Readers' Choice: Journeying Through the Quantum Froth

In his youth, there were two things that regularly competed for Giovanni Amelino-Camelia’s attention: his favorite soccer team, Napoli, and "anything that came close to being scientific." And since Napoli was struggling in the Italian soccer league in the summer of 1978, Amelino-Camelia found himself watching a series of programs on special relativity instead of soccer. "That was really the point of no return for me," he remembers. "Although I was 13-years old, nothing could have happened after that to keep me away from fundamental physics," he says. "It was lucky for me that those television shows were broadcast in a year when Napoli did very poorly!"

Lucky for him, and, in many ways, lucky for us, because almost 30 years later Amelino-Camelia, at La Sapienza University in Rome, Italy, is dedicated to pursuing answers to foundational questions. Armed with a $65,000 grant from FQXi, he is currently making a run at redefining the way physicists attack perhaps the most vexing and elusive puzzle of all, the problem of quantum gravity. Many lament that experimental tests of quantum gravity lie way beyond reach. However, Amelino-Camelia believes that cosmic rays may already be revealing clues about the frothiness of the fabric of the universe, and he is using these hints to probe the quantum nature of spacetime—one step at a time.

Physicists have been struggling to find an overarching theory that unites quantum mechanics with gravity for decades. Amelino-Camelia, by contrast, is taking a different approach. He isn’t searching for a single, all-encompassing theory of everything, such as promised by string theory—which suggests that elementary particles are made of tiny, vibrating strings—or the more recent loop quantum gravity—which posits that at the smallest scales spacetime is a woven fabric of quantum threads. Rather, he explains, he is searching for theories of not everything.

That doesn’t mean that Amelino-Camelia thinks that searches for a theory of everything should be abandoned. In fact, the majority of the simple models exploring portions of the quantum gravity problem that he works on were inspired by results obtained in the development of loop quantum gravity.

"Even assuming my concerns are correct—and only time will tell—research looking for a full solution of the quantum gravity problem would be very valuable because it provides guidance for the development of meaningful theories of not everything," says Amelino-Camelia. "But if all we did was research on the more ambitious theory-of-everything level, then inevitably we would be wasting some opportunities for valuable insight within the reach of the not everything approach."

Doubly Special Relativity

Okay, we now have a decent understanding of what Amelino-Camelia is not looking for. But what exactly is he looking for? The simplest example of a theory of not everything is one that can describe a "quantum spacetime." Both general relativity and quantum mechanics are formulated using the intuitive concept of a classical spacetime, he explains. But many arguments—including some based on theory-of-everything studies—suggest the correct microscopic description of spacetime should be based on a nonclassical geometry, that is, on a quantum spacetime.

The concept gels well with a theory that Amelino-Camelia introduced in 2000, which he dubs "doubly special relativity." While Einstein’s special relativity tells us that there is a maximum speed limit for light, Amelino-Camelia’s doubly special relativity posits that there is also a minimum length—the Planck length—below which space cannot contract. Spacetime, in this picture, is not continuous but can be thought of as a froth, made up of roiling pieces, or Planck-scale grains, that are just one hundred billion billionth the size of an atomic nucleus. This grainy spacetime is said to be quantized.

Instead of trying to fit this notion of a quantum spacetime straight into a larger quantum-gravity model, Amelino-Camelia argues that it will be fruitful to first fully examine its implications. Taking smaller bites out of the quantum gravity puzzle, in this way, gives physicists manageable chunks that they can chew on more easily.

Crucially, this opens up the possibility of examining each portion thoroughly and subjecting each portion to experimental scrutiny, explains Amelino-Camelia. Some aspects of the puzzle can be tested using today’s technology, while others will hopefully be testable in the coming years, as new technologies are developed.

Probing Spacetime

For example, different types of quantum spacetime could affect particles in slightly different ways. In most scenarios, these tiny effects would be tough to detect. But one place where the effects might show up is in the behavior of some high-energy particles, called cosmic rays, and bursts of high-energy radiation, known as gamma rays. Cosmic rays and gamma rays travel huge distances across the universe, and over the course of their long journey the tiny effects of spacetime quantization would have had a chance to accumulate to an observable level, Amelino-Camelia explains.

Cosmic rays have already helped to rule out some of the simplest quantum pictures of spacetime. Those models predicted that the maximum possible energy of cosmic rays, known as the GZK cutoff, would be significantly higher than currently thought. However, data recently gathered by the Pierre Auger cosmic-ray observatory does not support this. "In order to get to the level of probing more promising pictures, we still have some way to go," says Amelino-Camelia. "Although it is something we will realistically start doing within a few years."

In addition, some quantum-spacetime models predict a particular relationship between the speed of photons in gamma rays and their energy. There have already been tantalizing hints of these effects. In 2007, for instance, the MAGIC gamma-ray telescope collaboration based on La Palma in the Canary Islands announced that they had measured a 4-minute time difference between the arrival of high and low-energy gamma rays released at the same time in a flare from the Markarian 501 galaxy, some half a billion light years away (Physics Letters B, 668, 253-257, 2008). Standard theories suggest that the photons should have arrived simultaneously.

Along with Lee Smolin at the Perimeter Institute in Waterloo, Ontario, Amelino-Camelia has begun analyzing new gamma-ray data from NASA’s Fermi Telescope, launched in June 2008 (see image above right). The new data show similar delays in the arrival times of photons, which they believe will help physicists discriminate between these models. In this way, theories of not everything are falsifiable—giving them the edge over more ambitious theory-of-everything candidates.

John Stachel, a physicist at Boston University, Massachusetts, admires this drive to produce a falsifiable theory. "Amelino-Camelia grew impatient with the exclusively theoretical nature of most work on quantum gravity," he says. "The smallness of most predicted effects threatened to make the field more an area of speculative scholasticism than a part of science."

But it’s not just about being able to scrutinize the theory using experiments and observations. Ultimately for Amelino-Camelia, theories of not everything offer a greater sense of wonder than an all-encompassing theory of everything ever could. "I do not see so much beauty in the picture of having a theory of everything," he says. "It seems to me it is much more appealing to find partial answers, answers that close the door on a few open issues but actually open the door on many more puzzling issues."