Where gravity falls down

Discovering a new law of nature is the acme of scientific achievement, and one granted to few. Those who succeed are assured of a place in the pantheon of science, rubbing shoulders with the likes of Archimedes, Newton and Einstein. Now a new name may be destined to join their ranks, as evidence builds for his new view of one of the cornerstones of physics: the law of gravity.

For over 25 years Professor Mordehai Milgrom of the Weizmann Institute in Israel has been pursuing the possibility that both Newton and Einstein missed something when they devised their theories of this most ubiquitous of forces. Newton portrayed gravity as some kind of mysterious influence that allows masses to affect each other even through the vacuum of space. While declining to say exactly how this influence worked, Newton came up with a precise mathematical description of its effects, in the form of his celebrated "inverse-square law" of universal gravitation.

Supposedly inspired by watching an apple fall in his mother's garden almost 350 years ago, Newton's law remained the best description of gravity until 1915, when Albert Einstein published his general theory of relativity, which gave the first detailed account of what gravity actually is. According to Einstein, mass warps the very fabric of space and time around it, rather like a cannonball sitting on a vast rubber sheet. This creates the illusion that objects moving past some mass are accelerated by a mysterious "force" emanating from it. In reality, they are just responding to the distortion of space and time - the effect of which is described in detail by Einstein's theory, and captured pretty well even by Newton's simple formula.

Pretty well, but not perfectly: Einstein showed that Newton's formula starts to break down when gravitational fields become very strong - for example, close to stars or black holes. Since the early 1980s, Prof Milgrom has suspected there is another flaw in Newton's venerable formula - one which even Einstein failed to fix. And after decades of being ignored by the scientific establishment, there is mounting evidence that he is right.

Prof Milgrom's theory goes by the prosaic name of Modified Newtonian Dynamics or MOND, and is based the bizarre idea that Newton's law of gravity breaks down at low accelerations. And he means very low: around 100-billionth that generated by the Earth's gravity. Like Newton, Prof Milgrom was inspired by a simple observation - albeit a rather more esoteric one than the fall of an apple. During the 1970s, astronomers discovered something odd about the movement of stars in galaxies. Like the planets orbiting our sun, the stars should follow Newton's law of gravity, and travel ever more slowly the further out they are from the galactic centre. Yet beyond a certain distance, their speeds remained more or less constant - in flat contradiction of Newton's law.

Astronomers quickly proposed a solution: that there are huge amounts of invisible "dark matter" lurking in and around galaxies, whose gravitational pull invisibly affects the stars. But Prof Milgrom had a more radical proposal: that there is something wrong with the law of gravity itself. His calculations suggested that the anomalous motion of the stars could be explained if Newton's law breaks down for masses accelerating below a critical rate of around one ten-billionth of a metre per second per second.

If this were the only anomaly cleared up by MOND, few scientists would take it seriously. But over the years, Prof Milgrom and others have found other puzzles that MOND seems able to explain, such as unexpected connections between the brightness of galaxies and the motion of their stars, and the so-called Pioneer Anomaly. Named after the two Nasa probes launched in the early 1970s and now travelling far beyond the solar system, the Pioneer Anomaly is a very gradual slowing in the speed of the probes. While so far unexplained by conventional physics, the rate at which the probes are slowing just happens to match the critical acceleration found by Prof Milgrom from studies of galaxies.

A coincidence? Perhaps: certainly, many scientists still view MOND with suspicion - not least because there's no real understanding of why the universe should possess a critical acceleration. Most still prefer the standard explanation of the anomalous motion of stars, which calls for huge amounts of invisible "dark matter" lurking around galaxies. But now two international teams of astronomers have published the results of studies able to decide between the two explanations - and they seem to back Prof Milgrom and MOND.

The astronomers have been studying the motion of stars in small "satellite" galaxies that accompany our own Milky Way. These are expected to be devoid of any dark matter, and so shouldn't show any of the anomalous behaviour usually attributed to its presence. Yet the teams still found that the stars were travelling far faster than predicted by Newton's law of gravity - just as Prof Milgrom's MOND theory predicts.

Taken at face value, the findings - reported in two leading astrophysics journals last week - are the best evidence yet that there is something missing from the standard law of gravity. What's needed now is some theory of why the law breaks down. One possible culprit is an effect due to quantum theory, the laws of the sub-atomic world that were unknown to Newton and ignored by Einstein. Prof Milgrom himself has suggested that so-called vacuum effects may play a role. According to quantum theory, even supposedly empty space is seething with particles and energy, constantly popping in and out of existence. This vacuum energy is normally undetectable, but in certain circumstances its presence can be revealed. And that includes any attempt to accelerate through this invisible quantum sea.

Newton once said that he saw himself as "only like a boy playing on the seashore... whilst the great ocean of truth lay all undiscovered before me". Prof Milgrom may have given us our first glimpse of what lies beyond that shoreline. Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England