Physicists are not sure what time is or how it works. The dominant position is the one articulated recently by Carlo Rovelli, who reminds us that the equations of general relativity – one of the best-tested scientific theories of all time – are time-invariant, meaning that time is just one more dimension of spacetime, and it doesn’t ‘flow’ in the way most of us think it does.

Rovelli offers the scientific take about several common misconceptions about time: (i) it doesn’t tick uniformly everywhere in the universe, as clocks travelling faster give slower measures of time, a relativistic effect due to gravity; (ii) the concept of ‘now’ is also a bit shaky, scientifically, as everything we look at actually happened some time ago, specifically the time it took light to reach our eyes (which in the case of nearby galaxies can be millions of years); (iii) time has no direction in either general relativity or quantum mechanics, and the perception of directionality is given only by the macroscopic phenomena subject to the second principle of thermodynamics (entropy always increases, statistically speaking). And if he is right about his theory of quantum gravity, it may even turn out that time is not a seamless flowing thing, but comes in discrete bits, just like light turned out to be organised in quantum packets, even though it appears continuous to us.

Then again, there are dissenters. Lee Smolin is arguably the most vocal of them. In various papers and books, including The Singular Universe and the Reality of Time, co-authored with philosopher Roberto M. Unger, he claims that time is actually fundamental in physics, and we need to take it more seriously. As they put it: “Time is real. Indeed, it is the most real feature of the world, by which we mean that it is the aspect of nature of which we have most reason to say that it does not emerge from any other aspect. Time does not emerge from space, although space may emerge from time.” They point out that when fundamental physicists (like Rovelli) talk about the unreality of time they are in the same breadth denying not just our common perception, but the most spectacular discovery of modern cosmology: that the universe itself had a beginning, about 13.8 billion years ago, give or take a few years. Needless to say, ‘years’ is a measure of time, and things have very definitely been unfolding in one direction ever since, regardless of what general relativity or quantum mechanics may say about it.

There is one more difficulty for contemporary science’s view of time as an ‘illusion’: why do we have this illusion in the first place? This isn’t a question that physicists by themselves may be equipped to answer. It requires a number of steps. To start with, why is it that ‘macroscopic’ phenomena are subject to entropy-driven directional time, while this does not occur at the quantum level? In other words, how do we explain the transition between quantum and macroscopic phenomena, exactly? But that’s just the beginning. We then need an account, likely from a combination of evolutionary biology, developmental biology, and cognitive science, of why we perceive time as flowing linearly, with distinct past, present, and future.

What’s the adaptive advantage? Which neural mechanisms are responsible for this? As you can see the science and philosophy of time still have a lot of work to do.

But then there is the more mundane, and at the same time far more relevant for most of us, question: given that our ‘time’ is limited and appears to have certain characteristics (like, we can’t move bidirectionally along it, as we do in space), what implications does all of this have for our lives as we actually live them? Here the answers don’t come from science, but from philosophy. Or if not the answers, exactly, at least helpful ways to think about them. My favourite framework in this case is the Stoic one, inspired by the Pre-Socratic philosopher Heraclitus.

Plato, in Cratylus, summarises one of the most famous doctrines of the sage from Ephesus: “Heraclitus says somewhere that everything gives way and nothing is stable, and in likening things to the flowing of a river he says that one cannot step twice into the same river.” This notion is certainly popular in modern philosophy, and it is referred to as process metaphysics, as distinct to object-oriented metaphysics. What we deem to be stable entities (e.g., mountains, us) are actually dynamic processes. It’s just that our perception is often too coarse, or our time horizon too limited, to see how things are really unfolding. Interestingly enough, this sort of metaphysics without objects finds deep resonance with the latest from fundamental physics, as beautifully explained in James Ladyman and Don Ross’s Every Thing Must Go: Metaphysics Naturalized.

But what follows from the Heraclitean view, in practice? This is where the Stoics picked up the ball and ran with it. One thing that follows is that we shouldn’t get attached to anything, because everything is impermanent. The Stoics thought that we should think of everything we have, including our loved ones, as a loan from the universe, and be ready to give back that loan whenever the universal web of cause-effect demands it. As Epictetus puts it:

“People act like a traveller headed for home who stops at an inn and, finding it comfortable, decides to remain there. You’ve lost sight of your goal, man. You were supposed to drive through the inn, not park there.”

We don’t own the place, we are just travellers who stop by for a while and then move on to leave space for other travellers.

But it’s really Seneca who hits the nail on the head in terms of practicality. He reminds us that what’s important in our life is not how long it lasts, but what we do with it. And moreover, for him, like for many other philosophers since, it is precisely the fact that time is limited and we do not know when we are going to die that leads us to live life urgently and meaningfully. But he also reminds his friend Lucilius:

“Such is the foolishness of mortal beings: when they borrow the smallest, cheapest items, such as can easily be replaced, they acknowledge the debt, but no one considers himself indebted for taking up our time. Yet this is the one loan that even those who are grateful cannot repay.”

So by all means let us turn to physics, and perhaps to biology, for a better understanding of the nature of time. But it is philosophy that counsels us about what to do with the time we have. Perhaps time is, at a deep level, an illusion. But you still need to set your alarm clock for tomorrow morning and then decide how to spend your day well.