Antimatter takes a free gravitational fall.

Antimatter takes a free gravitational fall

A hammer and a feather dropped simultaneously from a given height in an airless environment will hit the ground at the same time--as vividly demonstrated in 1971 by an astronaut who performed this experiment while standing on the moon's surface. In other words, influenced only by gravity, different objects fall with the same acceleration. But what would happen if one of the two test objects consisted entirely of antimatter? Would it fall in exactly the same way as ordinary matter?

According to Einstein's general theory of relativity, a particle and its antiparticle should accelerate identically in a given gravitational field, an idea enshrined in the so-called equivalence principle. But the tremendous technical difficulties associated with measuring the gravitational acceleration of an antiproton or position in free fall have so far prevented experimenters from directly testing this prediction.

Two groups of researchers have now independently reinterpreted existing experimental data to provide indirect evidence that, within limits, antimatter really does fall with the same acceleration as ordinary matter. They report their conclusions in separate papers in the Feb. 18 PHYSICAL REVIEW LETTERS.

Richard J. Hughes and Michael H. Holzscheiter of the Los Alamos (N.M.) National Laboratory use the results of an experiment performed at Harvard University by Gerald Gabrielse and his team. It showed that a proton and an antiproton have the same mass to a precision of four parts in 100 million (SN: 7/21/90, p.38). The Harvard researchers determined the relative masses of the particles in that experiment by measuring the frequencies at which protons and antiprotons orbit around magnetic field lines.

Using the idea that gravity acts on energy as well as mass, Hughes and Holzscheiter argue that any differences between the frequencies observed for protons and antiprotons set an upper limit on the extent to which the gravitational behavior of antimatter can differ from that of ordinary matter. "These particles are in the gravitational field of the whole universe," Hughes says. "If the gravity of the rest of the universe pulls on the kinetic energies of particles and antiparticles with different strengths, then the two [types of] particles would orbit at different frequencies."

The Harvard results show that any such effect, if it exists at all, must be exceedingly small.

Eric G. Adelberger and his colleagues at the University of Washington in Seattle rely on the results of high-precision experiments originally designed to ferret out a nongravitational, "fifth" force (SN: 9/22/90, p.183). They argue that if antimatter falls with an acceleration different from that of ordinary matter because of the influence of additional, gravitational forces like those that arise in various theories of quantum gravity, this effect would show up in a special way in their experiments, even though they performed their experiments using ordinary matter.

"The very great precision achieved in our fifth-force type of experiments allows us to say something really quite significant about the predicted effects of antimatter," Adelberger says. The results suggest that additional gravitational forces, which would have anomalous effects on antimatter, can't exist unless they exert an influence over such short ranges -- less than a few centimeters or so -- that there are no measurable, macroscopic effects.

Despite these efforts to resolve the question of which way antimatter falls, some researchers believe the real test will come only with a direct measurement of an antiparticle's gravitational acceleration. "I don't feel completely comfortable with all this nifty theory," Gabrielse says. "As always in this business, you look for the unexpected. There could be surprises there that we don't know about."