December 12, 2000
Quantum Theory Tugged, and All of Physics Unraveled
By DENNIS OVERBYE
Quantum theory ignited a scientific revolution 100 years
ago, giving rise to paradoxical notions of lumpy light, wavelike particles
and the disconnection of cause and effect. Leading physicists were among
those who debated the theory at a 1927 congress in Brussels. The discussion
continues in Berlin this week.
1900 Max Planck proposes that atoms emit energy
in discrete amounts, called quanta, rather than in continuous waves.
1905 Albert Einstein explains the photoelectric
effect (light strikes an atom and dislodges an electron) by suggesting
that light is made of little energy bundles, which are later called photons.
Compact disc players work when the light (photons) from
a laser strikes a sensor (photodiode) to generate electrical current (electron
1913 Niels Bohr proposes a planetary model of the
atom in which electrons orbit the nucleus and jump between orbits as the
atom absorbs or emits energy.
1924 Louis de Broglie develops the idea that matter,
like light, can behave as waves. According to de Broglie's formula, the
wavelength of an electron is only about one-10,000th the wavelength of
a photon of light.
In electron microscopes, beams of matter, electron beams,
explore spaces far smaller than those accessible to light. At right, the
mouth of the common housefly.
1924 Einstein and Satyendra Nath Bose develop a
set of statistics recognizing a class of particles, called bosons, which
can collectively exist in the same state of energy.
Photons are bosons, so they can collectively occupy a
single state, allowing them to coalesce as an intense laser beam.
1925 Wolfgang Pauli develops the exclusion principle,
stating that no two electrons in an atom can occupy the same state of energy
simultaneously. This explains the movement of electrons into successive
orbits around the nucleus of an atom.
1926 Erwin Schrödinger proposes that an electron
is best described by the mathematical function of all its possible energy
states, a wave. Max Born later proposes that this wave is not the particle
itself, but the probability of finding the particle in a particular place.
1926 Enrico Fermi and Paul Dirac describe the statistical
properties of particles that obey the Pauli exclusion principle. Fermions,
as they are known, include protons, neutrons and electrons and are distinct
from particles that obey Bose-Einstein statistics.
Atoms in certain solids, semiconductors, will collectively
fill out their energy orbits with electrons. When excited from a burst
of energy, these electrons can move about. Semiconductors are at the heart
of the circuits in microprocessors.
1927 Werner Heisenberg arrives at his uncertainty
principle, theorizing that it is impossible to measure both the position
and momentum of a particle at the same time.
1928 Paul Dirac predicts antimatter by proposing
that fermions, including electrons, can inhabit states of negative energy.
When matter and antimatter collide, they annihilate each other and release
Positron emission tomography relies on collisions between
electrons and their anti-particles, positrons, that occur in chemical compounds
injected into the body.
Sources: Science, Dorling Kindersley
hey tried to talk Max Planck out
of becoming a physicist, on the grounds that here was nothing left to discover.
The young Planck didn't mind. A conservative youth from the south of Germany,
a descendant of church rectors and professors, he was happy to add to the
perfection of what was already known.
Instead, he destroyed it, by discovering what was in effect a loose
thread that when tugged would eventually unravel the entire fabric of what
had passed for reality.
As a new professor at the University of Berlin, Planck embarked in the
fall of 1900 on a mundane sounding calculation of the spectral characteristics
of the glow from a heated object. Physicists had good reason to think the
answer would elucidate the relationship between light and matter as well
as give German industry a leg up in the electric light business. But the
calculation had been plagued with difficulties.
Planck succeeded in finding the right formula, but at a cost, as he
reported to the German Physical Society on Dec. 14. In what he called "an
act of desperation," he had to assume that atoms could only emit energy
in discrete amounts that he later called quanta (from the Latin quantus
for "how much" ) rather than in the continuous waves prescribed by electromagnetic
theory. Nature seemed to be acting like a fussy bank teller who would not
make change, and would not accept it either.
That was the first shot in a revolution. Within a quarter of a century,
the common sense laws of science had been overthrown. In their place was
a bizarre set of rules known as quantum mechanics, in which causes were
not guaranteed to be linked to effects; a subatomic particle like an electron
could be in two places at once, everywhere or nowhere until someone measured
it; and light could be a wave or a particle.
Niels Bohr, a Danish physicist and leader of this revolution, once said
that a person who was not shocked by quantum theory did not understand
This week, some 700 physicists and historians are gathering in Berlin,
where Planck started it all 100 years ago, to celebrate a theory whose
meaning they still do not understand but that is the foundation of modern
science. Quantum effects are now invoked to explain everything from the
periodic table of the elements to the existence of the universe itself.
Fortunes have been made on quantum weirdness, as it is sometimes called.
Transistors and computer chips and lasers run on it. So do CAT scans and
PET scans and M.R.I. machines. Some computer scientists call it the future
of computing, while some physicists say that computing is the future of
"If everything we understand about the atom stopped working," said Leon
Lederman, former director of the Fermi National Accelerator Laboratory,
"the G.N.P. would go to zero."
The revolution had an inauspicious start. Planck first regarded the
quantum as a bookkeeping device with no physical meaning. In 1905, Albert
Einstein, then a patent clerk in Switzerland, took it more seriously. He
pointed out that light itself behaved in some respects as if it were composed
of little energy bundles he called lichtquanten. (A few months later Einstein
He spent the next decade wondering how to reconcile these quanta with
the traditional electromagnetic wave theory of light. "On quantum theory
I use up more brain grease than on relativity," he told a friend.
The next great quantum step was taken by Bohr. In 1913, he set forth
a model of the atom as a miniature solar system in which the electrons
were limited to specific orbits around the nucleus. The model explained
why atoms did not just collapse the lowest orbit was still some slight
distance from the nucleus. It also explained why different elements emitted
light at characteristic wavelengths the orbits were like rungs on a ladder
and those wavelengths corresponded to the energy released or absorbed by
an electron when it jumped between rungs.
But it did not explain why only some orbits were permitted, or where
the electron was when it jumped between orbits. Einstein praised Bohr's
theory as "musicality in the sphere of thought," but told him later, "If
all this is true, then it means the end of physics."
While Bohr's theory worked for hydrogen, the simplest atom, it bogged
down when theorists tried to calculate the spectrum of bigger atoms. "The
whole system of concepts of physics must be reconstructed from the ground
up," Max Born, a physicist at Göttingen University, wrote in 1923.
He termed the as-yet- unborn new physics "quantum mechanics."
The new physics was born in a paroxysm of debate and discovery from
1925 to 1928 that has been called the second scientific revolution. Wolfgang
Pauli, one of its ringleaders, called it "boy's mechanics," because many
of the physicists, including himself, then 25, Werner Heisenberg, 24, Paul
Dirac, 23, Enrico Fermi, 23, and Pascual Jordan, 23, were so young when
Bohr, who turned 40 in 1925, was their father-confessor and philosopher
king. His new institute for theoretical physics in Copenhagen became the
center of European science.
The decisive moment came in the fall of 1925 when Heisenberg, who had
just returned to Göttingen University after a year in Copenhagen,
suggested that physicists stop trying to visualize the inside of the atom
and instead base physics exclusively on what can be seen and measured.
In his "matrix mechanics," various properties of subatomic particles could
be computed but, disturbingly, the answers depended on the order of the
In fact, according to the uncertainty principle, which Heisenberg enunciated
two years later, it was impossible to know both the position and velocity
of a particle at once. The act of measuring one necessarily disturbed the
Physicists uncomfortable with Heisenberg's abstract mathematics took
up with a friendlier version of quantum mechanics based on the familiar
mathematics of waves. In 1923, the Frenchman Louis de Broglie had asked
in his doctoral thesis, if light could be a particle, then why couldn't
particles be waves?
Inspired by de Broglie's ideas, the Austrian Erwin Schrödinger,
then at the University of Zurich and, at 38, himself older than the wunderkind,
sequestered himself in the Swiss resort of Arosa over the 1925 Christmas
holidays with a mysterious woman friend and came back with an equation
that would become the yin to Heisenberg's yang.
In Schrödinger's equation, the electron was not a point or a table,
but a mathematical entity called a wave function, which extended throughout
space. According to Born, this wave represented the probability of finding
the electron at some particular place. When it was measured, the particle
was usually in the most likely place, but not guaranteed to be, even though
the wave function itself could be calculated exactly.
Born's interpretation was rapidly adopted by the quantum gang. It was
a pivotal moment because it enshrined chance as an integral part of physics
and of nature.
"The motion of particles follows probability laws, but the probability
itself propagates according to the law of causality," he explained.
That was not good enough for Einstein. "The theory produces a good deal
but hardly brings us closer to the secret of the Old One," Einstein wrote
in late 1926. "I am at all events convinced that he does not play dice."
Heisenberg called Schrödinger's theory "disgusting" but both
versions of quantum mechanics were soon found to be mathematically equivalent.
Uncertainty, which added to the metaphysical unease surrounding quantum
physics, was followed in turn in 1927 by Bohr's complementarity principle.
Ask not whether light was a particle or a wave, said Bohr, asserting that
both concepts were necessary to describe nature, but that since they were
contradictory, an experimenter could choose to measure one aspect or the
other but not both. This was not a paradox, he maintained, because physics
was not about things but about the results of experiments.
Complementarity became the cornerstone of the Copenhagen interpretation
of quantum mechanics or as Einstein called it, "the Heisenberg- Bohr
A year later, Dirac married quantum mechanics to Einstein's special
relativity, in the process predicting the existence of antimatter. (The
positron, the antiparticle to the electron, was discovered four years later
by Carl Anderson.)
Dirac's version, known as quantum field theory, has been the basis of
particle physics ever since, and signifies, in physics histories, the end
of the quantum revolution. But the fight over the meaning of the revolution
had just barely begun, and it has continued to this day.
The first and greatest counterrevolutionary was Einstein, who hoped
some deeper theory would rescue God from playing dice. In the fall of 1927
at a meeting in Brussels, Einstein challenged Bohr with a series of gedanken,
or thought experiments, designed to show that quantum mechanics was inconsistent.
Bohr, stumped in the morning, always had an answer by dinner.
Einstein never gave up. A 1935 paper written with Boris Podolsky and
Nathan Rosen described the ultimate quantum gedanken, in which measuring
a particle in one place could instantly affect measurements of the other
particle, even if it was millions of miles away. Was this any way to run
Einstein called it "spooky action at a distance."
Modern physicists who have managed to create this strange situation
in the laboratory call it "entanglement."
Einstein's defection from the quantum revolution was a blow to his more
conservative colleagues, but he was not alone. Planck also found himself
at odds with the direction of the revolution and Schrödinger, another
of "the conservative old gentlemen," as Pauli once described them, advanced
his cat gedanken experiment to illustrate how silly physics had become.
According to the Copenhagen view, it was the act of observation that
"collapsed" the wave function of some particle, freezing it into one particular
state, a location or velocity. Until then, all the possible states of the
particle coexisted, like overlapping waves, in a condition known as quantum
Schrödinger imagined a cat in a sealed container in which the radioactive
decay of an atom would trigger the release of cyanide, killing the cat.
By the rules of quantum mechanics the atom was both decayed and not decayed
until somebody looked inside, which meant that Schrödinger's poor
cat was both alive and dead.
This seemed to be giving an awful lot of power to the "observer." It
was definitely no way to run a universe.
Over the years physicists have proposed alternatives to the Copenhagen
Starting in 1952, when he was at Princeton, the physicist David Bohm,
who died in 1992, argued for a version of quantum mechanics in which there
was a deeper level, a so-called quantum potential or "implicate order,"
guiding the apparent unruliness of quantum events.
Another variant is the many- worlds hypothesis developed by Hugh Everett
III and John Wheeler, at Princeton in 1957. In this version the wave function
does not collapse when a physicist observes an electron or a cat; instead
it splits into parallel universes, one for every possible outcome of an
experiment or a measurement.
Shut Up and Compute
Most physicists simply ignored the debate about the meaning of quantum
theory in favor of using it to probe the world, an attitude known as "shut
up and compute."
Pauli's discovery that no two electrons could share the same orbit in
an atom led to a new understanding of atoms, the elements and modern chemistry.
Quantum mechanics split the atom and placed humanity on the verge of
plausible catastrophe. Engineers learned how to "pump" electrons into the
upper energy rungs in large numbers of atoms and then make them all dump
their energy all at once, giving rise to the laser. And as Dr. Lederman
said in an interview, "The history of transistors is the history of solving
Schrödinger's equation in various materials."
Quantum effects were not confined to the small. The uncertainty principle
dictates that the energy in a field or in empty space is not constant,
but can fluctuate more and more wildly the smaller the period of time that
one looks at it. Such quantum fluctuations during the big bang are now
thought to be the origin of galaxies.
In some theories, the universe itself is a quantum effect, the result
of a fluctuation in some sort of preuniversal nothingness. "So we take
a quantum leap from eternity into time," as the Harvard physicist Sidney
Coleman once put it.
Where the Weirdness Goes
Bohr ignored Schrödinger's cat, on the basis that a cat was too
big to be a quantum object, but the cat cannot be ignored anymore. In the
last three decades, the gedanken experiments envisioned by Einstein and
his friends have become "ungedankened," bringing the issues of their meaning
back to the fore.
Last summer, two teams of physicists managed to make currents go in
two directions at once around tiny superconducting loops of wire a feat
they compared to Schrödinger's cat. Such feats, said Wojciech Zurek,
a theorist at Los Alamos National Laboratory, raise the question of why
we live in a classical world at all, rather than in a quantum blur.
Bohr postulated a border between the quantum and classical worlds, but
theorists prefer that there be only one world that can somehow supply its
own solidity. That is the idea behind a new concept called decoherence,
in which the interaction of wave functions with the environment upsets
the delicate balance of quantum states and makes a cat alive or dead but
not in between.
"We don't need an observer, just some `thing' watching," Dr. Zurek explained.
When we look at something, he said, we take advantage of photons, the carriers
of light, which contain information that has been extracted from the object.
It is this loss of information into the environment that is enough to crash
the wave function, Dr. Zurek says.
Decoherence, as Dr. Zurek notes, takes the observer off a pedestal and
relieves quantum theory of some of its mysticism, but there is plenty of
weirdness left. Take the quantum computer, which Dr. Lederman refers to
as "a kinder, gentler interpretation of quantum spookiness."
Ordinary computers store data and perform computations as a series of
"bits," switches that are either on or off, but in a quantum computer,
due to the principle of superposition, so-called qubits can be on and off
at the same time, enabling them to calculate and store myriads of numbers
at a time.
In principle, according to David Deutsch, an Oxford University researcher
who is one of quantum computing's more outspoken pioneers, a vast number
of computations, "potentially more than there are atoms in the universe,"
could be superposed inside a quantum computer to solve problems that would
take a classical computer longer than the age of the universe.
In the minds of many experts, this kind of computing illuminates the
nature of reality itself.
Dr. Deutsch claims that the very theory of a quantum computer forces
physicists to take seriously the many-worlds interpretation of quantum
theory. The amount of information being processed in these parallel computations,
he explains, is more than the universe can hold. Therefore, they must be
happening in other parallel universes out in the "multiverse," as it is
"There is no other theory of what is happening," he said. The world
is much bigger than it looks, a realization that he thinks will have a
psychological impact equivalent to the first photographs of atoms. Indeed,
for Dr. Deutsch there seems to be a deep connection between physics and
computation. The structure of the quantum computer, he says, consists of
many things going on at once, lots or parallel computations. "Any physical
process in quantum mechanics," he said. "consists of classical computations
going on in parallel."
"The quantum theory of computation is quantum theory," he said.
The Roots of Weirdness
Quantum mechanics is the language in which physicists describe all the
phenomena of nature save one, namely gravity, which is explained by Einstein's
general theory of relativity. The two theories one describing a discontinuous
"quantized" reality and the other a smoothly curving space-time continuum
are mathematically incompatible, but physicists look to their eventual
marriage, a so-called quantum gravity.
"There are different views as to whether quantum theory will encompass
gravity or whether both quantum theory and general relativity will have
to be modified," said Lee Smolin, a theorist at Penn State.
Some groundwork was laid as far back as the 1960's by Dr. Wheeler, 89,
who has argued quantum theory with both Einstein and Bohr. Even space and
time, Dr. Wheeler has pointed out, must ultimately pay their dues to the
uncertainty principle and become discontinuous, breaking down at very small
distances or in the compressed throes of the big bang into a space-time
Most physicists today put their hope for such a theory in super- strings,
an ongoing and mathematically dense effort to understand nature as consisting
of tiny strings vibrating in 10-dimensional space.
In a sort of missive from the front, Edward Witten of the Institute
for Advanced Study in Princeton, N.J., said recently that so far quantum
mechanics appeared to hold up in string land exactly as it was described
in textbooks. But, he said in an e-mail message, "Quantum mechanics is
somehow integrated with geometry in a way that we don't really understand
The quantum is mysterious, he went on, because it goes against intuition.
"I am one of those who believes that the quantum will remain mysterious
in the sense that if the future brings any changes in the basic formulation
of quantum mechanics, I suspect our ordinary intuition will be left even
Intuition notwithstanding, some thinkers wonder whether or not quantum
weirdness might, in fact, be the simplest way to make a universe. After
all, without the uncertainty principle to fuzz the locations of its buzzing
inhabitants, the atom would collapse in an electromagnetic heap. Without
quantum fluctuations to roil the unholy smoothness of the big bang, there
would be no galaxies, stars or friendly warm planets. Without the uncertainty
principle to forbid nothingness, there might not even be a universe.
"We will first recognize how simple the universe is," Dr. Wheeler has
often said, "when we recognize how strange it is." Einstein often said
that the question that really consumed him was whether God had any choice
in creating the world. It may be in the end that we find out that for God,
the only game in town was a dice game.