Albert Einstein bestowed some marvelous gifts on the physicists of the world — a new understanding of space and time, an explanation of gravity, a way to prove the existence of atoms, and profound insights into the nature of matter and energy. But he also bequeathed them a major headache, in the form of the dream of finding a “unified field theory” that would join the math describing gravity with that describing electromagnetism, thereby revealing a deep unity in the laws governing the cosmos. In its modern form, that goal has become a theory unifying all of nature's basic particles and forces. Einstein's own quest failed. And so, two new books contend, has its modern counterpart, superstring theory.

After years of neglect by most physicists, superstring theory (string theory for short) emerged in 1984 as a leading candidate to solve the especially acute problem of reconciling general relativity — Einstein's theory of gravity — with quantum mechanics, the math describing the micro-realm of atoms. It posits the conceptually innocent but mathematically sophisticated idea that basic units of matter and force are more like tiny vibrating rubber bands than like the point-size tiny marbles envisioned by traditional particle physics. Math describing these vibrating “strings” incorporates gravity naturally, offering hope that string theory could realize Einstein's ambition. But string theory has its own problems: it cannot yet claim success in explaining any of nature's specific features, and does not even exist as a complete theory. Instead, it can be written in various dissimilar forms, though relationships among these different versions suggest the existence of a mysterious, deeper theory — known as M theory — that encompasses all proposed string theories.

So physicists now possess only fragments of the ultimate theory, pieces of a puzzle with no picture on the box to guide their assembly. String theorists generally say that is why they need to keep working on it. But in their new books, Peter Woit and Lee Smolin say enough is enough.

Woit, a lecturer in mathematics at Columbia University , dismisses string theory altogether, calling it a “failed program” and “seriously wrongheaded.” Smolin, a theoretical physicist at the Perimeter Institute near Toronto, complains that it “is believed by some of its adherents with a certainty that seems emotional rather than rational.” String theory's supporters, Smolin writes, have a “tendency to exaggerate results and minimize difficulties.” But his and Woit's own critiques exaggerate string theory's problems and minimize its results.

Woit does raise several technical concerns — well-studied by string theorists — with a principle known as supersymmetry, a component of superstring theory that predicts the existence of a new, and so-far undiscovered, set of subatomic particles. And he quotes interviews from secondary sources, many dating from the 1980's, reciting disapprovals of string theory. Woit's own main charge is that string theory makes no testable predictions, thus flouting the need for “falsifiability” articulated by the philosopher Karl Popper. If there's no way to prove string theory wrong, it fails to meet Popper's criterion, argues Woit. The string approach remains popular, Woit alleges, only because Edward Witten, one of the intellectual leaders of the field, has mesmerized other theorists.

Woit offers some intriguing ruminations on the relationship between physics and mathematics, but little intelligible insight into string theory itself. Smolin offers a much more substantial and serious exploration of the theory's vulnerabilities in “The Trouble With Physics.” He provides a full and fair discussion of its strengths, showing why so many theorists have been motivated to work on it (as Smolin himself did for a time). And his account is much more readable than Woit's, while covering much of the same ground, quoting many of the same sources, and expressing most of the same sociological complaints. String theorists, with their “tremendous self-confidence,” have a stranglehold on academic jobs in theoretical particle physics, discouraging young scientists from pursuing different approaches, Smolin asserts. And he contends that string theorists are afflicted by groupthink, blindly obsessed by the dogma that string theory is the only hope for unifying physics.

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