# Why doesn't gravity violate the 1st law of thermodynamic's?

Greetings all, I am a newbie here. I'm also a layman, as you probably guessed by my question. And I obviously am missing something about the 1st law and how it deals with Gravity. It hit me when I was thinking about Jupiter's moon, Io, and how it is volcanically active simply because of tidal forces. This seems to me an apparent free energy of sorts, a perpetual engine.
Can someone please offer an explaination why this doesn't violate the 1st law of thermodynamics (as I understood, energy cannot be created. So where's the energy coming from?)
Right now I'm juggling three options, the last two which redefine our concept of gravity.

1. If there are gravitons, then graviton energy is to blame, but they will eventually decay as photons supposedly might do (I don't like this explaination BTW)
2. Gravity is a form of the observed Casimir effect, a possible realization of vacuum energy.
3. the 1st law of thermodynamics applies only to the first three or possibly four dimensions, whereas gravity reaches into higher dimensions giving it additional energy as needed.

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Hurkyl
Staff Emeritus
Gold Member
Fourth answer; Io is so puny and insignificant compared to Jupiter that it doesn't even put a dent in the available energy.

Janus
Staff Emeritus
Gold Member
The energy comes from Jupiter's rotation. Jupiter slows down a bit as time goes by. But as Hurkyl has pointed out, Jupiter's energy store in terms of rotation is so great and Io so small, that it will be a long, long time before it's exhausted.

1LoT doesn't get violated. Feynman explained it in his Physics Lectures. If ever total energy comes up long or comes up short in a calculated situation, a new form of energy is hypothesized to make things come out right. Of course, finding out more about a new and strange form of energy might be challenging, but there's plenty of time.

Gravity, vacuum energy, and the 1st law of thermodynamics.

Great answers guys, thanks for the clarification. In retrospect, the slight slowing of Jupiter was so obvious as to have slapped me in the face. But I still have a little problem with the concept of gravity as a force(energy). If two bodies approach one another, their attraction increases. But where does this energy come from? Is it created as the influence of gravity increases, or is it potential energy from the total universal store that is activated? I suspect the latter, but if someone could say it in other terms for benefit of clarification.
On a similar note, if there is vacuum energy and we are able to 'tap' into it in the future, would this really be considered 'free energy?' If not, then could you explain (in theory) how conservation would apply. Much appreciated!

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Gravity as static force, as opposed to energy

malthis said:

I still have a little problem with the concept of gravity as a force(energy).
Force is different from energy. Force is pressure. Gravity is a static force (a static pressure). Energy is not static. Energy is power applied over time. Power is force * rate-of-application-of-force. Only if you could turn gravity on and off could you get energy from it, and then it would be violating the first law of thermodynamics.

But where does this energy come from?
Again, it is not energy, but force, and this force is (by definition) static. The reason why I say it is static is that it does not add or subtract anything from its interaction with bodies. As a small body approaches a large one, it "falls" and potential energy is translated into kinetic energy. No energy is gained or lost.

All in gravitation (classical) is a problem about central forces. When we send a probe to Jupiter, to give it gravitational help, the probe makes the same force to Jupiter, but Jupiter don't is altered. The energy lays constant. No violation.

Thermodynamics is a macroscopic science, which no makes any postulate about space. The principles are ok in one, two, three or n dimensions ... because the point is that:
- The balance of energy must be constant,
- The heat can not be transformed all into work ciclically,
- and heat can not go from a cold to a hot focus...

No presumptions about space, time, or even temperature.

That's because thermodynamics is so nice :)

Gravity's potential energy

Force is different from energy. Force is pressure. Gravity is a static force (a static pressure). Energy is not static. Energy is power applied over time. Power is force * rate-of-application-of-force. Only if you could turn gravity on and off could you get energy from it, and then it would be violating the first law of thermodynamics..
Does this mean if we find a way to create regions of higher gravity, or even antigravity, it must violate the first law? Or is it possible (in theory) that the static pressure of gravity could be 'mined' for local use, therefore conserving total gravitational force by diminishing its potential energy store? I know it's an "out there" proposition, but I want to test the feasability.

Again, it is not energy, but force, and this force is (by definition) static. The reason why I say it is static is that it does not add or subtract anything from its interaction with bodies. As a small body approaches a large one, it "falls" and potential energy is translated into kinetic energy. No energy is gained or lost.
This makes perfect sense to me, but also makes it impossible to calculate the amount of this force, as it would depend on how much energy is created over the entire timespan of the universe by the action of gravity. Is that correct?

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Force is different from energy. Force is pressure. Gravity is a static force (a static pressure). Energy is not static. Energy is power applied over time. Power is force * rate-of-application-of-force. Only if you could turn gravity on and off could you get energy from it, and then it would be violating the first law of thermodynamics.
Again, it is not energy, but force, and this force is (by definition) static. The reason why I say it is static is that it does not add or subtract anything from its interaction with bodies. As a small body approaches a large one, it "falls" and potential energy is translated into kinetic energy. No energy is gained or lost.
Force is not pressure and energy can be static (in a conserved system).

Intersting dicussion, it actually reminded me of a discussion i had about the earth-moon system, and how the tidal forces are slowing the earth down. Since the moons gravity pulls on the earth's oceans, it creates a bulge. Since this bulge is moving faster (bacause the earth rotates faster than the moon orbits), the bulge in a sense pulls on the moon because of the moon's gravity being attracted to it. The pulling actually speeds up the moon, making its orbit slowly get farther from the earth. At the same time the earth is slowing down beause of the moon's gravity pulling on the bulge. The earth will keep slowing down and the moon with keep speeding up until the earth's rotational speed will be lined up witht he moons orbiting speed. Anyways, just wanted to show a similar case.

Ian
I don't mean to interrupt the discussion but I don't believe that tidal forces actually slow down planetary rotation. I think tidal forces are the cause of the rotations of the planets.
Think on this;

At any one time as the earth revolves on it's axis there is one point on the sphere of the earth that is closest to the sun, and its opposite point is farthest away from the sun. these two points are subject to slightly different gravitational acceleration by the sun and therefore the nearer point will try to orbit with a greater velocity than the farther point.
If the difference in gravitational force between the two points is great enough to overcome the binding mechanism of the solid earth then the earth would shear. But since the difference is slight we observe a perpetual rotation of the planet.
I have a simple relation that I apply to planetary rotation: Divide the velocity of planetary rotation by the difference in orbital velocity between the nearest and farthest points to give the number or rotations per orbital revolution.
e.g:
The velocity of the earth's rotation (463.83 m/s) divided by (The orbital velocity at 1AU-earth radius) minus (the orbital velocity at 1AU+earth radius)
= 463.832 /1.26988 = 365.25 rotations for the earth per earth year.

This simple rule applies extremely well to the rotation of all the solid planets and holds well in the case of the gas giants which have a differential rotation. If the tidal forces slowed planets down there would be a great deviation of this rule among the planets that have moons due to the different mass combinations.

No comment on that....but in that case what does cause the earth to slow down. IT IS indeed slowing down, its a known fact. If its not cause of the moon, then why??

Ian said:
At any one time as the earth revolves on it's axis there is one point on the sphere of the earth that is closest to the sun, and its opposite point is farthest away from the sun. these two points are subject to slightly different gravitational acceleration by the sun and therefore the nearer point will try to orbit with a greater velocity than the farther point. If the difference in gravitational force between the two points is great enough to overcome the binding mechanism of the solid earth then the earth would shear. But since the difference is slight we observe a perpetual rotation of the planet.
And what about the fact that the moon's rotation is in in the same direction as its orbit rotation? Or the fact that Uranus' rotation axis lies more or less in the plane of ecliptic? That makes no sense at all.

krab
Ian said:
. I think tidal forces are the cause of the rotations of the planets...
Huh? This is pre-Newtonian thinking, where constant motion (rotation or linear) requires application of a constant force.

Yep, there can be some confusion in your explanation Ian. I dont know but i think what I say makes sense. Although it can get messy if you include the sun's gravitational forces on the earth, since its about as strong as the moon's from that distance (i think). :)

As a small body approaches a large one, it "falls" and potential energy is translated into kinetic energy. No energy is gained or lost.
This seemed like a pretty good explanation to me, until it hit me: potential energy. What potential energy is this? You see, I don't understand how we can this gravcitational potential energy as a form of 'normal energy' when it is the very thing under questioning.

What I mean is, we are defining gravity as something that converts a body's potential energy into kinetic energy as it gets closer to another mass. The problem is, that potential energy is defined by gravity itself, is it not?

I'm no expert, but I find it problemous to define item A by item B, while item B is already defined by item A.

In fact, gravitational potential energy, at least as we most often used it in our classes, was basically the same as saying 'how much kinetic energy does it have at the end?'

Do you see what I'm saying though?

no comment, but true true about "In fact, gravitational potential energy, at least as we most often used it in our classes, was basically the same as saying 'how much kinetic energy does it have at the end?'"

KingNothing said:
I'm no expert, but I find it problemous to define item A by item B, while item B is already defined by item A.

In fact, gravitational potential energy, at least as we most often used it in our classes, was basically the same as saying 'how much kinetic energy does it have at the end?'

Do you see what I'm saying though?

I do, and is the problem I'm having. I still need an explaination as to how we can measure the potential energy of gravity as a force when it is only recognized by its kinetic energy. Does this imply infinite potential energy, an amount of potential energy equal to all kinetic gravitational influence thoughout all time and all space (immesurable, and also possibly infinte) or is it definable in other terms?

How can we even apply such laws as thermodynamics to a force that introduces kinetic energy to a time and place (like when a passing meteor feels the sun's influence) where there wasn't any (or much) before?
Can dark energy be a 'cosmological constant' of sorts that keeps conservation laws intact (expansion as a result of rising attraction between bodies)? Plausible?

Ian
There is only one form of energy but our perception of it changes, it is either energy of matter (kinetic or potential) or energy of the vacuum (temperature).
Thermodynamics relates to the energy of a different medium than gravitation acts upon so gravity cannot violate any law of thermodynamics neither can thermodynamics violate any law of gravitation.
But, there must be an interface between the two since energy has units of mass length and time whatever the medium - we just don't know how to express that interface just yet.

arivero
Gold Member
Sorry to derail you guys, but thermodynamics is about closed systems. PV=nRT and all that. Of course energy preservation is beyond thermodynamics. But please be careful about what theories do you apply into a long range unstoppable force.

I don't think so. You can call "closed system" to whatever you want... obviously try to use thermodynamics to explain orbits... is a loose of time, to that sort of things is Mechanics, central forces, etc.

It is like if you want to "nail a nail with an screwdriver". The screwdriver is not thaught to nail ...

krab
Gravity is a conservative field. This may sound like a political statement, but it is in fact a mathematically loaded statement. It means that as the result of a distributed mass M, we can assign a quantity V to every point in space. This quantity has units of velocity squared. The force on a small test mass m is the gradient of V times m. This is all experimentally-verified info.

Now some math. If you allow mass m free movement in this field, with no friction, using F=ma, you can show by purely mathematical techniques that the sum of the kinetic energy of this mass plus mV is a conserved quantity. So we call mV "potential energy", and the sum of kinetic plus potential energy we call "total" energy. So by introducing this "potential" energy we find that total energy is conserved.

Ian
arivero said:
Sorry to derail you guys, but thermodynamics is about closed systems. PV=nRT and all that. Of course energy preservation is beyond thermodynamics. But please be careful about what theories do you apply into a long range unstoppable force.
The universe is a closed system isn't it? And we talk of the temperature of the background radiation don't we?
But I still agree with arivero in a way. But if thermodynamics is about pressure and volume with relation to the state of the systems matter content (a laymans translation of PV=nRT), then surely the universe having a temperature and therefore a pressure must expand - but if all the matter is in motion and that motion is angular there must exist a centripetal acceleration that provides a 'binding' force that counters either a of part or all the expansion due to the pressure.
Now that sounds like a description of gravitational forces acting in conjunction with energy due to temperature, so where does violation of thermodynamic principles come into it?