PLACE 20 coins, heads up, on a tray and film it as you give it a shake. Then play the film backwards. From a jumbled mess, the coins all jump and come to rest with the same side up – an unreal, slightly creepy sequence. “It seems like a mundane observation, but actually this is very profound,” says physicist at the University of Bristol, UK.
This little experiment illustrates the power of perhaps the most essential, implacable field within physics: thermodynamics, the science of heat, energy and, most crucially, entropy.
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The roots of thermodynamics lie in efforts to understand the steam engines that powered the industrial revolution of 18th and 19th-century Europe. The French engineer Sadi Carnot realised that their heat always tends to dissipate, moving to cooler regions. Anything that goes against this grain requires additional energy to power it.
This movement from hotter to cooler is an expression of a more fundamental drive in the universe: disorder, as measured by entropy, always increases. The specifics don’t matter – heat always flows, flipped coins always jumble, burning logs always turn to ash. “If we discover a new force tomorrow, thermodynamics will be fine,” says at University College London.
Entropy increase is so universal that many physicists propose it is why we see time flowing (see “How to think about… Time”). It is certainly why our hearts must constantly pump blood, supplying our cells with energy as a temporary stay against the inevitable onset of decay and disorder.
Is there any way out? Perhaps. The laws of thermodynamics only hold true as statistical averages. As a result, some see an escape route from entropy’s inevitable rise in the small-scale workings of the quantum world: rules based on statistics don’t mean much when you’re dealing with just a few particles.
There are concepts akin to entropy that tend to increase in the quantum world, says Oppenheim – uncertainty over a particle’s position, for one. The science of quantum thermodynamics is in its infancy, and any hopes of using its fuzzy rules to make batteries more efficient than conventionally possible will not be realised any time soon. Oppenheim is sceptical that we will ever override traditional thermodynamic restrictions. But one instance where quantum thermodynamics comes into play is at the event horizon of a black hole (see “How to think abouth… Black holes”) – so it could help solve the enduring riddle of how to unite general relativity with quantum theory.
“Our hearts pump blood to stay the rise of decay and disorder”
That’s unlikely to help out much with the bleak future predicted for the universe, in which it slides into a long, slow “heat death”, eventually turning all order to disorder. “Our present understanding is that things will become more and more disordered until life becomes very, very boring,” says Oppenheim.
Or will it? Even within that disordered soup at the end of the universe, in theory “all kinds of interesting things can still occur”, says Oppenheim. Perhaps the most bizarre of these was first expounded by Ludwig Boltzmann in 1896. Boltzmann argued that, given enough time in a large enough universe, fluctuations might randomly create a sub-universe that looks like ours. More plausibly, it might create a brain that thinks it exists in just such a universe – and that thinks entropy is always on the up.
This article appeared in print under the headline “How to think about… Entropy”
