# Ball does not bounce automatically because of entropy?

1. Jul 31, 2013

### Avichal

When reading about entropy it states that ball does not automatically bounce up because of entropy. It is just not favorable.
I don't get this. Isn't the correct reason gravity? Ball does not bounce up because of gravity! Where does entropy come from?

2. Jul 31, 2013

### Abhilash H N

Entropy and gravity are two very different topics. Entropy is the measure of randomness of the system. In the question it says that "the ball does not automatically bounce up". The only other way is that the ball must hit the surface and 'stick to it' so that it can't bounce. Since entropy is about randomness, it must have meant that the 'sticking up of ball and earth is nothing but the increase of randomness' (we must note that earth and ball are two very different things!).
However if could just give the whole context/paragraph of it (means more details..), then the discussion might become more easier.... :)

3. Jul 31, 2013

### Avichal

I read about entropy some time back. This thought occurred to me now. However I did find an instance where it gives a similar example. Here is the link.

It says that when we drop a ball it will fall down and this is because of second law of thermodynamics. I don't understand what second law of thermodynamics has to do with ball falling down. It falls down because of gravity.

4. Jul 31, 2013

### A.T.

The ball has enough thermal energy to lift the ball against gravity. So it wouldn't violate Conservation of Energy (1st law of thermodynamics) if the ball would jump up on its own. But it would violate the 2nd law, and constitute a perpetual motion machine of the second kind:
http://en.wikipedia.org/wiki/Perpetual_Motion_Machine#Classification

Thermal energy is kinetic energy with random movement of molecules. Converting that into kinetic energy of the ball as a whole would require a reduction of randomness (Entropy) of the movement .

5. Aug 20, 2013

### Avichal

If the ball is left in air - it falls down. Well if it stays, it's neither violating law of conservation of energy nor second law of thermodynamics.
Of course you'll say it falls because of gravity. But I'm still confused - it doesn't violate any law!

6. Aug 20, 2013

### neerajareen

Your confusion relies on the fact that gravity tends to be an entropic force. For example if you have a clump of gas in one area and you let the system evolve, by diffusion the clump will spread and entropy will increase.

In gravitational physics however entropy is different. If we start out with a almost evenly diffused system of gas molecule and only consider gravitational interactions, the molecules will start to clump together. This better be in accordance with the second law of thermodynamics. That must mean gravity behaves like an entropic force. Thinking about gravity and entropy in this way should help you understand your doubt

7. Aug 20, 2013

### Avichal

I googled 'entropy gravity' and found an article on Wikipedia. It seems to be an hypothesis and is intensely contested. I don't think I should think on the lines of that as it may not be true.

8. Aug 20, 2013

### Drakkith

Staff Emeritus
For it to stay in the air it would have to violate physics by not responding to the applied force of gravity. Note that something doesn't have to be called a law in order for it to be true.

9. Aug 20, 2013

### Avichal

Thanks, it makes sense now. Is the entropy gravity hypothesis a hot topic in physics? I am intrigued by the idea.

10. Aug 20, 2013

### Drakkith

Staff Emeritus
Based on my extremely limited reading about the subject, it appears that it isn't really that hot. It appears more as another "interpretation" of gravity, since it makes no predictions that GR doesn't already make.

11. Aug 20, 2013

### jfizzix

The big question touched on here is one that's still unresolved in physics. All the laws of motion from particles to galaxies are symmetric in time. So why do we see time having an obvious direction (the past is different than the future)?

On the level of single particles, we can't tell if a film of a trajectory is being played forward or backward. A planet orbiting a star would look the same except it would be moving in the opposite direction, still described accurately by the same laws.

The obvious exception to this is the second law of thermodynamics.
There is an obvious direction to time as we know it. We never see an egg unscramble, even though it's theoretically possible that every atom's trajectory could happen to be just so that it ends up coming back together again.
The reason we never see this is that it is so incredibly unlikely that every atom will happen to be arranged just so. There are much MUCH fewer ways that the atoms in the shell and yolk could be arranged to come back together again than there are for the shell and yolk to remain an icky mess on the floor.

This isn't entirely satisfactory because all that it means is that the entropy is always smaller in the past and larger in the future, and that the arrow of time points toward increasing entropy (kind of circular).

Getting a bit more speculative, if there were some point in the far distant past where the entropy was minimal (think.. big bounce scenario), then before that time, the arrow of increasing entropy would be pointing in the opposite direction in time that it does now. Alternatively, if there were some point in the far distant future, where the entropy was maximal (think.. heat death of the universe), there would be no meaningful distinction between past and future since the arrow of time in the direction of increasing entropy would no longer work as a concept.

I study information theory and entropy in grad school (for physics). Hope this helps:)