Feature - Don't put your money in perpetual motion, Mrs Worthington

Apparent perpetual motion machine on the cover
of a 1920 issue of Popular Science magazine
(image from Wikipedia)

Physicists dismiss perpetual motion machines and 'free energy' devices out of hand. Some consider this a lack of open-mindedness, but in reality it's just that the physicists understand the second law of thermodynamics.

The second law is often stated as 'in a closed system, heat moves from a hot to a cold body' (there's another definition using entropy, we'll come onto in a moment). That's the basis at some point in the chain of every way we source energy, from a clean, green wind turbine to a dirty diesel. And, for that matter, it applies to the way your body uses energy too. Such is the respect for the second law that one of the UK's top astrophysicists of the first half of the twentieth century, Arthur Eddington, wrote:

If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations [James Clerk’s masterpiece that describe how electromagnetism works] – then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation – well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in the deepest humiliation.

So there has been some excitement in the press since a paper from last November pointed out a circumstance where the second law appears to be broken. (It ought to be pointed out that the paper appears on the pre-print server arXiv, so has not been peer reviewed. I'm not saying there's anything wrong with it, just needs noting.)

Of itself, there's nothing odd about heat moving from a colder to a hotter body. It's what a fridge does, after all. But this can only happen if energy is supplied to make it happen - this is what the 'closed system' bit of the definition precludes. What was interesting in the  described experiment is that heat was transferred spontaneously from 'colder' to 'hotter'. (I'll come back to those inverted commas soon), which is what you need for perpetual motion and free energy.

What physicist Roberto Serra of the Federal University of ABC in Santo André, Brazil and the University of York, with his colleagues, did was to get molecules of chloroform - a simple organic compound where a carbon atom has one hydrogen and three chlorine atoms attached - into a special state. The hydrogen atom and the carbon atom in a molecule had one of their properties - spin - correlated, giving them a kind of linkage. The hydrogen atom was in a higher energy state than the carbon, making the hydrogen technically hotter. And without outside help, as the correlation decayed, heat was transferred from the carbon to the hydrogen. From colder to hotter.

To understand why this happened requires the alternative definition of the second law involving entropy. Entropy is a measure of the disorder in a system. The more entropy, the more disorder. And the second law can be stated as the entropy in a closed system will either stay the same or increase. If the entropy decreases it's like heat going from cold to hot.

Entropy is measured by the number of different ways the components of a system can be organised. So, for example, a book has much lower entropy than a version with all the words in a random scrambled form. There are far more ways to arrange the words randomly than to form the specific book. (Imagine dropping the words randomly on a page - they are far more likely not to be in the order in the book.) This is why the second law also says it's more likely to break something than to unbreak it.

In the case of the chloroform experiment, entropy decreases because there are more ways to arrange the quantum states when they are correlated than when the correlation goes away - it's a bit like there being more ways to throw a six with two dice together than with two dice individually.

But free energy enthusiasts don't need to get too excited. Although there does appear to have been a spontaneous reduction in entropy, getting the molecules into the right state to start with would have taken far more energy than could be extracted. It's not a free source of energy.

The moral still is - don't buy a perpetual motion machine.