AEROSPACE ENGINEER EUGEN SÄNGER hypothesized in the 1950s that if matter could be completely converted into light particles (called photons), the photons themselves could be a power source to thrust a rocket to intergalactic speeds. While he assumed a photon rocket could only ever be the stuff of science fiction, the seed of his idea has only continued to take root. To create the ultimate laser, a number of research teams are pursuing technology that could maintain coherent gamma rays, the most energetic form of light in our universe.

If we could produce coherent gamma rays just as an ordinary laser produces coherent rays of visual light, the technology could unlock interstellar travel—as well as blow missiles out of the sky and revolutionize cancer treatment. While the gamma ray laser (also known as a “graser”) is still conceptual, it’s considered one of the most important problems in physics.

Invisible to our eyes, gamma rays burst from supernova explosions as well as the hottest and most energy dense objects in the cosmos, like pulsars, those highly magnetized neutron stars that emit pulses of radiation. Gamma rays travel through the vacuum of space at the speed of light, with wavelengths so minuscule that they can pass through the space within the atoms of a detector. With the smallest wavelengths and the fastest frequency, gamma rays fall on one extreme end of the electromagnetic spectrum.

Visionaries have been trying to push laser technology into the farthest reaches of the electromagnetic spectrum since the invention of the first laser in 1961. Along the way, scientists have learned how to stabilize gamma rays into a coherent beam, a necessary step toward developing any laser technology.

A TRADITIONAL LASER excites electrons in a gas, liquid, or solid to emit coherent radiation. In other words, the photon emissions are in sync with each other, like soldiers marching in step, generating a stronger effect in combination. This is different from the light an incandescent bulb emits, because its radiation is incoherent, or random, depending on which atoms are excited at any given moment. To pull off this dance with gamma-level photons, scientists must manipulate a massive number of atomic nuclei into deformed, excited states known as isomers.

Going beyond current laser technology—which includes coherent X-ray lasers, just next to gamma rays on the electromagnetic spectrum—means scientists need to investigate what happens when dense bunches of speedy electrons collide with a strong laser field to emit high-energy light. That’s what University of Rochester researchers are doing in collaboration with colleagues from ELI Beamlines, a laser research center in the Czech Republic.

“The ability to make coherent gamma rays would be a scientific revolution in creating new kinds of light sources, similar to how the discovery and development of visible light and X-ray sources changed our fundamental understanding of the atomic world,” Antonino Di Piazza, Ph.D., a University of Rochester physics professor and lead investigator of the new work, says in a press release.

The first step toward building any working laser is to show that the science works, he says. “We are not the first scientists who have tried creating gamma rays in this way. But we are doing so using a fully quantum theory—quantum electrodynamics—which is an advanced approach to addressing this problem.”

The team will analyze how one or two electrons emit light. Eventually, they hope to work with many electrons in order to produce coherent gamma rays. If the team learns how to keep the beam coherent and stable for long periods of time, gamma rays could become a new source of energy for creating antimatter (like matter, but with the opposite electric charge). They could also provide a new way to study nuclear processes and scan the insides of dense objects like shipping containers.

FUNDED BY THE NATIONAL SCIENCE FOUNDATION, this project builds on earlier, ongoing coherent gamma ray research.

For instance, a 2012 article in the journal Acta Astronautica proposed rocket propulsion via gigaelectron volt gamma ray laser. “It is shown that the idea of a photon rocket through the complete annihilation of matter with antimatter, first proposed by Sänger, is not a utopian scheme as it is widely believed,” according to the authors.

It’s complicated, but the process would begin with protons and antiprotons annihilating each other, generating a massive surge of gamma rays. Next, a focused laser beam of concentrated gamma rays would launch inside the spacecraft, creating a “photon avalanche.” A magnetic field surrounding this process would absorb the recoil momentum of the beam and transmit it to the spacecraft, providing the entire spacecraft momentum.

While the researchers suggest in their paper that this scenario may initially work better with smaller space vehicles, other teams in Sweden and Iceland went further with their study of gamma rays for rocket propulsion in 2020, suggesting ways to use hydrogen fuel to generate the necessary power for larger rockets, as well.

Of course, before we can jump into our gamma ray-powered rockets to explore the neighboring galaxy, scientists have to overcome the beam coherence problem.

Back in 2019, University of California, Riverside scientists tried a promising approach. They successfully encapsulated a bubble of positronium—a collection of atoms that have positrons, antiparticle counterparts to electrons—within superfluid liquid helium. It’s a form of helium similar to a superconductor, with no resistance, viscosity, or friction. The helium provided a protective barrier between the outside world and the positronium. The interactions of the positrons generated gamma rays if it stayed in a quantum state known as a Bose-Einstein condensate.

One of the challenges of developing and stabilizing gamma-emitting isomers is that they release their energy too quickly to maintain a large population of isomers at once. Fortunately, some isotopes (forms of chemical elements with differing numbers of neutrons) can provide more energetic gamma rays, with less energy input to maintain their isomers, and scientists are pursuing them as potential solutions to the coherent beam problem.

A next-generation gamma ray laser may not propel us to the Andromeda galaxy in our lifetimes, but the underlying technology to make that journey possible very well could be.

Before joining Popular Mechanics, Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.