A California physicist believes he’s calculated a way to make a lot of the quantum state Bose-Einstein condensate required for a gamma ray laser.
We use gamma ray bursts in medicine and other fields, but a laser must be steady and stable, which so far has mystified scientists for decades.
Superfluid liquid helium, with no friction or viscosity, repels the condensate so it forms stable bubbles.
A scientist in California has taken steps toward a long-sought gamma ray laser by harnessing positronium bubbles in special liquid helium. Positronium is a volatile, short-lived atom that seems kind of like hydrogen but has a positron—an antiparticle considered opposite to an electron, sometimes even called an antielectron—instead of a proton.
Holding positronium in liquid helium extends its viable stability, a relationship that’s decades old: “Positronium's long lifetime in liquid helium was first reported in 1957,” says the press release, which links to a paper by physicist Richard A. Ferrell about the “reduced pickoff” positronium experiences when it can form a bubble inside liquid helium. The helium naturally repels the positronium, which forms a protective barrier between the bubble of positronium and the outside world.
The new study takes Ferrell’s observations a step further. Allen Mills, Jr. has calculated that the positronium isn’t just longer lived in the liquid helium—it’s stable. And in stable form, positronium can form what Dr. Mills calls a Bose-Einstein condensate, where the nature of the positronium pushes it into a quantum state, but the positronium atoms still adhere in one bulk.
The interactions of positronium in Bose-Einstein condensate form causes gamma rays. Those are the most energetic form of light, able to penetrate stone and concrete and outpace the speed of light. The secret is their tiny wavelength compared with other light. Gamma knife surgery uses gamma rays made from an isotope of cobalt that’s doctored to be radioactive. Cobalt gamma rays are also much of what we colloquially call “radiation” cancer treatment, because gamma rays are energetic enough to enter the body.
These applications of gamma rays are effective and important, but a hypothetical gamma ray laser must be stronger, more coherent, and more stable. Mills has determined a concrete theory that scientists can now test using real liquid helium and positronium. The helium must be not just liquid, but superfluid liquid helium, which, similar to a superconductor with no resistance, has no viscosity or friction.
The Bose-Einstein condensate phase is also a superfluid, first made in a lab in 1995. But to isolate it in superfluid helium is a big job, one that Mills’s lab will do by tuning a special antimatter beam. Teams around the world are always working on different solutions to and obstacles with the gamma ray laser problem, which is considered one of the most important unsolved problems in physics and even sometimes discussed as a pipe dream.
Any potential solution, like one proposed in 2018 that uses cesium gas, must combine extremely high-level knowledge of multiple disciplines and cutting-edge tools like supercooling and antimatter beams. After a hypothetical solution like Mills’s is proposed, scientists must then find a way to experimentally test the solution, which is really hard because of the extreme accommodations and delicate materials involved.
Observing these microscopic movements, even to demonstrate that what you have is a Bose-Einstein condensate, is also hard. To do so is a potential next goal of Mills’s gamma ray investigation team. “Near term results of our experiments could be the observation of positronium tunneling through a graphene sheet, which is impervious to all ordinary matter atoms,” he says in the press release.