The majority of atoms consist of positively charged protons, neutral neutrons, and negatively charged electrons. An exotic type of atom known as positronium comprises one negative electron paired with a positively charged antimatter positron. Although it has a very brief existence, scientists, including a team from the University of Tokyo, have successfully cooled and slowed down positronium samples using specially adjusted lasers. This breakthrough is expected to aid further exploration into unusual forms of matter and potentially reveal the mysteries surrounding antimatter.
There is a part of our universe that remains elusive. You might have encountered such an odd remark while reading about cosmology in recent years. Scientists make this claim because nearly everything we observe in the universe is composed of matter, including ourselves and the Earth beneath our feet. For quite some time, the existence of antimatter has been recognized, which, as its name indicates, serves as a counterpart to regular matter. Antimatter particles possess the same mass and other attributes as their matter counterparts, but have opposite charges. When matter and antimatter collide, they annihilate each other, and it is widely accepted that they were created in equal measures at the beginning of time. Yet, that balance does not reflect what we observe today.
“Current physics accounts for only a fraction of the total energy in the universe. Investigating antimatter might help us understand this gap, and our recent research marks a significant development in this area,” stated Associate Professor Kosuke Yoshioka from the Photon Science Center. “We have successfully managed to slow down and cool exotic atoms of positronium, which consists of 50% antimatter. For the first time, this allows us to investigate it in ways that were previously not possible, and this will fundamentally include a more thorough examination of antimatter.”
Positronium may sound like a concept from science fiction, and while it has a very fleeting existence, it is indeed real. Picture it as a hydrogen atom, which features a central, positively charged proton and a tiny, negatively charged electron in orbit. Now, replace the proton with its antimatter counterpart, the positron. This results in an exotic atom that is electrically neutral, yet it lacks a substantial nucleus; instead, the electron and positron orbit each other, forming a two-body system. In comparison, hydrogen is a multi-body system since a proton is actually composed of three smaller particles called quarks held together. Thanks to positronium’s nature as a two-body system, it can be accurately described using traditional mathematical and physical theories, making it an excellent candidate for precise experimental validation.
“For researchers engaged in what’s known as precision spectroscopy, being able to precisely assess the properties of cooled positronium allows us to compare our findings against rigorous theoretical calculations of its characteristics,” explained Yoshioka. “Positronium is among the few atoms formed entirely from just two fundamental particles, facilitating such exact computations. The notion of cooling positronium has existed for approximately three decades, but a casual remark made by undergraduate Kenji Shu, who is currently an assistant professor in my team, inspired me to tackle this challenge, and we have finally succeeded.”
Yoshioka and his team faced several challenges while working to cool positronium. The first hurdle was its short lifespan: only one ten-millionth of a second. The second challenge stemmed from its exceptionally light mass. Because it’s so lightweight, traditional cooling methods using cold surfaces couldn’t be utilized; instead, the team relied on lasers. You may think of lasers as hot, but in reality, they are simply packets of light, and the effect they have is determined by how the light interacts with the target. In this case, a weak and finely-tuned laser applies gentle pressure against a positronium atom, slowing it down and consequently cooling it. By performing this process repetitively in just one ten-millionth of a second, they managed to reduce the temperature of portions of positronium gas to nearly 1 degree above absolute zero (-273 degrees Celsius). Given that the positronium gas initially sat at 600 kelvins or 327 degrees Celsius before cooling, this represents a remarkable shift within a brief time frame.
“Our computer simulations, based on theoretical models, indicate that the positronium gas might be even cooler than our current measurements can capture. This suggests that our specialized cooling laser is remarkably effective at reducing positronium’s temperature, and these concepts might assist other researchers in studying different exotic atoms,” Yoshioka commented. “In this experiment, we utilized a laser in just one dimension, but if we can apply it in all three dimensions, we will be able to measure positronium’s properties with even greater precision. These experiments hold significance because they may provide insights into how antimatter interacts with gravity. If antimatter behaves differently than regular matter in the presence of gravity, this could offer explanations for the missing parts of our universe.”
