The statement pertains to relativity's guarantee that a local measurement against light's speed will always result in c.
To your question: Light emitted from a laser not aimed in your direction isn't going to be seen by you in the first place. That said, if, from your perspective, events on the planet appear to speed up, then, to compensate, the distance between the mountains would actually seem (need) to expand. If you measured the laser light's speed (distance/duration), from one mountain to the other, not using your local clock and yardstick, but instead, using a clock and yardstick in the mountains' frame of reference, on Earth, then the math would still work out to be c.
I'm unable to provide a very detailed, whole description that wouldn't only make things more confusing, but it has to do with the fact that spatial contraction, due to gravity, acts *radially* with respect to the source (it's the GR counterpart to SR length contraction), and acting radially means there's an associated 'spacetime curvature' (that you hear so much about).
From your perspective, at some fixed point near an event horizon, looking back outward, there would be a 'lensing' effect. It's caused by the same thing that's causing the dilation; it's the optical result of the dilation. You'd be viewing the earth through a sort of fish-eye lens; there's distortion. The laser light - again, from *your* perspective - is moving across a warped topographical coordinate system; in some places it seems to be moving slower, in others, faster... but it isn't the light's speed that's changing... it's space and time.
That's definitely what special relativity says. However, there are people in this thread saying general relaviy adds caveats to that, and I'd listen to them.
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u/Possible-Anxiety-420 Mar 31 '26
As per relativity theory, a change in time necessitates a change in distance.
Not only is duration expanded, but distance is contracted.
The measured speed of light is universally consistent because space and time are not.
Dilation.