# The Sun vanishes. Earth keeps orbiting just fine for 8 min 20 sec

See subject.

This is true, right? Hard to understand. The fact that the Sun could be gone but we wouldn’t see/know it for a while seems much easier to understand. Self-evident, really.

But gravity is information, too.

I just had a brainstorm, in real SD time : This is analogous to the slinky held loosely, free, and when released the bottom rung “hangs in mid-air” perfectly still until the collapsing upper coils reach it…

Is that analogy correct? If so, how far does it hold sort-of-true?

If the Sun vanished – the object, not just the light – we would not keep “orbiting,” but continue on a straight line, tangent to our position just before the Sun vanished. The light wouldn’t wink out for a few minutes, but we’d be heading off into space, only subject to other gravitational forces in our Solar System.

If magic things happen, then the rules of magic rather than physics apply. If gravity is moderated by virtual gravitons and the magic didn’t eliminate those, then the earth would keep orbiting for 8 minutes. I assume under general relativity, the disappearance of the sun would “unbend” space-time and this unbending would travel as a gravity wave which would disrupt the movement of the earth 8 minutes later when it arrived. I don’t think this would have too much effect above the simple “unbending” and the earth would traveling in straight line in the direction it was then moving (and not the direction 8 minutes later).

And yes it’s related to the slinky in the sense that a force cannot travel instantaneously. Gravity, as far as we know “travels” at the speed of light. The speed down the slinky must be related to the slinky’s elasticity I’d think.

What makes you think gravity propagates faster than light?

What is the speed of gravity?

This seems to address the question of the OP directly. According to that, you’d still feel the effects of gravity for about 8 minutes or so as it propagates through the system.

That, I think, is the OP’s question. It’s a good one too. Personally, I have no idea.

And is it relative to the medium?

In general relativity, gravity and gravitational interactions are propogated at c.

If the mass of the Sun suddenly vanished, it would be more than just the Earth now released from its orbit. The instantaneous release of gravitational potential energy would cause the crust and mantle to suddenly relax, which certainly result in massive seismic and volcanic effects.

Stranger

I can wait. Then we go flying off in space. Watch out for Jupiter, which is doing the same thing a few minutes later.

As Stranger points out, current evidence strongly suggests that the speed of gravity is c, same as for light in a vacuum.

So we miserable earthlings would probably never know anything was wrong until the light went out.

Evidence? Suggest?

Read the explanation I posted. Gravity has been shown through indirect observation of binary pulsars to travel at somewhere between 2.993 x 10^8 and 3.003 x 10^8 m/s, which is a range within 1% of c.

Plus, from what we understand of physics, it would make a real mess of everything if it didn’t travel at c. When you don’t have any particular reference frame to consider, and the question is “How fast?”, the only answer possible is “c”.

Freedom is never free…

Has anyone come up with an experiment to try and test this? Obviously, we can’t make stars or other massive objects blink out of existence and see what happens. Is there a way to test it on a smaller scale (that could be still measurable)?

There’s that giant satellite interferometer thing. Can’t think of the name, but it’s amazing. [count-down to when somebody posts the name…]

ETA: or does this experiment look for isotropy only?

Which, unfortunately, is deep into funding limbo. A much better prospect is LIGO.