The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next, by Lee Smolin, Boston: Houghton Mifflin, 392 pages, $26
Lee Smolin became a physicist in the 1970s amid heady expectations that the field was on the verge of breakthrough insights into how the universe works. Theoreticians had proposed, and experimenters were verifying, the standard model of particle physics, a detailed but incomplete picture of matter and its interactions. The next step, it seemed, would be a "theory of everything," a full accounting of nature's most fundamental laws.
Three decades later, he has written a fascinating and sobering lack-of-progress report. Smolin works at the Perimeter Institute for Theoretical Physics, a think tank in Ontario; his book The Trouble With Physics combines a pungent critique of the regnant views in theoretical physics with a broader meditation on how science works, or fails to work. Its prime target is string theory, the dominant avenue of research for theoretical physicists since the mid-1980s. He argues that string theory has turned out to be a "theory of anything," an ill-defined framework that lacks explanatory and predictive power, relies on excessive conjecture, and crowds out more promising lines of inquiry.
Much more is at stake here than which faction in physics departments will have the highest success rate in achieving tenure. Physics, as Smolin points out, made steady and definitive progress in understanding nature from the 1780s until the 1980s. The field thereby demonstrated science's efficacy and helped set high intellectual standards and a confident tone for science overall. Moreover, the quest for fundamental laws of nature overlaps with longstanding philosophical and religious concerns. String theory, for instance, has become entangled in the politically charged controversy over whether the natural world bears signs of intelligent design. If theoretical physics is slowing down, or on the wrong track, scientific and intellectual life more broadly may be damaged.
And then there are the technological implications. The early 20th century's theoretical breakthroughs-relativity and quantum mechanics-paved the way for lasers, transistors, nuclear weapons, and magnetic resonance imaging machines, among other things. More recently, cutting-edge physics theories have had far fewer technological consequences. This could mean, as some scientists suggest, that physics has probed to realms so removed from the human scale as to have no practical application. But it also could mean the theories are wrong, or that the relevant technologies have not been imagined yet. Nobody in the 19th century realized that James Clerk Maxwell's electromagnetism equations eventually would produce television.
Smolin identifies five key problems at the frontier of physics. One is the problem of quantum gravity-of how you combine quantum mechanics (which focuses on small-scale phenomena and nongravitational forces) and general relativity (which deals with large-scale objects and gravitational forces) into a coherent picture of nature. Another is figuring out how to make sense of quantum mechanics, with its counterintuitive phenomena such as particles that behave like waves. A third problem is determining whether nature's various particles and forces are all manifestations of a single entity (much as Maxwell showed electricity and magnetism to be types of the same force).
A fourth puzzle is why various numbers in the standard model of particle physics-constants such as the masses of particles and the strengths of forces-are what they are. The standard model has many such parameters that are derived from experiment but not logically required by the theory itself. Finally, there is the problem of dark matter and dark energy. This arises from surprising observations made by astronomers: Stars move in ways that suggest there is far more matter in galaxies than we can see, while measurements of supernovas and cosmic radiation show that the universe's expansion is accelerating as if driven by some mysterious energy.
String theory proposes some answers to these questions. The theory holds that nature's particles and forces are indeed manifestations of one underlying thing: "strings," which are infinitesimal strands of vibrating energy. A string vibrating one way is one type of particle; a string vibrating another way is a different particle. The theory offers what may turn out to be a solution to the problem of quantum gravity: The gravitational force, in this scenario, is one more manifestation of vibrating strings. String theorists celebrate as one of their theory's virtues that gravity arises readily from its mathematics, rather than being put in "by hand." That said, what they have is not a full answer to the quantum gravity question but more of an intriguing sketch of what the answer could be.
As for the other problems on Smolin's list, string theory's power is similarly limited. It generally has not addressed how to interpret wave-particle duality and other perplexities of quantum mechanics. The theory leaves room for various particles and forces that could account for dark matter and dark energy, but it gives little guidance on how to narrow down the many possibilities. Nor has it provided a principle that would explain why our universe's constants are what they are. In other words, it seems to apply to numerous possible universes, not just our own.
That's where strings enter the debate over intelligent design. While most of that argument focuses on biology, some of it revolves around the supposed "fine tuning" found in physics.
Proponents of intelligent design observe that life as we know it depends on nature's constants-if particle masses and force strengths were different, we would not be here -and argue that the universe's compatibility with life indicates a cosmic planner. Such claims are speculative, to say the least. It is hard to know what the universe really would be like if various constants were changed, even harder to know what forms life might take other than the carbon-based variety we are familiar with on Earth.
String theory has frequently been used to support a counterargument against the design claim-that multiple universes may exist, each with its own set of constants, and of these at least one happens to be suitable for life. According to some recent mathematical work, string theory is not a specific model of the universe but an entire "landscape" of models. In fact, there may be a stupendous number of universe types compatible with the theory-about 10500 by one estimate. Some scientists, including one of string theory's co-originators, the Stanford physicist Leonard Susskind, argue that these universes are not just mathematical constructs but real places.
Smolin is having none of this, seeing neither a cosmic designer nor Susskind's cosmological landscape as a scientifically productive path. He points out that there are other theoretical possibilities regarding the putative tuning of constants. One hypothesis, which Smolin elaborated in his earlier book The Life of the Cosmos, involves universes "reproducing" through black holes in a cosmological version of Darwinian natural selection. Smolin notes that certain predictions stemming from this scenario have held up, and he emphasizes that scientists should focus on ideas that are testable. String theory, including its extension into a multitude of universes, has been notoriously difficult to put to any experimental or observational test.
Smolin does not argue that string theory is wrong. He acknowledges that it may end up leading to, or being part of, the truth about nature. But he convincingly argues that physicists have awarded it disproportionate attention, given its limitations and uncertainties. He calls for a stepped-up effort to find and develop other ideas about fundamental physics-in particular, a "background-independent" theory of fundamental physics.
In such a theory, space and time are not a fixed background against which things happen. Rather, they are dynamic entities that can change in response to other physical phenomena. With general relativity, Einstein introduced such background independence to physics; it turned out that space and time could be curved and warped by matter and energy, an irregular geometry that we experience as gravity.