The ray-gun is no longer science fiction
The idea of the ray-gun has been central to science-fiction for decades. Now it’s science fact. Cathal O’Connell reports.
“However it is done, it is certain that a beam of heat is the essence of the matter. Heat, and invisible, instead of visible, light. Whatever is combustible flashes into flame at its touch, lead runs like water, it softens iron, cracks and melts glass …”
HG Wells, The War of the Worlds, 1897
The turret emerges from the ship and pivots, following its target with its telescopic eye. Silently it fires, and two kilometres away, a small, glowing spot appears on the fuselage of an aircraft—the engine bursts into flames.
Not the Millennium Falcon. Not the USS Enterprise. But the USS Ponce, an American Navy vessel in 2015. The ray-gun is here.
In War of the Worlds, the invading Martians devastated Earth’s forces with their ‘heat-ray’. Variants of the ray-gun have been a mainstay of science fiction ever since, from the phasers of Star Trek to the blasters of Star Wars.
But though the US military pursued the idea for decades—culminating in Reagan’s proposed Star Wars defence system for using satellites to blast missiles out of the sky—the research never bore fruit. Not, at least, until now.
In 1960, American physicist Ted Maiman built the first laser. He crafted a cylinder of ruby with reflective ends, and placed it inside a powerful xenon lamp. A flash of light sent the atoms of a ruby into an excited state. Some of these atoms emitted a photon—which themselves went on to hit other excited atoms, causing ‘stimulated emission’ of other photons—identical to the first.
The light beam that emerged was coherent, meaning all the peaks and troughs lined up. These properties minimised the tendency for a light beam to spread out and lose power over distance, making the ray-gun a possibility. Unbeknownst to Maiman, the fledgling Advanced Research Projects Agency was already considering a proposal by another physicist to create a laser weapon.
Over the decades, many kinds of powerful lasers were developed. The problem was all of them required huge equipment and massive amounts of energy. In the early 2000s, the US Air Force tried packing a chemical laser, the Airborne Laser YAL1, onto a Boeing 747. It worked, shooting down a couple of training targets, but cost billions of dollars and was ultimately canned.
In the last decade we’ve seen spectacular advances in laser technology that may make the ray-gun practical again.
The Laser Weapons System (LaWS) is one of the first of a new breed of more compact systems based on the fibre laser. Fibre lasers can generate laser beams at efficiencies of 40%, far higher than conventional lasers, and achieve kilowatt powers. High power fibre lasers are already used in industrial cutting and welding machines, some with laser power of 100 kW and capable of welding blocks of metal parts 30 cm thick.
A 100 kW infra-red laser is exactly the ‘heat-ray’ that Wells imagined—equivalent to using a giant, kilometre-wide magnifying glass to focus the sun’s heat energy onto a single point the size of your fingernail.
The objective for LaWS is to affordably shoot down cheaply made insurgent rockets and drones, without wasting absurdly expensive missiles. While an anti-air cruise missile might cost hundreds of thousands of dollars, a single shot from LaWS works out at about $1 in energy cost. In 2014, a LaWS prototype installed on the USS Ponce demonstrated it could shoot down drones and disable boats. The US Air Force plans to put a similar device, developed by Lockheed Martin, on a fighter jet by 2021.
One difference from movie sci-fi, these real ray-guns don’t emit exciting ‘Pew! Pew!’ sound effects when they fire. They’re silent. Wells’ ominous words are more apt: “this invisible, inevitable sword of heat.”
SENSORS: Sensors on the turret track the target’s movement. A radio frequency sensor provides range data, helping LaWS to focus its multiple beams on the same spot.
BEAM DIRECTOR: The laser itself is generated inside the ship, and then routed through the fibre optic cables to the ‘beam director’, which looks a bit like an ominous amateur telescope. This prototype combined six laser fibres, for a total optical energy of 30 kW. Other versions of up to 150 kW are in the pipeline.
Fibre LASERS: The core technology at the heart of LaWS is the optical fibre laser. The light beam is first generated in a diode laser, which is incredibly efficient, but only works at low power. This low-power laser beam is fed into a special optical fibre containing light-emitting atoms of ytterbium in its core. As the laser light bounces along the fibre, it continually excites more photons from the core—building up an intensely powerful beam.
COMBINING LASERS: While individual fibre lasers can reach tens of kilowatts, engineers realised it was more efficient to combine the power of up to 100 smaller laser beams into one fibre. To do it, engineers borrow a trick from the communications industry called “spectral beam combining”. The beams are designed with slightly different wavelengths so they wouldn’t interfere with one another. In telecoms, this means higher bandwidth internet. Here, it makes for a highly efficient and powerful ray-gun blast.