| From the New York Times...
By BRIAN GREENE
 UST
about a hundred years ago, Albert Einstein began writing a paper that
secured his place in the pantheon of humankind's greatest thinkers.
With his discovery of special relativity, Einstein upended the
familiar, thousands-year-old conception of space and time. To be sure,
even a century later, not everyone has fully embraced Einstein's
discovery. Nevertheless, say "Einstein" and most everyone thinks
"relativity."
What is less widely appreciated, however, is that physicists call
1905 Einstein's "miracle year" not because of the discovery of
relativity alone, but because in that year Einstein achieved the
unimaginable, writing four papers that each resulted in deep and
formative changes to our understanding of the universe. One of these
papers - not on relativity - garnered him the 1921 Nobel Prize in
physics. It also began a transformation in physics that Einstein found
so disquieting that he spent the last 30 years of his life in a
determined effort to repudiate it.
Two of the four 1905 papers were indeed on relativity. The first,
completed in June, laid out the foundations of his new view of space
and time, showing that distances and durations are not absolute, as
everyone since Newton had thought, but instead are affected by one's
motion. Clocks moving relative to one another tick off time at
different rates; yardsticks moving relative to one another measure
different lengths. You don't perceive this because the speeds of
everyday life are too slow for the effects to be noticeable. If you
could move near the speed of light, the effects would be obvious.
The second relativity paper, completed in September, is a three-page
addendum to the first, which derived his most famous result, E = mc2,
an equation as short as it is powerful. It told the world that matter
can be converted into energy - and a lot of it - since the speed of
light squared (c2) is a huge number. We've witnessed this equation's
consequences in the devastating might of nuclear weapons and the
tantalizing promise of nuclear energy.
The third paper, completed in May, conclusively established the
existence of atoms - an idea discussed in various forms for millenniums
- by showing that the numerous microscopic collisions they'd generate
would account for the observed, though previously unexplained, jittery
motion of impurities suspended in liquids.
With these three papers, our view of space, time and matter was permanently changed.
Yet, it is the remaining 1905 paper, written in March, whose legacy
is arguably the most profound. In this work, Einstein went against the
grain of conventional wisdom and argued that light, at its most
elementary level, is not a wave, as everyone had thought, but actually
a stream of tiny packets or bundles of energy that have since come to
be known as photons.
This might sound like a largely technical advance, updating one
description of light to another. But through subsequent research that
amplified and extended Einstein's argument (see Figures 1 through 3), scientists revealed a mathematically precise and thoroughly startling picture of reality called quantum mechanics.
Before the discovery of quantum mechanics, the framework of physics
was this: If you tell me how things are now, I can then use the laws of
physics to calculate, and hence predict, how things will be later. You
tell me the velocity of a baseball as it leaves Derek Jeter's bat, and
I can use the laws of physics to calculate where it will land a handful
of seconds later. You tell me the height of a building from which a
flowerpot has fallen, and I can use the laws of physics to calculate
the speed of impact when it hits the ground. You tell me the positions
of the Earth and the Moon, and I can use the laws of physics to
calculate the date of the first solar eclipse in the 25th century.
What's important is that in these and all other examples, the accuracy
of my predictions depends solely on the accuracy of the information you
give me. Even laws that differ substantially in detail - from the
classical laws of Newton to the relativistic laws of Einstein - fit
squarely within this framework.
Quantum mechanics does not merely challenge the previous laws of
physics. Quantum mechanics challenges this centuries-old framework of
physics itself. According to quantum mechanics, physics cannot make
definite predictions. Instead, even if you give me the most precise
description possible of how things are now, we learn from quantum
mechanics that the most physics can do is predict the probability that
things will turn out one way, or another, or another way still.
The reason we have for so long been unaware that the universe
evolves probabilistically is that for the relatively large, everyday
objects we typically encounter - baseballs, flowerpots, the Moon -
quantum mechanics shows that the probabilities become highly skewed,
hugely favoring one outcome and effectively suppressing all others. A
typical quantum calculation reveals that if you tell me the velocity of
something as large as a baseball, there is more than a
99.99999999999999 (or so) percent likelihood that it will land at the
location I can figure out using the laws of Newton or, for even better
accuracy, the laws of Einstein. With such a skewed probability, the
quantum reasoning goes, we have long overlooked the tiny chance that
the baseball can (and, on extraordinarily rare occasions, will) land
somewhere completely different.
When it comes to small objects like molecules, atoms and subatomic
particles, though, the quantum probabilities are typically not skewed.
For the motion of an electron zipping around the nucleus of an atom,
for example, a quantum calculation lays out odds that are all roughly
comparable that the electron will be in a variety of different
locations - a 13 percent chance, say, that the electron will be here, a
19 percent chance that it will be there, an 11 percent chance that it
will be in a third place, and so on. Crucially, these predictions can
be tested. Take an enormous sample of identically prepared atoms,
measure the electron's position in each, and tally up the number of
times you find the electron at one location or another. According to
the pre-quantum framework, identical starting conditions should yield
identical outcomes; we should find the electron to be at the same place
in each measurement. But if quantum mechanics is right, in 13 percent
of our measurements we should find the electron here, in 19 percent we
should find it there, in 11 percent we should find it in that third
place. And, to fantastic precision, we do.
Faced with a mountain of supporting data, Einstein couldn't argue
with the success of quantum mechanics. But to him, even though his own
Nobel Prize-winning work was a catalyst for the quantum revolution, the
theory was anathema. Commentators over the decades have focused on
Einstein's refusal to accept the probabilistic framework of quantum
mechanics, a position summarized in his frequent comment that "God does
not play dice with the universe." Einstein, radical thinker that he
was, still believed in the sanctity of a universe that evolved in a
fully definite, fully predictable manner. If, as quantum mechanics
asserted, the best you can ever do is predict probabilities, Einstein
countered that he'd "rather be a cobbler, or even an employee in a
gaming house, than a physicist."
This emphasis, however, partly obscures a larger point. It wasn't
the mere reliance on probabilistic predictions that so troubled
Einstein. Unlike many of his colleagues, Einstein believed that a
fundamental physical theory was much more than the sum total of its
predictions - it was a mathematical reflection of an underlying
reality. And the reality entailed by quantum mechanics was a reality
Einstein couldn't accept.
An example: Imagine you shoot an electron from here and a few
seconds later it's detected by your equipment over there. What path did
the electron follow during the passage from you to the detector? The
answer according to quantum mechanics? There is no answer. The very
idea that an electron, or a photon, or any other particle, travels
along a single, definite trajectory from here to there is a quaint
version of reality that quantum mechanics declares outmoded.
Instead, the proponents of quantum theory claimed, reality consists
of a haze of all possibilities - all trajectories - mutually
commingling and simultaneously unfolding. And why don't we see this?
According to the quantum doctrine, when we make a measurement or
perform an observation, we force the myriad possibilities to ante up,
snap out of the haze and settle on a single outcome. But between
observations - when we are not looking - reality consists entirely of
jostling possibilities.
Quantum reality, in other words, remains ambiguous until measured.
The reality of common perception is thus merely a definitive-looking
veneer obscuring the internal workings of a highly uncertain cosmos.
Which is where Einstein drew a line in the sand. A universe of this
sort offended him; he could not accept, as he put it, that "the Old
One" would so profoundly incorporate a hidden element of happenstance
in the nature of reality. Einstein quipped to his quantum colleagues,
"Do you really think the Moon is not there when you're not looking?"
and set himself the Herculean task of reworking the laws of physics to
resurrect conventional reality.
Einstein waged a two-front assault on the problem. He sought an
internal chink in the quantum framework that would establish it as a
mere steppingstone on the path to a deeper and more complete
description of the universe. At the same time, he sought a grander
synthesis of nature's laws - what he called a "unified theory" - that
he believed would reveal the probabilities of quantum mechanics to be
no more profound than the probabilities offered in weather forecasts,
probabilities that simply reflect an incomplete knowledge of an
underlying, definite reality.
In 1935, through a disarmingly simple mathematical analysis,
Einstein (with two colleagues) established a beachhead on the first
front. He proved that quantum mechanics is either an incomplete theory
or, if it is complete, the universe is - in Einstein's words -
"spooky." Why "spooky?" Because the theory would allow certain widely
separated particles to correlate their behaviors perfectly (somewhat as
if a pair of widely separated dice would always come up the same number
when tossed at distant casinos). Since such "spooky" behavior would
border on nuttiness, Einstein thought he'd made clear that quantum
theory couldn't yet be considered a complete description of reality.
The nimble quantum proponents, however, would have nothing of it.
They insisted that quantum theory made predictions - albeit statistical
predictions - that were consistently born out by experiment. By the
precepts of the scientific method, they argued, the theory was
established. They maintained that searching beyond the theory's
predictions for a glimpse of a reality behind the quantum equations
betrayed a foolhardy intellectual greediness.
Nevertheless, for the remaining decades of his life, Einstein could
not give up the quest, exclaiming at one point, "I have thought a
hundred times more about quantum problems than I have about
relativity." He turned exclusively to his second line of attack and
became absorbed with the prospect of finding the unified theory, a
preoccupation that resulted in his losing touch with mainstream
physics. By the 1940's, the once dapper young iconoclast had grown into
a wizened old man of science who was widely viewed as a revolutionary
thinker of a bygone era.
By the early 1950's, Einstein realized he was losing the battle. But
the memories of his earlier success with relativity - "the years of
anxious searching in the dark, with their intense longing, their
alternations of confidence and exhaustion and the final emergence into
the light" - urged him onward. Maybe the intense light of discovery
that had so brilliantly illuminated his path as a young man would shine
once again. While lying in a bed in Princeton Hospital in mid-April
1955, Einstein asked for the pad of paper on which he had been
scribbling equations in the desperate hope that in his final hours the
truth would come to him. It didn't.
Was Einstein misguided? Must we accept that there is a fuzzy,
probabilistic quantum arena lying just beneath the definitive
experiences of everyday reality? As of today, we still don't have a
final answer. Fifty years after Einstein's death, however, the scales
have certainly tipped farther in this direction.
Decades of painstaking experimentation have confirmed quantum
theory's predictions beyond the slightest doubt. Moreover, in a
shocking scientific twist, some of the more recent of these experiments
have shown that Einstein's "spooky" processes do in fact take place
(particles many miles apart have been shown capable of correlating
their behavior). It's a stunning finding, and one that reaffirms
Einstein's uncanny ability to unearth features of nature so
mind-boggling that even he couldn't accept what he'd found. Finally,
there has been tremendous progress over the last 20 years toward a
unified theory with the discovery and development of superstring
theory. So far, though, superstring theory embraces quantum theory
without change, and has thus not revealed the definitive reality
Einstein so passionately sought.
With the passage of time and quantum mechanics' unassailable
successes, debate about the theory's meaning has quieted. The majority
of physicists have simply stopped worrying about quantum mechanics'
meaning, even as they employ its mathematics to make the most precise
predictions in the history of science. Others prefer reformulations of
quantum mechanics that claim to restore some features of conventional
reality at the expense of additional - and, some have argued, more
troubling - deviations (like the notion that there are parallel
universes). Yet others investigate hypothesized modifications to the
theory's equations that don't spoil its successful predictions but try
to bring it closer to common experience.
Over the 25 years since I first learned quantum mechanics, I've at
various times subscribed to each of these perspectives. My shifting
attitude, however, reflects that I'm still unsettled. Were Einstein to
interrogate me today about quantum reality, I'd have to admit that deep
inside I harbor many of the doubts that gnawed at him for decades. Can
it really be that the solid world of experience and perception, in
which a single, definite reality appears to unfold with dependable
certainty, rests on the shifting sands of quantum probabilities?
Well, yes. Probably. The evidence is compelling and tangible.
Although we have yet to fully lay bare quantum mechanics' grand lesson
for the underlying nature of the universe, I like to think even
Einstein would be impressed that in the 50 years since his death our
facility with quantum mechanics has matured from a mathematical
understanding of the subatomic realm to precision control. Today's
technological wizardry (computers, M.R.I.'s, smart bombs) exists only
because research in applied quantum physics has resulted in techniques
for manipulating the motion of electrons - probabilities and all -
through mazes of ultramicroscopic circuitry. Advances hovering on the
horizon, like nanoscience and quantum computers, offer the promise of
even more spectacular transformations.
So the next time you use your cellphone or laptop, pause for a
moment. Recognize that even these commonplace devices rely on our
greatest, yet most puzzling, scientific achievement and - as things now
stand - tap into humankind's most supreme assault on the idea that
reality is what we think it is.
Brian Greene, a professor of physics and
mathematics at Columbia, is the author of
“The Elegant Universe,’’ and, most recently,
“The Fabric of the Cosmos.”
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