Light in a vacuum moves at a dizzying speed. But an impulse that passes through matter raises another question. At Harvard, Lene Hau’s team obtained a propagation of seventeen meters per second in a gas of ultracold atoms, a result published in 1999.
The medium and a control laser change the way the pulse propagates. Experience is not about changing the constant that describes the speed of light in a vacuum. It exploits a very special interaction between light and atoms.
The team also studied the storage of optical information in atoms and its restitution in the form of a pulse. Saying that the light is “stopped” then becomes a shortcut to explain. The most surprising result is precisely this controlled transformation: a message carried by light can wait in the matter before returning to its path.
What we know
Pulse propagation speed in a medium, no change in velocity in vacuum; optical storage distinguished from still photons.
To go to the source
References consulted on September 21, 2026. Linked pages may be in another language.
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