Saturday, February 22, 2014

Evolution and Entropy, Again

I just got an email from someone who had read my essay about evolution and the second law of thermodynamics and thought he had found a flaw in it. It made me realize that the discussion there is rather technical and mathematical, and I ought to write up the basic idea in a clear and non-mathematical way. So here goes.

The thermodynamic argument against evolution goes something like this:
  1. Evolution involves an increase of order, and therefore a decrease of entropy.
  2. The second law of thermodynamics says that entropy never decreases.
  3. Therefore, evolution contradicts the second law of thermodynamics.
Let's first consider (1.). In order to establish this, one would have to show that the body of a human being has less thermodynamic entropy than an equal mass of bacteria  (for instance). Now, this may in fact be true, but no one has ever proven such a thing, to my knowledge. Without proving (1.), the argument can't get off the ground.

Secondly, even if (1.) is true, the argument fails, because (2.) is wrong: the second law of thermodynamics doesn't say that entropy never decreases.

In fact, entropy decreases spontaneously in lots of natural settings: for instance, when a pond freezes over in winter. Ice has much less entropy than liquid water - if it were impossible for entropy to decrease then it would be impossible for ponds to freeze over.

So what does the second law of thermodynamics say? It says that there can't be a decrease of entropy in one place without a compensating increase of entropy somewhere else. In the case of the pond, the heat escaping from the water during the freezing process causes an increase of entropy of the air over the pond.

If you wanted to prove that the freezing of the pond violates the second law of thermodynamics, you would have to calculate the entropy decrease of the water, calculate the entropy increase of the air, and show that the latter is less than the former.

In the case of evolution, you would have to calculate the entropy decrease due to cells being "organized" into higher life forms, which, we already noted, has never been done. Then you would have to show there was no compensating increase of entropy elsewhere. This second step is what I addressed in my essay. If we take the whole Earth as our system, then we find there is an absolutely enormous increase of entropy due to the radiation of heat energy into space. This entropy increase is so large that no possible decrease of entropy due to evolution would cause a violation of the second law of thermodynamics.

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