Ghostbusters Science: Gamma-Ray Lasers Could Be Made From Positronium Antimatter

Scientists from the University of Maryland Joint Quantum Institute have calculated that it might be possible to build gamma-ray lasers from mixing antimatter and matter together to make a compound called "positronium", which would be used to turn ordinary light into a laser beam.

This might sound a bit like it comes from the movies, such as Ghostbusters (positron colliders and particle accelerators, anyone?), but the concept actually comes from a 20-year-old theory.

In 1994, two Bell Laboratories physicists Philip Platzman and Allen Mills Jr proposed that a gamma-ray laser could be made from a Bose-Einstein condensate (BEC) of positronium, which is the simplest atom made of both matter and antimatter.

A Bose-Einstein condensate is an unusual chilled gas state, and 20 years ago, a BEC of any kind of atom did not yet exist.

Today however, there are now BECs made from 13 different elements and scientists Yi-Hsieh Wang, Brandon Anderson and Charles W Clark have found that that building such a gamma-ray laser from positronium might indeed be possible, according to their study, entitled "Spinor Bose-Einstein condensates of positronium", published in the journal Physical Review A.

What is positronium?

Each atom of positronium is made up of an ordinary electron and a positron (the antimatter equivalent of an electron).

Positrons are positively charged, while electrons are negatively charged, and if the two touch, they mutually annihilate within a billionth of a second, releasing two photons, i.e. light, at high energies which move in opposite directions in the gamma-ray range.

Atoms are formed by electrons and protons orbiting each other, and similarly, electrons and positrons are able to spin around each other, but positrons are much lighter than photons.

So, in order to make a gamma-ray laser out of anti-matter and matter, the positronium would have to be chilled as close to absolute zero, which is -273 degrees Celsius, so that it becomes stable.

Once chilled, the positronium becomes a BEC and the electrons and positrons pair up, becoming a single atom which is called "Ps".

The speed that the electrons and positrons spin at in BEC adds up to 1, so that means the positronium is numbered spin-1 because it takes a fraction of a nanosecond longer to annihilate itself.

Mutual annihilation

In order for the gamma-ray laser to function properly, a light pulse with far-infrared wavelengths will be needed to convert positronium to spin-0, so that the atom can annihilate itself when the electrons and positrons touch, thus forming a bidirectional laser beam.

"The idea to try and make a Ps BEC, and from this an annihilation laser, has been around for a long time, but nobody has really thought about the details of how a dense Ps BEC would actually behave, until now," explained Dr David B Cassidy, a positronium experimentalist at University College London, who was not involved in the study.

"This work neatly shows that the simple expectation that increasing the Ps density in a BEC would increase the amount of stimulated annihilation is wrong! Although we are some years away from trying to do this experimentally, when we do eventually get there the calculations in this paper will certainly help us to design a better experiment."