We like to think of time in distinct intervals: we have seconds, hours, and years. It’s not too difficult, then, to imagine that according to quantum mechanics, time is not a continuous flow, but is instead made of tiny, discrete intervals. The smallest possible time interval, called Planck time, is 10^{-43} seconds.

Now, a group of physicists has proposed that the smallest possible time interval is actually an order of magnitude larger than Planck time. A larger discrete time would have measurable effects on all quantum mechanical systems.

Planck time is part of a system of natural units, which are based exclusively on universal physical constants. Currently, there is no physical significance related to this time scale, but due to the way it is defined, it doesn’t make sense to talk of a smaller time scale.

Neither quantum mechanics nor special relativity requires a discrete time, but recent theories that are trying to bridge the gap between the two (like quantum gravity and string theory) need space to be split up into tiny intervals defined by the Planck length, which is the distance a photon covers in a Planck time.

The research therefore presents a new theory: If the same principle that’s applied to space is applied to time, then the universe must be split into (discrete) intervals of time, the Planck time. Surprisingly, the calculations indicate that time is not only discrete, but the size of the time intervals depends on the energy of the system. This discovery has an impact throughout the theory, and equations need to be modified to accommodate for discrete time.

The team has devised a possible test for their idea, which involves the spontaneous emission of a hydrogen atom. If one electron is in an excited state, it can decay back to the ground state by emitting a photon.

The rate of decay of the hydrogen atom is well modelled, so when the modifications due to the quantized time are applied, we obtain two different values. They are currently within the range of uncertainty from the experimental value, but future experiments will be able to distinguish between the two scenarios.

The paper is published in The European Physical Journal with the title "Time crystals from minimum time uncertainty." Here the term "time crystal" is an indication that time can be pictured as a structure made of regular, repeating blocks.

The smallest interval of time that we are capable of measuring is 1.2 × 10^{−17} seconds, which is still 100 million billion billion times larger than the Planck time, but maybe not as far from the smallest time interval.