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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.
"How
Do We Interpret Our Data? Typical Detector Components"
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.
"Laser
Tutorial"
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.
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