The Second Law and its misuses


An OrbitCon session brought to my attention to the fact that Steven Pinker spouts a lot of bullshit about the Second Law of Thermodynamics. (The Second Law is, “entropy cannot decrease over time in a closed system.”) In Pinker’s book, Enlightenment Now, he begins by refuting the creationist argument that the Second Law contradicts the theory of evolution. This is easy to do, you just say that the earth isn’t a closed system, and dramatically point at the sun. But Pinker then proceeds to forget about the sun, and argues that the Second Law of Thermodynamics explains poverty. I don’t have the book available, but Pinker has written an essay along similar lines:

Poverty, too, needs no explanation. In a world governed by entropy and evolution, it is the default state of humankind. Matter does not just arrange itself into shelter or clothing, and living things do everything they can not to become our food. What needs to be explained is wealth.

Here’s the thing: creationists are really really wrong about the Second Law. There’s plenty of room to be less wrong than creationists, but still really really wrong. For those of us who have taken an interest in fighting creationism, we know we can just point at the sun and be done with it. But just because you’re familiar with this argument, please don’t mistake that for an understanding of the Second Law. Don’t be like Steven Pinker.

Here I will state and explain a few basic principles about entropy, with the goal of going beyond a mere refutation of creationist arguments.


“Entropy” is a measure of our ignorance of microscopic configurations.

Usually when physicists speak to the public, they say that entropy is a measure of disorder. This isn’t wrong exactly, but “disorder” is an imprecise concept understood differently by different people, while “entropy” is a very precise concept. So here’s a more precise explanation.

Let’s consider the example of a cup of water. There are some “macroscopic” properties of the water that are very easy to measure, such as the volume and temperature. But it’s practically impossible to measure the “microscopic” state of the water–the position, velocity, and rotation of every single water molecule. So given what we know about the macroscopic state of the water, how many possible microscopic states are there? Entropy is the log of the number of possible microscopic states.

Microscopic disorder is by far the largest source of entropy.

If you consider a person’s desk, that desk might be very organized or very messy. We could consider “messy” and “organized” to be two macroscopic states of the desk. And we could say that the “microscopic” state of the desk is the configuration of each individual object on the desk. It is intuitive to say that a messier desk has more possible microscopic states, and thus more entropy.

But how many objects are on this desk anyway? A hundred? Compare to the number of molecules in my cup of water. Millions of billions of billions! The large-scale entropy of a messy desk doesn’t even begin to compare to the microscopic entropy of a cup of water.

So this is an important point. The most obvious kinds of order and disorder, the things that relate to us most on the human level, barely register in the Second Law. You want to know the entropy of poverty, I’d have to ask, what’s the mass and temperature of the poor people? You say you’re interested in how poor they are? Sounds like the Second Law isn’t the relevant principle then.

Both natural and artificial processes follow the Second Law.

The creationist argument goes that if evolution involves decreasing entropy, the only way to get there is through a designer. While a supernatural designer might be able to decrease entropy, this is definitely not true of designers in the real world. If you find a watch in the desert, you might infer that it was made by a watchmaker, but the process of making watches does not involve breaking the Second Law by any means!

To understand why this is, recall that the arrangement of macroscopic objects (such as watch parts) barely registers in the Second Law, and the Second Law cares far more about processes like turning electricity into heat, or turning food into poop. All macroscopic naturalistic processes–including ones that involve intelligent intervention–obey the Second Law.

So when Pinker says poverty is the natural state of things because of the Second Law, and then says that we need to intervene to create wealth, this is complete bullshit.  Intervention is not a way to “resist” the Second Law.

Complexity and entropy are not correlated.

Although “complexity” is an imprecise concept, a more rigorous definition of “complexity” is possible. Suppose that you’re a graphics artist, and you want to create a 3D model of an object; “complexity” is basically how much code you have to write. And this has nothing to do with entropy.

Sean Carroll (video) talks about the example of a cup of coffee. At the start, the cup is neatly separated into layers of coffee and cream. As they begin to mix, there are turbulent swirls of coffee and cream at the boundary. Eventually, they are evenly mixed. The entropy of the system is highest at the end of the process, but complexity is highest in the middle. So even in this very simple system, complexity has a nonlinear relationship with entropy.

(Note that Sean Carroll hopes to eventually establish a general principle, that complexity rises and then falls over time. I am skeptical that there is any such principle, although admittedly it applies to a few very important things such as cups of coffee and the universe.)

There is no rule that says things fall apart.

Intuitively, we understand that when we create a piece of technology, the tendency is for the technology to run down and fall apart. It should be clear by now that this is not related to the Second Law, since even processes of creation and repair obey the Second Law. But you could easily wonder, does this reflect some sort of deeper principle, even if it’s not the Second Law?

No. No it does not. And that’s basically the end of the story. There are lots of patterns in life that have little to do with fundamental laws of physics, why would we expect this one to be different?

I mean, you could just make up your own rule.  Things fall apart in absence of intervention (except when they don’t).  Food and shelter don’t spontaneously appear without effort (except when they do).  We might even come to an agreement that these rules are generally true.  But bringing in the Second Law is a pointless exercise in scientism.

Comments

  1. wereatheist says

    Modern human artifacts tend to consist of material rarely/never found in nature. Let’s say a machine made of steel. It will rust away over some centuries. Things made out of massive aluminum may last far longer, but in the end these will be all oxidised, too. So there is a ‘rule’ that things fall apart, eventually.
    But Pinker seems to be a shallow thinker, to me 🙂

  2. Hj Hornbeck says

    Whew, good to hear I didn’t screw up that description of entropy. I embraced the “order”/”disorder” interpretation pretty strongly, so I was worried I’d slip up during the presentation.

    Thanks for that link, too, it crystallised what my next post about Pinker will be about.

  3. Hj Hornbeck says

    wereatheist @1:

    Let’s say a machine made of steel. It will rust away over some centuries. Things made out of massive aluminum may last far longer, but in the end these will be all oxidised, too. So there is a ‘rule’ that things fall apart, eventually.

    Actually, rusted iron has less entropy than non-rusted iron. You need to look at the entire system when applying the Second Law, it says nothing about individual things.

  4. sonofrojblake says

    Matter does not just arrange itself into shelter

    One word disproof: caves.

    The best definition I’ve heard of for entropy is “how spread out the heat is”. It’s a nice, simple definition with depth to it that requires a bit of thinking about. But boiled down to that, it becomes obvious how little it has to do with poverty or the state of my desk.

  5. says

    @HJ Hornbeck,
    Rusting increases entropy because it’s an exothermic reaction. While the entropy of the metal itself decreases, the heat produced by the rusting process creates a greater offset in entropy. Sometimes creationists think that while the Second law allows for entropy to transfer out of a system, it must take a very unusual setup for this to occur. But it really doesn’t. Processes that locally decrease entropy occur all the time with or without intelligent intervention.

    @sonofrojblake,
    I’m not a fan of “how spread out the heat is”. Heat is “spread out” when temperatures are equilibriated, and temperature is defined in terms of entropy rather than the other way around. I would also say there’s more to entropy than just heat distribution.

  6. enkidu says

    So Pinker berates his audience of (non scientists, presumably) for not being able to describe the Second Law of Thermodynamics, while simultaneously demonstrating that he doesn’t understand it himself.

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