NYTimes.com Article: Quantum Theory Tugged, and All of Physics Unraveled

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Quantum Theory Tugged, and All of Physics Unraveled

http://www.nytimes.com/2000/12/12/science/12QUAN.html

 

December 12, 2000

 

By DENNIS OVERBYE

 

They 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 it.

 

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 quantum theory.

 

" 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 invented relativity.)

 

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. "

 

Boy's 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 it began.

 

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 calculations.

 

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 other.

 

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 tranquilizing philosophy. "

 

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.

 

Quantum Wars

 

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 a universe?

 

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 superposition.

 

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 view.

 

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 sometimes called.

 

" 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 " foam. "

 

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 yet. "

 

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 farther behind. "

 

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.

 

 

 

 

 

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