5 ms·

> Doesn't gravity propagate at the speed of light This is technically true, but not in a way that matters for practical purposes. Changes in the gravitational

> Doesn't gravity propagate at the speed of light This is technically true, but not in a way that matters for practical purposes. Changes in the gravitational field -- that is, the metric structure of spacetime -- do propagate at the speed of light. So for instance, if a massive object were to suddenly appear in space, then nearby objects would only experience a gravitational attraction to it after a delay. But of course that can't actually happen because of mass-energy conservation. It turns out that if you use general relativity to calculate a more realistic scenario, where conservation laws are obeyed -- say, a pair of massive objects moving at a constant relative velocity -- then no matter how far apart they are, the speed-of-light delay is almost exactly canceled out by velocity-dependent effects, so that from each object's perspective it appears to be attracted to the other's instantaneous position, without any delay. (If this weren't true, orbits would be extremely unstable. If the Earth was feeling a gravitational force corresponding to the relative position where the sun appeared to be 8.5 minutes ago, instead of where it currently is, then it would lose orbital angular momentum and spiral into the sun on a timescale of millennia.) In the case of objects that are accelerating (not moving at a constant velocity) this cancellation is not quite perfect, which is what leads to the existence of gravitational waves. But the difference is exceedingly small under almost all circumstances. See also: https://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html https://math.ucr.edu/home/baez/physics/Relativity/GR/grav_sp... https://en.wikipedia.org/wiki/Speed_of_gravity#Static_fields https://en.wikipedia.org/wiki/Speed_of_gravity#Static_fields https://physics.stackexchange.com/questions/458136/does-gravity-sometimes-get-transmitted-faster-than-the-speed-of-light https://physics.stackexchange.com/questions/458136/does-grav...

krisoft 4y ago
> If the Earth was feeling a gravitational force corresponding to the relative position where the sun appeared to be 8.5 minutes ago But that is exactly it. Earth is experiencing gravitational force from where the sun appears to be. Which is already “corrected” for speed of light effect, since both the light (where the sun appears to be) and the gravity (where it appears to pull from) is delayed by the same amount. If we would be living in a simulation which neglects to take delay in gravity into account we would be experiencing a pull from the direction where the sun will appear in 8.5 minutes. Now, I can totally believe you that this effect is not significant, or can be neglected for other reasons. But that sentence where you say that orbits would be extremely unstable if gravity would not be delayed indicates otherwise.
- sebzim4500 4y ago
  >If we would be living in a simulation which neglects to take delay in gravity into account we would be experiencing a pull from the direction where the sun will appear in 8.5 minutes. This is actually pretty much what happens. The point that the earth accelerates towards is, to first (second?) order, the predicted point that the sun would be assuming it continued accelerating at its current rate. It is not accelerating towards where the sun was 8 minutes ago, nor where the sun appears now or 8 minutes ago.
  - pyinstallwoes 4y ago
    If the sun disappeared, would it take 8 minutes to know?
    - petschge 4y ago
      Yes.
      - pyinstallwoes 4y ago
        What would happen during this 8 minutes from beginning to end? Genuinely curious at a high level
egeozcan 4y ago
Every time there is a discussion about general relativity, my brain stops working and I feel very stupid. Doesn't matter how many youtube videos I watch or articles I read. So it's very nice that Newtonian physics work to some scale in a limited context. I consider it a fast-inverse-square-root style shortcut the universe has given to people like me.
midasuni 4y ago
> nstead of where it currently is Surely where the sun currently is depends entirely on the position and frame of reference of the observer?
- the__alchemist 4y ago
  I'm suspicious here lies the crux of the issue. And, here be dragons.
- SiempreViernes 4y ago
  No, the sun is where it is, that's what having physical reality means. What you are talking about is how that point is named by different observers, and there is indeed great ambiguity there. But ultimately, if two persons are starting from different places towards the same goal following a map, the fact that they don't both eventually arrive at the same place has implications about the map they used, not about where the place is.
  - wruza 4y ago
    But where is it then? Like in (x,y,z,t).
    - SiempreViernes 4y ago
      In Heliocentric coordinates it is at (0,0,0,0).
  - QuadmasterXLII 4y ago
    Two observers at the same time, same point, and different velocoties will agree on the sun’s trajectory, but not on where along its trajectory it ‘currently’ is.
wruza 4y ago
If the Earth was feeling a gravitational force corresponding to the relative position where the sun appeared to be 8.5 minutes ago, instead of where it currently is, then it would lose orbital angular momentum I can’t understand why. If we imagine a completely dark universe of pointlike massive bodies, orbits still should work. Light and gravity not in sync should not change gravity? If that was obvious, why LIGO(iirc?) wanted to detect gravitational waves?
tenthirtyam 4y ago
Would an easier answer be that (in simple two-body Earth-Sun orbit) the Earth is sitting in the Sun's gravitational field. Apart from tiny negligible wobbles due to the Sun orbiting the earth-sun centre-of-mass, this gravitational field is static and always points directly towards the Sun. And so the Earth feels, right now, a pull towards where the Sun will visually appear to be in 8 minutes. Likewise in a galaxy, the gravitational field always points to the centre of the galaxy. Lots of tiny wobbles due to stars moving around, but mostly just points directly to the centre (and the giant black hole lurking there maybe?).

krisoft 4y ago

> If the Earth was feeling a gravitational force corresponding to the relative position where the sun appeared to be 8.5 minutes ago But that is exactly it. Earth is experiencing gravitational force from where the sun appears to be. Which is already “corrected” for speed of light effect, since both the light (where the sun appears to be) and the gravity (where it appears to pull from) is delayed by the same amount. If we would be living in a simulation which neglects to take delay in gravity into account we would be experiencing a pull from the direction where the sun will appear in 8.5 minutes. Now, I can totally believe you that this effect is not significant, or can be neglected for other reasons. But that sentence where you say that orbits would be extremely unstable if gravity would not be delayed indicates otherwise.

sebzim4500 4y ago
>If we would be living in a simulation which neglects to take delay in gravity into account we would be experiencing a pull from the direction where the sun will appear in 8.5 minutes. This is actually pretty much what happens. The point that the earth accelerates towards is, to first (second?) order, the predicted point that the sun would be assuming it continued accelerating at its current rate. It is not accelerating towards where the sun was 8 minutes ago, nor where the sun appears now or 8 minutes ago.
- pyinstallwoes 4y ago
  If the sun disappeared, would it take 8 minutes to know?
  - petschge 4y ago
    Yes.
    - pyinstallwoes 4y ago
      What would happen during this 8 minutes from beginning to end? Genuinely curious at a high level

egeozcan 4y ago

Every time there is a discussion about general relativity, my brain stops working and I feel very stupid. Doesn't matter how many youtube videos I watch or articles I read. So it's very nice that Newtonian physics work to some scale in a limited context. I consider it a fast-inverse-square-root style shortcut the universe has given to people like me.

midasuni 4y ago

> nstead of where it currently is Surely where the sun currently is depends entirely on the position and frame of reference of the observer?

the__alchemist 4y ago
I'm suspicious here lies the crux of the issue. And, here be dragons.
SiempreViernes 4y ago
No, the sun is where it is, that's what having physical reality means. What you are talking about is how that point is named by different observers, and there is indeed great ambiguity there. But ultimately, if two persons are starting from different places towards the same goal following a map, the fact that they don't both eventually arrive at the same place has implications about the map they used, not about where the place is.
- wruza 4y ago
  But where is it then? Like in (x,y,z,t).
  - SiempreViernes 4y ago
    In Heliocentric coordinates it is at (0,0,0,0).
- QuadmasterXLII 4y ago
  Two observers at the same time, same point, and different velocoties will agree on the sun’s trajectory, but not on where along its trajectory it ‘currently’ is.

wruza 4y ago

If the Earth was feeling a gravitational force corresponding to the relative position where the sun appeared to be 8.5 minutes ago, instead of where it currently is, then it would lose orbital angular momentum I can’t understand why. If we imagine a completely dark universe of pointlike massive bodies, orbits still should work. Light and gravity not in sync should not change gravity? If that was obvious, why LIGO(iirc?) wanted to detect gravitational waves?

tenthirtyam 4y ago

Would an easier answer be that (in simple two-body Earth-Sun orbit) the Earth is sitting in the Sun's gravitational field. Apart from tiny negligible wobbles due to the Sun orbiting the earth-sun centre-of-mass, this gravitational field is static and always points directly towards the Sun. And so the Earth feels, right now, a pull towards where the Sun will visually appear to be in 8 minutes. Likewise in a galaxy, the gravitational field always points to the centre of the galaxy. Lots of tiny wobbles due to stars moving around, but mostly just points directly to the centre (and the giant black hole lurking there maybe?).