**The Quantum of Time**

If I ask you for the smallest unit of time you can possibly think of, you might suggest a second, or a millisecond, or a nanosecond if you’re clever. But while these units are small enough to measure everyday events, physicists have to deal with cosmological forces and events on incredibly tiny scales, so they need to use appropriately tiny units to measure them. In 1899, German physicist Max Planck (who was also, incidentally, the founder of quantum theory) proposed a system of natural units of measurement called Planck units, stated in terms of five universal physical constants: the Gravitational constant, the Reduced Planck constant, the speed of light in a vacuum, the Coulomb constant, and Boltzmann’s constant. The system is based on the idea that space and time aren’t continuous—they’re quantised, which means that there’s a shortest possible measurable length (called Planck length) and a shortest possible measurable time (called, surprise, Planck time). Planck length is roughly 1.616 × 10^{-35} metres, and Planck time is the amount of time it takes for a photon to travel a single Planck length, i.e. 5.391 × 10^{−44} seconds. This is an unimaginably small quantity, but it helps to define the unimaginable small scale at which current physical theories break down—and helps physicists to study the beginning of the Universe, where the sequence of events in its early evolution was crammed into minute fractions of time.