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This would be an awesome xkcd video, they're killing it on YouTube
I'm really hoping Randall makes some new material soon - I enjoy the videos but I've read all the What-Ifs on his site and own both the books.
Earlier quoted context omitted.
i walked around and it involved force and energy. i will clarify. in the absence of other forces, matter is indeed pinned to the space it is in. you talk about gravity as if its something distinct from what is driving the expansion of the universe. we have that model, its what newton proposed.
I believe all of the current theories being seriously considered hold that whatever is causing expansion isn't gravity, so yes I do talk as if gravity is distinct from what is driving the expansion of the universe. Regarding being "pinned," that still fails to account for inertia. The idea that there's a specific piece of space that we're stuck to implies there's a rest state at which there is no motion independent o…
In many interpretations of GR it is believed that mass deforms spacetime, but is also influenced by the spacetime it moves through. In the absence of mass, spacetime expands. Carrying along any mass with it. When mass is present this default expansion is overpowered by its influence which causes a contraction. Hence black holes.
Gravity is still colored by classical definitions so most assume its power is unidirectional. However there is a quantitative factor for expansion in the very same Einstein equation which defines GR.
We need to stop thinking of gravity as a force acting on objects, and rather something that acts on spacetime. Many people advocate this but stop at the rubber sheet analogy, which is completely devoid of the idea that the sheet actually moves as well.
Gravitational waves move at the speed of light, doubtful you can "look" at it. If it's that strong it'll just seem like a shake. Like an earthquake. Except it's the universe that's quaking.
> Gravitational waves move at the speed of light Do we know if there are mediums (sp? media?) where gravitational waves move slower than the speed of light? Like light does in glass?
I'm firmly of the opinion there's no "spooky action at a distance". All waves propagate gradually (if rapidly) through a medium, even if we decide to call that medium "vacuum" and define it as empty. Or more modernly, a "field". Waves don't propagate in nothing by definition.
Gravitational waves move at the speed of light, doubtful you can "look" at it. If it's that strong it'll just seem like a shake. Like an earthquake. Except it's the universe that's quaking.
If everything is quacking then would anyone “feel” it?
If light speed was infinite. If sound speed was infinite. Or even if water waves were infinite, we'd never feel or see anything waving.
We feel a wave by seeing the differential effect of it propagating. Which requires speed with a detectable progression. Compression, decompression. Excitation, de-excitation.
So when everything is quaking, we detect it by neighboring regions not being in sync as to the direction, amplitude, phase etc. of where they're quaking at any given moment.
Just like with normal earthquakes. The ground is shaking. If everything atop shook in perfect sync, you'd miss the earthquake. But otherwise, you feel it, you're destabilized. Not in sync with the ground.
For gravitational waves, it's much harder for us to feel it without special, very sensitive equipment. Normally. But if it was strong enough, we would.