Atomic Clocks are the most precise way to keep time

Credit: Paola Mathus/ Credit: Paola Mathus/

Atomic clocks are the most precise way to keep time ever devised. What makes them so accurate, and how do they work?

All clocks, wristwatches, and pocket watches work on the same basic principle: counting regular intervals of time, and keeping the intervals consistent. This principle applies to any way of keeping time, mechanical or not; this is what we do when we count “one Mississippi, two Mississippi…” to keep track of passing seconds. But clocks are better than we are at accurately counting the seconds, because clocks rely on the laws of physics to measure the seconds using material objects.

A traditional mechanical clock — like a grandfather clock — uses a pendulum to keep time. The pendulum has a period of one second, meaning it takes one second to make one cycle. Inside the clock, gear mechanisms ensure that the pendulum keeps swinging at the same rate. Clocks like this are accurate under ideal conditions. However, it’s easy for them to be thrown off by damaged gears or dislodged mechanisms.

Quartz clocks keep time using a small quartz crystal shaped like a tuning fork. An electric current makes the crystal vibrate at a regular frequency like a tuning fork does when struck. The period of this vibration is used to keep time. Because they are inexpensive and compact, quartz clocks are the most widely used type of timekeeping device in the world, and is, on average, ten times as accurate as mechanical clocks.

They aren’t perfect, though: quartz is somewhat sensitive to temperature changes. If the temperature of the quartz crystal fluctuates too extremely, the period will change, making the timekeeping inaccurate.
Atomic clocks were designed to meet a new standard of accuracy —not just being accurate, but remaining accurate over long periods of time. To avoid the effects that threaten the accuracy of other types of clocks, atomic clocks keep time with one of the most reliable structures known: the atom. Atomic clocks contain a tiny capsule filled with gas. Electric pulses are fed through the gas, causing the gas to oscillate between energy states. The regular oscillation of the gas creates a resonance (like the resonance of the crystal in quartz clocks) that is used to keep time.

Since their invention in the 1960s, atomic clocks have become a crucial part of ensuring the accuracy and consistency of timekeeping, both in scientific disciplines and everyday life. Hundreds of atomic clocks in dozens of laboratories around the world are synchronized to keep International Atomic Time, an incredibly precise global time standard.

The extreme accuracy of atomic clocks has made major technological advances possible, including the reliability of GPS satellites, the refinement of radio astronomy, and the synchronization of the internet.