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Observation is a spacetime event.Icarus said:On the constancy of c, there are some concerns to be brought up:

1) Constant with respect to what? Relativity requires c to be constant with respect to observer, butnotwith respect to space or time. Relativity (Special or General) only says that if you have two observers at the same event, they will measure the same speed for light, regardless of any relative motion. It does not require that they measure the same speed as observers at other events.

This is a point almost entirely ignored (and very seldom even realized). While extensive evidence supports special relativity and the constancy of light with respect to observer, none of this implies that the speed of light is also constant with respect to spacetime event. The speed in other galaxies or around other stars may differ significantly from what it is here. The speed may have been different in earth's past than it is now. Our best evidence for constancy with respect to event is simply that the error intervals for all measurements made so far overlap. (Some have suggested that early measurements show a different value - but those measurement were so inaccurate that the difference is more readily explanable as simple error). By Fermat, changes in the value of c along a light ray's path should produce bending, so presumably, if the value of c varies in space, we should see lensing affects. Might some of the "gravitational" lensing seen actually be caused by variation in "c"?

Show me that conjecture of Fermat's(i think i might have heard something simliar but i ahve not read it, and will not be able to reply to it accurately until having done so) and its support.

You can only to that conclusion about gravitational lensing through a gross misunderstanding and misuse of the relevant equations. If you maintain your frame of reference you will get no such possible interpretation, because you cannot get a variational speed of light in the same equation if you do. Gravitational lensing is the result of a curved geometry and the lgiht ray following the path of least action, the solution to the Euler-Lagrange equation, namely, a geodesic.