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 Originally Posted by Eric
Thought Experiment: The black hole at the center of the Milky Way instantly disappears.
How long does it take for things to change for our solar system and our planet such that they no longer orbit around the center of the galaxy? How are gravity waves part of the change?
The solar system, i.e. everything inside the sun's sphere of influence, wouldn't really notice the difference aside from the dramatic changes in the appearance of the Milky Way as it expands and diffuses across the sky. The Earth is orbiting the sun, and so it doesn't really feel the gravity of the galactic center. There will be tidal forces, but at the distance the sun is from the galactic core, those are negligible. I mean, the moon's tidal forces are quite noticeable on Earth, but the tidal forces on Earth due to the sun are difficult to measure.
The sun will cease to orbit the galactic center ~27,000 years after the black hole annihilates, as it is ~27,000 light years from the center and gravity waves propagate at a rate of 1 light year per year (I.e. the speed of light).
Gravity waves are kinda like this:
Think of a trampoline. (Yeah, this again.)
I'm not re-stating all the pros and cons of this thought experiment; be aware that this is just a basic visual aid,
The galactic core is like a huge weight in the center of the trampoline. The weight sags down and sits at the bottom of a very curved pit in the trampoline. This curvature represents the effect of mass on space-time.
For the purposes of this visualization, replace the weight with a taught rope pulling down on the trampoline from the other side.
Then cut the rope.
The trampoline sheet will spring up. The wave of this up-springing will travel out from the center at the speed of sound in the trampoline sheet.
This is like the way the curvature of space-time would respond to the annihilation of mass. It springs back to the state of having no source of curvature. It overshoots that equilibrium position and then experiences a decaying oscillation as it settles down.
This creates ripples moving spherically outward.
I can't help but think of the ripples in a still pond, if I were to throw a rock into it. At first, the rock displaces the surface into a bowl shape, but then it releases that pull. The resulting waves (not of the impact, but of the release) are probably not a terrible way to visualize 3-d waves in 2-d. You just have to remember that space-time is 3-d and these waves are how the space-time stretches and compresses.
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