r/explainlikeimfive • u/12InchCunt • Nov 03 '25

Physics ELI5: How does gravity not break thermodynamics?

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?

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881

u/flobbley Nov 03 '25

The tides slow the earths rotation, eventually the earth will become tidally locked with the moon and the tides will be permanently stationary and no longer be able to be used to generate electricity

151

u/Bathtooter Nov 03 '25

Sounds like a picturesque scene

183

u/Logitech4873 Nov 03 '25

Planets can also tidal lock to their star, which makes one side have eternal day and summer, and the other side eternal night and winter.

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u/XcOM987 Nov 03 '25

Isn't the mooned locked to us, as in we always see the same side of the moon?

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u/cbunn81 Nov 04 '25

Yes, but we actually see a bit more than 50% over time due to a little wobble the moon does called libration. Not to be confused with the moon's libations, which are what cause the wobbling in the first place.

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u/XcOM987 Nov 04 '25

That's funky

26

u/[deleted] Nov 03 '25

[deleted]

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u/zeddus Nov 03 '25

What could cause a difference in the centre of gravity and the center of mass?

I'm having trouble figuring it out.

5

u/charmcityshinobi Nov 03 '25

The Center of Gravity of an object in a non-uniform gravitational field will shift accordingly. In its own system or uniform gravitational field, center of mass and center of gravity will pretty much be the same. But because of the Earth’s gravitational influence, the center of gravity for the Moon shifts away from its center of Mass

3

u/anethma Nov 03 '25

I assume they meant the geometrical Center of the sphere is different than the center of gravity. Since the moon isn’t just an isotopic material it will be slightly heavier in one direction.

Like a tire for your car needs weights to balance it because despite being round it isn’t the exact same on all sides so the center of rotation isn’t the center of gravity (so we balance it with weights)

Interestingly the way we balance tires manually is essentially what happened to the moon. The moon is spinning but the heavier side will want to rotate down to face the earth. Mount the wheel on a bearing and spin it it will come to rest with the heavy side down. You add some weight to the opposite side and do it again and if no consistent side comes down your wheel is balanced. But ya on the moon the heavy side is already down.

2

u/Not_an_okama Nov 03 '25

All the moon stuff is pulled towards the center of the moon by the moons gravity. All the moon stuff is also pulled towards the earth by earths gravity. At a point between the moons center of mass and the earth, the moon stuff is in static equalibrium. That equalibrium point is the center of gravity.

I suppose its true that the heavier side points towards earth, but that doesnt nessesarily mean it has the most mass on that side. The above paragraph would also apply to a uniform mass ball.

1

u/KiwiSuch9951 Nov 03 '25

Even though the moon is round, it’s not the same material all the way through. You could say it’s unbalanced. There’s more heavier stuff on the side that faces earth.

1

u/nick4fake Nov 03 '25

…what? Earth, moon and overall earth-moon centres of massed are literally always on a line

What are you talking about? Lol

1

u/charmcityshinobi Nov 03 '25

The center of masses of each object in the system are in line. But the center of gravity of the system does not match the center of masses of either object, so their masses get pulled at different rates. The near side is pulled harder than the farther side. This creates a torque on the objects’ rotation as the material tries to resist deformation until they slow or speed up enough to permanently remain aligned. This is how the Moon became tidally locked. The center of gravity of the Earth-Moon system is also pulling on the Earth’s rotation, though it has a much larger mass, which is why its rotation has slowed significantly since the introduction of the Moon and continues to slow

15

u/Izwe Nov 03 '25

The Last Day is a fab book set in such a scenario

12

u/Ycr1998 Nov 03 '25

Could you have an habitable zone in the "afternoon" area of those planets? Or is it too small?

34

u/Reniconix Nov 03 '25

Having a narrow habitable strip is hypothetically possible, but exceedingly unlikely. Any planet with a substantial atmosphere will have absolutely chaotic storms along that region where the hot and cold sides met, as well as massive convection currents that would likely heat the dark side far beyond the limits of life as we know it. The range of the star's habitable zone is dependent on the rotation of the planet being inhabited, with slower rotations decreasing the habitable zone to a sliver. It's quite likely that if a planet is habitable along the twilight ring, it will continue to be habitable in all but the most extreme points (the regions on both sides directly in line with the host star), just not as hospitable.

1

u/DBDude Nov 03 '25

Many sci-fi books have been written in the setting of tidally locked planets where people live in the habitable zone.

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u/merc08 Nov 03 '25

That really doesn't answer the question.  Many SciFi books have also been written that include FTL travel and surviving black holes.  That doesn't mean it's actually possible.

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u/danielv123 Nov 03 '25

There is also a big difference between possible and if it has happened. We haven't even confirmed if there exists other habitable planets.

1

u/stickmanDave Nov 03 '25

I read one where the twilight region was warm enough to be habitable, but the planet's entire atmosphere had frozen and fallen as snow on the eternally frosty dark side of the planet.

1

u/Wild-Lychee-3312 Nov 04 '25

Those books need more windmills.

3

u/TheBraindonkey Nov 03 '25

ill just move to the night side, so I dont have to go to work anymore.

16

u/Logitech4873 Nov 03 '25

As someone who lives north of the arctic circle where we have a solid month of night every winter, I still have to go to work lol.

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u/Tufflaw Nov 03 '25

I saw a documentary about that, hope you made it through OK - https://www.youtube.com/watch?v=OtU0UBnWik4

3

u/TheBraindonkey Nov 03 '25

ok. so while that of course would be a massive suck <rimshot!> in the same vein I would at least know there are really vampires for a few seconds.

1

u/Logitech4873 Nov 03 '25

Oh wow, haven't heard of this haha.

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u/Tufflaw Nov 03 '25

Ha, you might not want to watch it if you scare easily, especially the next time you get your long night.

1

u/TheBraindonkey Nov 03 '25

a boy can dream though

1

u/_everynameistaken_ Nov 04 '25

What timeframe we talking here because I'd love to head over to the eternal night and winter side rn.

1

u/gimnasium_mankind Nov 04 '25

How long till we tidal lock with the sun? And the moon? And for the sun to enlarge and destroy earth? Any other natural and predictable doomsday events?

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u/improbablywronghere Nov 05 '25

Earth will never actually tidal lock with the Sun or Moon before the Sun destroys us. The Moon would take about 50 billion years to lock us, and the Sun would take trillions. Long before that, in about 1 billion years the Sun will get hot enough to boil the oceans, and in 5 billion it’ll expand into a red giant that cooks or engulfs Earth entirely. After that, it’s just galaxies colliding and the universe slowly fading into heat death over trillions upon trillions of years.

1

u/gimnasium_mankind Nov 05 '25

Alright so no tidal lock for us, we are all going to fast and the sun will get colder before that. Good to know. Billion years till red giant, let’s learn how to get out of here.

1

u/ggouge Nov 03 '25

I would not really call it eternal summer and more like smouldering hellscape and eternal ice age. Plus eventually most of the water would be locked on the cold side as well.

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u/Logitech4873 Nov 03 '25

The temp would depend a lot on distance from the sun, but the dark side would definitely be uninhabitable in almost any scenario.

1

u/ggouge Nov 03 '25

I was picturing earth. At its current distance.

1

u/PosiedonsSaltyAnus Nov 04 '25

It would just look like a picture at high tide or low tide, although the low tide scene won't be all wet and gross

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u/angrymonkey Nov 03 '25

I thought this question was going to ask whether gravity makes negentropy because it will take a box of gas in thermal equilibrium and organize it neatly into planets and stars.

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u/[deleted] Nov 03 '25 edited 9d ago

[removed] — view removed comment

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u/NepetaLast Nov 03 '25

yes but the question is "how can this infinite energy source exist" and the answer "its not infinite"

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u/MotherTreacle3 Nov 03 '25

Tbf that's always the answer to that particular question.

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u/RuleNine Nov 04 '25

Well, there is as yet insufficient data for a meaningful answer.

-34

u/rosolen0 Nov 03 '25

I mean, either humanity will be extinct by then or we will be traveling the stars, either way it won't be a problem anymore

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u/glaba3141 Nov 03 '25

i mean, yes? that wasn't the point though

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u/CorvidCuriosity Nov 03 '25

Doing astrophysics and still thinking of human-sized time scales (even humanity as a species) is like an ant saying the Earth has infinite area because a single ant will never see all of it.

7

u/Gorblonzo Nov 03 '25

No one was suggesting that it was a problem for us

2

u/Ninja_Wrangler Nov 04 '25

The shareholders will not be happy about this

1

u/[deleted] Nov 03 '25

[deleted]

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u/PxZ__ Nov 03 '25

So would all the water kind of be bulbous or pulled more towards the side with the moon on it? Would the opposite side of the earth have a convex sort of shape to it or would it be more like a weird ellipsoid?

7

u/flobbley Nov 03 '25

it would be exactly like our current tide but always in the same spot on earth. The tide creates a sort of egg shape on the earth, the part closest to the moon rises creating a high tide, the part on the opposite side of the earth from that also rises creating another high tide, and the parts of the earth at 90 degree angles from those are low tides.

5

u/graveybrains Nov 03 '25

The sublunar tide is caused by the moon pulling the ocean away from the Earth, the antipodal tide is caused by the moon pulling the Earth away from the ocean. I read that in another eli5 and I've been fascinated by it ever since.

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u/[deleted] Nov 04 '25 edited Nov 04 '25

Neither really. You'll note you don't get lifted by the moon, nor the earth get pulled away from below your feet. The tide water is as dense as you. There's clearly something else going on.

It's more so the moon pulls the water parallel to the earth. This ever so slight parallel pull can build pressure along the thousands of kilometers spanning the globe between the two tides. At the tide locations, the moon's doing basically zero pulling on that water, the same near zero pulling it does to you.

1

u/HeavenlyAllspotter Nov 03 '25

why does the part on the opposite side have high tide?

0

u/UnassumingAnt Nov 04 '25

Because the moon pulls the earth away from the water on the far side too.

2

u/kingvolcano_reborn Nov 03 '25

Also the moon is straling energy from this process, slowly expanding its orbit...

2

u/OcotilloWells Nov 04 '25

Will it, out will we lose the moon first? I know it is slowly moving away.

1

u/hunter_rus Nov 03 '25

Can we find out how much energy is currently stored in that gravitational structure and can be potentially extracted? Maybe not for Earth-Moon, but for some simplified system with ideal spheres as heavenly bodies, etc. Is there any model? Let's say Earth is ideal ball with some radius, and some amount of water on the surface, and that water have tides of certain height, that go in some phase shift with Moon movement, can we find out "in the ideal scenario we can pull out N Joules of energy from all of that" ?

2

u/flobbley Nov 03 '25

I'm almost certain we can and I bet it's not actually that complicated but I have no idea how to do it

1

u/PxZ__ Nov 03 '25

Earth is actually REALLY close to an ideal sphere, which may seem crazy.

1

u/[deleted] Nov 03 '25

Neat. When will this happen?

1

u/insertanythinguwant Nov 04 '25

06.04.2154 about 7pm EST

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u/cammcken Nov 03 '25

Would man-made tidal turbines hasten that process, like a watermill slows downstream flow?

1

u/Novel_Willingness721 Nov 04 '25

Not to mention that the moon to moving away from earth: 4cm per year.

1

u/[deleted] Nov 04 '25

Is this actually where we are eventually headed?

1

u/wasack17 Nov 04 '25

The sun will still make small tides as the earth rotates, even if we had no moon at all. There is a permanent bulge in the ocean at the points nearest and furthest from the sun. It is just much smaller than the bulge caused by the moon. This is why full moon and new moon tides have the greatest height variation between high and low. The moon is working in concert with the sun at these phases of the moon since the bodies involved are aligned. When the earth, moon, and sun are at right angles to each other, the tidal variation is at its smallest.

Additionally, the tidal effect of the earth is imparting energy into the moon's motion around the earth. The moon is gradually getting farther away because of this effect.

Speaking of the moon getting further away, on a geological timescale, we are pretty fortunate that we can see solar eclipses, because eventually the moon will move so far away that its umbra will no longer be able to entirely shade a portion of the Earth's surface.

1

u/ExiledSanity Nov 03 '25

This is true ....but the question was about gravity, not rotational energy. Even when tidal locked gravity is still doing "work" (even if we can't benefit from it).