Skip to main content

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 (It ought to be pointed out that the paper appears on the pre-print server arXiv, so has not been peer reviewed.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.



Comments

  1. I'm puzzled. If the transition from correlated to uncorrelated occurs spontaneously, does it release energy? If so, that energy will warm the environment and increase its temperature. If not, why does it happen?

    I remain confident that any claim to have demonstrated a spontaneous decrease in the total entropy of the universe will be refuted on closer analysis.

    ReplyDelete
    Replies
    1. As I mention at the end, there's inevitably lots of energy required to get things into the right state, so the universe is just fine.

      Delete

Post a Comment

Popular posts from this blog

Beyond Weird - Philip Ball *****

It would be easy to think 'Surely we don't need another book on quantum physics.' There are loads of them. Anyone should be happy with The Quantum Age on applications and the basics, Cracking Quantum Physics for an illustrated introduction or In Search of Schrödinger's Cat for classic history of science coverage. Don't be fooled, though - because in Beyond Weird, Philip Ball has done something rare in my experience until Quantum Sense and Nonsense came along. It makes an attempt not to describe quantum physics, but to explain why it is the way it is.

Historically this has rarely happened. It's true that physicists have come up with various interpretations of quantum physics, but these are designed as technical mechanisms to bridge the gap between theory and the world as we see it, rather than explanations that would make sense to the ordinary reader.

Ball does not ignore the interpretations, though he clearly isn't happy with any of them. He seems to come clo…

Mercury - William Sheehan ****

Driving to work one morning several years ago, I spotted a tiny white dot close to the rising sun. ‘That’s Venus,’ I said to myself. Almost immediately I saw another, much brighter dot a few degrees away. ‘No, that’s Venus – the first one must be, um ... Mercury.’ Even with a lifelong interest in astronomy, I always manage to forget Mercury.

With eight planets in the Solar System, one of them has to be the least interesting – and Mercury got the short straw. That’s a relative statement, though, and a diligent author could still dig up enough fascinating facts about that tiny dot by the Sun to fill a short book. William Sheehan has done a brilliant job of doing just that.

One of the reasons Mercury is so easy to forget is that it’s almost impossible to get a good view of it from Earth. Even after the invention of the telescope, which turned planets like Mars and Jupiter into explorable worlds, Mercury remained a mystery – and the subject of some pretty wild speculations. In 1686, for exa…

Everything You Know About Planet Earth is Wrong - Matt Brown ****

This is the latest of a series of 'Everything You Know About... is Wrong' books from Matt Brown. Although I always feel slightly hard done by as a result of the assertion in the title, as there are certainly things here I know that aren't wrong (I mean, come on, the first corrected piece of 'knowledge' is that 'The Earth is only 6,000 years old' and I can't imagine many readers will 'know' that), it's a handy format to provide what are often surprisingly little snippets of information that are very handy for 'did you know' conversations down the pub (or showing up your parents if you're a younger reader).

Some of the incorrect statements that head each article are well-covered, if often still believed (for example, people thought that world was flat before Columbus), some are a little tricksy in the wording (such as seas have to wash up against land) and some are just pleasantly surprising (countering the idea that gold is a rar…