Light and matter exhibit wave-particle duality, meaning they can behave both as particles and as waves. Recent experiments have demonstrated that antimatter also exhibits this duality. But what exactly is antimatter?
While antimatter may seem like a concept from science fiction, it is a genuine and intriguing phenomenon. Every particle of matter has a corresponding antimatter counterpart, which mirrors its properties but has an opposite charge. When a particle and its antiparticle come into contact, they annihilate each other, resulting in a release of energy.
The theoretical foundation for antimatter was established nearly a century ago through mathematical models, and it became a part of scientific reality four years later with the identification of the positron, the antimatter counterpart of the electron.
Although it was theoretically established that antimatter would also demonstrate wave-particle duality, this was not experimentally confirmed until 2019. A collaborative team of researchers from Switzerland and Italy conducted a double-slit experiment with individual positrons, revealing evidence of wave interference.
The original double-slit experiment, conducted in 1801, provided proof of light’s wave characteristics. In this experiment, light was directed through two closely spaced narrow slits, creating a pattern of alternating bright and dark areas on a screen rather than producing two distinct spots. This outcome indicated that light behaves as two interfering waves. A century later, Albert Einstein validated light’s particle nature through his explanation of the photoelectric effect.
Subsequent studies demonstrated that electrons also exhibited wave interference. However, to extend this research to antimatter, a suitable particle needed to be identified. The answer lies in positronium, an atom composed of an electron and its antimatter counterpart, the positron. Rather than annihilating each other, these two particles orbit around a common center for a brief duration, forming a matter-antimatter bound state.
What sets positronium apart from other atoms is its unique symmetry; it consists of two particles of identical mass. For example, hydrogen contains a heavy proton at its nucleus, with a lighter electron in orbit. In contrast, positronium is perfectly balanced, with both particles equally orbiting a shared center before ultimately annihilating.
This symmetry piqued scientists’ interest in how positronium would behave if it were transformed into a beam and subjected to a double-slit experiment. A research team from the Tokyo University of Science, led by Professor Yasuyuki Nagashima, along with Associate Professor Yugo Nagata and Dr. Riki Mikami, sought to explore this question. Their findings, recently published in Nature Communications, demonstrate for the first time that a positronium beam diffracts, exhibiting wave-like behavior similar to electrons.
The creation of this beam was an impressive technical achievement. The researchers first produced negatively charged positronium ions and then applied a precisely timed laser pulse to remove the extra electron. What remained was a high-speed, electrically neutral stream of positronium atoms, sufficient to generate clear interference patterns.
This beam was directed toward a layer of graphene, consisting of carbon atoms arranged in a honeycomb structure. The spacing between the carbon atoms closely matched the quantum wavelength of positronium, making it an ideal diffraction surface. As the positronium traversed the two-to-three-layer graphene sheet, a clear diffraction signal was detected, confirming its wave-like characteristics.
The experiment was conducted in an ultra-high vacuum environment, ensuring the graphene surface remained uncontaminated and the outcomes were clear.
What is particularly remarkable about this result is that positronium, made up of an electron and a positron, exhibited unified behavior rather than acting independently. Instead of diffracting separately, the two particles acted as a single quantum entity, creating one coherent wave.
Professor Yasuyuki Nagashima stated in an interview with Science Daily, “For the first time, we have observed quantum interference of a positronium beam, which can pave the way for new research in fundamental physics using positronium.”
This discovery, in addition to its theoretical significance, holds practical implications. Since positronium is electrically neutral, it can examine material surfaces without causing damage—unlike charged particles, which may disrupt sensitive materials. This property makes positronium particularly valuable for researching insulators or magnetic materials.
Dr. Nagata emphasized the significance of the experiment in advancing fundamental physics, stating, “It not only demonstrates positronium’s wave nature as a bound lepton-antilepton system but also opens pathways for precision measurements involving positronium.”
The observable universe initially emerged with matter slightly prevailing over antimatter. Continued exploration into the properties of antimatter could potentially reshape our understanding of the cosmos.
This article was compiled by Nityanjali Bulsu, an intern at The Indian Express.
