Several theoretical physicists have proposed a new theory for understanding why the force of gravity is so weak. The answer to this perplexing question may lie in dimensions beyond our own.

Gravity in the Fifth Dimension

The force exerted by a small magnet, say one that can be held between your index finger and thumb, should be no match for the collective gravitational force operating within our universe. But place a nail near a magnet and we all know what happens--the magnet prevails. This 'David and Goliath' physics phenomenon has baffled theoreticians ever since the advent of modern physics a century ago. It's a puzzle that not even Einstein could solve.

Now a number of physicists have proposed a new theory for understanding the weakness of gravity in our universe. The notion is elegant in its simplicity: They suggest that most gravitons--'messenger' particles programmed by nature to carry the gravitational force (much like photons carry light impulses)--don't live in the same dimension in which we live. In fact, such particles may reside in a vast fifth dimension that remains largely separate from our own. Gravity, they contend, may be a powerful force, but its agents just don't live in our neighbourhood.

Lisa Randall, who is one of the chief architects of this theory, spoke at ICTP's Conference on Physics Beyond Four Dimensions, held between 3-6 July.

Lisa Randall

Randall, professor of physics at Princeton University, notes that "the theories that I and my colleague, Raman Sundrum, a physicist at Stanford University, have presented draw on ideas which have been central to the study of string theory over the past two decades. Like string theory, the intellectual construct on which we base our space-time concept depends on the existence of extra-dimensions. And like string theory, our theory suggests that the physical world is configured by strings or bands that anchor the particles and forces that fill our universe."

But Randall expands upon string theory in this way: Whereas string theorists suggest that gravitons and the force that they carry may be wrapped within tightly bound strings residing in as many 11 dimensions, Randall suggests that gravity--or at least most of it--is found on strings or bands that reside largely in a vast fifth dimension. As a result, gravity exerts a weak force on us not because it is tightly bound in strings but because it is largely insulated from our known reality. In Randall's words, "geography, not compaction, accounts for gravity's weak force."

"Think of ourselves as living in a bubble consisting of three dimensions plus time that floats within a vast multidimensional universe," explains Randall. "Within this physical landscape, three of nature's four elementary forces--electromagnetism and the weak and the strong force--are attached to strings inside the bubble, exerting a force that can be detected and analysed. Nature's fourth elemental force--gravity--is largely attached to strings in another dimension. We can only detect it when gravitons leak into our dimension through the surface or 'branes' of our universe's strings. In short, our multidimensional world consists of bubbles floating within bubbles that are comprised of constituent elements that rarely violate the constituent elements found in other space-time dimensions."

The theory, which was first presented in Physical Review Letters in 1999, has generated a great deal of excitement in both scientific publications and the popular press. In fact it has been examined extensively in Nature and Science and discussed at length in The New York Times and The Economist.

This unusual convergence of professional and public interest is based on several factors.

First, the theory offers a possible answer to one of physics' greatest mysteries: how gravity relates to nature's other elementary forces. Solving this mystery would mark a critical step forward in the unification of all the forces of nature.

Second, if gravitons exist in a much larger dimension than theorists have previously thought, and if these particles are not bound in tightly wound strings that require absurdly high energies to untangle, then it may be possible to test the theory in the near future.

At this point, the theory is just that--intelligent, well-formed speculation based on years of study and insight, as well as mathematical calculations that seem to point in this direction. The good news is that the theory is likely to be put to the test within the next few years. That, in turn, could bring theories and experiments in high energy physics closer together than they have been in more than two decades.

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