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NY Times, May 8, 2018
What Does Quantum Physics Actually Tell Us About the World?
By James Gleick
WHAT IS REAL?
The Unfinished Quest for the Meaning of Quantum Physics
By Adam Becker
370 pp. Basic Books. $32.
Are atoms real? Of course they are. Everybody believes in atoms, even
people who don’t believe in evolution or climate change. If we didn’t
have atoms, how could we have atomic bombs? But you can’t see an atom
directly. And even though atoms were first conceived and named by
ancient Greeks, it was not until the last century that they achieved the
status of actual physical entities — real as apples, real as the moon.
The first proof of atoms came from 26-year-old Albert Einstein in 1905,
the same year he proposed his theory of special relativity. Before that,
the atom served as an increasingly useful hypothetical construct. At the
same time, Einstein defined a new entity: a particle of light, the
“light quantum,” now called the photon. Until then, everyone considered
light to be a kind of wave. It didn’t bother Einstein that no one could
observe this new thing. “It is the theory which decides what we can
observe,” he said.
Which brings us to quantum theory. The physics of atoms and their
ever-smaller constituents and cousins is, as Adam Becker reminds us more
than once in his new book, “What Is Real?,” “the most successful theory
in all of science.” Its predictions are stunningly accurate, and its
power to grasp the unseen ultramicroscopic world has brought us modern
marvels. But there is a problem: Quantum theory is, in a profound way,
weird. It defies our common-sense intuition about what things are and
what they can do.
“Figuring out what quantum physics is saying about the world has been
hard,” Becker says, and this understatement motivates his book, a
thorough, illuminating exploration of the most consequential controversy
raging in modern science.
The debate over the nature of reality has been growing in intensity for
more than a half-century; it generates conferences and symposiums and
enough argumentation to fill entire journals. Before he died, Richard
Feynman, who understood quantum theory as well as anyone, said, “I still
get nervous with it...I cannot define the real problem, therefore I
suspect there’s no real problem, but I’m not sure there’s no real
problem.” The problem is not with using the theory — making
calculations, applying it to engineering tasks — but in understanding
what it means. What does it tell us about the world?
From one point of view, quantum physics is just a set of formalisms, a
useful tool kit. Want to make better lasers or transistors or television
sets? The Schrödinger equation is your friend. The trouble starts only
when you step back and ask whether the entities implied by the equation
can really exist. Then you encounter problems that can be described in
several familiar ways:
Wave-particle duality. Everything there is — all matter and energy, all
known forces — behaves sometimes like waves, smooth and continuous, and
sometimes like particles, rat-a-tat-tat. Electricity flows through
wires, like a fluid, or flies through a vacuum as a volley of individual
electrons. Can it be both things at once?
The uncertainty principle. Werner Heisenberg famously discovered that
when you measure the position (let’s say) of an electron as precisely as
you can, you find yourself more and more in the dark about its momentum.
And vice versa. You can pin down one or the other but not both.
The measurement problem. Most of quantum mechanics deals with
probabilities rather than certainties. A particle has a probability of
appearing in a certain place. An unstable atom has a probability of
decaying at a certain instant. But when a physicist goes into the
laboratory and performs an experiment, there is a definite outcome. The
act of measurement — observation, by someone or something — becomes an
inextricable part of the theory.
The strange implication is that the reality of the quantum world remains
amorphous or indefinite until scientists start measuring. Schrödinger’s
cat, as you may have heard, is in a terrifying limbo, neither alive nor
dead, until someone opens the box to look. Indeed, Heisenberg said that
quantum particles “are not as real; they form a world of potentialities
or possibilities rather than one of things or facts.”
This is disturbing to philosophers as well as physicists. It led
Einstein to say in 1952, “The theory reminds me a little of the system
of delusions of an exceedingly intelligent paranoiac.”
So quantum physics — quite unlike any other realm of science — has
acquired its own metaphysics, a shadow discipline tagging along like the
tail of a comet. You can think of it as an “ideological superstructure”
(Heisenberg’s phrase). This field is called quantum foundations, which
is inadvertently ironic, because the point is that precisely where you
would expect foundations you instead find quicksand.
Competing approaches to quantum foundations are called
“interpretations,” and nowadays there are many. The first and still
possibly foremost of these is the so-called Copenhagen interpretation.
“Copenhagen” is shorthand for Niels Bohr, whose famous institute there
served as unofficial world headquarters for quantum theory beginning in
the 1920s. In a way, the Copenhagen is an anti-interpretation. “It is
wrong to think that the task of physics is to find out how nature is,”
Bohr said. “Physics concerns what we can say about nature.”
Nothing is definite in Bohr’s quantum world until someone observes it.
Physics can help us order experience but should not be expected to
provide a complete picture of reality. The popular four-word summary of
the Copenhagen interpretation is: “Shut up and calculate!”
For much of the 20th century, when quantum physicists were making giant
leaps in solid-state and high-energy physics, few of them bothered much
about foundations. But the philosophical difficulties were always there,
troubling those who cared to worry about them.
Becker sides with the worriers. He leads us through an impressive
account of the rise of competing interpretations, grounding them in the
human stories, which are naturally messy and full of contingencies. He
makes a convincing case that it’s wrong to imagine the Copenhagen
interpretation as a single official or even coherent statement. It is,
he suggests, a “strange assemblage of claims.”
An American physicist, David Bohm, devised a radical alternative at
midcentury, visualizing “pilot waves” that guide every particle, an
attempt to eliminate the wave-particle duality. For a long time, he was
mainly lambasted or ignored, but variants of the Bohmian interpretation
have supporters today. Other interpretations rely on “hidden variables”
to account for quantities presumed to exist behind the curtain. Perhaps
the most popular lately — certainly the most talked about — is the
“many-worlds interpretation”: Every quantum event is a fork in the road,
and one way to escape the difficulties is to imagine, mathematically
speaking, that each fork creates a new universe.
So in this view, Schrödinger’s cat is alive and well in one universe
while in another she goes to her doom. And we, too, should imagine
countless versions of ourselves. Everything that can happen does happen,
in one universe or another. “The universe is constantly splitting into a
stupendous number of branches,” said the theorist Bryce DeWitt, “every
quantum transition taking place on every star, in every galaxy, in every
remote corner of the universe is splitting our local world on earth into
myriads of copies of itself.”
This is ridiculous, of course. “A heavy load of metaphysical baggage,”
John Wheeler called it. How could we ever prove or disprove such a
theory? But if you think the many-worlds idea is easily dismissed,
plenty of physicists will beg to differ. They will tell you that it
could explain, for example, why quantum computers (which admittedly
don’t yet quite exist) could be so powerful: They would delegate the
work to their alter egos in other universes.
Is any of this real? At the risk of spoiling its suspense, I will tell
you that this book does not propose a definite answer to its title
question. You weren’t counting on one, were you? The story is far from
When scientists search for meaning in quantum physics, they may be
straying into a no-man’s-land between philosophy and religion. But they
can’t help themselves. They’re only human. “If you were to watch me by
day, you would see me sitting at my desk solving Schrödinger’s
equation...exactly like my colleagues,” says Sir Anthony Leggett, a
Nobel Prize winner and pioneer in superfluidity. “But occasionally at
night, when the full moon is bright, I do what in the physics community
is the intellectual equivalent of turning into a werewolf: I question
whether quantum mechanics is the complete and ultimate truth about the
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