The future of time measurement

Vacuum chambers containing free-floating atoms are at the heart of today's most precise instruments for measuring time. Laser pulses function as a timing generator for the newest generation of atomic clocks. They are incredibly precise.

What is time? Neither physicists nor philosophers can answer this question. But this does not stop people from dividing it into intervals and measuring it as precisely as possible. Timekeepers like the sun dials of antiquity or the mechanical tower clocks of the middle ages were important stages of development for the way we measure time. One key technical advancement was achieved by the mathematician and physicist Christian Huygens in the 17th century. His pendulum clock precisely divided time into hours, minutes and seconds for the very first time. Since then, increasingly precise measurement tools have been developed. Today, caesium atomic clocks are the basis for official standardized universal time and lose only one second every 30 million years.

Caesium atomic clocks keep the world's time

These modern timekeepers do not have pendulums or hands. Instead, atoms of caesium "tick" the time away. They are heated until they boil off and are then passed into a tube with a high vacuum. The low pressure ensures that the atoms do not bump into each other and pass through the chamber with high energy. The atoms are then exposed to microwave radiation. The right frequency, referred to by physicists as the resonant frequency, changes the energy state of some of the atoms. They are "excited" and counted by a detector.

Now the microwave frequencies are adjusted until the energy state of the largest possible number of caesium atoms changes. This happens reliably at 9,192,631,770 Hertz ֠so over nine billion oscillations per second. Inversely, one second passes when this number is reached. Thus is this the basis for defining the length of a second in the SI International System of Units.

Even more precision with lasers

In addition to caesium radiation clocks, caesium fountain clocks are also used as reference clocks for universal time today. In these clocks, caesium atoms are introduced into the vacuum chamber ֠in a way that resembles a fountain ֠before they are radiated with microwaves. Because the resonant frequency of the atoms can be even more precisely determined using this trick, fountain clocks will, at most, only lose one second in 40 million years.

Optical atomic clocks using strontium or ytterbium atoms, for example, are even more precise. Instead of using microwaves, the atoms are excited with lasers, also in an ultra high vacuum. The light frequency that results when one atom changes its energy state serves as the reference for the length of a second. The frequency is higher and the electron transition frequency is faster, such that time can be divided into even smaller intervals. The optical clocks "tick" much faster and at more exact intervals than caesium clocks. They may become the new timekeepers for universal time, making it necessary to redefine the unit of time we know as a second. Regardless of the process, all types of atomic clocks require a vacuum chamber for their reference atoms.