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View Full Version : Is light considered to "slow down", in gravitational fields?



Frog march
2009-Aug-22, 02:11 AM
Does mainstream physics consider that light slows down if, and while it passes through a gravitational field, as for example, passing close to a neutron star, or even being beamed down to a neutron star's surface, ie due to time dilation?

undidly
2009-Aug-22, 02:22 AM
Does mainstream physics consider that light slows down if, and while it passes through a gravitational field, as for example, passing close to a neutron star, or even being beamed down to a neutron star's surface, ie due to time dilation?

An observer outside the gravity well will see it slow down.
An observer inside the gravity well will see no change.
An observer inside the gravity well can look up and see EVERYTHING happening
more quickly.

publius
2009-Aug-22, 03:08 AM
Depends on what you mean by "slow down". This is a subtle business. Light always travels a null path through space-time. However, because of curvature (of both space-time and non-inertial coordinates used to map it), the coordinate speed of a null path will vary.

However, locally, light speed is always the constant 'c'. Indeed that is matter of how local rulers and clocks are defined. But globally, using those same rulers and clocks, the coordinate speed will differ from 'c'.

With that in mind, in Schwarzschild, the distant observer will say the coordinate speed of light slows down in the well. However, to any observer inside the well, the speed of light is faster up high.

But the path is always null. In the domain of SR, the coordinate speed of a null path is globally the constant 'c'. In GR, that doesn't hold. However, null is null, and that is what is invariant. These things are best thought of as coordinate effects.

-Richard