Some 15 years ago, as a young researcher trying to push a new idea, I was accused publicly by a much more eminent fellow physicist of not understanding entropy. Ever since then, I’ve been fixated with trying to really understand entropy. Personal pride, and the emerging scholarly consensus, require me to point out that I have since been proved right in what I was trying to say back then, but the intervening 15 years have taught me that neither I nor my eminent colleague had a full grasp of the mysteries of entropy, still less its mystical (what I might now refer to as ‘theological’) overtones.
Entropy is what makes life worth living. Arguably the most important of all the laws of physics (at least where biological life is concerned), the Second Law of Thermodynamics declares that the entropy (disorder) of a closed system (one that is thermally insulated from its surroundings) can never decrease, but must always increase (or at the very least stay the same). This turns out to provide a reasonably straightforward way into an otherwise notoriously-difficult problem – how to define life.
Erwin Schroedinger explained in his classic book What is Life? that death is the point at which an organism begins to come to a state of maximum entropy, reaching equilibrium with its surroundings by cooling and decaying until it’s indistinguishable with them. Life is the process of resistance to this inevitable decay, the maintenance of a state of negative entropy with respect to the surroundings, far from equilibrium with them. An organism can only do this by taking in negative entropy itself, in the form of complex foods and concentrated energy. It then makes use of the Second Law by processing the negative entropy and expelling it in the form of positive entropy. So entropy increases overall, while the organism remains a little bubble of negative entropy in a sea of disorder. But importantly, the Second Law is still obeyed, and the organism lives on.
The Second Law is not only a useful way of thinking about life (at least, for a physicist like me), it also has a rather unusual status as the most ‘theological’ of all physical laws. Taken universally, the Second Law suggests a kind of anti-purpose to the world, that whatever we do, the world will inexorably run down as all order slowly turns to disorder. For sure, it is entropy – and entropy alone – that seems to allow us to tell the direction of time. The laws of physics look the same in reverse time as in forward time. The only way we can tell which direction time is travelling in is because of entropy. So if you were to see a movie clip of a pile of shards of glass assembling themselves into a wine glass you would know that the movie was running in reverse. Entropy defines time’s arrow for us.
I recently discovered an intriguing statement of this idea in Sir Arthur Eddington’s Gifford Lectures, delivered at Edinburgh in 1927. He said:
The direction of time’s arrow could only be determined by that incongruous mixture of theology and statistics known as the second law of thermodynamics; or, to be more explicit, the direction of the arrow could be determined by statistical rules, but its significance as a governing fact making sense of the world could only be deduced on teleological assumptions.
My first take on reading this was to conclude that Eddington was just trying to shock. How could one of the most important laws of physics – the one that defined the direction to time, no less – be a mixture of theology and statistics? Further thought suggested that he might have hit on something rather important.
To be continued…