The Physicist Who Stopped Light in Its Tracks

Light travels at 299,792,458 meters per second in a vacuum—fast enough to circle Earth 7.5 times in one second. It's the cosmic speed limit, the one constant Einstein built his theories around. So when Harvard physicist Lene Hau announced in 1999 that she'd slowed light to just 17 meters per second—about 38 miles per hour, the speed of a car in a school zone—the physics world stopped in its tracks.

Two years later, she did something even more remarkable: she stopped light completely.

The Coldest Place in the Universe

Hau's experiments didn't happen in ordinary conditions. To manipulate light like this, she needed to create one of the coldest places in the universe—a Bose-Einstein condensate (BEC), a bizarre state of matter that exists at temperatures just 50 billionths of a degree above absolute zero.

At these extreme temperatures, atoms lose their individual identities and behave as a single quantum entity. Hau's team used a device that wicked sodium atoms from molten metal, then hit them with carefully tuned lasers to drain away nearly all their energy. What resulted was a cloud of ultracold atoms acting in perfect quantum harmony.

How Do You Stop Light?

The technique is called electromagnetically induced transparency. Normally, dense clouds of atoms absorb light. But by hitting the BEC with a "coupling" laser, Hau's team changed the optical properties of the atom cloud, making it transparent to a specific wavelength of light while simultaneously slowing it down.

In the 1999 experiment, a pulse of light entered the BEC traveling at normal speed, then decelerated to 17 m/s—slow enough that you could outrun it on a bicycle. The light pulse compressed from a kilometer long to less than a millimeter as it crawled through the condensate.

In 2001, Hau took it further. By turning off the coupling laser at precisely the right moment, she trapped the light pulse inside the condensate, storing its information in the quantum states of the atoms. The light vanished. Then, by turning the coupling laser back on, she released it—the light pulse continued on its way as if nothing had happened, like pressing pause and play on the universe's remote control.

Why Does This Matter?

Beyond being mind-bending, these experiments opened doors to revolutionary technologies:

• Quantum computing: Manipulating light at quantum speeds could enable ultra-fast information processing
• Optical memory storage: Stopping light means storing information encoded in photons, creating "quantum hard drives"
• Quantum cryptography: Perfect security through quantum communication networks
• Precision measurement: New ways to study fundamental physics and test the limits of quantum mechanics

Hau's later work achieved storage times of more than one second—an eternity in quantum physics—with remarkable coherence. Recent research has proposed extending these techniques using microgravity environments in space, potentially storing light for minutes rather than seconds.

The Woman Behind the Breakthrough

Lene Vestergaard Hau, born in Denmark, became one of the few women leading cutting-edge physics research at Harvard. Her work didn't just slow light—it challenged our understanding of what's possible when matter reaches its quantum limits.

The experiments prove that even the universe's "constants" aren't quite so constant when you know how to bend the rules. Light—the fastest thing in existence—can be tamed, stopped, stored, and released. All you need is the coldest place imaginable and a deep understanding of quantum mechanics.

Einstein said nothing could travel faster than light. Hau showed that sometimes, light doesn't have to travel at all.