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 Question 1563: If the speed of light is constant, how can light be slowed to a stop?

 I've recently heard about an experiment by Harvard that 'stopped' light in 2002. Is this really what happened? If the speed of light is supposed to be a constant c, how is it appearing stopped? — CR, Dallas, Texas
The speed of light in vacuum, as denoted by the letter c, is truly a constant of nature and one of its most influential constant at that. Even if light didn't exist, the speed of light in vacuum would. It is a key component of the relationship between space and time known as special relativity.

But while the speed of light in vacuum is a constant, the speed of light in matter isn't. Light is an electromagnetic wave and consists of electric and magnetic fields. Electric fields push on electric charge and matter contains electric charges, so light and matter interact. That interaction normally slows light down; the light gets delayed by the process of shaking the electric charges. In air, this slowing effect is tiny, less than 1 part in a thousand. In glass, plastic, or water, light is slowed by about 30 or 40%. In diamond, the interaction is strong enough to slow light by 60%. In silicon solar cells, light is slowed by 70%. And so it goes.

To really slow light down, however, you need to choose a specific frequency of light and let it interact with a material that is resonant with that light. Because a resonant material responds extremely strongly to the light's electric field, it delays the light by an enormous amount. And by choosing just the right wavelength of light to match a particular collection of resonant atoms, Lene Hau and her colleagues managed to bring light essentially to a halt. The light lingers nearly forever with the atoms in their apparatus and it barely makes any headway.