People often have an unpleasant experience with gravity even in infancy: when their first attempts to walk end in a flop. We know from experience that gravity is always attractive, i.e. attractive – but it is nevertheless explicitly taught in physics courses. This makes gravity different from electromagnetism, another fundamental interaction. There are two possibilities: attraction and repulsion.
But couldn’t the gravitational force also have a repelling effect? What happens when antimatter enters the Earth’s gravity field? In a magnetic field, for example, positrons fly in the opposite circular direction as their antiparticles, the electrons. So could it be that instead of falling to the ground like a flopping baby, antimatter is tumbling into the sky?
Such antigravity has long existed in science fiction novels and films. The force allows vehicles or platforms to float above the surface of planets without contact with the ground. But a coalition of researchers is now rejecting such dreams. According to them, it can be ruled out that there is such a thing as antigravity. Like the team led by physicist Jeffrey Hangst from Aarhus University in the journal Nature reported, resulted in an experiment called “Alpha-g”. Nuclear Research Center CERN that antimatter obeys gravity in the same way as matter.
The researchers produced antihydrogen for their experiment
Alpha stands for “Antihydrogen Laser Physics Apparatus”, the suffix “g” is the symbol for gravitational acceleration. Alpha is a facility where experiments are carried out with antimatter, the mysterious twin of elementary particles, atoms and molecules. Antimatter includes all antiparticles such as antineutrinos or positrons. Even anti-atoms can be created, and that’s exactly what the Alpha collaboration did: By combining positrons and antiprotons, the scientists produced so-called antihydrogen. The antiprotons for this were provided by CERN’s “Antiproton Decelerator” and cooled down using the “Elena” deceleration ring, as physicist Patrick Mullan, who was involved in the study, explains.
In an upright column, the antiparticles were trapped in a section of the column using two magnetic traps. Antihydrogen has a neutral electrical charge and cannot be manipulated using electric fields. However, it can be held in place using magnets because the antihydrogen’s own magnetic moment interacts with external magnetic fields. After antihydrogen atoms were enriched in several cycles between the two magnetic traps, the scientists reduced the magnetic field strength of the traps and observed what happened.
“To put it simply, we are conducting a kind of Leaning Tower of Pisa experiment,” Jonathan Wurtele, a researcher involved, is quoted as saying in a press release. According to legend, Galileo Galilei suggested that two objects with identical volumes but different masses should fall from the tower in Pisa at the same time. Such free fall experiments are now a classic in physics lessons: If the free fall takes place in a vacuum, i.e. in an airless glass tube, for example, a metal ball and a spring hit the ground at the same time.
Around 80 percent of the antihydrogen particles released fell downwards in the alpha-g experiment. Because of the weak influence of gravity on the particles, this corresponds exactly to the behavior that normal hydrogen would have shown under the same conditions.
When matter meets antimatter, the particles destroy each other
On the one hand, the experiment confirms what was already theoretically expected: “Einstein’s general theory of relativity says that antimatter should behave exactly the same as matter,” says Wurtele. On the other hand, the researchers succeeded in a difficult experiment. Because antiparticles are extremely volatile. Positrons, for example, which are used in medicine as diagnostic tracers, are obtained through radioactive decay – as was the case with the alpha-g experiment. Once created, antiparticles immediately disappear again; the effect is called annihilation. For example, as soon as a positron hits an electron, the two particles destroy each other, leaving only energy in the form of radiation. Although this is useful in positron emission tomography (PET), the volatility of antimatter causes problems for particle physicists.
In order to reduce the probability of pair annihilation, the Alpha collaboration used antiprotons that hardly moved at all. At the same time, scientists used annihilation to be able to measure anything at all. If the antihydrogen is let out of the magnetic trap, it hits particles of matter and “irradiation” occurs: flashes of light are created, which were recorded using detectors arranged cylindrically around the column. The location of the matter-antimatter extinction could be reconstructed from the flashes of light – and thus it could be determined that antimatter is subject to gravity as predicted by Albert Einstein.
But there is one downside: if it had been shown that there is a repulsive form of gravity, physicists might now be smarter about other questions. Because the Big Bang was supposed to have created equal amounts of matter and antimatter. But why haven’t the two destroyed each other long ago – and why can so much matter be observed in the universe today and hardly any antimatter? A previous theory explained this so-called baryon asymmetry with gravity: This may have a slightly different effect on antimatter than on matter and thus ensure an excess of matter. But that is apparently not the case. So asymmetry remains a mystery.