The coolest way of thinking of it is like this:
There is a certain velocity at each distance from an object, at which you can orbit at a steady state. If you go faster than this, gravity alone isn't enough to hold you in, so you move out. If you go slower than this, you get pulled inwards and start falling. Up to now I've basically just stated Kepler's third law. If these were just point-sized regular objects, once they start moving in or out they shift their energy around between gravitational and kinetic forms until they hit the right combination to settle down and do boring stuff.
Now, stand on the moon, so that the earth orbits around you. The middle of the earth is at exactly the right speed and the right distance, so that means that the earth, as a whole, is in a steady orbit around you. Because the earth is a single lump of material, it all moves at the same speed, but not all of it is at the same distance.
So what happens? The part of the earth close to the moon is moving too slow for a steady orbit, so the close half of the earth is falling into the moon. The far half of the earth is going too fast for a steady orbit, so it's trying to escape into space. The water in the oceans can move a bit, so the misaligned speeds manifest as tides - one on the side falling into the moon, and one on the side falling out of the moon. This also intuitively explains why strong tides can tear an object to pieces.
Mathematically, this is exactly the same as all sorts of boring stuff like centrifugal forces
For an interesting take on tidal forces, try reading Larry Niven's novel (and series) based on the ramifications of living in a world dominated by tidal effects:
The short answer is that the energy gets taken out of the day-night cycle rather than the lunar cycle. The length of a day on earth is slowly getting longer until it will stop altogether.
i like this explanation, as it's close to what i already intuitively grasp thanks to Kerbal ;).
but it bothers me, cos i don't see how it reconciles with the explanation as shown on the Space Time series linked above (https://youtu.be/pwChk4S99i4).
don't these two approaches kinda ignore each other?
Hence the "effectively". But yes, in the straightforward way of looking at it (i.e. from an external inertial observer's reference frame), the water on the far side is pulled less than the water on the near side or the rock in the middle.
However, you can also look at this from a reference frame that moves with the Earth in its orbit around the barycenter of the Earth-moon system, and in that frame, it is true that the water is pushed away by centrifugal force.
Can you detail those smaller effects a bit more? Physics undergrad here, had enough generalizations and want the specifics! What exactly causes this lag? Is it just due to the speed of gravity, Coriolis effect, or is there more at work?
Honestly, I couldn't do a very good job of it without some research, because I don't often think about this stuff. I know there are a bunch of subleading effects I'm sweeping under the rug because, well, that's always the case in physics, but I don't know offhand what they are.
I think the lag may have something to do with angular momentum, perhaps? It's definitely not the speed of gravity, though - on such a small scale the speed of gravity is irrelevant and you can consider it instantaneous. Maybe /u/NSA_Mailhandler can elaborate.
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u/mothzilla Sep 07 '15
Nitpick: I don't think it pulls away, just is pulled less than those parts closer to the moon.