In 1859, scientist Robert Kirchhoff introduced an interesting
problem into the world of physics: the question of blackbody radiation. A
"blackbody" is basically a black box that absorbs all the radiation
that is directed toward it. The amount of energy that it emits
is independent of the size or shape of the box; it depends only
on temperature.

For decades, physicists worked to figure out the relationship between
the temperature of the blackbody and the distribution of the emitted
energy along the electromagnetic spectrum. This was of particular
interest to theorists because finding the relationship could yield
valuable physical constants that could then be applied to other
physics concerns. However, there was a more concrete and technical
reason to search for a formula relating energy to temperature.
Such an equation could be used as a standard for rating the strength
of electric lamps.

For this reason, the imperial bureau of standards–the
Physikalisch- Technische Reichsanstalt–took a special interest in
finding the formula. And, in 1896, a young German physicist working there,
Wilhelm Wien, seemed to have stumbled onto an equation that worked.
With the knowledge of the spectral distribution of the energy at
one temperature, Wien's equation would produce the distribution
for any other temperature. It was an experimentally accurate theory,
but Wien had no explanation for *why* his equation worked;
he knew only that it did.

Meanwhile, Planck was hired to take Kirchhoff's old job
at the University of Berlin. Planck spent much of the 1890s studying
problems of chemical thermodynamics,
specifically entropy. His work in this field led him to the puzzle
of blackbody radiation, and he set himself the goal of finding
a workable theory that would yield Wien's equation.

But just as Planck thought he'd found the answer, a series
of experiments proved that Wien's equation was actually incorrect. Rather
than assuming his theory was correct and hoping the empirical data
would eventually prove him right, Planck chose to trust the experimental
results: Wien's theory was wrong, which mean Planck's was, too.
So, in 1900, Planck was forced to start all over again.

At this point, Planck took a revolutionary step, although
he didn't realize it at the time. Unable to get the numbers to
work any other way, he made a bold assumption: Planck posited that
energy was emitted by the black box in tiny, finite packets. This
was an unprecedented move, as it had always been assumed that energy came
in an unbroken continuous wave, not in a series of discrete energy
packets. But the assumption led Planck to an equation that worked,
the equation that would make him famous: *E = hv*.