IT IS the invisible presence that governs your world. Trailing you like an unshakeable shadow, it ticks and tocks incessantly – you can sense it in your heartbeat, in the rising and setting of the sun, and in your daily rush to make meetings, trains and deadlines. It brings order to our lives through the categories of past, present and future.

Time. There is nothing with which we are so familiar, and yet when you try to pin it down you find only a relentless torrent of questions. Why does time appear to flow? What makes it different from space? What exactly is it? It’s enough to make your neurons misfire, then sizzle and smoke.

You are not alone. Physicists have long struggled to understand what time really is. In fact, they are not even sure it exists at all. In their quest for deeper theories of the universe, some researchers increasingly suspect that time is not a fundamental feature of nature, but rather an artefact of our perception. One group has recently found a way to do quantum physics without invoking time, which could help pave a path to a time-free “theory of everything”. If correct, the approach suggests that time really is an illusion, and that we may need to rethink how the universe at large works.

For decades, physicists have been searching for a quantum theory of gravity to reconcile Einstein’s general relativity, which describes gravity at the largest scales, with quantum mechanics, which describes the behaviour of particles at the tiniest scales. One reason it has been so difficult to merge the two is that they are …