Most astronomers refuse to embrace MOND. Milgrom understands: "I think the science community should give any such idea a hard time. If you really want to shake the principles, it shouldn't be an easy matter."
Milgrom also acknowledges that MOND has a serious flaw: It has no connection to any deeper theory. MOND works, but no one has explained why. Newton's description of gravity was part of an enormous theoretical edifice. So was Einstein's.
MOND may turn out to be a piece of something larger, but for now it stands alone. "MOND is, I like to think, in an early stage of its development," says Milgrom. "The concept is there. The basic idea is there. Now I'm trying to understand where MOND comes from, because I see it as a derived concept. I think there should be some more basic underlying theory, some idea from which it will follow."
Maybe physicists shouldn't be too surprised if Milgrom turns out to be onto something. After all, Newton's laws have been modified in the past, most famously by Albert Einstein. According to Newton, gravity is a force that acts invisibly and instantaneously between any objects with mass.
Einstein had a different thought: There is no gravitational force. What we perceive as gravity is just a geometric effect, a consequence of the way massive objects distort the shape of space-time. That might seem abstract and impossible to visualize—after all, we can't see four dimensions.
But look at the graceful parabola traced by water squirting from a sprinkler, or watch a football arcing through the air. Both are following the curves of space-time created by Earth's mass. If Milgrom is right, Newton's and Einstein's laws will be in for some major tweaking. Milgrom suspects that a large part of that tweaking will have something to do with the value of his transition acceleration, a0.
"It turns out that this value has a cosmological significance," says Milgrom. "It is the acceleration you get by dividing the speed of light by the lifetime of the universe. If you start from zero velocity, with this acceleration you will reach the speed of light roughly in the lifetime of the universe. This is a cornerstone of all my searches for an underlying theory. It must be telling us something about the origin of the theory."
There's another mystery MOND might be able to explain—the mystery of Pioneer 10 and 11.
John Anderson, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, California, was analyzing data from Pioneer 10 and 11 in 1980 when he noticed that both spacecraft were decelerating in a way that violated Newton's laws, as if the sun's gravity had increased in strength.
"Something was exerting a force on the spacecraft that we didn't understand," says Anderson. "We thought we'd be able to explain it terms of forces generated by the spacecraft. And I really thought eventually it would go away as we got farther and farther from the sun. But it did not go away."
Anderson continued to monitor the signals, but it wasn't until late last year that he and five colleagues, including Michael Martin Nieto of Los Alamos, published an exhaustive 50-page analysis of the Pioneer mystery. They looked for evidence of onboard gas leaks, software errors—anything that might explain the anomaly. They found nothing. "We tried, and our friends tried, and our enemies tried. And nobody could come up with a smoking gun," says Nieto. "And we've really looked hard."
The Pioneer effect is real. For decades, something has been decelerating the spacecraft—or accelerating it toward the sun—at a rate that is eerily consistent with what MOND would predict: The acceleration is about one 10-billionth the acceleration we feel from gravity on Earth.
"I've been interested in Milgrom's stuff for a long time simply because it's a simple idea, a clever idea," says Nieto. "And I don't care if it's totally wrong; it's been totally useful. MOND has forced people to look much more closely at galaxy composition, dark matter, and gravity than they would have otherwise."
Nieto and Anderson would dearly love to see the launch of a mission dedicated to rigorously testing the Pioneeranomaly. Nieto is already drawing up plans for one and looking for backers. Over lunch at a café in Los Alamos, he talks excitedly about the mission and whether its discoveries might rock the foundations of physics. At one point he flings his arms out to describe the solar sails the spacecraft might have and accidentally whacks a waitress. I ask if he has come up with a name for the project.
"Hey, if she'll fund it, I'll call it the Anna Kournikova," he says. "Seriously, if God came down and said, 'OK now, bet your soul and tell me what's causing the Pioneer effect,' I'd say a systematic error. I would be surprised if we measured something that shows us gravity is acting differently, but I'd be very happy if it was.
A physicist has to keep very clear in his mind what he knows, what he thinks he knows, what he's suspicious of, and what he wants. I want Pioneer to be different. Who wouldn't? Of course, I'd love it to be something new. I'd love it. I'd definitely go out and stick my tongue out at my enemies. But that's different than saying I believe it is."
[This article originally appeared in print as "Nailing Down Gravity."]