Theories · 1916 – 1917
Stimulated Emission
Working out how atoms and light exchange energy, Einstein found a third process nobody had noticed. Forty years later it became the laser.
The Question
By 1916 physicists knew, from Niels Bohr's model, that atoms have discrete energy levels and that light is emitted or absorbed when an atom jumps between them. Einstein, taking a break from gravitation, asked a careful question: what must be true of these jumps for a gas of atoms and a bath of light to remain in balance at a steady temperature?
Three Processes
Two processes were already familiar.
- Absorption. An atom in a lower level takes in a light quantum and jumps up.
- Spontaneous emission. An atom in an upper level drops down by itself, at a random moment, sending out a quantum in a random direction.
Einstein found that these two alone cannot produce the correct balance. A third process is required.
- Stimulated emission. A passing light quantum of the right frequency can prompt an excited atom to drop down and release a second quantum, identical to the first: same frequency, same direction, in step with it.
He described the rates of the three processes with what are now called the Einstein A and B coefficients, and showed that they must be related as in the equation above. With that, Planck's law for the radiation from hot objects follows directly.
A Bonus Result
In the same paper Einstein showed that a light quantum carries momentum, , in a definite direction. An atom recoils when it emits one, like a rifle. This made light quanta look much more like real particles, years before experiments confirmed it.
It also troubled him. Spontaneous emission happens at a time and in a direction that the theory leaves to chance. He called this a weakness of the theory, an early sign of the unease about chance that he would later voice about quantum mechanics.
From Paper to Device
One identical copy can trigger another. If more atoms are held in the upper level than the lower, light passing through is amplified in a chain reaction of copies. That is the working principle of the maser, built in 1954, and the laser, first operated in 1960. The "SE" in the word laser stands for stimulated emission.
What People Get Wrong
Einstein did not invent the laser, or foresee one. He identified the physical process on which it depends. The engineering insight, how to get light to amplify itself, came from Charles Townes, Arthur Schawlow, Theodore Maiman, and their contemporaries four decades later. See Lasers.