Preface: Why Does Love Cool?

Almost everyone who has loved for a long time knows the curve. At the start, a message from the other person can change the colour of a whole day. A few years later, the same message is read between two other notifications and answered with a thumbs-up. Nothing has gone wrong, exactly. No one has betrayed anyone. The warmth has simply drained away, the way a cup of tea goes cold on the desk.

The comparison with the cup of tea is the starting point of this book, and it is more than a figure of speech. A cooling cup obeys the second law of thermodynamics: left to itself, any isolated system moves towards its most probable state, in which its energy is spread out as evenly as it can be. Physicists call the measure of that spreading entropy. A hot cup in a cool room is an improbable arrangement; a lukewarm cup in a slightly warmer room is a probable one. Time runs from the first to the second, and never back, unless someone does work.

The question of this book is whether the same kind of reasoning can say anything useful about desire, attention and intimate relationships, and if so, where its reach ends. It reads four bodies of knowledge side by side:

1. Thermodynamics and information theory. Boltzmann’s entropy, $S = k_B \ln \Omega$, Shannon’s entropy of a message, and Schrödinger’s observation that living things keep themselves ordered by “feeding on negative entropy” from their surroundings.
2. The neuroscience of reward. Wolfram Schultz’s discovery that dopamine neurons signal not pleasure but prediction error, the difference between what was expected and what arrived, and the learning rules that turn every repeated delight into an expectation.
3. The physics of self-organisation. Ilya Prigogine’s dissipative structures, patterns such as convection cells and living cells that exist only while energy flows through them, and Karl Friston’s free-energy principle, which describes a mind as a system that keeps itself in shape by predicting its world.
4. Two contemplative traditions. The Daodejing’s warnings about the senses that “blind the eye” and “deafen the ear”, and the Buddhist notion of āsrava (漏), the outflows through which, in the old teaching, the mind’s energy leaks away.

The argument, in one sentence, is this: desire cools for the same reason the cup does, because it is a form of order kept up by a flow, and when the flow stops, the order relaxes towards its most probable state. The practical consequence is not to fight the second law, which no one can do, but to understand what kind of flow a relationship and a mind need, and what makes that flow leak.