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Re: [Phys-l] starlight deflection



On 10/12/2007 06:22 PM, David Ward asked:

.... does the starlight tug ever-so-slightly on the sun?

Yes.

Photons lack mass, so I suspect they don't warp spacetime...

Bad guess. Mass is not the only source term for the
gravitational field.

general relativity .... I suspect the answer may lie
there.

That's exactly correct.

On 10/12/2007 06:32 PM, Rauber, Joel wrote:
Photons lack mass, but they do not lack mass-energy, so they do behave
as a source for the bending of space.

The right answer is much more complicated than that.

Let's be clear:
-- Total energy is not "the" source term for the gravitational field.
-- Rest energy (aka mass) is not "the" source term for the gravitational field.

Here's a nice example that drives home this point. This is based
on a question that a student asked me:
http://www.av8n.com/physics/gravity-source.htm

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

The right answer is Einstein's field equation
http://en.wikibooks.org/wiki/General_relativity:Einstein's_equation

Alas most people need a year-long course in general relativity to
get up to speed with what this means. In some *simple* cases it
has an easy-to-visualize representation, but the field induced by
a photon is not one of the easy cases (AFAIK).

Here's one of the easy cases:
http://www.av8n.com/physics/geodesics.htm
I know it doesn't directly answer the question. But it lays some of
the groundwork (a small part of the groundwork).