In the dark and distant past, I called myself a laser physicist. I would speak with pride of lasers that produced incredible power: the thought of a petawatt laser system would bring a tear to my remaining eye.

But I have to admit that our best hardware is relatively wimpy when compared to natural sources of energy that output far more power. Of course, it is really hard to convince a neutron star to sit in the lab and not destroy the planet. But now, out of the minds of theorists and into a lab hopefully not-too-near you, we may have the chance to match astronomical radiation sources at the press of a button.

Our petawatt laser systems involve collecting a lot of photons (about 1018 of them) and then releasing them all at once (in about 10-15 s) to make one. For comparison, a simple nuclear decay can release a photon pretty damn close to the same power. If you could convince all the nuclei in a nanogram of material to decay simultaneously, you'd hit the same power flow.

Of course, nuclei don't pay much attention to each other, so they're hard to coordinate. Plus, you end up limited to the energy range produced by the physics of the radioactive decay. If you want higher-energy gamma rays, then you're back to needing a neutron star that shakes the living daylights out of the nearest electrons. These high-energy electrons will then produce gamma rays.

It turns out, though, that we do similar things all the time. Many facilities around the world, including the predecessor to the LHC, accelerate electrons. Some of these electrons hit very high energies. But we're not using any of them to produce gamma rays. So, what does a physicist have to do to get some gamma rays?

Bring me a shrubbery laser

With the advent of excessively large lasers, new ways to accelerate electrons became available. Instead of sending a single laser pulse into a material and accelerating the electrons by dragging them along with the light, you can trap electrons at a focus. The idea is to send in 12 laser beams to a single focal point. Six beams form a cone of light approaching from the left, and the remaining six form a cone that approaches from the right. Together, these meet in the middle to create a high-intensity spot. For a variety of reasons, an electron that is nearby will move to the brightest part of the spot and start oscillating back and forth along the axis of the two cones.

The motion of the electrons results in some high-energy photons, but not gamma rays. To make matters worse, the trap isn't that good, so electrons get flung out quickly. So, you get a short burst of disappointing X-rays, and then it's all over.

If you turn the laser power up even more, space-time starts to hate your laser. The intensity is large enough that electrons and their antiparticle, positrons, are produced from the vacuum. These are oppositely charged, so they head off in different directions in response to the local field. This has two effects: first, it replenishes the electrons in the trap, and, second, it strengthens the field.

The first part is good. You need to have charged particles in the trap to produce any radiation at all. But having the field strengthen is like the Universe deciding that it was on your side for once. As the field strength increases, the electrons oscillate even more violently, so the energy of the photons that they radiate goes up. And the increased field strength creates pairs of electrons and positrons even faster. So, while the laser pulse is on, this becomes a self-amplifying effect.

Gamma ray laser? Check. Bring on the mutants

Out of that hot mess comes a directed blast of gamma rays with low-end energies of 10 Mega-electronVolts, which is about twice the energy that you get from radioactive decay. At the high end, it reaches a few GeV, ranges normally produced by astronomical events. The researchers predict that, depending on the photon energy, a 40-petawatt laser would produce 108 photons in 10-15s. This would produce a gamma ray beam with laser-like qualities and a power of 40 terawatts.

Even more incredible, this sort of thing can be done in the (big) basement of your local physics building, as long as they line the walls with lead bricks. So, yes, it's a facility-sized laser, but not like a synchrotron or a nuclear reactor, which needs an entire campus to itself.

The perennial question: what is it good for? I've no clue, but I'll take two.

Physical Review X, 2017, DOI: 10.1103/PhysRevX.7.041003 (About DOIs.)