In October 1984 I arrived at
Oxford University, trailing a large steamer trunk containing a couple
of changes of clothing and about five dozen textbooks. I had a freshly
minted bachelor’s degree in physics from Harvard, and I was raring to
launch into graduate study. But within a couple of weeks, the more
advanced students had sucked the wind from my sails. Change fields now
while you still can, many said. There’s nothing happening in fundamental
physics.
Then, just a couple of months later, the prestigious (if tamely titled) journal Physics Letters B published
an article that ignited the first superstring revolution, a sweeping
movement that inspired thousands of physicists worldwide to drop their
research in progress and chase Einstein’s long-sought dream of a unified
theory. The field was young, the terrain fertile and the atmosphere
electric. The only thing I needed to drop was a neophyte’s inhibition to
run with the world’s leading physicists. I did. What followed proved to
be the most exciting intellectual odyssey of my life.
That was 30 years ago this month, making the moment
ripe for taking stock: Is string theory revealing reality’s deep laws?
Or, as some detractors have claimed, is it a mathematical mirage that
has sidetracked a generation of physicists?
Unification has become synonymous
with Einstein, but the enterprise has been at the heart of modern
physics for centuries. Isaac Newton united the heavens and Earth,
revealing that the same laws governing the motion of the planets and the
Moon described the trajectory of a spinning wheel and a rolling rock.
About 200 years later, James Clerk Maxwell took the unification baton
for the next leg, showing that electricity and magnetism are two aspects
of a single force described by a single mathematical formalism.
The next two steps, big ones at that, were indeed
vintage Einstein. In 1905, Einstein linked space and time, showing that
motion through one affects passage through the other, the hallmark of
his special theory of relativity. Ten years later, Einstein extended
these insights with his general theory of relativity, providing the most
refined description of gravity, the force governing the likes of stars
and galaxies. With these achievements, Einstein envisioned that a grand
synthesis of all of nature’s forces was within reach.
But by 1930, the landscape
of physics had thoroughly shifted. Niels Bohr and a generation of
intrepid explorers ventured deep into the microrealm, where they
encountered quantum mechanics, an enigmatic theory formulated with
radically new physical concepts and mathematical rules. While
spectacularly successful at predicting the behavior of atoms and
subatomic particles, the quantum laws looked askance at Einstein’s
formulation of gravity. This set the stage for more than a half-century
of despair as physicists valiantly struggled, but repeatedly failed, to
meld general relativity and quantum mechanics, the laws of the large and
small, into a single all-encompassing description.
Such was the case until December 1984, when John
Schwarz, of the California Institute of Technology, and Michael Green,
then at Queen Mary College, published a once-in-a-generation paper
showing that string theory could overcome the mathematical antagonism
between general relativity and quantum mechanics, clearing a path that
seemed destined to reach the unified theory.
The idea underlying string unification is as simple as
it is seductive. Since the early 20th century, nature’s fundamental
constituents have been modeled as indivisible particles—the most
familiar being electrons, quarks and neutrinos—that can be pictured as
infinitesimal dots devoid of internal machinery. String theory
challenges this by proposing that at the heart of every particle is a
tiny, vibrating string-like filament. And, according to the theory, the
differences between one particle and another—their masses, electric
charges and, more esoterically, their spin and nuclear properties—all
arise from differences in how their internal strings vibrate.
Much as the sonorous tones of a cello arise from the
vibrations of the instrument’s strings, the collection of nature’s
particles would arise from the vibrations of the tiny filaments
described by string theory. The long list of disparate particles that
had been revealed over a century of experiments would be recast as
harmonious “notes” comprising nature’s score.
Most gratifying, the mathematics revealed that one of
these notes had properties precisely matching those of the “graviton,” a
hypothetical particle that, according to quantum physics, should carry
the force of gravity from one location to another. With this, the
worldwide community of theoretical physicists looked up from their
calculations. For the first time, gravity and quantum mechanics were
playing by the same rules. At least in theory.
I began learning
the mathematical underpinnings of string theory during an intense
period in the spring and summer of 1985. I wasn’t alone. Graduate
students and seasoned faculty alike got swept up in the potential of
string theory to be what some were calling the “final theory” or the
“theory of everything.” In crowded seminar rooms and flyby corridor
conversations, physicists anticipated the crowning of a new order.
But the simplest and most important question loomed
large. Is string theory right? Does the math explain our universe? The
description I’ve given suggests an experimental strategy. Examine
particles and if you see little vibrating strings, you’re done. It’s a
fine idea in principle, but string theory’s pioneers realized it was
useless in practice. The math set the size of strings to be about a
million billion times smaller than even the minute realms probed by the
world’s most powerful accelerators. Save for building a collider the
size of the galaxy, strings, if they’re real, would elude brute force
detection.
Making the situation seemingly more dire, researchers
had come upon a remarkable but puzzling mathematical fact. String
theory’s equations require that the universe has extra dimensions beyond
the three of everyday experience—left/right, back/forth and up/down.
Taking the math to heart, researchers realized that their backs were to
the wall. Make sense of extra dimensions—a prediction that’s grossly at
odds with what we perceive—or discard the theory.
String theorists pounced on an idea
first developed in the early years of the 20th century. Back then,
theorists realized that there might be two kinds of spatial dimensions:
those that are large and extended, which we directly experience, and
others that are tiny and tightly wound, too small for even our most
refined equipment to reveal. Much as the spatial extent of an enormous
carpet is manifest, but you have to get down on your hands and knees to
see the circular loops making up its pile, the universe might have three
big dimensions that we all navigate freely, but it might also have
additional dimensions so minuscule that they’re beyond our observational
reach.
In a paper submitted for publication a day after New
Year’s 1985, a quartet of physicists—Philip Candelas, Gary Horowitz,
Andrew Strominger and Edward Witten—pushed this proposal one step
further, turning vice to virtue. Positing that the extra dimensions were
minuscule, they argued, would not only explain why we haven’t seen
them, but could also provide the missing bridge to experimental
verification.
Strings are so small that when they
vibrate they undulate not just in the three large dimensions, but also
in the additional tiny ones. And much as the vibrational patterns of air
streaming through a French horn are determined by the twists and turns
of the instrument, the vibrational patterns of strings would be
determined by the shape of the extra dimensions. Since these vibrational
patterns determine particle properties like mass, electric charge and
so on—properties that can be detected experimentally—the quartet had
established that if you know the precise geometry of the extra
dimensions, you can make predictions about the results that certain
experiments would observe.
For me, deciphering the paper’s equations was one of
those rare mathematical forays bordering on spiritual enlightenment.
That the geometry of hidden spatial dimensions might be the universe’s
Rosetta stone, embodying the secret code of nature’s fundamental
constituents—well, it was one of the most beautiful ideas I’d ever
encountered. It also played to my strength. As a mathematically oriented
physics student, I’d already expended great effort studying topology
and differential geometry, the very tools needed to analyze the
mathematical form of extra-dimensional spaces.
And so, in the mid-1980s, with a small group of
researchers at Oxford, we set our sights on extracting string theory’s
predictions. The quartet’s paper had delineated the category of
extra-dimensional spaces allowed by the mathematics of string theory
and, remarkably, only a handful of candidate shapes were known. We
selected one that seemed most promising, and embarked on grueling days
and sleepless nights, filled with arduous calculations in higher
dimensional geometry and fueled by grandiose thoughts of revealing
nature’s deepest workings.
The final results that we
found successfully incorporated various established features of particle
physics and so were worthy of attention (and, for me, a doctoral
dissertation), but were far from providing evidence for string theory.
Naturally, our group and many others turned back to the list of allowed
shapes to consider other possibilities. But the list was no longer
short. Over the months and years, researchers had discovered ever larger
collections of shapes that passed mathematical muster, driving the
number of candidates into the thousands, millions, billions and then,
with insights spearheaded in the mid-1990s by Joe Polchinski, into
numbers so large that they’ve never been named.
Against this embarrassment of riches, string theory
offered no directive regarding which shape to pick. And as each shape
would affect string vibrations in different ways, each would yield
different observable consequences. The dream of extracting unique
predictions from string theory rapidly faded.
From a public relations standpoint, string theorists
had not prepared for this development. Like the Olympic athlete who
promises eight gold medals but wins “only” five, theorists had
consistently set the bar as high as it could go. That string theory
unites general relativity and quantum mechanics is a profound success.
That it does so in a framework with the capacity to embrace the known
particles and forces makes the success more than theoretically relevant.
Seeking to go even further and uniquely explain the detailed properties
of the particles and forces is surely a noble goal, but one that lies
well beyond the line dividing success from failure.
Nevertheless, critics who had bristled at string
theory’s meteoric rise to dominance used the opportunity to trumpet the
theory’s demise, blurring researchers’ honest disappointment of not
reaching hallowed ground with an unfounded assertion that the approach
had crashed. The cacophony grew louder still with a controversial turn
articulated most forcefully by one of the founding fathers of string
theory, the Stanford University theoretical physicist Leonard Susskind.
In August 2003,
I was sitting with Susskind at a conference in Sigtuna, Sweden,
discussing whether he really believed the new perspective he’d been
expounding or was just trying to shake things up. “I do like to stir the
pot,” he told me in hushed tones, feigning confidence, “but I do think
this is what string theory’s been telling us.”
Susskind was arguing that if the mathematics does not
identify one particular shape as the right one for the extra dimensions,
perhaps there isn’t a single right shape. That is, maybe all of the
shapes are right shapes in the sense that there are many universes, each
with a different shape for the extra dimensions.
Our universe would then be just one of a vast
collection, each with detailed features determined by the shape of their
extra dimensions. Why, then, are we in this universe instead of any
other? Because the shape of the hidden dimensions yields the spectrum of
physical features that allow us to exist. In another universe, for
example, the different shape might make the electron a little heavier or
the nuclear force a little weaker, shifts that would cause the quantum
processes that power stars, including our sun, to halt, interrupting the
relentless march toward life on Earth.
Radical though this proposal may be, it was supported
by parallel developments in cosmological thinking that suggested that
the Big Bang may not have been a unique event, but was instead one of
innumerable bangs spawning innumerable expanding universes, called the
multiverse. Susskind was suggesting that string theory augments this
grand cosmological unfolding by adorning each of the universes in the
multiverse with a different shape for the extra dimensions.
With or without string theory, the multiverse is a
highly controversial schema, and deservedly so. It not only recasts the
landscape of reality, but shifts the scientific goal posts. Questions
once deemed profoundly puzzling—why do nature’s numbers, from particle
masses to force strengths to the energy suffusing space, have the
particular values they do?—would be answered with a shrug. The detailed
features we observe would no longer be universal truths; instead, they’d
be local bylaws dictated by the particular shape of the extra
dimensions in our corner of the multiverse.
Most physicists, string theorists among them, agree
that the multiverse is an option of last resort. Yet, the history of
science has also convinced us to not dismiss ideas merely because they
run counter to expectation. If we had, our most successful theory,
quantum mechanics, which describes a reality governed by wholly peculiar
waves of probability, would be buried in the trash bin of physics. As
Nobel laureate Steven Weinberg has said, the universe doesn’t care about
what makes theoretical physicists happy.
This spring,
after nearly two years of upgrades, the Large Hadron Collider will
crackle back to life, smashing protons together with almost twice the
energy achieved in its previous runs. Sifting through the debris with
the most complex detectors ever built, researchers will be looking for
evidence of anything that doesn’t fit within the battle-tested “Standard
Model of particle physics,” whose final prediction, the Higgs boson,
was confirmed just before the machine went on hiatus. While it is likely
that the revamped machine is still far too weak to see strings
themselves, it could provide clues pointing in the direction of string
theory.
Many researchers have pinned their hopes on finding a
new class of so-called “supersymmetric” particles that emerge from
string theory’s highly ordered mathematical equations. Other collider
signals could show hints of extra-spatial dimensions, or even evidence
of microscopic black holes, a possibility that arises from string
theory’s exotic treatment of gravity on tiny distance scales.
While none of these predictions can properly be called
a smoking gun—various non-stringy theories have incorporated them too—a
positive identification would be on par with the discovery of the Higgs
particle, and would, to put it mildly, set the world of physics on
fire. The scales would tilt toward string theory.
But what happens in the event—likely, according to some—that the collider yields no remotely stringy signatures?
Experimental evidence is the final
arbiter of right and wrong, but a theory’s value is also assessed by the
depth of influence it has on allied fields. By this measure, string
theory is off the charts. Decades of analysis filling thousands of
articles have had a dramatic impact on a broad swath of research cutting
across physics and mathematics. Take black holes, for example. String
theory has resolved a vexing puzzle by identifying the microscopic
carriers of their internal disorder, a feature discovered in the 1970s
by Stephen Hawking.
Looking back, I’m gratified at how far we’ve come but
disappointed that a connection to experiment continues to elude us.
While my own research has migrated from highly mathematical forays into
extra-dimensional arcana to more applied studies of string theory’s
cosmological insights, I now hold only modest hope that the theory will
confront data during my lifetime.
Even so, string theory’s pull remains strong. Its
ability to seamlessly meld general relativity and quantum mechanics
remains a primary achievement, but the allure goes deeper still. Within
its majestic mathematical structure, a diligent researcher would find
all of the best ideas physicists have carefully developed over the past
few hundred years. It’s hard to believe such depth of insight is
accidental.
I like to think that Einstein would
look at string theory’s journey and smile, enjoying the theory’s
remarkable geometrical features while feeling kinship with fellow
travelers on the long and winding road toward unification. All the same,
science is powerfully self-correcting. Should decades drift by without
experimental support, I imagine that string theory will be absorbed by
other areas of science and mathematics, and slowly shed a unique
identity. In the interim, vigorous research and a large dose of patience
are surely warranted. If experimental confirmation of string theory is
in the offing, future generations will look back on our era as
transformative, a time when science had the fortitude to nurture a
remarkable and challenging theory, resulting in one of the most profound
steps toward understanding reality.
Source: http://www.smithsonianmag.com/science-nature/string-theory-about-unravel-180953637/?all
