by Michael Irving
GENEVA, SWITZERLAND—Physics tells us that a hammer and a feather, dropped in a vacuum, will fall at the same rate—as famously demonstrated by an Apollo 15 astronaut on the Moon. Now, CERN scientists are preparing to put a spooky new spin on that experiment, by dropping antimatter in a vacuum chamber to see if gravity affects it the same way it does matter—or if antimatter falls upward instead.
For every particle of matter there’s a corresponding antimatter particle, which is identical in every way except that it has the opposite charge. That means that if matter and antimatter touch, they annihilate each other in a flash of energy—which understandably makes it tricky to study. Scientists at CERN first managed to trap and study the stuff back in 2010, albeit only for a fraction of a second. The following year that time was increased to a more useful 16 minutes.
Predictions say that antimatter particles should mostly follow the same rules as their normal counterparts, but it’s worth double-checking to be sure—after all, any other differences could bring into question the entire Standard Model of particle physics. A few years ago, the CERN team trapped and studied the optical spectrum of antihydrogen for the first time and, breathing a big sigh of relief, found that it was identical to that of hydrogen.
Another fundamental question is whether antimatter reacts to gravity the same way. Again, predictions say it should fall like regular matter, but there’s about a one-in-a-million chance that it actually falls up instead. So far, antimatter has only been studied while suspended in an electromagnetic trap, since letting it fall to the bottom (or top?) of any normal container will destroy it.
Two new experiments at CERN are ready to test out the problem. In both cases, after the antimatter is created, the scientists will switch off the electromagnetic traps holding it, then examine where in the tube the annihilations occur. That will allow them to measure the effects of gravity on antiatoms, and see if there are any discrepancies.
The main difference between the two experiments is how they go about creating the stuff, and getting it ready for the drop. The first, known as ALPHA-g, is based on the existing ALPHA equipment that allows scientists to create and trap antimatter, but turns it vertically. Antiprotons are collected from the Antiproton Decelerator (AD) and bound to positrons (or anti-electrons) to create neutral anithydrogen atoms. That neutrality is important, since carrying a charge could have an effect on the results and obscure the influence of gravity.
The second experiment, known as GBAR, gets its antiprotons from the ELENA deceleration ring, and combines them with positrons from a small linear accelerator. Together, that makes antihydrogen ions, which are then ultracooled to 10 microkelvin and made neutral by stripping them of a positron by hitting them with a laser. The resulting neutral antihydrogen is then subjected to the drop test.
Unfortunately, both experiments are racing the clock. In a few weeks’ time, CERN’s accelerators are due to be shut down for two years as the facility undergoes an intensive upgrade known as the High-Luminosity Large Hadron Collider (HL-LHC). Ideally, one or both will be able to be conducted before that happens.