Does Gravity reduce Entropy ?

[Widdekind]

Consider the classical cartoon, describing entropy -- a state where all the molecules of gas, confined to a box, happen to occupy only half of that available volume, is said to have "lower entropy", than a state where the same molecules are spread out more evenly.

 

Now, make that "box" into the whole cosmos. And, make "half the box" into the gravitationally condensed over-densities, associated with Large Scale Structure. By "ordering" the matter, which was once spread out evenly across the cosmos (at the Big Bang), into Galaxies, Clusters, Super-Clusters, and the Cosmic Web, does gravity reduce the entropy of the cosmos ???

[rigney]

Consider the classical cartoon, describing entropy -- a state where all the molecules of gas, confined to a box, happen to occupy only half of that available volume, is said to have "lower entropy", than a state where the same molecules are spread out more evenly.

 

Now, make that "box" into the whole cosmos. And, make "half the box" into the gravitationally condensed over-densities, associated with Large Scale Structure. By "ordering" the matter, which was once spread out evenly across the cosmos (at the Big Bang), into Galaxies, Clusters, Super-Clusters, and the Cosmic Web, does gravity reduce the entropy of the cosmos ???

Be less pragmatic.

A closed thermodynamic system, a quantitative measure of the amount of thermal energy not available to do work.

A measure of disorder and randomness in a closed system.

The tendency for all matter and energy in the universe to evolve toward a state of inert uniformity?

"An inevitable and steady deterioration of a society". (I like this one).

But:Take your pick.

[IM Egdall]

Consider the classical cartoon, describing entropy -- a state where all the molecules of gas, confined to a box, happen to occupy only half of that available volume, is said to have "lower entropy", than a state where the same molecules are spread out more evenly.

 

Now, make that "box" into the whole cosmos. And, make "half the box" into the gravitationally condensed over-densities, associated with Large Scale Structure. By "ordering" the matter, which was once spread out evenly across the cosmos (at the Big Bang), into Galaxies, Clusters, Super-Clusters, and the Cosmic Web, does gravity reduce the entropy of the cosmos ???

 

As I understand it, the net effects of gravity increase entropy. Say a bunch of encycopedias are all over your room. Then you come in and straigtens out, putting the encycopedias in proper alphabetical order in the bookcase. The "after" encylopedias are more ordered than the "before" encyopedias, so a decrease in entropy. But this is more than compensated for by the fat burned and heat generated by you in cleaning up the room. The net result is an increase in entropy.

Similarly say a duffuse cloud of stellar gas in space condenses due to gravity to form a star. The entropy decrease from the intially diffuse cloud of gas to the more orderly star is more than compensated by the heat generated as the gas compresses, and ultimately by the enormous amount of heat and light produced when nuclear processes begin to take place within the star's core. The net result is an increase in entropy due to the effects of gravity. (REF: Brian Greene, The Fabric of the Cosmos, pp. 172,173)

Edited August 10, 2010 by I ME

[swansont]

Put a slightly different way: Gravitational collapse causes temperatures to rise, and hot things radiate more than cold things. Without the radiation you could argue that the process was reversible, but with the emission of thermal radiation it is not. The entropy increases.

[ydoaPs]

Put a slightly different way: Gravitational collapse causes temperatures to rise, and hot things radiate more than cold things. Without the radiation you could argue that the process was reversible, but with the emission of thermal radiation it is not. The entropy increases.

With enough gravity, the heat can't escape.

 

And what about dark matter? I thought it doesn't interact via EM, and as such would be subject to the organization without the generated heat due to friction.

[swansont]

With enough gravity, the heat can't escape.

 

Inside of the Schwarzschild radius, and not accounting for Hawking radiation.

 

 

 

And what about dark matter? I thought it doesn't interact via EM, and as such would be subject to the organization without the generated heat due to friction.

 

At best that would leave you with constant entropy, if the process were reversible. But you also presumably have gravitational radiation.

[Widdekind]

Put a slightly different way: Gravitational collapse causes temperatures to rise, and hot things radiate more than cold things. Without the radiation you could argue that the process was reversible, but with the emission of thermal radiation it is not. The entropy increases.

 

Why does the entropy increase, on the emission of (thermal, heat) radiation ? Is it because the entropy, of a "larger" system, having more "particles", is larger ?

 

Is entropy a measure of the "volume of state space" available to the system -- the number of microstates? In crude numbers, for classical particles, that would be proportional to the spatial volume of the system (~R³) times the momentum-space "volume" available to the system (~<p>³). Now, for classical particles, the square of the characteristic momentum, being the (characteristic) KE of the system, scales with the inverse spatial size of the system:

 

<p>

²

/2m ~ k

_B

T ~ GM

²

/R

 

<p>

²

~ R

^-1

If so, then w.h.t. R³ x <p>³ ~ R^3/2, so that as the system shrinks, its entropy decreases. What am I missing ?

[swansont]

Without photon emission, why would the system shrink? The collisions are elastic.

[Widdekind]

(thanks for clarifying that concept.)

 

Including radiation, could we say, that gravity reduces the entropy of the matter (whilst "shunting" (increased) entropy "out" into the radiation field, of photons) ?

Edited August 13, 2010 by Widdekind

[Martin]

(thanks for clarifying that concept.)

 

Including radiation, could we say, that gravity reduces the entropy of the matter (whilst "shunting" (increased) entropy "out" into the radiation field, of photons) ?

 

 

Some people, I understand, approach the question you have raised by assigning an entropy to the gravitational field, as well as to the matter.

 

The entropy of the gravitational field is calculated in a way which at first seems odd or unintuitive. The lowest entropy state is the most evenly spread out!

 

Because while a gas wants to spread out evenly, by its random molecular bouncings, the gravitational field wants to cluster and curdle and collapse. So for him, being uniformly spread out is low entropy. And being in various coagulated states is higher entropy.

 

I remember attending a lecture by Roger Penrose where he was explaining this. There was a pretty woman sitting next to me. But I cannot satisfactorily recall Sir Roger's discussion for you.

 

Swansont is certainly right about the radiation. For things to coagulate some gravitational energy must be blown off. Also in an expanding universe that radiation energy that gets blown off (to allow things to stick together) will eventually get redshifted to zero. There should be a source on entropy in cosmology. I don't have one handy. Someone mentioned Wald in this context. Do you have a book by Wald handy so you can check in the index.

Edited August 16, 2010 by Martin

[phoenix2]

Gravity increases entropy. The potential energy is transfered to the matter the force is applied to. (increasing heat)

[michel123456]

Gravity increases entropy. The potential energy is transfered to the matter the force is applied to. (increasing heat)

 

From Here, how some explanations produce confusion.

 

"A continuing theme within this intricately detailed biography of the Universe is the underlying conflict between the attractive force of gravity and the tendency for physical systems to evolve toward more disorganized conditions. Entropy provides a measure of disorder within a physical system-whenever entropy is generated, the amount of disorder increases. In the broadest sense, gravity pulls things together and thereby organizes physical structures. Entropy production opposes this order and acts to make physical systems more disorganized and spread out. The interplay between these two competing tendencies provides much of the drama in astrophysics and ultimately drives the evolution of the entire Universe, including the life cycles of its constituent stars and galaxies."

 

And further in the text:

"As time unfolds, a series of natural astronomical disasters punctuates this cosmic newsreel and shapes the subsequent development of the Universe. And throughout this evolutionary history, both past and future, the force of gravity continually engages in a cosmic battle with entropy."/

 

And all along the text.

Edited November 11, 2010 by michel123456

[Widdekind]

I am grateful for the linked & cited source. In some sense, the struggle between "Chaos & Order (Structure)" [set vs. Osiris, as it were] is deeply rooted in the fabric of physics.