C. More direct evidence
As we've noted, relativity theory implies that a quantum of energy ought to be a quantum of momentum as well. While Einstein's analysis showed that this idea was consistent with known facts about light and matter, modern experiments with individual light quanta and subatomic particles demonstrate the existence of momentum quanta more directly.
One early demonstration was an effect studied by Arthur Holly Compton. When x-rays a high-frequency form of light-collide with atoms, the x-rays scatter in all directions accompanied by electrons from the atoms. The wavelength of the scattered x-rays and the momentum of the electrons both vary with their direction of motion. While the classical theory of light and electricity doesn't explain the variation actually observed, the quantum theory accounts for it easily: the x-ray quanta have momenta proportional to their frequencies. When an x-ray quantum strikes an electron in an atom, some of its momentum goes into the electron as it knocks the electron out of the atom. The electron gains some of the x-ray quantum's initial forward momentum; it can also gain a positive momentum in a sideways direction as the x-ray quantum gains the same-sized negative momentum in the opposite sideways direction. The final momenta of the electron/x-ray pair total up to the x-ray quantum's initial momentum.
Another demonstration occurs in experiments with subatomic particles, especially those involving particles from high-energy accelerator beams or cosmic rays. These experiments often result in the original particles' energy turning into showers of other particles. Sometimes a single particle will decay into other particles, which may in turn decay into still others; other times two particles will collide, turning some of their energy into other particles. Examination of the particles produced, or their effects on appropriate particle-detection devices, show that in many cases some of these particles are light quanta. The energies and momenta of these quanta are consistent with those of the other particles involved in the reaction, so that the total energy and total momentum before and after each particle reaction are the same.
The third process Einstein considered -stimulated emission-is the basis for the increasingly ubiquitous laser. Whereas an atom in a low energy state, exposed to light whose absorption could put it into a higher energy state, has some chance of absorbing a light quantum, the same atom, in the same environment but in the higher energy state, has some likelihood of emitting a light quantum under the stimulus of the surrounding light. (The atom might also spontaneously emit a light quantum with the same result-Einstein's first process-but it's likely to emit one sooner when exposed to similar light quanta.) When the atom emits one light quantum under the stimulus of another, the two quanta's light waves will vibrate in step with each other, making a single wave of twice the intensity. In effect, the stimulating light wave is amplified when the atom emits its own light wave.
The laser multiplies this effect on a large scale. Whenever many atoms are exposed to light quanta of an appropriate frequency, whether more quanta are added than removed depends on whether more atoms are in a high energy state or a low one. Putting more atoms in the higher energy state produces the amplification. The name laser is an acronym describing the result of this process and the mechanism that produces it: "light amplification by stimulated emission of radiation".
Next article: The General Theory of Relativity
References, Links, and Comments:
"Zur Quantentheorie der Strahlung" by Albert Einstein
First printed in Mitteilungen der Physikalishen Gesellschaft Zurich, No. 18, 1916, and later in Physikalische Zeitschrift 18 (1917), p. 121. Also available in The Collected Papers of Albert Einstein, Volume 6: The Berlin Years: Writings, 1914-1917, edited by A.J. Kox, Martin J. Klein, and Robert Schulmann.
English translations ("On the Quantum Theory of Radiation") are available in:
Source of Quantum Mechanics edited by B.L. van der Waerden, which is Volume V in the Classics of Science series under the general editorship of Gerald Holton.
The Old Quantum Theory, edited by Dirk ter Harr.
The Collected Papers of Albert Einstein, Volume 6: The Berlin Years: Wrintings, 1914-1917. (English translation supplement); Alfred Engel, translator; Englebert Schucking, consultant.
One page from a sequence about subatomic-particle detectors. From The Particle Adventure, a website about the most elementary known particles that constitute all matter.
"How Lasers Work" by Matthew Weschler
From the "How Stuff Works" website.
[N.B.: The website originally referenced in the below has either moved or is no longer being maintained. Links now point to archived versions of these pages at the Internet Archive. (Note added 3/1/2012)]
From Web Science Resources. Covers the amplification of light, the energizing of the amplifying medium, laser oscillators (which amplify already-amplified light), the absorption and emission of light by atoms, and how more atoms are put into a higher-energy state than a lower-energy state to make the amplification possible.