Mordehai Milgrom never wanted to be a heretic. Twenty-five years ago, while poking around for a meaty research problem, he found one that changed the course of his career—and that might yet transform our most fundamental understanding of the universe. His ideas, long relegated to the fringes of physics, where all but cranks fear to tread, have finally become too intriguing for his mainstream colleagues to ignore.
Milgrom's heresy? He denies the existence of dark matter, the shadowy and thoroughly hypothetical stuff generally held to make up 80 percent or more of all matter in the universe. Even though dark matter has eluded all attempts at detection, most cosmologists are convinced it must be out there. Without it, there's no explanation for much of what they see in the cosmos.
Or at least there hadn't been until Milgrom's break with orthodoxy. His alternative not only eliminates dark matter, it strikes at the heart of modern physics. In short, Milgrom thinks that Isaac Newton's laws of gravity are incomplete. Unlike many radical alternatives to conventional physics, Milgrom's brainchild—known as modified Newtonian dynamics, or MOND—has not withered under scrutiny. Attacked? Yes. Ridiculed? Certainly. Refuted? No.
A little more than a year ago, Milgrom, a professor of physics at the Weizmann Institute in Rehovot, Israel, gained new support for his ideas when his longtime collaborator, Jacob Bekenstein, published a new, more powerful version of the theory, one fully consistent with Einstein's general theory of relativity. With this advance, MOND is poised to go head-to-head with dark-matter theories in describing how galaxies form and evolve. Should MOND prove successful, thousands of papers in mainstream cosmology will become obsolete overnight. And that is just half the story. If Milgrom can claim victory, he will have wrought the most dramatic revision of our understanding of gravity since Einstein's work of almost a century ago.
Mordehai Milgrom began his career studying objects called ultracompact neutron stars in binary star systems. In 1979, sensing the need for a new challenge, he bundled his family and headed on sabbatical to Princeton University, one of the world's leading centers for the study of galaxies. Even after half a century of unraveling the structure and evolution of galaxies, astronomers still had much to learn about them. Milgrom became particularly intrigued by one intractable problem. "I had heard there was this trouble understanding the so-called galactic rotation curves, which describe the way stars rotate around the centers of galaxies," he says. "I thought I would apply myself and try to think about this problem."
