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> For example, we see light coming from regions that have very different gravity, but it still behaves the same. Blackholes bend light for example, isn't that

by _8j50 3y ago

> For example, we see light coming from regions that have very different gravity, but it still behaves the same. Blackholes bend light for example, isn't that light behaving differently due to gravity? Couldn't pulsars and quasars do the same? > That property of spacetime (a better name for it would be local Lorentz invariance) is the same everywhere no matter how weak or strong gravity is. Isn't gravity a product of spacetime curvature? Normal objects' "speed" changes when they curve, can observational evidence froma distance, especially considering relativistic effects be enough to tell observers on earth thay even during curvatures of spacetime, the speed of light in vaccume locally at that point won't be affected?

pdonis 3y ago
> Blackholes bend light for example, isn't that light behaving differently due to gravity? No. Light "bending" due to gravity isn't a change in the behavior of light. The light is still traveling on the straightest possible path and obeying the same Maxwell's Equations. It's just doing it in a curved spacetime instead of a flat one.
pdonis 3y ago
> Isn't gravity a product of spacetime curvature? See my response to rkagerer downthread.
pdonis 3y ago
> Normal objects' "speed" changes when they curve The fact that you put "speed" in scare-quotes shows that, even if you don't realize it, you intuitively grasp that the concept you are trying to use doesn't work the way you are saying it does. Normal objects in free fall, i.e., moving only under the influence of "gravity", move on straight lines in spacetime, just like light does. (They are different straight lines because normal objects have mass while light is massless. But they're still straight lines in both cases.) The apparent "curvature" comes from the curvature of spacetime, not any change in the behavior of the objects. The objects have no way of telling locally whether the spacetime they are in is flat or curved. > the speed of light in vaccume locally at that point won't be affected? No. You can't tell anything about spacetime curvature or gravity by local measurements of the speed of light in vacuum.
- _8j50 3y ago
  That all makes much more sense now, thanks for taking the time to patiently explain it all.

by _8j50 3y ago

pdonis 3y ago

> Blackholes bend light for example, isn't that light behaving differently due to gravity? No. Light "bending" due to gravity isn't a change in the behavior of light. The light is still traveling on the straightest possible path and obeying the same Maxwell's Equations. It's just doing it in a curved spacetime instead of a flat one.

pdonis 3y ago

> Isn't gravity a product of spacetime curvature? See my response to rkagerer downthread.

pdonis 3y ago

> Normal objects' "speed" changes when they curve The fact that you put "speed" in scare-quotes shows that, even if you don't realize it, you intuitively grasp that the concept you are trying to use doesn't work the way you are saying it does. Normal objects in free fall, i.e., moving only under the influence of "gravity", move on straight lines in spacetime, just like light does. (They are different straight lines because normal objects have mass while light is massless. But they're still straight lines in both cases.) The apparent "curvature" comes from the curvature of spacetime, not any change in the behavior of the objects. The objects have no way of telling locally whether the spacetime they are in is flat or curved. > the speed of light in vaccume locally at that point won't be affected? No. You can't tell anything about spacetime curvature or gravity by local measurements of the speed of light in vacuum.

_8j50 3y ago
That all makes much more sense now, thanks for taking the time to patiently explain it all.