2nd Law of Thermodynamics vs Gravity

[ydoaPs]

Does gravity negate the 2nd Law?

 

I remember that when we did our heat engine problems that the entropy of the system only went down when we took energy out.

 

Gravity is negative energy, right? Wouldn't that effect large scale entropy and thus the 2nd Law's application to the universe as a whole?

[insane_alien]

it only has a negative sign because there isn't a limit to the depth of a gravitational potential well so it makes sense to set the zero point at an infinite distance as this will be consistent for all gravitational fields.

 

you could in theory work it for earth sea level as the zero point but then you're going to have to come up with new equations for every gravitationally significant body you come across. there'd be fudge factors all over the place. taking the zero point at infinity all these fudge factors go to zero and disappear making everything nice and simple.

[ydoaPs]

it only has a negative sign because there isn't a limit to the depth of a gravitational potential well so it makes sense to set the zero point at an infinite distance as this will be consistent for all gravitational fields.

 

you could in theory work it for earth sea level as the zero point but then you're going to have to come up with new equations for every gravitationally significant body you come across. there'd be fudge factors all over the place. taking the zero point at infinity all these fudge factors go to zero and disappear making everything nice and simple.

I guess I was confused with the whole Hawking radiation thing. Two virtual particles outside of a black hole. One falls in, losing energy which gives the other enough to become real....or something like that.

 

 

 

I still think gravity might have a similar effect to removing heat from a system(iirc, laws of thermodynamics were formulated for heat engines, not the universe in general, anyway). I mean, thinking of entropy as "disorder", doesn't gravity lessen it? Take, for example, a nebula. Doesn't the gravity pull the wandering unorganized molecules together into a more organized form?

[insane_alien]

not really, in a nebula the gasses are cold, when they collapse into a star there is a LOT of heat generated(the energy from gravity doesn't just disappear) and this is eventually radiated out as photons. if you work through the calculations considering everything(including hte photons released) you'll see an increase in overall entropy.

[D H]

The gravitational potential energy between two objects rises toward some horizontal asymptote as the distance between these objects approaches infinity. Calling this horizontal asymptote zero is convenient, but not necessary. In gravitational potential energy, as in any other form of potential energy, it is only the change in potential energy that matters. Gravitational potential energy is not "negative energy" in the sense that it represents an over-unity source of energy.

 

The sole reason gravity can be represented in the form of a potential is because gravity is a conservative force. You can draw energy out of a gravitational system, but that means the total energy of the system, potential+kinetic, has decreased. There is no free lunch with gravity, or any other conservative force.

 

In Newtonian mechanics, a pair of isolated gravitating objects will maintain a constant total energy. However, Newtonian gravity is not quite right. In general relativity, two gravitating objects will emit gravity waves because gravity makes the objects accelerate. The total energy of the two objects will decrease over time.

[ydoaPs]

not really, in a nebula the gasses are cold, when they collapse into a star there is a LOT of heat generated(the energy from gravity doesn't just disappear) and this is eventually radiated out as photons. if you work through the calculations considering everything(including hte photons released) you'll see an increase in overall entropy.

 

I actually thought of that right after I clicked submit reply.

 

 

It seems as though(via my attempts at asking the Oracle) that the relation of gravity and thermodynamics is not very well settled.

[insane_alien]

all i'm going to say is that a LOT of crazies come up when you talk about gravity and thermodynamics. most of them still trying to overbalance wheels.

[ydoaPs]

it only has a negative sign because there isn't a limit to the depth of a gravitational potential well so it makes sense to set the zero point at an infinite distance as this will be consistent for all gravitational fields.

 

you could in theory work it for earth sea level as the zero point but then you're going to have to come up with new equations for every gravitationally significant body you come across. there'd be fudge factors all over the place. taking the zero point at infinity all these fudge factors go to zero and disappear making everything nice and simple.

Krauss seems to indicate that I was onto something here with gravity being negative energy.

[swansont]

To add to what D H said — in a Newtonian view, gravity represents a reversible process, so it doesn't change the entropy in an of itself. It's the non-gravitational interactions the objects undergo that increase entropy.

[cypress]

Krauss seems to indicate that I was onto something here with gravity being negative energy.

 

I don't see how Krauss implied anything that could be seen as support for your initial posit that different or even more accurate treatments (though it is not known if gravity is negative energy in an absolute sense) of gravity would have any influence on entropy laws. Can you give me an indication where your posit could be seen as supported?