(dramatic music) - [Narrator] Over the centuries, we've tracked time a variety of ways, but since the 1960s, (clocks ticking) time has been officially determined using atomic clocks and the quantum characteristics of atoms.

The consistency of atomic clocks arises from the very nature of atoms.

- Atomic clocks depend crucially on the quantum physics of atoms.

You have a nucleus (dramatic music continues) around which there are electrons in certain energy levels, and these energy levels of possible energy states that the electron can have inside the atom.

- [Narrator] Since an electron can only be at certain energy levels and not in between, to get to a higher level, it needs to encounter a very specific helping hand, such as a particular photon.

- So if it were to absorb an incoming photon, it would have to be of just the right energy to jump from one level to a higher level.

- [Narrator] That special relationship between the electrons of a particular atom and a photon of a specific energy level is a unique signature for that atom.

It's called a resonant frequency.

- So this characteristic signature of this atom gives us a very specific frequency standard that we can use to build a timekeeping device.

(air whooshes) (dramatic music continues) - [Narrator] Atomic clocks work in different ways, but they all use a specific type of atom or molecule as a reference to lock in the frequency of an electromagnetic wave whose oscillations provide the ticking of the clock.

(waves ticking) (air whooshes) Today, a second is officially defined by counting the oscillations of the primary resonant frequency of a Cesium-133 atom.

That's over 9 billion oscillations per second.

(air warbling)