John Moffat is not crazy. It's important to mention this, because many of those who claim to have improved on Einstein's theory of gravity , known as general relativity, probably are. Or if not crazy, at least misguided. Their error-riddled and often incoherent papers land regularly on the desks of physics journal editors (where they are promptly discarded or filed under "cranks") and clog certain corners of the Internet.
But Moffat is, in fact, a respected physicist. Born in Denmark, he earned his PhD from Cambridge and taught at the University of Toronto for more than three decades before moving on to his current position at the Perimeter Institute for Theoretical Physics in Waterloo, Ont. (I've met Moffat many times over the years, profiling him for this newspaper and others; recently, we've shared the same stage on the "physics book circuit," each of us promoting our take on rather fundamental topics; for him, gravity, for myself, time.)
General relativity is one of the great triumphs of 20th-century physics. It explains the motion of planets, stars and galaxies with astonishing precision, and even gives a framework for understanding the expansion of the universe itself, providing a foundation for the Big Bang model of cosmic evolution. And while general relativity involves some difficult mathematics, in "everyday" situations — working out the Earth's orbit around the sun, for example — it reduces down to Newton's simple formula for universal gravitation.
So why fiddle with a perfectly good theory? The problem is that, in certain situations, one has to make some disturbingly ad hoc assumptions to make sense of the data. Take, for example, the motion of galaxies. These giant agglomerations of stars spin very slowly; our own Milky Way takes about 200 million years to make a complete revolution. (It sounds sluggish, but it still means that our solar system is whizzing along at about 220 kilometres per second in its journey around the galactic centre.)
But our observations show that galaxies are spinning faster than they ought to, based on the amount of matter — stars, gas and dust — that they appear to contain. For several decades, astronomers have postulated the existence of unseen "dark matter" to make up the difference: If there is enough of this dark matter in each galaxy, then Einstein's equations can be made to fit the observations. Unfortunately, as Moffat points out, no one has yet detected this mysterious dark matter.
Moffat's theory, known as modified gravity theory (MOG), developed in spurts over about 30 years, is superficially similar to Einstein's theory, but with some key differences. Crucially, it predicts a stronger gravitational pull at large distances than we would expect from Newton's or Einstein's formulation, thus explaining the faster galaxy rotations without the need to invoke dark matter.
But MOG, according to Moffat, has other advantages. In Einstein's theory, a collapsing body — such a star that has exhausted its nuclear fuel supply — can shrink down to nothingness; the result of this unstoppable gravitational crunch is called a black hole. At the centre of the black hole, matter is infinitely squished; mathematicians call this a "singularity."
Physicists, however, wonder what it means for any real, measurable quantity to be infinite. Singularities, they suspect, are a sign that Einstein's equations are not giving the whole picture; that some more sophisticated approach is needed. With MOG, matter can become extremely dense, but never infinitely so; the singularity is avoided. (A star still "collapses to a very dense object, which is not exactly black, but possibly 'grey,'" Moffat says. While nothing can escape from a black hole, some light would always be able to escape from such a "grey star.")
And there is more: Recently, astronomers have discovered that the universe is not only expanding, but also accelerating. Physicists have suggested that some kind of unknown force is acting against gravity, pushing galaxies apart at ever-increasing speeds. For now, this mysterious entity is known as "dark energy." Moffat says MOG's equations "predict a repulsive, anti-gravity component within gravity itself. ... Its effect is to push the fabric of space-time to expand faster and faster" — an explanation, perhaps, for one of the deepest mysteries in physics today.
No dark matter, no singularities and an explanation for dark energy — these are bold claims. The big question is whether MOG can make specific predictions that can be confirmed or refuted by observation or experiment. Moffat insists it can, and lists a series of tests that might be carried out in the future, involving microwaves emitted in the early universe, gravitational waves (think of these as ripples in space-time itself — if you can) and laboratory experiments that would investigate force and acceleration at small scales. (These latter experiments, Moffat suggests, could be performed on board the International Space Station.)
If evidence for MOG is eventually found, Moffat will be hailed as one of the great physicists of our age. If not, MOG — like so many other attempts to go beyond Einstein — will be just another dead end. As Moffat points out, the many previous efforts at such a breakthrough make up a "graveyard" of failed theories. Paradigms do not shift easily.
At the very least, Moffat has shown remarkable perseverance and an exceptional ability to think "outside the box." And his book — elegantly written, though occasionally challenging — provides a compelling insider's view of modern physics.
Dan Falk's latest book, In Search of Time: Journeys along a Curious Dimension, was published in October.
