Evolution and the second law of thermodynamics


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When I wrote my first piece about Roger Penrose‘s Cycles of Time – one of the links offered to me was to a Christian fundamentalist blogger who claimed that the second law of thermodynamics showed evolution could not have taken place:

E. Evolution contradicts the Second law of Thermodynamics

In the Theory of Evolution it is proposed that “simple life” evolved into more complex life forms by Spontaneous Generation. Both Darwin’s Theory of Evolution and the Spontaneous Generation model both directly contravene the Law of Biogenesis.

The Second Law Of Thermodynamics

The Second Law of Thermodynamics states that the disorder in a system increases, rather than decreases. 

The problem for this argument’s advocates is that Penrose demolishes it in brilliant style. I won’t quote from him directly, but I will try to summarise his argument.

Penrose begins by actually restating the fundamentalist argument in a much wider sense – it is life itself that on a naive view would appear to violate the second law – after all our bodies do not melt away (so long as we live), but remain highly ordered and as we grow (eg our hair or nails) appear to create order out of disorder.

The key to this is the Sun. The first thing a secondary school science student is taught is that the Sun supplies the Earth with energy but, in fact, this is not true in the sense that the Sun does not provide a net increase in energy on Earth – if it did then our planet would continually heat up until it reached an equilibrium. The Earth re-radiates the Sun’s energy back into space at a equal rate to which it is recieved.

What the Sun is, though, is much hotter than surrounding space and so it sends the Earth a number of high energy (yellow light) photons. When the Earth re-radiates the Sun’s energy it does so at a lower temperature than the Sun – essentially at infra-red frequencies – so many more photons are radiated back into space than are received. More photons means a greater phase space and hence it means a higher entropy. So the Sun continually supplies the Earth with low entropy energy which processes on the Earth – including life – convert into high entropy energy.

For instance when we eat food we convert that low entropy food source (eg an egg) into high entropy heat energy. The food source itself ultimately derived its energy from the low entropy energy source that is the Sun, and so on.

Of course, all the time, the Sun’s own entropy is increasing, but we don’t need to worry about the consequences of that for a few billion more years.

It’s a brilliant, beautiful, argument though it is also one that is seldom, if ever, taught in schools.

The second law of thermodynamics and the history of the universe


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I had to go on quite a long plane journey yesterday and I bought a book to read – Roger Penrose‘s work on cosmology: Cycles of Time: An Extraordinary New View of the Universe

I bought it on spec – it was on the popular science shelves: somewhere I usually avoid at least for the physical sciences, as I know enough about them to make hand waving more annoying than illuminating, but it seemed to have some maths in it so I thought it might be worthwhile.

I have only managed the first 100 pages of it so far, so have not actually reached his new cosmology, but already feel it was worth every penny.

Sometimes you are aware of a concept for many years but never really understand it, until some book smashes down the door for you. “Cycles of Time” is just such a book when it comes to the second law of thermodynamics. At ‘A’ level and as an undergraduate we were just presented with Boltzmann’s constant and told it was about randomness. If anybody talked about configuration space or phase space in any meaningful sense it passed me by.

Penrose gives both a brilliant exposition of what entropy is all about in both intuitive and mathematical form but also squares the circle by saying that, at heart, there is an imprecision in the law. And his explanation of why the universe moves from low entropy to high entropy is also brilliantly simple but also (to me at least) mathematically sound: as the universe started with such a low entropy in the big bang a random walk process would see it move to higher entropy states (volumes of phase space).

There are some frustrating things about the book – but overall it seems great. I am sure I will be writing more about it here, if only to help clarify my own thoughts.

In the meantime I would seriously recommend it to any undergraduate left wondering what on earth entropy really is. In doing so I am also filled with regret at how I wasted so much time as an undergrad: university really is wasted on the young!

(On breakthrough books: A few years ago I had this experience with Diarmaid MacCulluch’s Reformation and protestantism. People may think that the conflict in the North of Ireland is about religion – but in reality neither ‘side’ really knows much about the religious views of ‘themuns’. That book ought to be compulsory reading in all Ireland’s schools – North and South. Though perhaps the Catholic hierarchy would have some issues with that!)