The Evolution Debate: Thermodynamics

Yes, this really is relevant. Credit: JJ Harrison (Wikipedia).

I’m moving away from the main thrust of the creation-evolution debate in biology and geology to a topic that is still mentioned a lot, but is much more physics oriented: thermodynamics.

One of the best ways that creationists could disprove evolution is not some new piece of scientific evidence, but rather if they found some contradiction that made it impossible even in principle. And in fact, many of them think they have found one:

“The Second Law of Thermodynamics disproves evolution.”

This argument says that evolution is against the laws of physics, so it can’t be true. Unfortunately for creationists, this argument is wrong—and if you think about it, it’s pretty obviously wrong.

The Second Law of Thermodynamics states that the entropy (disorder) of any isolated physical system can only increase over time, never decrease. Things in an organized state will become disorganized—will fall apart or otherwise turn messy—but they won’t spontaneously organize themselves if no outside forces intervene. One example is that a bunch of oranges will never stack themselves up on their own into a nice, neat pyramid. But a pyramid of oranges might fall own on its own into a disordered pile if it’s bumped or jostled too much.

Life, creationists say, is a highly ordered system that, according to “evolution,” spontaneously arose from a highly disordered system—the primordial soup (or whatever the current theory is—it doesn’t matter). This is a violation of the Second Law of Thermodynamics, they say so it couldn’t have happened this way.

The problem with this, as any physicist will tell you, is that the Second Law of Thermodynamics only applies to closed systems—systems that have no interaction with the universe outside them. And Earth is not a closed system. We get energy from the Sun.

But let’s back up and dig into it in more detail. First off, the Second Law of Thermodynamics isn’t exactly a law like gravity is a law. It’s a statistical result that says when you apply Newton’s laws of motion to an enormous number of molecules, you will almost always get a disordered result. It’s not actually impossible for order to increase. There is still something like a—no joke—one in a googolplex* chance that the oranges will spontaneously stack themselves up again because the random motions of the molecules all hit them just right, but it is so outrageously unlikely that there’s basically no chance that you’ll ever see it.

Of course, we know that the oranges get stacked because there’s a greengrocer doing the stacking. The greengrocer doesn’t have to be God in this metaphor. It can be a robot with no mind of its own, in fact. The problem is that the oranges together with the greengrocer who’s stacking them is also a physical system. The greengrocer is made of molecules that are subject to the same Second Law of Thermodynamics that the oranges are. If we naively apply the Second Law to this system, it should be physically impossible for him or her to stack the oranges!

As you can probably guess, this isn’t true because we’re not talking about a closed system. There is energy going into the greengrocer-oranges system in the form of food and oxygen. This energy can be used to do work, and this work can be increasing order in part of the system—the oranges. Meanwhile, the system as a whole is getting more disordered because the heat produced by the greengrocer’s body (heat is the most “disordered” form of energy) represents more disorder than the order in the oranges.

Before we take this up to the level of the Earth, I want to demonstrate that it does not require any intelligence or even life to achieve this (local) increase in order. Water freezing into ice is an increase in the order of the water molecules—from a random soup to an organized crystal structure. Water freezes because heat leaks out of it into the (colder) environment. But even if it freezes because it was poured onto solid pavement on a very cold winter day, so that nothing moved or melted or grew apparently more disordered, the disorder of the water-pavement system will still increase in the form of random vibrations of the molecules in the pavement, which count toward total entropy.

In contrast, if pure water is put in a completely closed system, or even a nearly closed one like a thermos, it will never spontaneously freeze because there is nowhere for the heat to go. Its order cannot increase while it is isolated.

Now, what about Earth? Well, Earth is not a closed system. Energy is going into it in the form of sunlight (and a few other things, but sunlight is by far the most important). This sunlight is captured by plants in photosynthesis and drives an increase in local order—namely, the growth of plants and increase in the animal population. In thermodynamic terms—in terms of the total order and entropy of the system—evolution is no different from this ordinary growth and reproduction. Any evolutionary change, in the structure of DNA or the physical complexity of organisms, is so tiny as to be a rounding error in this larger cycle of energy and nutrients.

And that’s not even getting to the fact that order and complexity are not the same thing. Complexity is not order. Rather, complexity is what happens between order and chaos, but that would take a whole other post to explain.

However, if we back up from our Earth-centric view one more time, what about the universe as a whole? How can we have the ordered, complex universe we see around stemming us from the utter disorder of the hot Big Bang? This isn’t an evolution question, but it’s still one you might hear from time to time in creationism debates. But the simple answer is that the Big Bang wasn’t disordered! The Big Bang theory tells us that the early universe was in an extremely ordered state, in relative terms. A uniform soup of matter sounds pretty disordered, but entropy starts to get weird when gravity gets involved. A uniform soup of matter is unstable on universe-wide scales, and small variations will grow over time under the influence of gravity, eventually collapsing into dense knots like the galaxies. In fact, for weird thermodynamic reasons I don’t fully understand myself, the highest entropy state you can have is actually a black hole, which is one of the simplest objects in the universe!

So the universe began with a hot Big Bang in an ordered state. Over time, gravity disrupted that state and turned it into disorder, but that disorder created complexity, and that complexity allowed the local increase in order that is life to come about in certain places, and at no point was the Second Law of Thermodynamics broken.

My question for creationists on this topic is: Will you admit that the existence of the Sun voids the claim than evolution is against the laws of thermodynamics, just as much as plants growing here and now do not violate them despite representing a local increase in order?

*Actually, it’s more like 1 in 10^(10^20), which is a lot less than a googolplex, but still so much huger than any numbers that make sense to us that it doesn’t matter.

About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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