Harvard's Nima Arkani-Hamed Ponders New Universes, Different Dimensions

Physicists have had admirable success since the 1970s explaining the forces and particles at work in the cosmos, but at several fundamental levels their description requires some absurdly delicate adjustments, according to a leading young theorist at Harvard University.

Nima Arkani-Hamed said such "fine tuning" is required at distances around 1/1000 of the diameter of a proton to help explain the "weak" force that governs some forms of radioactive decay. At an even smaller distance scale, the so-called Planck scale, still greater amounts of fine tuning are required to explain the minute amounts of energy that permeate even a vacuum.

In a 28 April lecture sponsored by the AAAS's Dialogue on Science, Ethics and Religion, Arkani-Hamed said the usual hypothesis is that such fine tunings—in which huge contributions to two key parameters must, in effect, be cancelled out in the math—are justified because they reflect some yet-undiscovered mechanisms that will be shown to be natural and reasonable.

While some may argue the universe appears finely tuned because there is a God who makes it so, Arkani-Hamed said, most scientists reject the need for divine intervention to get them out of their dilemma.

Instead, they have been looking for alternative hypotheses to explain why things that appear to be finely, even heroically, tuned actually are not. One possibility, he said, is that our universe is not unique but is only part of a vast "landscape" of universes.

If there are huge numbers of universes, perhaps 10 to the 500th power by one estimate, then it is no great stretch to imagine that at least one of them—ours—wound up having extremely small amounts of observed vacuum energy and a weak force that operates on a scale much smaller than expected.

Arkani-Hamed said a landscape of many low-energy worlds is compatible with string theory, which describes the cosmos in terms of tiny vibrating strings and higher dimensional surfaces called branes rather than point-like particles. The theory posits multiple dimensions beyond the three dimensions of space and the fourth dimension of time with which we are familiar.

String theory, which has been causing unusual excitement in physics for more than a decade, has been criticized as intellectual hand-waving without much experimental data to back it up.

But Arkani-Hamed said that may be about to change. There will be ways to probe some of the fundamental questions indirectly, he said, at a new particle accelerator called the Large Hadron Collider, scheduled to come on line in 2007 at CERN, the European center for particle physics near Geneva, Switzerland.

The new collider should be able to readily probe phenomena at the distance scale of the weak force. If his argument is correct, Arkani-Hamed said, "we will see the weak scale is not unnatural" and some reasonable new principles may emerge to help explain the current need for fine tunings.

There is ample precedent in physics for such faith that a natural explanation for deep problems can lie just around the corner. He cited the dilemma physicists faced when they computed the strength of the electrical field surrounding the electron. It seemed to be infinite. "You would say there is something wrong, this isn't natural," Arkani-Hamed said. "Therefore there has to be some kind of new physics that comes in and removes this problem." It turned out that the existence of an anti-matter twin for the electron, called the positron, did the trick.

The current excitement goes beyond the likelihood of probing the weak scale to new depth at CERN. Physicists hope that gravity, which long has eluded their efforts to unify all forces and particles, may ultimately be explainable in that realm as well.

With his colleagues Savas Dimopoulos of Stanford University and Gia Dvali of New York University, Arkani-Hamed has suggested that the extreme weakness of gravity can be attributed to the existence of extra dimensions of space. We may experience only part of its impact, while the true strength of the force is hidden from us in another dimension.

Such seemingly bizarre concepts are taken seriously by theorists. Dimopoulos has argued that gravity is weak because the particle that mediates the gravitational force (called the graviton) lives far away from us in new spatial dimensions. At the new CERN collider, he argues, physicists may be able to indirectly study the nature of gravity at small distances by searching for hints of energy that may be radiating into extra dimensions. He compares it to viewing a billiards game—which is played in two dimensions on a table top—while listening for sounds of the colliding balls in a third dimension above and beyond the flat table.

Jim Miller, senior program associate for the AAAS Dialogue on Science, Ethics and Religion, said that just because string theory has been viewed by many as "mathematicians at play" and has not lent itself to experimental demonstration does not disqualify it from serious consideration.

Since the days of Galileo, Miller said, provocative theories have often preceded science's ability to experimentally validate them. "The early scientists, including Galileo, were convinced of the truthfulness of their views," Miller said, even though they lacked empirical demonstrations.

Miller also said that any decision to reject a role for God in the universe is "not a scientific judgment" based on experimental data but rather "grows out of the presuppositions that one brings to the observation of the way the world is."

Earl Lane

11 May 2005