A `crazy' idea that has prompted some useful investigation
EPHRAM FISCHBACH has done physics a favor. He's come up with the kind of ``crazy'' idea that spurs research. He suggests that gravity is not quite what textbooks say it is -- a force that depends solely on the mass of interacting bodies and not on their composition. In fact, Fischbach and his colleagues speculate that a previously undetected atomic force slightly counteracts gravity at short distances. That force would depend on a body's composition.
Experimenters in a number of laboratories have been scrambling to test this notion since it was detailed in Physical Review Letters last January. Meanwhile, Shu-yuan Chu of the University of California, Riverside, and Robert F. Dicke of Princeton University recently published a critique in Physical Review Letters that discounts Fischbach's force. But the results of the new experiments aren't yet in. And the question is unsettled.
Fischbach's force nags at physicists the way hints of a poorly suppressed scandal nag at politicians. Indeed, to borrow a phrase from the editor of the journal Nature, physicists have been ignoring ``a minor scandal'' over gravity for decades. They can't determine G -- the constant of gravitation they use to calculate gravity's force -- to a precision better than one part in 100,000. That's rather poor compared to the precision of other basic physical constants. The uncertainties in measuring G could well hide the action of a weak antigravitational force that operates only over distances of less than about 1,000 feet.
Such a force would be a fifth basic force of nature. The other four are gravity (as we now know it), electro-magnetism, and the strong and weak forces that hold atoms together and regulate some forms of radioactivity.
The experimenters -- Fischbach of Washington University in St. Louis; Daniel Sydarsky, Aaron Szafer, and Carrick Talmadge of Purdue University; and S. H. Aranson of the Brookhaven National Laboratory -- drew inspiration from the errors of one of physics' classic series of experiments. From 1888 to 1922, Hungarian physicist Roland von Eotvos and various colleagues published studies that showed no difference in the gravitational action of materials as diverse as tallow and copper.
To a precision of better than one part in a million, they confirmed, in effect, that a feather should fall as fast as a brick, except for air resistance. Later results refined that precision a thousandfold. (These measurements did not, however, help determine G more accurately.) For over half a century, physicists have accepted these results as solid proof that it is only the mass of a body, and not its composition, that matters where gravity is concerned.
Fischbach and his colleagues took a new look at the old data and said these were ``sensitive to the composition of the material used'' after all. Eotvos had discounted small discrepancies between what he observed and what standard gravitational theory predicted. Fischbach and his co-workers now consider those ``experimental errors'' significant. They could be accounted for, they suggest, by the fifth force.
Physicists have speculated about this force for a couple of decades. It would arise from the subtleties of atomic composition. Atomic nuclei are made up of two kinds of related particles -- protons and neutrons. These belong to a particle family physicists call baryons. Baryons account for almost all -- but not quite all -- of an atom's mass. The rest of the mass is contributed by electrons orbiting the nucleus and by the mass of the so-called binding energy of the nucleus itself. The relationship between the baryons in the nucleus and the total mass varies among the elements and could give rise to the fifth force.
Many theorists are comfortable with this concept since they consider the basic material forces to be aspects of an ultimate single force. In fact, they anticipate that the other forces should ``chip in'' to moderate gravitational interactions over short distances.
But it's a big step from such theorizing to saying that the old Eotvos data actually reflect this effect, as Chu and Dicke now point out. They show that Eotvos's discrepancies could well be due to temperature variations that caused air currents in his laboratory. Such measurements were -- and still are -- delicate.
Fischbach says he has taken this into account. Nevertheless, he concedes that the objection has to be looked into seriously.
At this point, physicists can only wait hopefully for more definitive results from the many experiments now underway. No one knows what they will show about a fifth basic force. But the biggest payoff of this renewed interest in gravity may be a more precise number for G.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.