Please forgive: I am a layman when it comes to physics and cosmology, and have tried finding an answer to this that I can understand, with no luck.

As I understand it, the solar system evolved from a massive molecular cloud. To me, this seems to break the second law of thermodynamics, as I think it suggests order from disorder.

I know there must be something wrong with my logic, but am really stuck.

Can anyone explain this one in layman's terms?

(Posting to both "Astronomy" and "Physics", as it seems to overlap these subjects)

  • $\begingroup$ The trick is not to confuse the precise definition of "entropy" with the layman's simplified understanding that it is somehow related to "disorder." $\endgroup$ – Larry Gritz Jun 9 '14 at 18:47

The total entropy actually increases, as the molecular cloud shrinks under gravity.

It may seem that as the molecules are getting closer, they are more ordered, which means less entropy. That is however only one part of the process. The second (important) part is: when the molecules are closer, they also have higher kinetic energy (since they descended into lower gravitational potential). So the gas is getting hotter, as it shrinks.

The increase in temperature of the gas is increasing its entropy, because the molecules occupy more momentum space. This increase in entropy via temperature is bigger than the decrease in entropy via the shrinking itself.

Later on, the hot condensed gas (or hot planet) radiates the heat into the space and cools down. You end up with a cold planet which indeed has lower entropy than the original gas cloud, because it is no longer hot. But the entropy increase was carried away by the radiated photons. So in total - entropy of the universe increased (the radiated photons are out there somewhere).

You can find some more detailed discussion of this topic on the excellent web page by John Baez or here.

  • 1
    $\begingroup$ The statement "the radiated photons are out there somewhere" really does make it clear. $\endgroup$ – dotancohen Jun 10 '14 at 5:20

This comes from a misunderstanding of local and absolute.

There is nothing to prevent a local increase in order - overall, order still decreases (or in common terminology, entropy increases)

From Wikipedia:

According to the second law of thermodynamics the entropy of an isolated system never decreases, because isolated systems spontaneously evolve towards thermodynamic equilibrium, the configuration with maximum entropy. Systems which are not isolated may decrease in entropy.

So the universe is considered an isolated system, but our local solar system is not isolated, so our local decrease in entropy does not breach the 2nd law of thermodynamics as the overall entropy of the universe does not decrease.

  • $\begingroup$ So where was the corresponding increase in entropy to offset the creation of the solar system? Heat loss? $\endgroup$ – dotancohen Jun 10 '14 at 5:19

This is a question that is fundamental to our understanding of how order can emerge from disorder. So its worth considering the ways that this can happen:

  1. Local entropy reduction by random fluctuation.

  2. There is an attractor for the dynamics (point, cycle or strange) giving rise to self-organisation.

  3. The system is dissipative and open, local order is sustained by energy crossing the system boundary (eg your local library/information store is kept ordered through continual energy input).

Clearly 2. of the list is the reason for accretion disks to form stable rings. Then random collisions of the bits does the rest. If the bits are small particles you get Saturn, if they are large you get rocky planets.


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