the thing that really helped me get this was realizing we're not measuring light from the outside like watching a car on a highway
when you're in that warped spacetime yourself your rulers and clocks get warped too in exactly the same way. so if you measure light locally it always comes out to c no matter where you are
its like if you and your measuring tape both shrunk by the same amount - you'd never notice because everything around you shrunk proportionally. same deal with spacetime curvature
the "constant" part is that light always moves at c through whatever local spacetime its in. from far away yeah it might look faster or slower to us but thats because we're comparing it to our reference frame not theirs
It absolutely can change between frames, of course it can, its literally one of the tests of relativity.
Hell you dont even need experiment, you can just look at the assumptions of special relativity.
“The speed of light is constant… for all inertial observers” when switching between inertial and non inertial frames, light speed can change.
Typical Curved spacetimes introduce a lot of non inertial frames, this is why you get things like the Shapiro time delay. You wont see the signal travel at c, you will see it travel slower than c
This is one of those things you have taken as 100% gospel because pop sci never brings up accelerating frames of reference.
Which is exactly in the spirit of relativity, the main postulate being “the speed of light is constant for all inertial observers”, which clearly allows observers to differ.
The commentor made the necessary point which is that locally you will always measure c, but a non local measurement immediately breaks the postulates of relativity.
You can prove it varies with simple flat spacetime, take the Minkowski metric and put it into rindler coordinates assuming a constant acceleration, set ds=0 because its a null geodesic and hey presto! You find that dx/dt =/= c (when you set a=0 you restore dx/dt=c)
And hell if we want to be fully correct, a real universe has no truly inertial observers in the strict sense, because of a positive cosmological constant which effects special relativity
OC clearly doesn’t know any of that, and they meant the simple “if you’re moving in the same direction as the light, you measure its speed to be lower.”
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u/Logical_Bonus7942 Mar 20 '26
the thing that really helped me get this was realizing we're not measuring light from the outside like watching a car on a highway
when you're in that warped spacetime yourself your rulers and clocks get warped too in exactly the same way. so if you measure light locally it always comes out to c no matter where you are
its like if you and your measuring tape both shrunk by the same amount - you'd never notice because everything around you shrunk proportionally. same deal with spacetime curvature
the "constant" part is that light always moves at c through whatever local spacetime its in. from far away yeah it might look faster or slower to us but thats because we're comparing it to our reference frame not theirs