203
See also: Ludwig Boltzmann 139 ■ Albert Einstein 214–21 ■ Erwin Schrödinger 226–33
air in between it and us is cold.
Every object absorbs and emits
electromagnetic radiation. If its
temperature rises, the wavelength
of the radiation it emits decreases
while its frequency increases. For
example, a lump of coal at room
temperature emits energy below
the frequency of visible light, in the
infrared spectrum. We cannot see
the emission, so the coal appears
black. Once we set the coal alight,
however, it emits higher-frequency
radiation, glowing a dull red as the
emissions break into the visible
spectrum, then white-hot and
finally a brilliant blue. Extremely
hot objects, such as stars, radiate
even shorter-wavelength ultraviolet
light and X-rays, which again we
cannot see. Meanwhile, in addition
to producing radiation, a body also
reflects radiation, and it is this
reflected light that gives objects
color even when they do not glow.
In 1860, German physicist
Gustav Kirchhoff thought of an
idealized concept he called a
“perfect black body.” This is a
theoretical surface that, when at
thermal equilibrium (not heating
up or cooling down), absorbs every
frequency of electromagnetic
radiation that falls on it, and does
not itself reflect any radiation.
The spectrum of thermal radiation
coming off this body is “pure,”
since it is not mixed with any
reflections—it will only be
the result of the body’s own
temperature. Kirchhoff believed
that such “black-body radiation” is
fundamental in nature—the Sun,
for example, comes close to being aA PARADIGM SHIFT
A new scientific truth
does not triumph by
convincing its opponents and
making them see the light, but
rather because...a new
generation grows up that is
familiar with it.
Max Planckblack-body object whose emitted
spectrum is almost entirely a result
of its own temperature. Studying
the distribution of a black body’s
light would show that emission of
radiation depended only on a body’s
temperature, and not its physical
shape or chemical composition.
Kirchhoff’s hypothesis kick-started
a new experimental program
designed to find a theoretical
framework that would describe
black-body radiation.Entropy and black bodies
Planck arrived at his new quantum
theory through the failure of
classical physics to explain the
experimental results of black-body
radiation distribution. Much of
Planck’s work focussed on the
second law of thermodynamics,
which he had identified as an
“absolute.” This law states that
isolated systems move over time
toward a state of thermodynamic
equilibrium (meaning that all
parts of the system are at the same
temperature). Planck attempted to ❯❯Radiation is not continuous,
but is emitted in discrete quanta of energy.But nonsense results are reached
for the distribution of black-body radiation,
assuming a continuous range.The problem is solved by treating radiation as if it were
produced in discrete “quanta.”Classical mechanics treats radiation as if it were emitted across a
continuous range.