it and come
back to "does a photon have a rest mass"); you ignore the most relevant
parts of my replies; you mistake "because I say so" for an argument.

If you don't think a photon has a gravitational field, try finding an
actual serious reference that supports you. If you really think that
only rest mass, and not the stress-energy tensor, is the source of
spacetime curvature, show us a set of field equations -- they won't
be Einstein's -- that predict this. If you don't think energy gravitates,
go read up on the Kreuzer experiment and explain why Kreuzer's
conclusions, and those of many physicists who have followed, are
wrong. If you don't believe the radiation-dominated stage of the
standard cosmological model -- in which the main source of gravity
was photons

It was? What about all the matter?

-- makes sense, point us to a reference that shows how
you can reproduce observations without it. If you don't believe that
the Earth follows a spacetime geodesic, tell us how to modify general
relativity to change this.

No need to - the Earth is in orbit, its path is limited by orbital
mechanics.

If you think a two-body problem in which
one of the bodies is a photon doesn't take into account the photon's
gravity,

I can't. The photon has no mass. Without mass there is no gravity.

point us toward a paper with such a calculation. If you don't
believe Aichelburg and Sexl's paper, show us the flaw in it (and publish
-- if you find a real error, you'll be famous!).

Steve Carlip

--
Bob.

.

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