Asked by: Steve Collins, by email
The smallest viable measurement of time is derived from a unit of distance called the Planck length. It’s where dimensions become so tiny that the classical laws of physics break down and quantum effects kick in.
It works out at about 1.6 x 10-35m. From here, physicists speak of the smallest theoretical period of time as being the interval required for a photon travelling at the speed of light to cover the Planck length, or just 0.5 x 10-43 seconds.
Don’t worry if values like 10-43 seem a bit abstract and meaningless, they’re right at the extreme end of physics.