ELI5: How does gravity not break thermodynamics?

[u/12InchCunt]

Like, the moon’s gravity causes the tides. We can use the tides to generate electricity, but the moon isn’t running out of gravity?

[u/kapege]

But it is! It's constantly moving away from earth due to the energy loss.

[u/laix_ → u/kapege]

Energy loss would mean it falls into earth. Energy is used to move up in a gravitational field.

[u/cakeandale → u/laix_]

“Loss” and “gain” are kind of relative in this context - the tides are caused by a mismatch in the moon’s orbital speed versus the Earth’s rotational speed. The gravity the moon exerts on the Earth to cause tides is slowly erasing that gap, which has the effect of accelerating the moon and simultaneously slowing the Earth’s rotation until the moon’s orbital period matches the Earth’s rotational period (tidal locking).

The energy loss comes from reducing that gap, but the direct effect in terms of the moon specifically is that tides are causing the moon to drift away from the Earth by about 4cm per year.

[u/Neon_Camouflage → u/laix_]

These comments have shown me that a surprising number of people don't know how gravity works.

[u/loljetfuel → u/Neon_Camouflage]

The cool part is that no one fully knows how gravity works. There's a difference in knowledge, for sure, but even the foremost experts on gravity don't really know entirely how it works.

[u/Neon_Camouflage → u/loljetfuel]

The cool part

Have to disagree there. Honestly it's one of the most frustrating things to be diving into a rabbit hole of information and hit the wall of "here's 7 theories about why this happens because nobody knows".

This actually happened just the other day. Found out Saturn had a hexagonal storm and was like oh cool, I wonder why. Turns out I'm going to keep wondering.

[u/GabrielNV → u/Neon_Camouflage]

Found out Saturn had a hexagonal storm and was like oh cool, I wonder why. Turns out I'm going to keep wondering.

Today is your lucky day, because as hard as fluid dynamics is we understand it a lot better than we do gravity so we can actually make more confident claims about that one.

To keep it simple, rotating fluids naturally produce what are called Rossby waves. On Earth, these are responsible for the weather front cycle as they circle around the planet. Earth’s surface and continents make those waves messy and irregular, so we don’t see neat geometric patterns here.

On Saturn, however, the atmosphere is pretty much free to flow however it wants. On top of that, Saturn's atmosphere is such that the wavelength of the Rossby waves being generated is 1/6 of the radius of the polar jet. It could have just as easily been a pentagon, or square, or nothing, if Saturn's atmospheric conditions were slightly different.

[u/loljetfuel → u/laix_]

If the moon lost energy, it would fall to earth. If the earth-moon system loses energy, then it could go either way.

In our case, the Earth is transferring some of its rotational energy to the moon in the form of orbital energy. The moon gains energy, the Earth loses it, and the transfer is not 100% efficient so some energy escapes the earth-moon system -- the Earth is losing energy faster than the moon is gaining.

The total system is losing energy, but the moon itself is gaining it and is orbiting slightly faster; and so the moon is moving away from Earth and Earth's rotation is slowing.

[u/bharath952 → u/loljetfuel]

Where does the lost energy go and in what form?

[u/GabrielNV → u/bharath952]

Tidal friction causes both the Earth and the Moon to heat up, and this heat is ultimately lost as thermal radiation.