How did the redefinition of the second in 1967 change the way time is measured?

In 1967, the second, the base unit of time in the International System (SI), was redefined using the constant electromagnetic radiation emitted during ground state transitions in the cesium atom. Instead of relying on astronomical observations, time could be measured by counting oscillations of electric fields that cause atoms to change state. This made measurements infinitely more accurate.

What technologies rely on atomic clocks, and how has this changed society?

Atomic clocks underpin many technologies, including global navigation satellite systems, mobile telephones, and smart grids. Their accuracy is essential for guiding commutes, connecting people across continents, and powering homes. The redefinition of the second, enabled by atomic clocks, ushered in this era of precision.

Why were atoms considered a more reliable timekeeping standard compared to earlier methods involving physical standards?

James Clerk Maxwell suggested using the "period of vibration of a piece of quartz crystal of specified shape and size and at a stated temperature" as a better absolute standard of time. However, he noted the challenges in replicating physical standards. Atoms, with their identical properties, offered a more reliable solution, as Thomson and Tait described, because atoms like hydrogen or sodium are "ready made in infinite numbers, all absolutely alike in every physical property."

What role did Isidor Isaac Rabi and his atomic and molecular beam resonance method play in the development of atomic clocks?

Isidor Isaac Rabi's invention of the atomic and molecular beam resonance method in 1937, for which he won the Nobel Prize in Physics in 1944, laid the groundwork for atomic clocks. By 1939, Rabi considered using his technique as a time standard and, in 1945, called atomic clocks "the most accurate clock in the universe."

Besides atomic properties, what specific technologies were crucial in making atomic clocks a reality?

Key enabling technologies include Quantum Mechanics (to manipulate and measure atomic behavior), Microwave Electronics (to interact with atoms at their resonant frequencies), and Molecular Beam Technology (to isolate and study atoms). Quantum Mechanics provided the tools to manipulate and measure atomic behavior with unprecedented accuracy, and Molecular Beam Technology, pioneered by Isidor Isaac Rabi, allowed scientists to isolate and study atoms with remarkable precision.

Futuristic atomic clock towering over a modern city

Tick-Tock: How Atomic Clocks Redefined Time and Shape Our Modern World

Reese Marlowe in Science & Nature August 2025 • 4 min read.

"A look at the history and U.S. contributions to the atomic redefinition of the second, revolutionizing timekeeping in 1967 and beyond."

Time, the most fundamental of measures, received a radical upgrade in 1967. The redefinition of the second, the base unit of time in the International System (SI), marked a pivotal moment. This wasn't just a minor tweak; it was a paradigm shift from astronomical observations to the unwavering precision of atomic behavior.

Before atomic clocks, our concept of time was tethered to the heavens, dividing the solar day or tropical year into smaller, somewhat inconsistent parts. Then came the atomic clock, grounded in the constant electromagnetic radiation emitted during ground state transitions in the cesium atom. This new definition meant time could be measured by counting oscillations of electric fields that cause atoms to change state, making seconds, minutes, and hours infinitely more accurate.

Today, atomic timekeeping underpins countless technologies we often take for granted. From global navigation satellite systems guiding our commutes to the mobile telephones connecting us across continents, and even the smart grids powering our homes, atomic clock accuracy is essential. It's easy to forget that this era of precision is relatively recent. In 2017, we celebrated the 50th anniversary of this atomic revolution. This article explores the U.S. contributions to the atomic redefinition of the SI second in 1967, highlighting the people and organizations that ushered in this new age.

Why Atomic Clocks? The Quest for Unwavering Precision

Futuristic atomic clock towering over a modern city

The idea of using atoms to measure time isn't new; it was first suggested in Europe in the 19th century. James Clerk Maxwell, a Scottish physicist, was among the first to recognize the potential of atoms as timekeepers. He proposed that the "period of vibration of a piece of quartz crystal of specified shape and size and at a stated temperature" would be a better absolute standard of time than the mean solar second.

However, Maxwell recognized challenges in making exact copies of physical standards and keeping them in good condition. Atoms, with their identical properties, offered a more reliable solution. As Thomson and Tait noted in their Elements of Natural Philosophy, atoms of elements like hydrogen or sodium are "ready made in infinite numbers, all absolutely alike in every physical property," offering time measurements independent of their position in the universe.

Quantum Mechanics: The quantum revolution gave us the tools to manipulate and measure atomic behavior with unprecedented accuracy.
Microwave Electronics: Advances in microwave technology allowed us to interact with atoms at their resonant frequencies.
Molecular Beam Technology: Pioneered by Isidor Isaac Rabi, this technique allowed scientists to isolate and study atoms with remarkable precision.

Isidor Isaac Rabi and his team at Columbia University laid much of the groundwork for atomic clocks, beginning in the 1930s. Rabi's invention of the atomic and molecular beam resonance method in 1937 earned him the Nobel Prize in Physics in 1944. As early as 1939, he considered using his technique as a time standard. World War II temporarily halted his research, but in 1945, Rabi publicly discussed the possibility of atomic clocks, calling them "the most accurate clock in the universe."

A Lasting Legacy

The redefinition of the second was not just a scientific milestone but a transformative moment for society. The work of these scientists and engineers has had a profound impact on our world, and their contributions to atomic timekeeping should not be forgotten.