[Paleopsych] NYT: Quantum Trickery: Testing Einstein's Strangest Theory
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Mon Jan 9 15:39:55 UTC 2006
Quantum Trickery: Testing Einstein's Strangest Theory
By DENNIS OVERBYE
Einstein said there would be days like this.
This fall scientists announced that they had put a half dozen
beryllium atoms into a "cat state."
No, they were not sprawled along a sunny windowsill. To a physicist, a
"cat state" is the condition of being two diametrically opposed
conditions at once, like black and white, up and down, or dead and
These atoms were each spinning clockwise and counterclockwise at the
same time. Moreover, like miniature Rockettes they were all doing
whatever it was they were doing together, in perfect synchrony. Should
one of them realize, like the cartoon character who runs off a cliff
and doesn't fall until he looks down, that it is in a metaphysically
untenable situation and decide to spin only one way, the rest would
instantly fall in line, whether they were across a test tube or across
The idea that measuring the properties of one particle could
instantaneously change the properties of another one (or a whole
bunch) far away is strange to say the least - almost as strange as the
notion of particles spinning in two directions at once. The team that
pulled off the beryllium feat, led by Dietrich Leibfried at the
National Institute of Standards and Technology, in Boulder, Colo.,
hailed it as another step toward computers that would use quantum
magic to perform calculations.
But it also served as another demonstration of how weird the world
really is according to the rules, known as quantum mechanics.
The joke is on Albert Einstein, who, back in 1935, dreamed up this
trick of synchronized atoms - "spooky action at a distance," as he
called it - as an example of the absurdity of quantum mechanics.
"No reasonable definition of reality could be expected to permit
this," he, Boris Podolsky and Nathan Rosen wrote in a paper in 1935.
Today that paper, written when Einstein was a relatively ancient 56
years old, is the most cited of Einstein's papers. But far from
demolishing quantum theory, that paper wound up as the cornerstone for
the new field of quantum information.
Nary a week goes by that does not bring news of another feat of
quantum trickery once only dreamed of in thought experiments:
particles (or at least all their properties) being teleported across
the room in a microscopic version of Star Trek beaming; electrical
"cat" currents that circle a loop in opposite directions at the same
time; more and more particles farther and farther apart bound together
in Einstein's spooky embrace now known as "entanglement." At the
University of California, Santa Barbara, researchers are planning an
experiment in which a small mirror will be in two places at once.
Niels Bohr, the Danish philosopher king of quantum theory, dismissed
any attempts to lift the quantum veil as meaningless, saying that
science was about the results of experiments, not ultimate reality.
But now that quantum weirdness is not confined to thought experiments,
physicists have begun arguing again about what this weirdness means,
whether the theory needs changing, and whether in fact there is any
This fall two Nobel laureates, Anthony Leggett of the University of
Illinois and Norman Ramsay of Harvard argued in front of several
hundred scientists at a conference in Berkeley about whether, in
effect, physicists were justified in trying to change quantum theory,
the most successful theory in the history of science. Dr. Leggett said
yes; Dr. Ramsay said no.
It has been, as Max Tegmark, a cosmologist at the Massachusetts
Institute of Technology, noted, "a 75-year war." It is typical in
reporting on this subject to bounce from one expert to another, each
one shaking his or her head about how the other one just doesn't get
it. "It's a kind of funny situation," N. David Mermin of Cornell, who
has called Einstein's spooky action "the closest thing we have to
magic," said, referring to the recent results. "These are extremely
difficult experiments that confirm elementary features of quantum
mechanics." It would be more spectacular news, he said, if they had
come out wrong.
Anton Zeilinger of the University of Vienna said that he thought, "The
world is not as real as we think.
"My personal opinion is that the world is even weirder than what
quantum physics tells us," he added.
The discussion is bringing renewed attention to Einstein's role as a
founder and critic of quantum theory, an "underground history," that
has largely been overlooked amid the celebrations of relativity in the
past Einstein year, according to David Z. Albert, a professor of
philosophy and physics at Columbia. Regarding the 1935 paper, Dr.
Albert said, "We know something about Einstein's genius we didn't know
The Silly Theory
From the day 100 years ago that he breathed life into quantum theory
by deducing that light behaved like a particle as well as like a wave,
Einstein never stopped warning that it was dangerous to the age-old
dream of an orderly universe.
If light was a particle, how did it know which way to go when it was
issued from an atom?
"The more success the quantum theory has, the sillier it seems,"
Einstein once wrote to friend.
The full extent of its silliness came in the 1920's when quantum
theory became quantum mechanics.
In this new view of the world, as encapsulated in a famous equation by
the Austrian Erwin Schrödinger, objects are represented by waves that
extend throughout space, containing all the possible outcomes of an
observation - here, there, up or down, dead or alive. The amplitude of
this wave is a measure of the probability that the object will
actually be found to be in one state or another, a suggestion that led
Einstein to grumble famously that God doesn't throw dice.
Worst of all from Einstein's point of view was the uncertainty
principle, enunciated by Werner Heisenberg in 1927.
Certain types of knowledge, of a particle's position and velocity, for
example, are incompatible: the more precisely you measure one
property, the blurrier and more uncertain the other becomes.
In the 1935 paper, Einstein and his colleagues, usually referred to as
E.P.R., argued that the uncertainty principle could not be the final
word about nature. There must be a deeper theory that looked behind
the quantum veil.
Imagine that a pair of electrons are shot out from the disintegration
of some other particle, like fragments from an explosion. By law
certain properties of these two fragments should be correlated. If one
goes left, the other goes right; if one spins clockwise, the other
That means, Einstein said, that by measuring the velocity of, say, the
left hand electron, we would know the velocity of the right hand
electron without ever touching it.
Conversely, by measuring the position of the left electron, we would
know the position of the right hand one.
Since neither of these operations would have involved touching or
disturbing the right hand electron in any way, Einstein, Podolsky and
Rosen argued that the right hand electron must have had those
properties of both velocity and position all along. That left only two
possibilities, they concluded. Either quantum mechanics was
"incomplete," or measuring the left hand particle somehow disturbed
the right hand one.
But the latter alternative violated common sense. Such an influence,
or disturbance, would have to travel faster than the speed of light.
"My physical instincts bristle at that suggestion," Einstein later
Bohr responded with a six-page essay in Physical Review that contained
but one simple equation, Heisenberg's uncertainty relation. In
essence, he said, it all depends on what you mean by "reality."
Enjoy the Magic
Most physicists agreed with Bohr, and they went off to use quantum
mechanics to build atomic bombs and reinvent the world.
The consensus was that Einstein was a stubborn old man who "didn't
get" quantum physics. All this began to change in 1964 when John S.
Bell, a particle physicist at the European Center for Nuclear Research
near Geneva, who had his own doubts about quantum theory, took up the
1935 E.P.R. argument. Somewhat to his dismay, Bell, who died in 1990,
wound up proving that no deeper theory could reproduce the predictions
of quantum mechanics. Bell went on to outline a simple set of
experiments that could settle the argument and decide who was right,
Einstein or Bohr.
When the experiments were finally performed in 1982, by Alain Aspect
and his colleagues at the University of Orsay in France, they agreed
with quantum mechanics and not reality as Einstein had always presumed
it should be. Apparently a particle in one place could be affected by
what you do somewhere else.
"That's really weird," Dr. Albert said, calling it "a profoundly deep
violation of an intuition that we've been walking with since caveman
Physicists and philosophers are still fighting about what this means.
Many of those who care to think about these issues (and many prefer
not to), concluded that Einstein's presumption of locality - the idea
that physically separated objects are really separate - is wrong.
Dr. Albert said, "The experiments show locality is false, end of
story." But for others, it is the notion of realism, that things exist
independent of being perceived, that must be scuttled. In fact,
physicists don't even seem to agree on the definitions of things like
"locality" and "realism."
"I would say we have to be careful saying what's real," Dr. Mermin
said. "Properties cannot be said to be there until they are revealed
by an actual experiment."
What everybody does seem to agree on is that the use of this effect is
limited. You can't use it to send a message, for example.
Leonard Susskind, a Stanford theoretical physicist, who called these
entanglement experiments "beautiful and surprising," said the term
"spooky action at a distance," was misleading because it implied the
instantaneous sending of signals. "No competent physicist thinks that
entanglement allows this kind of nonlocality."
Indeed the effects of spooky action, or "entanglement," as Schrödinger
called it, only show up in retrospect when the two participants in a
Bell-type experiment compare notes. Beforehand, neither has seen any
violation of business as usual; each sees the results of his
measurements of, say, whether a spinning particle is pointing up or
down, as random.
In short, as Brian Greene, the Columbia theorist wrote in "The Fabric
of the Cosmos," Einstein's special relativity, which sets the speed of
light as the cosmic speed limit, "survives by the skin of its teeth."
In an essay in 1985, Dr. Mermin said that "if there is spooky action
at a distance, then, like other spooks, it is absolutely useless
except for its effect, benign or otherwise, on our state of mind."
He added, "The E.P.R. experiment is as close to magic as any physical
phenomenon I know of, and magic should be enjoyed." In a recent
interview, he said he still stood by the latter part of that
statement. But while spooky action remained useless for sending a
direct message, it had turned out to have potential uses, he admitted,
in cryptography and quantum computing.
Nine Ways of Killing a Cat
Another debate, closely related to the issues of entanglement and
reality, concerns what happens at the magic moment when a particle is
measured or observed.
Before a measurement is made, so the traditional story goes, the
electron exists in a superposition of all possible answers, which can
combine, adding and interfering with one another.
Then, upon measurement, the wave function "collapses" to one
particular value. Schrödinger himself thought this was so absurd that
he dreamed up a counterexample. What is true for electrons, he said,
should be true as well for cats.
In his famous thought experiment, a cat is locked in a box where the
decay of a radioactive particle will cause the release of poison that
will kill it. If the particle has a 50-50 chance of decaying, then
according to quantum mechanics the cat is both alive and dead before
we look in the box, something the cat itself, not to mention cat
lovers, might take issue with.
But cats are always dead or alive, as Dr. Leggett of Illinois said in
his Berkeley talk. "The problem with quantum mechanics," he said in an
interview, "is how it explains definite outcomes to experiments."
If quantum mechanics is only about information and a way of predicting
the results of measurements, these questions don't matter, most
quantum physicists say.
"But," Dr. Leggett said, "if you take the view that the formalism is
reflecting something out there in real world, it matters immensely."
As a result, theorists have come up with a menu of alternative
interpretations and explanations. According to one popular notion,
known as decoherence, quantum waves are very fragile and collapse from
bumping into the environment. Another theory, by the late David Bohm,
restores determinism by postulating a "pilot wave" that acts behind
the scenes to guide particles.
In yet another theory, called "many worlds," the universe continually
branches so that every possibility is realized: the Red Sox win and
lose and it rains; Schrödinger's cat lives, dies, has kittens and
scratches her master when he tries to put her into the box.
Recently, as Dr. Leggett pointed out, some physicists have tinkered
with Schrödinger's equation, the source of much of the misery, itself.
A modification proposed by the Italian physicists Giancarlo Ghirardi
and Tullio Weber, both of the University of Trieste, and Alberto
Rimini of the University of Pavia, makes the wave function unstable so
that it will collapse in a time depending on how big a system it
In his standoff with Dr. Ramsay of Harvard last fall, Dr. Leggett
suggested that his colleagues should consider the merits of the latter
theory. "Why should we think of an electron as being in two states at
once but not a cat, when the theory is ostensibly the same in both
cases?" Dr. Leggett asked.
Dr. Ramsay said that Dr. Leggett had missed the point. How the wave
function mutates is not what you calculate. "What you calculate is the
prediction of a measurement," he said.
"If it's a cat, I can guarantee you will get that it's alive or dead,"
Dr. Ramsay said.
David Gross, a recent Nobel winner and director of the Kavli Institute
for Theoretical Physics in Santa Barbara, leapt into the free-for-all,
saying that 80 years had not been enough time for the new concepts to
sink in. "We're just too young. We should wait until 2200 when quantum
mechanics is taught in kindergarten."
The Joy of Randomness
One of the most extreme points of view belongs to Dr. Zeilinger of
Vienna, a bearded, avuncular physicist whose laboratory regularly
hosts every sort of quantum weirdness.
In an essay recently in Nature, Dr. Zeilinger sought to find meaning
in the very randomness that plagued Einstein.
"The discovery that individual events are irreducibly random is
probably one of the most significant findings of the 20th century,"
Dr. Zeilinger wrote.
Dr. Zeilinger suggested that reality and information are, in a deep
sense, indistinguishable, a concept that Dr. Wheeler, the Princeton
physicist, called "it from bit."
In information, the basic unit is the bit, but one bit, he says, is
not enough to specify both the spin and the trajectory of a particle.
So one quality remains unknown, irreducibly random.
As a result of the finiteness of information, he explained, the
universe is fundamentally unpredictable.
"I suggest that this randomness of the individual event is the
strongest indication we have of a reality 'out there' existing
independently of us," Dr. Zeilinger wrote in Nature.
He added, "Maybe Einstein would have liked this idea after all."
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