I have never seen this really explained in details to the general public which I belong to. Maybe that's a sign I'm completely misunderstanding the subject though.
Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it. See the river model of general relativity for a more thorough explanation.
Analogy, take two attracted magnets, or two opposing electrodes, and expand the space between them. Things change
If you are asking hypothetically, if a human body were floating in the intergalactic medium, then yes, the accepted answer you quoted would apply.
Surely expansion is happening at every scale but locally other factors dominate right? To what degree? Is it mainly gravity? Electromagnetic attraction between atoms and/or molecules?
The naive mental model that I have is of two balls tied with a rubber band, each on a treadmill going in opposite directions. Since the rubber band attraction dominates, they slip on the surface of the treadmill and their distance is barely affected even if the surface underneath “expands” outwards quite quickly. Is this a reasonable analogy or is it too simplistic?
Of course the rubber band force is proportional to the distance, while the attraction forces we are talking about are inversely proportional. And I have no idea if the expansion of the universe can be reasonably modeled as an outwards “drag force” on matter. How “sticky” is matter with respect to space?
So for our bodies, planets, solar systems, even galaxies and clusters, because these are bound (either electromagnetically or gravitationally), the influence of the expansion of the universe on them is not just negligible, it's non-existent.
It's a little different when wo do include dark energy and other mechanisms more complicated than a simple matter or light content. For your intuition, you can think of this as a constant omnipresent negative pressure. We have no idea how it works on scales smaller than those of the observable universe, but if we imagine it works the same on every scale, then it's an extremely tiny force constantly pulling your body apart.
"Brooklyn is not expanding"
But if you will, think about it like that. All life adapts to its environment. All life. All the time. Everywhere. And the expansion is not that fast that a single generation of anything from a one day fly to a centenarian turtle, or a millennia old tree has to bother about it. It's invisible at our timescale.
I've always thought why animals were so huge at the beginning. Not just dinosaurs, but insects the size of a large dog. Maybe it's the oxygen rich environment. Maybe it's evolutionary processes shrinking size in time to optimize energy needs and improve survival. Maybe gravity somehow changed in time, or it was the expansion of the Universe or a myriad of sci-fi reasons we can come up with. But life adapted and moved on.
So that's what expansion means for us. One of millions of variables we constantly adapt to. If it's slow enough, no problem. But if it's hitting us fast like the accelerating climate change or technological progress, that... we may have problems with. That's when you see unrest, violence, crime. Wars. Famine. Suffering.
The Universe is not our problem. We... are our own problem.
> One of millions of variables we constantly adapt to. If it's slow enough, no problem. But if it's hitting us fast(...), that... we may have problems with. That's when you see unrest, violence, crime. Wars. Famine. Suffering.
No. Famine, starvation, disease, suffering, mass deaths - and even wars - are exactly the process through which life adapts to slow changes. That's what it means for ecosystems to thrive, for nature to be in balance - that balance is held dynamically, by constant cycles of excessive slaughter followed by mass starvation.
We brought a lot of new problems on the table, both for ourselves and all other life, the latter of which can't even keep pace. But senseless suffering and comically painful death - that one came from nature, and we're actually successfully reducing it.
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.
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.
In fact, everyone on the planet would hear the same chirp. Someone should comb the historical records (or even, mythologies) for a birdless chirp heard by many people.
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.
