If Antimatter Fell Up
Physicist studies the gravity of antimatter's situation
Friday, June 2, 2000
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So he plans to make a beam of it in a mile-diameter underground trillion-electron-volt particle accelerator, fire the beam through a system of gratings, and measure how the alien matter falls. Naturally.
Antimatter may seem like the strangest stuff of science fiction. It is made up of antiparticles with charges opposite those of ordinary protons, neutrons and electrons  making them antiprotons, antineutrons and positrons.
Stranger still, when even a tiny amount of antimatter meets ordinary matter, the collision causes both to "annihilate," exploding with great force. On Earth, antimatter exists only in the rarefied environment of accelerator labs like CERN in Switzerland and Fermilab in Illinois. But some theorists have suggested that there may be entire antimatter galaxies somewhere out in the far reaches of space.
Like any good high-energy particle physicist, Phillips readily ventures into such exotic realms to design his experiments. The project he proposes uses the tools of his trade  particle accelerators, magnetic traps, transmission gratings  to test assumptions physicists make about gravity, including the assumption that gravity attracts antimatter just as it does ordinary matter.
Despite antimatter's oppositely charged particles, the theory of general relativity demands that it behave the same as regular matter under the influence of gravity. According to general relativity, gravity is actually just a consequence of Earth's mass warping the fabric of space-time, much as a bowling ball placed on a rubber sheet will warp the sheet. If antimatter fell up, that would either mean it has negative e
nergy in the context of general relativity, or that our theory of gravity is fundamentally flawed.At first, Phillips, an associate research professor whose "day job"  exploring particle physics  often takes him to Fermilab, was simply drawn to the idea of doing an experimental test of general relativity using the tools of high-energy physics. As he progressed, however, his motivation shifted.
"Antimatter falling up would be a huge surprise, but I've come to realize that our understanding of gravity may not be as good as we think it is," Phillips said. "We know that there are some problems with it, and I think it's valuable to test the assumptions on which general relativity is built, wherever we can."
One problem with general relativity is its incompatibility with quantum mechanics, which seeks to explain the behavior of fundamental particles.
For example, quantum theory says that there is enough energy in a vacuum that the universe should have enough mass to force it to collapse under its own gravity and disappear almost instantly. "We don't see that," Phillips deadpanned.
Another big question is why the universe seems to contain only matter and no antimatter. Although physicists have some theories about how this matter-antimatter asymmetry came about, the details of these theories don't quite work. If antimatter fell up, however, there could be equal amounts of matter and antimatter in the universe, but they would stay separated because of the gravitational repulsion between them.
"There's a potential to solve some real problems in cosmology if our theory of gravity is incorrect in ways that would be determined by my proposed experiment," Phillips said. "If you started out with even a tiny separation between matter and antimatter during the Big Bang that began the universe, gravitational repulsion would amplify and maintain the separation to keep all the matter and antimatter from annihilating."
Phillips proposes to begin his experiment by making antimatter, specifically "antihydrogen" from its building blocks, antiprotons and positrons. To make these antiparticles, he plans to use a large particle accelerator at CERN or Fermilab.
Next, he will isolate the antiprotons and positrons in magnetic traps  small rings placed inside long, cylindrical coils of superconducting wire and insulated by a very low-temperature vacuum.
By adjusting the voltage on the rings and currents in the coils, which act as powerful magnets, Phillips can use the facts that charged particles travel in circles in magnetic fields and that like charges repel to keep the antimatter suspended in the vacuum. Once the antiprotons are caught inside this positively-charged trap, Phillips plans to tweak the trap's voltage just enough so that a few antiprotons spill out and collide with the positrons in the other trap to form antihydrogen, a neutral atom.
"Since antihydrogen is neutral, it isn't trapped by the voltages  it just goes sailing right out the end of the magnet," Phillips said.
Once created, the antihydrogen beam will shoot through a set of three transmission gratings  Phillips compares these to the bars over sewer drains  and create a pattern of "hits" and "misses" on the other side, much as light shone through a narrow slit creates a pattern of light and dark lines on the opposite wall. Phillips can then analyze this pattern to determine how much  and in what direction  the antimatter fell.
He then plans to compare the antihydrogen measurement to a similar one performed on ordinary hydrogen to determine whether there is a difference between the two. This difference measurement will allow Phillips not only to find out whether antimatter falls up, but also to look for new, hypothetical forces that are much weaker than gravity, and which might affect matter and antimatter in different ways.
Phillips adds that there is an educational purpose to his proposed experiment, not just a scientific one.
"Most people think antimatter is science fiction, and it's kind of neat for them to find out that it really exists. High-energy and particle physics are hard for people to understand, but they can understand antimatter at some level, and they can certainly understand gravity at some level.
"So I think the experiment has value in educating people about physics, even if it just confirms that gravity acts on antimatter in the way our theories predict."
Written by Margaret Harris.
