system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the speed of gravity limited to the speed of light?

I've been taught 4 things:

1. The speed of light is the maximum speed that anything can travel through space
2. Unlike light, gravity is not a force carried by particles traveling *through* space, it's caused by the distortion of spacetime itself
3. The expansion of the universe can happen faster than the speed of light, because the maximum speed limit only applies to things moving through space, not space itself distorting
4. Gravity/gravity waves travel at the speed of light

Number 4 doesn't seem to follow from the first three, can someone explain why gravity can't propagate faster than the speed of light? For example, I've heard it said that the earth doesn't orbit the Sun's current location, it orbits where the sun was 8 minutes ago. Why couldn't the curvature of spacetime be "updated" faster? Why can spacetime expand faster than light, but not bend faster than light?
gpt
> Unlike light, gravity is not a force carried by particles traveling through space, it's caused by the distortion of spacetime itself

These things don't exclude each other.

> The expansion of the universe can happen faster than the speed of light

It doesn't make sense to call it a speed. The expansion is "speed per distance". The distance between things with a large distance can increase faster than the speed of light. The distance between things nearby (where absolute speed values would be a more useful concept) does not.

> Gravity/gravity waves travel at the speed of light

Gravitational waves. Gravity waves are things like waves on the surface of an ocean.

> Number 4 doesn't seem to follow from the first three, can someone explain why gravity can't propagate faster than the speed of light?

For the same reason as (1), the speed of light is the limit on every actual speed of propagation of anything.

> For example, I've heard it said that the earth doesn't orbit the Sun's current location, it orbits where the sun was 8 minutes ago.

That is not really true. The gravitational influence comes from the Sun as it was 8 minutes ago, but if you care about this then you also have to take into account the effect of its motion. General relativity tells you we are attracted to the position the Sun would have now if it wouldn't have accelerated within the last 8 minutes.