A kiss is an event – where is it now? – but then so, ultimately, is a stone, or a planet. A freshly emitted electron is just a cloud of possibilities until it slams into a screen. “The difference between past and future, between cause and effect, between memory and hope, between regret and intention … in the elementary laws that describe the mechanism of the world, there is no such difference.” In a quantum world, with neither what we understand as substance or order, there are only events, which crowd around chaotically. Reality is not what it seems – the British title of Rovelli’s popular book from 2016 – and at this level there is no evidence of time, let alone direction. “What makes the world go round are not sources of energy, but sources of low entropy.” That suggests we can be sure of a before and an after, but that is only because we see the world as an approximation, and not as a buzz of its ultimate constituents, a ferment of atoms, and beyond the atoms, a fizzing of subatomic particles that in turn may be thought of as quivering fields embedded in yet more gravitational and electromagnetic fields. We observe this journey because heat flows towards the cold things and one day all the heat will have dissipated, and we will experience neither past nor future. There is, according to Rovelli, just one basic equation that points to an arrow of time: the second principle of thermodynamics, which says that entropy is always increasing, that the journey from order to disorder is down a one-way street. At the deepest level of mathematical physics, time does not exist at all. There is a more profound puzzle underlying this spacetime confusion. People who live at altitude really do have more time, as do people who stay still Time shifts with both mass and velocity, to be different at every point in the universe: we live, Rovelli says, in a “spiderweb of time”. But everything in the universe is in motion everything feels gravity’s grasp. The difference as measured on an atomic clock is counted only in billionths of a second, and we on Earth can conveniently ignore this temporal untidiness. People who live at altitude really do have more time, as do people who stay still. In 1972, physicists sent a quartet of caesium clocks jetting around the planet in different directions to confirm Einstein’s special relativity.

Forget about universal time, a simultaneous now across the cosmos.

Rovelli the order of time