SCIENTIFIC revolutions are two-a-penny these days. Every development in
science seems to come with the promise that it will be “revolutionary”. Decoding
the human genome, discovering stem cells and building carbon nanotubes are
clearly important developments. But they’re not scientific revolutions.
Next week sees the centenary of a real revolution, the most profound in
20th-century science. It began in Berlin on 14 December 1900—decreed by
the Kaiser himself to be the first year of the 20th century, not the last year
of the 19th. On that day, the physicist Max Planck publicly announced a result
that marked the beginning of what we now call quantum theory.
Why was the advent of quantum theory a real revolution? Scientific
revolutions happen when scientists change the way they think about the
world—when they fashion new tools to do their job. These revolutions don’t
happen overnight. They take time. And you invariably find that the effects of a
revolution in one branch of science cascade into other related areas and
generate whole new fields. All this is certainly true of quantum theory, as it
is of other genuine scientific revolutions, such as plate tectonics, the
discovery of DNA as the carrier of inherited characteristics, and the idea of
the big bang.
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Quantum theory overturned the universally accepted notion that energy was
smooth and continuous, and replaced it with the realisation that it is
fundamentally granular and discrete. This led scientists to dub the previous
framework of physics as “classical”—an exquisite example of retro
labelling, like “snail-mail” and “acoustic guitar”. Quantum theory gave rise to
hundreds of new lines of research and is today the basis of fundamental physics
and semiconductor technology.
Though scientific revolutions aren’t driven by some Lenin-like figure at
their head, I’m willing to bet that we’ll be hearing a lot this week about how
the revolutionary Max Planck single-handedly founded quantum theory. This is a
myth. Planck was no revolutionary, but a profoundly conservative scientist
deeply respectful of the classical laws of physics. He once told his research
supervisor that he wished only to deepen the foundations laid by his
predecessors and had no wish to make new discoveries. Planck struggled for years
to make his new ideas fit with the classical ones that were so dear to his
heart.
What, then, did Planck do a hundred years ago to make us remember him, and
not others, as leader of the quantum revolution? And who else deserves
credit?
A 42-year-old theoretical physicist at the peak of his career, Planck had set
himself a task that sounds laughably obscure. He wanted to derive, from first
principles, the comparative brightness—known as relative
intensity—of each colour in the electromagnetic spectrum from a tiny hole
in a special sealed oven heated to a range of different temperatures. In the
trade, this is called “black-body” or “cavity” radiation.
Planck was intrigued by black-body radiation. Physicists had already proved
theoretically that at any given temperature, a black-body oven will radiate
energy at all wavelengths with the same spectrum of intensities, regardless of
the size or shape of the cavity or the material from which its walls are made.
Planck was trying to find a formula that would accurately fit this
experimentally observed distribution of intensities.
In early October 1900, Planck came up with a formula. He was liaising with
experimenters at the Imperial Institute of Physics and Technology in
Charlottenberg, which had been founded jointly by the German government and the
industrialist Werner von Siemens to refine the art of making precise
measurements. One hope was that the black-body radiation experiments would help
the heating and gas-lighting industries understand how lights radiate heat and
so how to make them more efficient, to the benefit of the German economy.
Planck’s equation fitted the experimenters’ data perfectly. But he wanted to
derive his formula in terms of what was going on inside the ovens. To explain
the formula, he found that he had to assume that matter came in discrete
lumps—atoms—an idea he had long disbelieved. And he had to accept
that you couldn’t say for sure how an atom would behave: the best you could hope
for was a statistical probability, a notion that Planck had long resisted.
After eight of the most strenuous working weeks of his life, Planck
brilliantly cobbled together a way of accounting for the formula. It involved
dividing up the total energy of all the atoms vibrating in the oven’s walls into
discrete amounts for each frequency. The result was the equation E = hf,
where E is energy, f is frequency and h was a new
fundamental constant, now called Planck’s constant.
Here was the first appearance of the discrete energy “quantum”, a Latin word
meaning “how much”. Many physics textbooks give the impression that Planck
cottoned on immediately to the significance of the quantum idea, but they’re
wrong. If anything, he downplayed it. With his trademark caution, he wrote, “I
did not give it much thought except for this: that I had to obtain a positive
result, under any circumstances and at whatever cost.”
Statements like this convinced the late philosopher and historian of science
Thomas Kuhn that Planck did not initially appreciate the significance of energy
quanta and that he did not believe that atomic energies came in discrete values.
Rather, Kuhn powerfully argued that Planck later believed, along with everyone
else, that the individual atoms could have any energy they liked.
Kuhn is well known for his theory that science proceeds by a series of
revolutions. Each revolution gives rise to a new way of working, which is
followed by a period of what Kuhn called “normal science”, when scientists work
to fit their world picture into the new framework. Kuhn trained as a physicist
and began his research on Planck believing that a moment would come when Planck
the revolutionary would light up with the new idea, and begin the revolution.
But that turned out not to be the case at all. Kuhn argued that it was Einstein,
not Planck, who first properly understood the quantum idea and its
implications.
Five years after Planck’s discovery, Einstein proposed that the quantum
energy idea did not just apply to the energy of atoms in Planck’s black-body
ovens. Einstein postulated that light energy, too, is transferred in discrete
amounts, later called photons. Einstein suggested that the energy of every light
quantum, in any situation, is also given by the simple formula, E = hf.
Einstein often said that was his only truly revolutionary work. The Nobel
committee seems to have agreed—this work was cited when Einstein was
awarded his long-overdue prize in 1921.
But for 14 years, Einstein’s fellow physicists almost universally regarded
his photon concept as an aberration, a blemish on an otherwise brilliant CV. The
idea came to be widely accepted only in 1924, thanks to the American physicist
Arthur Compton, who first demonstrated that X-radiation can behave like
particles as well as waves. Compton found that when he passed X-rays through
certain materials, their wavelength increased. He could explain this only by
modelling the collision of X-rays and the electrons in the materials as a
collision between two microscopic billiard balls—both were particles.
Astonishingly, Planck did not fully sign up to the light particle concept
even then. Indeed, he went to his grave in 1947 not quite believing in the
photon. He was, however, more comfortable with the quantum theory of matter that
was developed soon after his initial discovery, when Heisenberg and
Schrödinger produced a theory that gave fundamental insights into the
structure of atoms.
Although Planck did not at first realise the implications of quantum theory,
he immediately appreciated the importance of the new fundamental constant
h. It was the first new constant to be introduced in the 20th century and,
as it turned out, the last. What especially excited the normally imperturbable
Planck was that h could be combined with the two other fundamental
constants—the speed of light, c, and the gravitational constant,
G—to give absolute measures of energy, time and length. Today’s
cosmologists studying the early Universe routinely use these measures when they
are studying the limits of their theories.
Planck, always sceptical of revolutionary talk, much preferred to think of
science in terms of gradual change, of evolution. And he was right when he
wrote, characteristically, that the goal of scientists should be “the constant
improvement of the world picture by reducing the real elements contained in it
to reach a higher reality of a less naive character.” But, he added, “a
demonstrable attainment of this goal will—or can—never be ours.”
There’s a lesson here for those who misguidedly preach that we’re seeing the end
of science.
During the celebrations of the quantum centenary, the hype-peddlers would do
well to honour Planck’s memory by celebrating his work as a scientist, not as a
revolutionary. Continuing in the spirit of goodwill, how about a century-long
moratorium on predicting scientific revolutions?
