No Small Matter
Is theoretical physics stuck? And should you worry?
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.
String theory, as normally formulated, is "background-dependent," treating space and time as an unchanging stage (albeit a rather exotic one-the theory requires multiple dimensions, which are assumed to be unseen because they are too small). This means, according to Smolin, that string theory falls short as a foundation for general relativity and relies too much on assuming, rather than explaining, the nature of space and time.
Smolin surveys various background-independent alternatives to string theory. Some of these involve dissolving space and time into cause-and-effect relations among events, or using geometry that is equivalent to A times B not equaling B times A. Loop quantum gravity, an approach in which Smolin has been a key figure, recasts space and time in terms of field lines similar to those one might use to diagram a magnetic field; these lines can become loops if there is no matter present. Ultimately, Smolin ventures, progress will be made by "unfreezing" time-thinking of it as something more dynamic than physicists thus far have contemplated.
The Trouble With Physics criticizes not just string theory but string theorists. This community numbers more than 1,000 researchers worldwide, compared to a couple hundred focused on loop quantum gravity and other alternatives. Smolin portrays string theorists as tending toward arrogance, insularity, and groupthink; they value technical ability over original thought, follow faddishly the ideas of a few top physicists, and look down on adherents of other theories. This culture, in Smolin's telling, eschews the philosophical bent of Einstein and quantum theory's founders, preferring the "shut up and calculate" attitude of later particle physicists. That latter approach, Smolin suggests, was valuable when new experimental data abounded in the 1960s and '70s but is far less productive now that theory has run ahead of experiment.
Smolin's negative description of string theory's practitioners is probably overblown. Some of their writings, such as Brian Greene's The Fabric of the Cosmos, evince considerable philosophical interest in space, time, and matter. Nonetheless, Smolin is right that science needs both "craftspeople" and "seers," the former focused on technical problems, the latter on deeper meanings and new ideas. He makes a plausible argument that physics institutions have become too geared toward producing craftspeople rather than seers. The way for young physicists to get jobs, tenure, and grants, he notes, is to fill in the details of research lines established by their elders.
One reason for this, as Smolin points out, is that universities are no longer growing as fast as they did for decades after World War II, so there is more competition for physics posts and less room for nonconformists. Furthermore, theoretical physicists rely heavily on financial support from just a handful of federal agencies, with some private foundation money thrown into the mix. These limited funding options provide further incentives for conventional thinking. Observing that such incentives are not limited to physics, Smolin warns that intellectual sclerosis could be developing throughout the sciences.
Against all this, Smolin advocates that scientists embrace an ethos of multiple approaches to open questions, avoiding consensus until evidence is decisive. He also calls on administrators and grant officers to promote such diversity. This seems too limited a solution to fix the problems he identifies. Probably what is needed is greater institutional diversity, with more and different organizations involved in theoretical physics. Smolin's employer, the Perimeter Institute, is a step in that direction, avoiding academic hierarchies and drawing on varied public and private support, including donations from individuals and companies. Perhaps the private sector will develop a broader role in supporting theoretical physics, financing quantum gravity research and the like for prestige, tax breaks, or the chance of some future tech breakthrough.
Although Smolin does not thoroughly explore the possible solutions to the problems he cites, The Trouble With Physics is ultimately an optimistic book. If the recent difficulties of theoretical physics arose from flaws in scientific culture and institutions, rather than from the sheer abstruseness of natural laws, our progress in understanding the universe just might resume.
Kenneth Silber is a science writer and editor based in New York City.
