Antimatter also suffers the effect of gravity – 09/27/2023 – Science

Antimatter, the very rare “twin sister” of the common matter that makes up stars, planets and our bodies, appears to be affected by gravity in the same way as an apple falling from the tree, reveals a new study. The conclusion comes from an experiment in which researchers were able to measure the “free fall” of antimatter atoms in the laboratory for the first time.

The work has just been published in the scientific journal Nature, one of the most important in the world. Led by Jeffrey Hangst, from the University of Aarhus, in Denmark, and with the majority participation of European scientists, the work is also signed by a quartet of Brazilians: Cláudio Lenz Cesar, Daniel de Miranda Silveira, Rodrigo Lage Sacramento and Álvaro Nunes de Oliveira.

Antimatter is characterized, among other things, by being a “sign-switched” version of the so-called ordinary matter that we find in the Universe. The electrons that are behind the electricity we use in everyday life, for example, have a negative charge. Protons, present in the nucleus of the atoms of all chemical elements, such as the carbon in graphite or the hydrogen and oxygen in water, have a positive electrical charge.

In the case of antimatter, all this is reversed. Instead of electrons, it has positrons (as the name suggests, their charge is positive) and antiprotons (with a negative charge). This means that, when matter and antimatter meet, the particles of both annihilate each other, leading to a large release of energy.

The problem is that models about the origin of the Universe predict that, strictly speaking, the Cosmos should not have “discriminated” against antimatter in its emergence. In other words, at this moment of genesis, equal amounts of so-called ordinary matter and antimatter should have appeared.

However, as this would have led to the annihilation of all particles, the Universe would have disappeared before it even began. Since ordinary matter ended up predominating by a huge margin, giving rise to all the structures we know, it is necessary to explain why antimatter became such a minority.

“There were many discussions about the issue of ‘antigravity’, which would resolve the issue of the apparent non-existence of antimatter in our Universe”, says Lenz Cesar, who works at the Physics Institute at UFRJ.

According to this idea, assuming that antimatter did not behave in the same way as common matter under the action of gravity, one would have “repelled” the other away – in other words, a type of “antigravity”, since the common version of this Force involves the attraction between objects with mass. Therefore, in the early days of the Cosmos, antimatter would have been completely separated from ordinary matter.

Another possibility has to do with the fact that the most consolidated theory today to describe gravity, the so-called theory of general relativity, has not yet been unified with quantum mechanics, which describes the behavior of subatomic particles.

“In this way, it would not be excluded that we might find a small difference in the gravitational attraction between matter and antimatter in relation to that between matter and matter”, summarizes the UFRJ researcher.

The new experiment faced these doubts by creating a kind of magnetic trap in which antihydrogen atoms were trapped – that is, the “antimatter twin” of the hydrogen present in water. While the most common form of hydrogen has a single proton in its nucleus, around which an electron rotates, antihydrogen atoms are formed by an antiproton and a positron.

The magnetic trap, when turned on, prevented the antimatter atoms from colliding with the wall of the structure and annihilating each other. By slowly decreasing the action of magnetism on the atoms, scientists could observe whether they tended to “fall downwards” – exactly what is expected from the action of gravity, of course, since the structure of the trap is vertical, like that of a bottle. Under the circumstances of the experiment, it would not be expected that all the atoms would fall. And, of course, it is possible to predict that the fall, if it happened, would occur at the rate expected by the Earth’s gravitational attraction, which produces an acceleration of 9.81 meters per second squared in the case of ordinary matter.

Result: around 80% of the antimatter atoms suffered annihilation by “falling” – a value that matches what is expected from a common hydrogen cloud. “This result is historic because it definitively buries the idea of ​​antigravity and ushers in an era of direct measurement of the action of gravity on antimatter,” summarizes Lenz Cesar.