Leslie Rosenberg's attempt to understand the universe resembles a makeshift home hot-water heater tank, capped with some wires and shoved into a large, underground refrigerator. The experiment, housed in a laboratory adjacent to his office at the University of Washington, is a supercooled, magnetized vacuum chamber equipped with a sensitive detector that listens for the microwave “ping” of passing particles called axions. These particles are invisible and, so far, entirely hypothetical.
Rosenberg has been on the trail of this particle ever since he was a postdoctoral researcher at the University of Chicago in the early 1990s. In that time he has performed experiment after experiment, achieving ever greater precision and yet always the same old empty results, hoping for the positive detection that could rescue Albert Einstein's biggest—and most star-crossed—idea.
Physicists call it the unified field theory, but it is more popularly and evocatively known as the theory of everything. The idea has been to devise a single formulation that sums up the behavior of all the known forces of physics. Einstein started this quest nine decades ago. It bothered the great theorist that the two fundamental forces guiding the behavior of the universe—gravity and electromagnetism—appeared to play by different rules. He wanted to demonstrate that all types of matter and energy are governed by the same logic.
Rolling up the universe into a single formula was a formidable ambition, even for Einstein. “I want to know how God created this world,” he wrote in an oft-cited 1920 letter to a German physics student. “I am not interested in this or that phenomenon, in the spectrum of this or that element. I want to know his thoughts. The rest are details.”
But as the Yiddish proverb goes, “Man plans and God laughs.” Einstein pursued God's thoughts for three decades to no avail, running down one blind alley after another. When he died, in 1955, he left behind a set of unsolved unified field equations scrawled on his blackboard.
The task of unification has fallen to subsequent generations of physicists, who have broken the problem into myriad parts. What started as the grand vision of a singular genius has morphed into slow, grinding labor carried out by different teams of physicists, each trying to solve a small piece of a vast cosmic puzzle. Rosenberg, for instance, does not obsess over an all-encompassing theory of everything. He is focused on his one vexing and specific problem: the axion. It has theoretical properties that could wipe away the need to modify Einstein's equations of gravity. “We'll see what the data say,” Rosenberg notes. “I don't want to look into the mind of God.”
Despite their narrow focus, Rosenberg and his compatriots have not taken their eyes off the prize. They are engaged in a broader effort to hammer out flaws in the theoretical edifice that Einstein created and to build a more complete model of particle physics from the ground up, rather than from the top down. They seek to push the science forward by finding out how nature really behaves, not how scientists think it should (an approach that Rosenberg dismisses as “navel gazing”). Other researchers are designing experiments to reveal a hidden aspect of physics called dark energy or to detect two-dimensional quantum units that could be building blocks of our apparently three-dimensional existence. Their hard data may be just what today's physicists need to succeed where Einstein failed.
“We could actually test some of these crazy ideas about the evolution of the universe,” says physicist Joshua Frieman of the University of Chicago. Almost certainly, he believes, physicists will not get to a theory of everything without them.
Dark side of the universe
A look at Rosenberg's Axion Dark Matter eXperiment (ADMX) reveals the power of the small-is-beautiful approach. In its seemingly modest search for just one particle and one new set of physics rules, ADMX could also refute concerns about general relativity and solve a major cosmology puzzle in the process.
That puzzle dates back to the 1930s, when astronomers began to realize that the universe appeared to be full of some unseen component that makes its presence known only by its gravitational pull on the visible stars. The discovery turned even stranger in the 1980s, when new models of the big bang showed that the invisible (or “dark”) stuff—whatever it is—could not consist of ordinary atoms. That left two unsettling possibilities. Perhaps gravity does not work the way Einstein thought it did at large scales, or perhaps the universe contains an unknown class of particles that are invisible to all our telescopes.
Possibility number one is shunned by the vast majority ofphysicists because it is ad hoc, and it is also difficult to reconcile with measurements of how galaxies move. The scientific mainstream has therefore lined up behind possibility number two, instigating dozens of crafty efforts to unmask the unseen dark particles. Which is where ADMX comes in.
Axions nicely match the inferred properties of dark matter, so if Rosenberg and his ADMX team detect them, they would provide a more complete picture of how galaxies have formed and evolved. They would also do away with the need to make ugly modifications to some of Einstein's gravity equations. Above all, axions would force a revision of the Standard Model of particle physics. That model is a comprehensive, yet clearly incomplete, theory of fundamental particles and fields. Finding the axion would validate a much debated elaboration of the Standard Model, bringing physicists one step closer to a true theory of everything.
Until recently, axions were considered a long shot in the search for dark matter. Most of Rosenberg's colleagues were focusing their attention on another class of particles called WIMPs (weakly interacting massive particles), which were considered more theoretically attractive. “I was always a little odd duck out,” Rosenberg admits cheerily. Then the various WIMP detectors kept getting better and better, without finding anything. The watershed moment came last year, when an ultrasensitive WIMP finder called Large Underground Xenon (LUX), beneath the hills of South Dakota, switched on. So far it, too, has come up empty.
Now is the make-or-break moment for Rosenberg to prove that axions are the answer and to shore up general relativity—Einstein's idea that gravity comes from a curvature of spacetime—in the process. The concept behind ADMX is deliciously straightforward. If dark matter really consists of particles, there must be a continuous wind of them blowing through the earth and everything on it (including you) all the time. And if those particles are axions, theoretically they will very occasionally decay. The particles themselves are invisible, but in that rare decay process, they should turn into microwaves, which would produce a weak but detectable signal. Straightforward, yes, but difficult to execute in practice.
“We have a cavity the size of an oil drum,” Rosenberg says, “and it's cooled to 100 millikelvins,” which is 0.1 degree above absolute zero. The extremely low temperature ensures that the detector itself produces almost no microwave noise. Next the cavity is magnetized to stimulate the decay of axions. Then a small, pencil-shaped probe listens in for some microwaves that should not be there. Adding to the challenge, nobody knows exactly what kind of microwaves to listen for; the frequency of the signal depends on the mass of the axion, which is of course unknown.
The only way around this problem is to hop through the microwave band frequency by frequency; the entire ADMX endeavor is essentially a process of flipping channels on a CB radio. Rosenberg lights up when I offer that analogy: “I've always had this interest in radio electronics. I played with the radio as a kid, bouncing signals off the moon. Now we're looking at signals using receivers so sensitive they could get four bars of cell-phone reception on Mars!” He is also proud that ADMX, unlike Einstein's endless explorations of the unified field theory, is guaranteed to yield a concrete answer.
“By 2018 we will have completely covered the definitive search region for the axion,” Rosenberg says. “At that point, it's either there, or it isn't.” In other words, we will have either a big new clue about how to build a theory of everything or one more idea to scratch off the list.
Energy of empty space
While Rosenberg whittles away at the problem of dark matter, other researchers are working toward a complete picture of physics by going after the other major unseen aspect of the universe: dark energy. It is the opposite of dark matter in its effect, producing a repulsive force rather than a gravitational attraction. Because dark energy counters the action of gravity, it has direct implications for how to interpret the equations of general relativity. More profoundly, dark energy cannot be explained within the current model of particle physics. It therefore provides a critical test for any would-be theory of everything.
One such test is being run by Chicago's Frieman. It uses a custom-built camera strapped to the Blanco four-meter telescope atop Cerro Tololo, a towering peak in Chile rising more than two kilometers above sea level. The idea is to gather a vast number of pictures of distant galaxies. Each image from the camera contains 570 megapixels, a huge amount of data, and it will collect about 400 images a night, 105 nights a year, over five years total. The project is called—not surprisingly—the Dark Energy Survey, and when it is complete in February 2018, the survey will have examined 300 million galaxies and about 4,000 supernova explosions. (For comparison, a state-of-the-art automated supernova search conducted at the University of California, Berkeley, from 1998 to 2000 turned up a grand total of 96.)
Like Rosenberg, Frieman used to work as a theorist but got pulled over to the observational side by the idea of designing actual tests. Now he has to confront the realities of the task. “Taking the data is hard,” he says. “Processing the data is hard.”
Frieman and his team pick apart observations from the survey four different ways, each one designed to capture a specific aspect of how dark energy behaves. One analysis zeroes in on a class of exploding stars called type Ia supernovae, which act as mile markers in space. Their brightness indicates their distance, and their color indicates how quickly they are moving away from us. Put together a bunch of those mile markers, and you get a sense of how the expansion of the universe has been changing over time. The other three kinds of analyses explore various patterns of how galaxies cluster. Gravity tends to pull everything together, and dark energy tends to push everything apart. Mapping how galaxy clusters change over cosmic time therefore reveals the intensity of the dark energy effect.
In the simplest models of dark energy, it is an unchanging and ubiquitous feature of empty space. It turns out that the standard theories of particle physics can account for the existence of such an energy; they just predict a value 10120 times too large. (It is sometimes called the worst prediction in all of physics.) Accounting for the real, drastically smaller value of dark energy is one of the most important tests for a prospective theory of everything. Astronomers also do not know yet whether dark energy is truly constant. If Frieman finds that it changes over time, that is another thing that a theory of everything must explain.
Before we reach that point, though, there is a more basic issue to settle. “Our assumption is that dark energy is what's driving the accelerated expansion, but we don't know that for sure. It could be that on the largest scales, general relativity just isn't the correct theory,” Frieman says. A modified version of relativity could potentially mimic the dark energy effect, something that he will be investigating closely. One way or another, there must be a theory that goes beyond Einstein's, and the Dark Energy Survey will help find it.
Is life a hologram?
Weird as they are, dark matter and dark energy can still be thought of as garnishes on the universe as we know it: an icing of additional particles or fields atop the kind of reality that Einstein would have readily recognized. But what if that reality needs adjustment to make progress toward a more sweeping theory? What if spacetime itself has new, undetected properties that are not described by general relativity?
Craig Hogan, director of the Center for Particle Astrophysics at Fermi National Accelerator Laboratory, is exploring that head-scratcher with an experiment he calls the Holometer. His quest is to find out whether space and time are constructed out of fundamental units: a universe inherently built around ticks of time and marks on the ruler. In this alternative view, our sense of living in a three-dimensional universe is an illusion. If you could magnify space sufficiently—down to 10 trillion trillion times as small as an atom—you would see two-dimensional pixels that look three-dimensional only when viewed from a large-scale perspective, like the dots on a television screen.
Each of those units would follow quantum rules, such as having an amount of built-in uncertainty about its location. At large scales, space would appear continuous, as Einstein believed, but it would have an underlying quantum structure. In this way, a pixelated universe would force quantum mechanics into relativity, removing a key obstacle to creating a unified theory of physics.
The idea of the apparent 3-D universe emerging from a 2-D reality is known as the holographic principle, hence the name of Hogan's experiment. “Holometer” is also something of a pun, riffing on the name of a 16th-century precision surveying device. Hogan's instrument, now collecting data at Fermilab, is similarly designed to measure the lay of the land with unprecedented accuracy. It consists of a laser beam that is split in two, sent down different tunnels, bounced off a mirror and then recombined. If space has a quantum structure, the uncertainty of the location associated with each pixel should create a jitter within the device; that jitter would shift the two halves of the beam and knock them out of sync. In principle, the Holometer can measure movements at the attometer scale: 10–18 meter!
That may not be small enough, however. Any underlying quantum structure of space could be even more minuscule, far too subtle to detect experimentally, some of Hogan's colleagues have warned him. He took their skepticism as a dare. As we talk, he seems especially tickled by how acutely his experiment irks Leonard Susskind of Stanford University, one of the primary developers of the holographic universe concept. “Lenny has an idea of how the holographic principle works, and this isn't it. He's pretty sure that we're not going to see anything. We were at a conference last year, and he said that he would slit his throat if we saw this effect,” Hogan recalls.
Their dispute should be settled soon. After collecting one hour of data, the Holometer is approaching Planck sensitivity, the scale at which Hogan thinks the graininess of space might show up. A full answer could come within a year, he predicts, and then something will happen—he is just not sure what: “If we don't see something, or we do see something, either way it's going to constrain people's ideas. Nobody knows what the hell to expect.”
Einstein’s dream, continued
After Hogan's comments, I was eager to speak with Susskind to hear his take. Contrary to the stereotype of the pensive, math-obsessed theorist, Susskind quickly launches into a discussion about testable concepts. “People bitch about theoretical physicists' being frivolous with their ideas because they don't face the issues of falsifiability. That's nonsense. We all are very concerned about falsifiability,” he says. But if there is a laboratory test, he contends, the Holometer is not how to do it.
A better bet, Susskind says, is to look to the edge of the observable universe for behavior that supports string theory. In this theory, all particles and forces are different modes of vibrations in wiggling strings of energy, which makes it a unified explanation for all of them. (These strings are different from cosmic strings, which may be defects in spacetime.) It also makes predictions about physical conditions at the time of the big bang. More remarkable, some versions—the ones Susskind works on—make predictions about conditions at an even earlier stage, before our universe was born. Susskind believes astronomers might be able to identify evidence of that prior existence imprinted on radiation from the far reaches of our universe.
More likely, though, he believes the next strides toward the unification of physics will come not from experiment or observation but from intense mathematical explorations of black holes and space and time. “Important things are going to happen over the next five to 10 years,” Susskind predicts. “I don't say we're going to have a complete theory of everything; we're not even close. But there are going to be major insights into the connection between gravity and quantum mechanics.”
When that connection is revealed, Susskind—like most of today's theorists—expects that quantum mechanics will come out on top, with gravity and general relativity forced to live within its framework. But because Einstein was the one who started us down this path, it seems only fair to give the final word to one of today's leading Einsteinians, physicist Lee Smolin of the Perimeter Institute for Theoretical Physics in Ontario.
Smolin is convinced that many of his quantum-obsessed colleagues are literally thinking too small in their pursuit of an ultimate theory. “Quantum mechanics is only sensible as a theory of a subsystem,” he says, “but general relativity is not a description of subsystems. It is a description of the universe as a closed system.” If you want to understand the universe as a whole, then you have to think of it as Einstein did, in relativistic terms.
That approach has led Smolin to the startling hypothesis that the laws of physics may evolve over time and that the universe has a memory of its own history—what he calls the principle of precedence. In this way, he envisions moving beyond specific, unexplained details of quantum mechanics (the strength of this particular field or the mass of that particular particle) and regarding them all as developmental aspects of the single, closed-system universe. He even has a notion of how to test his idea.
“If we could evolve a system that is large and complex but still described by a pure quantum state, we would force nature to invent some novel systematics. We could imagine doing that with quantum devices,” Smolin says. After creating the same system over and over in the lab, nature might start to develop a preference for a certain quantum state. “It would be hard to distinguish from the noises of experimental practice. But not impossible.”
Smolin does not intend to sound mystical, but in some way he seems to be talking not about the physical universe but about the spirit of Einstein. One century ago a single man revealed a novel way to think about the universe. Sixty years ago that life was snuffed out, as all human lives are sooner or later. But the mind of Einstein still leaves a distinct imprint on today's researchers. They run new experiments in the service of an old ideal. The impulse seems unstoppable, as they keep recapitulating his search for a deeper truth, a higher enlightenment.