# Thread: entropy - isolated system - Confused!!

1. Hi,

we have a system of two balls, one (the smaller) orbiting the other, with the following characteristics:
-The system is totally isolated (no external forces whatsoever..)
-The system is in space (no air resistance..)
-The balls are perfect spheres, homogeneous, and the orbit is a perfect circle.

Then theoretically:
1-will the balls orbit each other forever?
2-will they orbit forever according to both classical physics and general relativity?
3-such a system will have a constant total energy, its entropy will be constant too. but shouldn't entropy increase over time? or is it allowed to stay constant? what does the second law of thermodynamics say abt it?

Thx.

2.

3. Entropy will increase if anything happens - but in the perfect frictionless etc. system you describe, nothing is happening. There's no heat flow happening with two masses orbiting each other.

4. Originally Posted by sam
Hi,

we have a system of two balls, one (the smaller) orbiting the other, with the following characteristics:
-The system is totally isolated (no external forces whatsoever..)
-The system is in space (no air resistance..)
-The balls are perfect spheres, homogeneous, and the orbit is a perfect circle.

Then theoretically:
1-will the balls orbit each other forever?
2-will they orbit forever according to both classical physics and general relativity?
3-such a system will have a constant total energy, its entropy will be constant too. but shouldn't entropy increase over time? or is it allowed to stay constant? what does the second law of thermodynamics say abt it?

Thx.
1. yes

2. no, as there will be energy loss due to radiation of gravitational waves (this will take a very long time)

3. Entropy and thermodynamics are fundamentally statistical and are not applicable in ths situation.

5. 3) the second law of thermodynamics is . Notice the "or equal" part. It can also be worded: "entropy does not decrease"

6. Originally Posted by Bender
3) the second law of thermodynamics is . Notice the "or equal" part. It can also be worded: "entropy does not decrease"
That is true. But entropy doesn't mean anything of importance in a two body system.

7. Originally Posted by Bender
3) the second law of thermodynamics is . Notice the "or equal" part. It can also be worded: "entropy does not decrease"
Right, but if the balls are solids, then entropy doesn't increase either. It just stays right where it is. If the balls had liquid or gaseous components, then the sloshing around of those components would create friction over time, and heat also.

8. Originally Posted by DrRocket
Originally Posted by sam
Hi,

we have a system of two balls, one (the smaller) orbiting the other, with the following characteristics:
-The system is totally isolated (no external forces whatsoever..)
-The system is in space (no air resistance..)
-The balls are perfect spheres, homogeneous, and the orbit is a perfect circle.

Then theoretically:
1-will the balls orbit each other forever?
2-will they orbit forever according to both classical physics and general relativity?
3-such a system will have a constant total energy, its entropy will be constant too. but shouldn't entropy increase over time? or is it allowed to stay constant? what does the second law of thermodynamics say abt it?

Thx.
1. yes

2. no, as there will be energy loss due to radiation of gravitational waves (this will take a very long time)

3. Entropy and thermodynamics are fundamentally statistical and are not applicable in ths situation.

Woooow, very interesting what you answered on point 2!!!..so in other words:
- any given mass will be lost over very long time due to the radiation of gravity?
- in the case of the 2 balls: when mass will be lost attraction will be weaker (m1xm2/d^2) and so the orbit will be bigger until the rotating ball will be lost in space

9. Originally Posted by sam
Woooow, very interesting what you answered on point 2!!!..so in other words:
- any given mass will be lost over very long time due to the radiation of gravity?
- in the case of the 2 balls: when mass will be lost attraction will be weaker (m1xm2/d^2) and so the orbit will be bigger until the rotating ball will be lost in space
No, this is not what he meant. Gravitational waves are not the same thing as Gravitational force. Gravity waves are produced by accelerating an massive object, and the energy for them comes at the cost of the kinetic energy of the object. In the case of the orbiting balls, the acceleration is due to their traveling in a circular path and the energy to produce these waves comes at the cost of their orbital energy.

As they lose orbital energy, they have to move in closer to each other to compensate. So the generation of gravity waves cause them to slowly spiral in towards each other not away. This production of gravity waves has no effect on the gravity produced by each object.

IOW, objects do not radiate away their gravity.

10. Originally Posted by Janus
Originally Posted by sam
Woooow, very interesting what you answered on point 2!!!..so in other words:
- any given mass will be lost over very long time due to the radiation of gravity?
- in the case of the 2 balls: when mass will be lost attraction will be weaker (m1xm2/d^2) and so the orbit will be bigger until the rotating ball will be lost in space
No, this is not what he meant. Gravitational waves are not the same thing as Gravitational force. Gravity waves are produced by accelerating an massive object, and the energy for them comes at the cost of the kinetic energy of the object. In the case of the orbiting balls, the acceleration is due to their traveling in a circular path and the energy to produce these waves comes at the cost of their orbital energy.

As they lose orbital energy, they have to move in closer to each other to compensate. So the generation of gravity waves cause them to slowly spiral in towards each other not away. This production of gravity waves has no effect on the gravity produced by each object.

IOW, objects do not radiate away their gravity.
Precisely

11. Originally Posted by Janus
Originally Posted by sam
Woooow, very interesting what you answered on point 2!!!..so in other words:
- any given mass will be lost over very long time due to the radiation of gravity?
- in the case of the 2 balls: when mass will be lost attraction will be weaker (m1xm2/d^2) and so the orbit will be bigger until the rotating ball will be lost in space
No, this is not what he meant. Gravitational waves are not the same thing as Gravitational force. Gravity waves are produced by accelerating an massive object, and the energy for them comes at the cost of the kinetic energy of the object. In the case of the orbiting balls, the acceleration is due to their traveling in a circular path and the energy to produce these waves comes at the cost of their orbital energy.

As they lose orbital energy, they have to move in closer to each other to compensate. So the generation of gravity waves cause them to slowly spiral in towards each other not away. This production of gravity waves has no effect on the gravity produced by each object.

IOW, objects do not radiate away their gravity.

thx for the explanation.
So acceleration in any isolated (etc. etc.) system is transformed into gravitational waves, so any curved movement will end up linear and non accelerated etc.
So we will end up with a system that is either at rest or in a uniform movement.
I can't but see a kind of similarity with entropy where there is only one way for things to evolve.
p.s: what's wrong wiz this text format?! is it my browser or am i overlooking some checkbox or smthg!

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