Two bodies with mass attract each other. This is the law of gravity, the weakest of the four fundamental forces of nature. But what if one of the objects is made of antimatter instead of matter? Nothing changes. The gravitational force exerts its influence just the same. The results of an international experiment, conducted with participation from Brazilian scientists at the European Organization for Nuclear Research (CERN), near Geneva, Switzerland, has just confirmed this principle.
When released from the magnetic device that confined them inside a vertical trap half a meter high, antihydrogen atoms began to free fall towards Earth in the same way as hydrogen atoms. Antimatter did not “fall upwards” due to the effect of supposed antigravity, as has been speculated in the past.
“In physics, you don’t really know something until you observe it,” said American physicist Jeffrey Hangst, from Aarhus University, Denmark, in a press release. Hangst is a spokesman for the international collaboration Antihydrogen Laser Physics Apparatus (ALPHA), one of CERN’s largest experiments, which conducted the research with antihydrogen atoms. “This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter.”
The results of the research were published in an article in the science journal Nature on September 27. “It’s very difficult to isolate and measure the action of gravity on these particles,” comments physicist Cláudio Lenz, from the Federal University of Rio de Janeiro (UFRJ), one of the study’s authors and the coordinator of Brazilian participation in the ALPHA project. “Any noncontrolled magnetic field has a greater effect on them than gravity and can interfere with the measurement.” Three other Brazilians are on the list of 70 researchers who signed the paper. Brazil’s participation in the collaboration is funded by the National Council for Scientific and Technological Development (CNPq) and the Rio de Janeiro State Research Support Foundation (FAPERJ).
Originally proposed by Lenz in 1997, the experiment’s design is relatively simple to understand. Its implementation, however, is complex. The ALPHA collaboration researchers created and stored antihydrogen atoms within a tightly controlled environment in order to determine the influence in isolation of gravity on antimatter.
Inside a magnetic trap, consisting of a vertically positioned cylindrical structure, around a thousand antihydrogen atoms were kept in extreme vacuum conditions at very low temperatures, on the order of 4º Kelvin (-269º Celsius). Almost completely frozen, the antiatoms were still moving with small, but not negligible, amounts of kinetic energy. To ensure that the antimatter particles could not escape before the appropriate moment, two identical magnetic fields, one at the top of the trap and the other at the bottom, were applied to the antihydrogen atoms and acted as barriers.
“We then slowly reduced these magnetic fields and let the antihydrogen escape,” explains Lenz. Those that ascended were recorded by a detector at the top of the trap. Those that descended were captured by another detector at the bottom of the cylindrical container. This procedure was repeated many times, with samples of approximately one thousand antiatoms.
Maximilien Brice / Alpha Collaboration
Recording the annihilation process of loose antihydrogen atoms on the inner surface of a magnetic trapMaximilien Brice / Alpha CollaborationAt the end of all the rounds, the experiment concluded that around 80% of the antihydrogen atoms descended, that is, they went into free fall. If subjected to the same conditions, hydrogen atoms present identical behavior in a similar proportion, due to the action of gravity.
It is expected that a certain number of both atoms and antiatoms will rise rather than fall, due to the presence of kinetic energy. The margin of error of the study’s final data is relatively high, up to 29%. According to the researchers, however, the results indicate that the action of gravity on antimatter is compatible with that exerted on matter and eliminates the possibility of antigravity.
The influence of gravity on any type of matter was predicted by Albert Einstein (1879–1955). “The result of this new experiment is important because it confirms the equivalence principle, one of the pillars of the theory of general relativity, according to which all forms of energy, whether matter or antimatter, respond to the effect of gravity in the same way,” explains George Matsas, from the Institute of Theoretical Physics at Paulista State University (IFT-UNESP), who did not participate in the work conducted at the LHC.
The concept of what is now known as antimatter began to emerge after the formulation of general relativity, through the work of English theoretical physicist Paul Dirac (1902–1984) beginning in 1928. However, the predominant view has always considered that the equivalence principle, one of the fundamental laws of physics, also applies to antiparticles. There were signs this was true, but there was a lack of more precise data such as that now provided by the international collaboration.
The experiment at CERN worked with antihydrogen because it is the simplest type of antimatter atom that exists and the easiest to produce, store, and manipulate in the laboratory. According to the standard model of physics, every particle has a corresponding antiparticle of equal mass and energy, but with an inverted electrical charge. The same occurs with atoms and antiatoms of an element.
Like the hydrogen atom, which consists of one electron (with a negative charge) and one proton (positive), the antihydrogen atom is composed of one positron (the counterpart of the electron, but with a positive charge) and one antiproton (the negative equivalent of a proton). The Big Bang, the primordial explosion that originated the Universe 13.8 billion years ago, would have produced the same amount of matter and antimatter since they are generated simultaneously. When an atom meets its antiatom, they annihilate each other. They cease to exist.
However, this did not occur perfectly throughout the evolution of the Universe. For unknown reasons, perhaps more particles than antiparticles were created, and this excess matter ended up not being annihilated. Cosmologists call this problem matter-antimatter asymmetry.
“Preventing the annihilation of antihydrogen inside the magnetic trap was one of the biggest challenges we faced in the experiment,” says physicist Rodrigo Sacramento, from UFRJ, another of the ALPHA study’s authors. “We couldn’t let the antiatoms come into contact with hydrogen atoms, or they would be destroyed. Therefore, we needed an environment with a highly controlled vacuum.” The ALPHA collaboration scientists hope to improve their results with new versions of the experiment that will allow them to reduce the margin of error.
Scientific article
ANDERSON, E.K. et al. Observation of the effect of gravity on the motion of antimatter. Nature. Sept. 27, 2023.
