Copyright 2006 The New York Times Company

The New York Times
October 20, 2006 Friday

Late Edition - Final

Reprinted without permission.

Universe on a String

By Brian Greene.

Brian Greene, a professor of physics and mathematics at

Columbia, is the author of ''The Elegant Universe'' and ''The

Fabric of the Cosmos.''

SEVENTY-FIVE years ago this month, The New York Times reported that

Albert Einstein had completed his unified field theory - a theory of

nature's forces into a single, tightly woven mathematical

tapestry. But as had happened before and would happen again, closer

scrutiny revealed flaws that sent Einstein back to the drawing

board. Nevertheless, Einstein's belief that he'd one day complete the

unified theory rarely faltered. Even on his deathbed he scribbled

equations in the desperate but fading hope that the theory would

finally materialize. It didn't.

In the decades since, the urgency of finding a unified theory has only

increased. Scientists have realized that without such a theory,

critical questions can't be addressed, such as how the universe began

or what lies at the hart of a black hole. These unresolved issues have

inspired much progress, with the most recent advances coming from an

approach called string theory. Lately, however, string theory has come

in for conciderable criticism. And so, this is an auspicious moment to

reflect on the state of the art.

First , some context. For nearly 300 years, science has been on a path

of consolidation. In the 17th century, Isaac Newton discovered laws of

motion that apply equally to a planet moving through space and to an

apple falling earthward, revealing that the physics of the heavens

later, Michael Faraday and James Clerk produce magnetic fields, and

moving magnets can produce electric currents, establishing that these

two forces are as united as Midas' touch and gold. And in the 20th

century, Einstein's work proved that space, time and gravity are so

entwined that you can't speak sensibly about one without the others.

This striking pattern of convergence, linking concepts once thought

unrelated, inspired Einstein to dream of the next and possibly final

move: merging gravity and electromagnetism into a single, overarching

theory of nature 's forces.

In hindsight, there was almost no way he could have succeeded. He was

barely aware that there were two other forces he was neglecting - the

strong and weak forces acting within atomic nuclei. Furthermore, he

willfully ignored quantum mechanics, the new theory of the microworld

that was receiving voluminous experimental support, but whose

probabilistic framework struck him as deeply misguided. Einstein

stayed the course, but by his final years he had drifted to the fringe

of a subject he had once dominated.

After Einstein's death, the torch of unification passed to other

hands. In the 1960's, the Nobel Prize-winning works of Sheldon

Glashow, Abdus Salam and Steven Weinberg revealed that at high

energies, the electromagnetic and weak nuclear forces seamlessly

combine, much as heating a cold vat of chicken soup causes the

floating layer of fat to combine with the liquid below, yielding a

homogeneous broth. Subsequent work argued that at even higher energies

the strong nuclear force would also meld into the soup, proposed

consolidation that has yet to be confirmed experimentally, that there

is no fundamental obstacle to unifying three of nature's four forces.

For decades, however, the force of gravity stubbornly resisted joining

the fold. The problem was the very one that so troubled Einstein: the

disjunction between his own general relativity, most relevant for

extremely massive objects like stars and galaxies, and quantum

mechanics, the framework invoked by physics to deal with exceptionally

small objects like molecules and atoms and their constituents.

Time and again, attempts to merge the two theories resulted in

ill-defined mathematics, much like what happens on a calculator if you

try to divide one by zero. The display will flash an error message,

reprimanding you for misusing mathematics. The combined equations of

general relativity and quantum mechanics yield similar problems. While

the conflict rears its head only in environments that are both

extremely massive and exceptionally tiny - black holes and the Big

Bank being two primary examples - it tells of a fissure in the very

foundations of physics.

Such was the case until the mid-1980's, when a new approach, string

theory, burst onto the stage. Difficult and complex calculations by

the physicists John Schwarz and Michael Green, who had been toiling

for years in scientific obscurity, gave compelling evidence that this

new approach not only unified gravity and quantum mechanics, as well

as nature's other forces, but did so while sweeping aside previous

mathematical problems. As word of the breakthrough spread, many

physicists dropped what they were working on and joined a global

effort to realize Einstein's unified vision of the cosmos.

String theory offers a new perspective on matter's fundamental

constituents. Once viewed as point-like dots virtually no size,

particles in string theory are minuscule, vibrating, string-like

filaments. And much as different vibrations of a violin string produce

different musical notes, different vibrations of the theory's strings

produce different kinds of particles. And electron is a tiny string

vibrating in one pattern, a quark is a string vibrating in a different

pattern. Particles like the photon that convey nature's forces in the

quantum realm are strings vibrating in yet other patterns.

Crucially, the early pioneers of string theory realized that one such

vibration would produce the gravitational force, demonstrating that

string theory embraces both gravity and quantum mechanics. In sharp

contrast to previous proposals that cobbled gravity and quantum

mechanics uneasily together, their unity here emerges from the theory

itself.

While accessibility demands that I describe these developments using

familiar words, beneath them lies a bedrock of rigorous analysis. We

now have more then 20 years of painstaking research, filling tens of

thousands of published pages of calculations, which attest to string

theory's deep mathematical coherence. These calculations have given

the theory countless opportunities to suffer the fate of previous

proposals, but the fact is that every calculation that has ever been

completed within string theory is free from mathematical

contradictions.

Moreover, these works have also shown that many of the prized

breakthroughs in fundamental physics, discovered over the past two

centuries through arduous research using a wide range of approaches,

can be found within string theory. It 's as if one composer, working

in isolation, produced the greatest hits of Beethoven, Count Basie and

the Beatles. When you also consider that string theory has opened new

areas of mathematical research, you can easily understand why it's

captured the attention of so many leading scientists and

mathematicians.

Nevertheless, mathematical rigor and elegance are not sufficient to

demonstrate a theory's relevance. To be judged a correct description

of the universe, a theory must make predictions that are confirmed by

experiment. And as a small but vocal group of critics of string theory

justly emphasize, string theory has yet to do so. This is a key point,

so it's worth serious scrutiny.

We understand string theory much better now than we did 20 years

ago. We've developed powerful techniques of mathematical analysis that

have improved the accuracy of its calculations and provided invaluable

insights into the theory's logical structure. Even so, researchers

worldwide are still working toward an exact and tractable formulation

of the theory's equations. And without that final formulation in hand,

the kind of detailed a definitive predictions that would subject the

theory to comprehensive experimental vettingremain beyond our reach.

There are, however, features of the theory that maybe open to

examination even with our incomplete understanding. We may be able to

test the theory's predictions of particular new particle species, of

dimensions of space beyond the three we can directly see, and even its

prediction that microscopic black holes may be produced through highly

energetic particle collisions. Without the exact quotations, our

ability to describe these attributes with precision is limited, but

the theory gives enough direction for the Large Hadron Collider, a

gigantic particle accelerator now being built in Geneva and scheduled

to begin full operation in 2008, to search for supporting evidence by

the end of the decade.

Research has also revealed a possibility that signatures of string

theory are imprinted in the radiation left over from the Big Bang, as

well as in gravitational waves rippling through space-time's

fabric. In the coming years, a variety of fabric. In the coming years,

a variety of experiments will seek such evidence with unprecedented

observational fidelity. And in a recent, particularly intriguing

development, data now emerging from the Relativistic Heavy Ion

Collider at the Brookhaven National Laboratory appear to be more

accurately described using string theory methods than with more

traditional approaches.

To be sure, no one successful experiment would establish that string

theory is right, but neither would the failure of all such experiments

prove the theory wrong. If the accelerator experiments fail to turn up

anything, it could be that we need more powerful machine; if the

astronomical observations fail to turn up anything, it could mean the

effects are too small to be seen. The bottom line is that it's hard to

test a theory that not only taxes the capacity of today's technology

but is also still very much under development.

Some critics have taken this lack of definitive predictions to mean

that string theory is a protean concept whose advocates seek to step

outside the established scientific method. Nothing could be further

from the truth. Certainly, we are feeling our way through a complex

mathematical terrain, and no doubt have much ground yet to cover. But

we will hold string theory to the usual scientific standard: to be

accepted, it must make predictions that are verified.

Other detractors have seized on recent work suggesting that one of

string theory's goals beyond unification of the forces - to provide an

explanation for the values of nature's constants, like the mass of the

electron and the strength of gravity - may be unreachable (because the

the4ory may be compatible with those constants having arrange of

values). But even if this were to prove true, realizing Einstein's

unified vision would surely be prize enough.

Finally, some have argued that if, after decades of research involving

thousands of scientists, the theory is still a work in progress, it's

time to give up. But to suggest dropping research on the most

promising approach to unification because the work has failed to meet

an arbitrary time-table for complete success is, well, silly.

I have worked on string theory for more than 20 years because I

believe it provides the most powerful framework for constructing the

long-sought unified theory. Nonetheless, should an inconsistency be

found, or should future studies reveal an insuperable barrier to

making contact with experimental data, or should new discoveries

reveal a superior approach, I'd change my research focus, and I have

little doubt that most string theorists would too.

But this hasn't happened.

String theory continues to offer profound breadth and enormous

potential. It has the capacity to complete the Einsteinian revolution

and could very well be the concluding chapter in our species' age-old

quest to understand the deepest workings of the cosmos.

Will we ever reach that goal? I don't know. But that's both the wonder

and the angst of a life in science. Exploring the unknown requires

tolerating uncertainty.