Posted on 02/15/2003 7:40:45 AM PST by Phaedrus
If Newton saw today's astronomical evidence, would he come up with a different law of gravity? A growing number of people think so, says Marcus Chown
There's something wrong with our understanding of spiral galaxies such as our own Milky Way. The stars in their outer parts are being whirled around far too fast. Like children on a speeded-up roundabout, they should be flung into intergalactic space.
To explain why this does not happen, astronomers have been forced to propose that the visible stars and nebulae are supplemented by at least 10 times more invisible stuff. The gravity of this "dark matter" holds on to the fast-orbiting stars and stops them going AWOL. But not everyone is happy with this picture.
"If Newton were alive today and saw the evidence," says Mordechai Milgrom, of Israel's Weizmann Institute, "he would have come up with a different law of gravity." In Newton's absence, Milgrom has obliged. And a sizeable minority of astronomers think he may be on to something. For Milgrom, it began at Princeton's Institute for Advanced Study in the early 1980s.
He became intrigued by the mystery of the fast-orbiting stars. According to Newton, the force of gravity weakens with an "inverse-square law" - stars twice as far away from the concentration of mass at the centre of a spiral galaxy should be experiencing gravity four times as weak; three times as far out, nine times as weak, and so on. Consequently, the stars should be orbiting ever more slowly at greater distances from the centre.
This was not, what observations showed. The orbital speed of stars remained constant as far out from the centre as it was possible to see. The explanation that gained acceptance among astronomers was that every spiral galaxy was embedded in an enormous spherical "halo" of non-luminous matter.
The gravity of this dark matter enhances the gravity in the outer parts of spiral galaxies, enabling them to hold on to their stars. Nobody knows what the dark matter is made of, though possible candidates are hypothetical "subatomic" particles left over from the Big Bang. "The dark matter explanation was perfectly reasonable," admits Milgrom.
"But it was the assumption of minimum daring." Milgrom's dissatisfaction led him to explore a radical alternative. Newtonian gravity applies in the laboratory and in the solar system. If it breaks down, it must do so under conditions unlike those in the solar system. Milgrom therefore made a list of physical quantities that differ markedly between the domain of the solar system and that of spiral galaxies, and looked to see whether a change in the law of gravity when crossing a threshold in any of them could explain the puzzling observations.
The obvious thing was distance. Maybe gravity is different over very large distances. Milgrom discovered that this did not work. He tried other quantities. Finally, he came to acceleration. Perhaps gravity is different at low accelerations.
Say there is a critical acceleration, and that at much greater accelerations, gravity falls off with the familiar Newtonian 1/r<+>2 but at, much smaller accelerations, it weakens more slowly, with a 1/r law? When Milgrom applied his formula to the observations of spiral galaxies, he was jubilant.
"If the critical acceleration was a ten-billionth of a g, my formula fit all the existing observations," he says. "The dark-matter people have to have a different amount of dark matter in each galaxy."
Milgrom called his idea modified Newtonian dynamics, or Mond. The idea has been developed by a number of people, including Stacy McGaugh, of the University of Maryland at College Park, Bob Sanders, of the University of Groningen, and Jacob Bekenstein, of the Hebrew University. Others are sceptical. Says Craig Hogan, of the University of Washington in Seattle: "I bet it's not right."
Mond has made predictions that have been confirmed by subsequent observations of galaxies. But it is yet to come up with the killer prediction that will cause a stampede of theorists to the Mond camp. Milgrom doubts such a prediction exists. He believes it more likely that Mond will be accepted because of general dissatisfaction with dark matter - which wrongly predicts stellar motion in the centre of galaxies.
"The dark matter paradigm is so bad that truly radical ideas, like the breakdown of inverse-square law at large distances, deserve careful examination," says Matt Visser, of Victoria University in Wellington, New Zealand.
The main objection to Mond is that it is merely empirical and has no funda mental basis in physics. Milgrom admits this is a problem. But he counters that theories such as quantum theory had no basis in fundamental theory for decades after they were firstproposed.
"I am sure there is deep theory underlying Mond. We just haven't found it yet." But that may be about to change. This week, Marc Soussa and Richard Woodard, two physicists at the University of Florida in Gainesville, have shown that Mond can be derived from ideas in fundamental physics.
"This has been a major sticking point for most people," says Bruce Bassett, of the University of Portsmouth. The "field equations" the two physicists obtained reduce to Einstein's theory of gravity when gravity is strong and Mond when gravity is weak. There's a way to go. But things appear to be looking up for what could be the most radical shake-up of gravitational theory since Einstein.
· Marcus Chown's book, The Universe next door (Headline, £7.99) was published in January.
This is amazing. I had no idea.
No surprise there.
'If the theory don't fit - ain't havin' none of it!'
Einstein said I have deep faith that the principle of the universe will be beautiful and simple.
This theory strikes me as beautiful and simple and therefore I am drawn to it. The dark matter theory, OTOH, feels "kluged" - like plugging in a variable just so the equation will work. Moreover, we have much to learn about matter - even the existence of the Higgs boson is yet to be confirmed. The Higgs boson is the presumptive explanation for most matter.
It intrigues me that standing, doing nothing, is identical to experiencing 1 "g" of acceleration. Why would this be?
I suspect it is the same reason we only see in 3 dimensions. Here's what a four dimensional perception would "look like." BTW, that website has excellent graphics and animations to help understand the theory of special relativity!
"But it was the assumption of minimum daring."
What's the opposite of Occam's Razor? Occam's Flatiron? Occam's Bludgeon? Help me out here.
Easier to understand if galaxies are "seeded" by black-holes. Otherwise, this dark matter should be all around us, so massive as it would seem detectable. But alas....
"Theory doesn't have to be right. It only has to be interesting." -- Gregory Benford.
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