Antimatter remains a puzzle in modern physics. While matter forms everything visible in the universe, antimatter is much less apparent despite theoretical predictions that matter and antimatter were created in equal amounts during the early universe. This discrepancy leaves many questions unanswered about what happened to antimatter.
"Modern physics only accounts for a part of the total energy of the universe. The study of antimatter might help us account for this discrepancy, and we've just taken a big step in this direction with our latest research," explained Yoshioka. "We have successfully slowed and cooled down exotic atoms of positronium, which is 50% antimatter. This means that for the first time, it can be explored in ways previously impossible, and that will necessarily include a deeper study of antimatter."
Positronium, though short-lived, is an exotic atom similar to hydrogen but with a positron replacing the proton. It forms a two-body system consisting of an electron and positron, making it ideal for precise experimental tests due to its simple structure. Unlike regular atoms like hydrogen, which are more complex due to their internal quark structure, positronium can be fully described by traditional physical theories.
"For researchers like us, involved in what is called precision spectroscopy, being able to precisely examine the properties of cooled positronium means we can compare them with precise theoretical calculations of its properties," said Yoshioka. "Positronium is one of the few atoms made up entirely of only two elementary particles, which allows for such exact calculations. The idea of cooling positronium has been around for around 30 years, but a casual comment by undergraduate student Kenji Shu, who is now an assistant professor in my group, prompted me to take on the challenge of achieving it, and we finally did."
The research team faced several challenges in cooling positronium. Due to its short lifespan - lasting only one-ten millionth of a second - and lightweight nature, traditional cooling methods couldn't be used. Instead, the team relied on laser cooling. Lasers may seem hot, but they are actually just light, and depending on how the light is applied, it can exert a cooling effect. The team used a carefully calibrated laser to slow the movement of positronium atoms, cooling them to just 1 degree above absolute zero (-273 degrees Celsius).
"Our computer simulations based on theoretical models suggest that the positronium gas might be even colder than we can currently measure in our experiments. This implies that our unique cooling laser is very effective at reducing the temperature of positronium and the concepts can hopefully help researchers study other exotic atoms," Yoshioka added. "This experiment used a laser in just one dimension, however, and if we utilize all three, we can measure the properties of positronium even more precisely. These experiments will be significant because we may be able to study the effect of gravity on antimatter. If antimatter behaves differently to regular matter due to gravity, it could help explain why some of our universe is missing."
The implications of this cooling technique go beyond just positronium. By applying the laser cooling method in multiple dimensions, researchers could unlock new insights into the behavior of antimatter and its interaction with fundamental forces like gravity. This could help solve the mystery of why the universe seems to lack the antimatter that was thought to exist in equal quantities alongside matter.
Research Report:Cooling positronium to ultra-low velocities with a chirped laser pulse train
Related Links
Department of Applied Physics at TokyoU
Space Technology News - Applications and Research
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