Antimatter Behaves Like Matter, Confirming Einstein's Theory of Gravity

In a groundbreaking experiment, scientists conducted a unique investigation involving antihydrogen atoms, observing their descent and concluding that gravity, contrary to repelling antimatter, actually attracts it toward Earth. This discovery serves as a confirmation of a fundamental aspect of Albert Einstein's general theory of relativity known as the weak equivalence principle. According to this principle, gravity exerts an equal force on all objects, regardless of their composition. Physicist Ruggero Caravita, who was not part of this new study, emphasized the central role of this concept in our understanding of gravitational phenomena.

Antimatter is essentially the counterpart of conventional matter, sharing the same mass but bearing an opposite electric charge. For instance, the antiparticle of an electron is the positively charged positron, while the antiproton serves as the counterpart to a proton, carrying a negative charge, and so forth.

 The notion that antimatter might fall upward instead of downward was not taken seriously by most physicists. However, until now, there had been no direct means of testing this hypothesis. Jeffrey Hangst, representing the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration, which published these findings, emphasized the enigmatic nature of antimatter and the need to substantiate its behavior.

 The results of the experiment not only revealed that antimatter behaves as anticipated but also indicated that it falls with nearly identical acceleration as regular matter.

 Published in the September 28 issue of Nature, these results highlight the growing mastery of scientists over antimatter, particularly antihydrogen. Working with antimatter can be a complex endeavor because it swiftly annihilates upon contact with any form of ordinary matter. Hangst stated that it took decades of research to even detect any gravitational effect on antimatter.

 In this experiment, conducted at CERN, the European laboratory near Geneva, researchers confined antihydrogen atoms using powerful magnetic fields. These antihydrogen atoms were created by combining antiprotons generated at CERN with positrons sourced from a radioactive material. Subsequently, the researchers released the antihydrogen from its magnetic confinement and recorded the number of atoms that moved upward versus those that descended. As per the expectation that gravity affects antimatter and matter similarly, the majority of atoms should have fallen downward, with a few ascending due to the initial disturbances within the atom group, which is precisely what the research team observed.

 Theoretical physicist Yunhua Ding of Ohio Wesleyan University, who was not involved in the study, described this experiment as an elegantly simple and compelling concept.

 To further validate the expected behavior of antihydrogen, the scientists modified the magnetic fields to induce upward movement in the atoms, thereby neutralizing the influence of gravity. In this scenario, roughly equal numbers of atoms moved upward and downward. Varying the magnetic fields further conformed to these expectations.

 Previous experiments had already suggested that gravity exerts the same pull on matter and antimatter. In 2022, the BASE experiment at CERN reported that oscillations of confined antiprotons indirectly confirmed that both matter and antimatter respond identically to gravity. However, the ALPHA experiment marks the first direct observation of antimatter particles descending under the influence of gravity.

One of the early accounts of Galileo's scientific endeavors highlights his renowned experiment involving the release of iron balls with varying weights from the summit of the iconic Leaning Tower of Pisa. Galileo's aim was to demonstrate that all objects descended at the same rate, irrespective of their mass.

Historically Aristotle's scientific framework proposed that objects fell to Earth because they were inherently driven to reach their designated natural locations. Several experiments were conducted at various locations with matter. However the experiment with antimatter had never been conducted until now.

Although physicists did not anticipate antimatter falling upward, some researchers had proposed the possibility of antimatter falling with a slightly different acceleration than regular matter. Caravita, representing the AEgIS collaboration at CERN, stressed that even the slightest difference would indicate the presence of new and unexplored phenomena. While the current experiment lacks the precision to detect such subtle distinctions, future tests employing innovative techniques, such as cooling antihydrogen atoms with lasers, could potentially enhance the accuracy of these investigations. This ongoing research aims to elucidate whether gravity maintains its impartiality when it comes to matter and antimatter.

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Source:

E.K. Anderson et al. Observation of the effect of gravity on the motion of antimatter. Nature. Vol. 621, September 28, 2023, p. 716

Credit:

Video: Keyi "Onyx" Li/U.S. National Science Foundation