(This is the final part of a series on Cherenkov radiation — the “light boom.” Read Part 1, Part 2, and Part 3 first.)

So we know what Cherenkov radiation is. We know how it works. We know that Pavel Cherenkov spent three years poking a glowing bottle of water before anyone believed him.

Now: what is it good for?

The answer, it turns out, is quite a lot. Cherenkov radiation shows up in some of the most dramatic, extreme, and important contexts in modern physics. And also, wonderfully, in hospitals.

Let’s start with the most visceral image in all of nuclear physics.

You’ve probably seen photographs of nuclear reactors — the ones where the fuel rods are submerged in a deep pool of water, and the water glows. That electric, otherworldly blue. It looks almost supernatural. Like something from a science fiction film. Like the reactor has a soul, and it’s blue.

That glow is Cherenkov radiation.

The reactor’s fuel rods are constantly releasing high-energy electrons and other decay products that travel through the surrounding water faster than light moves in water. And each of those particles drags a cone of blue light behind it. Billions of them, constantly, all producing that steady cold impossible-looking glow.

What makes this particularly striking is that it’s one of the very few places in all of physics where a genuinely relativistic phenomenon is directly visible to the naked eye. Most of the deep results of modern physics are invisible to human perception. You can’t see an electron. You can’t watch a quark change flavor. You can’t directly perceive spacetime curving around a massive object. You have to trust your instruments, trust your colleagues, trust the math.

But the Cherenkov glow in a reactor pool? You just look at it. That’s the light wake of particles outracing light. That’s a consequence of Maxwell’s equations and special relativity, visible and blue, right in front of you. No mediation required.

That’s Brad Bradington, sprinting through water, leaving light in his wake. The reactor’s heartbeat, made visible.

Here’s something humbling: we didn’t invent Cherenkov radiation. The universe has been doing this constantly, everywhere, for billions of years, completely without our input or appreciation.

The upper atmosphere of Earth is continuously bombarded by cosmic rays — high-energy particles streaming in from supernovae, neutron stars, black hole jets, and other extreme corners of the universe. When these particles slam into the atmosphere, they create cascades of secondary particles, many of which are moving faster than light moves in air.

The result: brief, faint, downward-pointing cones of blue and ultraviolet Cherenkov light, flashing constantly in the upper atmosphere, all over the planet, day and night, right now. You can’t see them from the ground — they’re too faint, and the sky is too bright. But they’re there. They’ve been there since long before there was anyone to notice them, or care, or build experiments around them.

Once we knew the universe was doing this, we decided to watch.

A class of instrument called an Imaging Atmospheric Cherenkov Telescope — IACT — does exactly what the name suggests. These are large mirror arrays built at high-altitude, dark-sky sites, pointed upward. They’re not looking for light from stars or galaxies. They’re watching for the faint Cherenkov flashes produced when very-high-energy gamma rays from space hit the upper atmosphere.

When an extreme-energy gamma ray enters the atmosphere, it creates a narrow, intense cascade of secondary particles — all of them moving faster than light in air — all producing Cherenkov radiation in a tight downward cone. The flash lasts only a few nanoseconds. The telescope has to catch it instantly and reconstruct the direction and energy of the original gamma ray from the shape of the flash.

The major instruments are MAGIC on La Palma in the Canary Islands, H.E.S.S. in Namibia, and VERITAS in Arizona. Between them, they’ve mapped the gamma ray sky in extraordinary detail — finding the remnants of supernovae, the jets of active galactic nuclei, the neighborhoods of pulsars — because the atmosphere itself is the detector, and the Cherenkov flash is the signal. We took a phenomenon we didn’t create and turned it into one of the most powerful tools in high-energy astrophysics.

The most audacious application of Cherenkov radiation isn’t a telescope pointed at the sky. It’s buried in the ice beneath the South Pole.

IceCube is a neutrino detector. Neutrinos are extraordinarily difficult to detect — they have no charge, almost no mass, and interact with matter so rarely that trillions of them pass through your body every second without leaving a trace. Catching one requires either enormous patience, enormous volumes of material, or both.

IceCube chose enormous volumes. It contains over 5,000 optical sensors embedded in a full cubic kilometer of Antarctic ice, monitoring the permanent darkness for flashes of blue light.

Here’s how it works. Occasionally — very occasionally — a high-energy neutrino passing through the ice will interact with an atomic nucleus and produce a charged particle, usually a muon. That muon, if it’s energetic enough, travels faster than light moves in ice. And when it does, it produces Cherenkov radiation: a faint cone of blue light, spreading outward through the ice as the muon moves.

The sensors catch those photons. The timing and pattern of hits across thousands of sensors allows physicists to reconstruct the direction the muon was traveling — and therefore the direction the neutrino came from — and therefore the location in the universe where something violent enough to produce such an energetic neutrino must have happened.

The most elusive particles in the universe, detected not by catching them but by the light wake they leave when they’re not quite elusive enough. Brad Bradington, moving through a cubic kilometer of Antarctic ice, leaving footprints made of light.

And then there are hospitals.

PET scanning — positron emission tomography — works by injecting a patient with a radioactive tracer that emits positrons as it decays. A positron is the antimatter partner of an electron. When a positron meets an electron inside the patient’s body — which happens almost immediately, because electrons are everywhere — the two annihilate and produce a pair of high-energy gamma ray photons flying off in exactly opposite directions.

Those gamma rays travel faster than light moves through human tissue.

They produce Cherenkov radiation. The direction and timing of those faint flashes can be used to reconstruct exactly where inside the patient the annihilation happened — which tells doctors where the radioactive tracer accumulated — which reveals where the metabolically active tissue is — which can identify tumors, measure blood flow, and map neurological activity.

Brad Bradington, in a very real and non-metaphorical sense, is helping diagnose cancer.

Pavel Cherenkov’s glow in a bottle of water in 1934 has become: the visible heartbeat of a nuclear reactor. The constant invisible light show in our upper atmosphere. The foundation of gamma ray astronomy across three continents. A cubic kilometer of Antarctic ice bristling with sensors hunting the universe’s most elusive particles. A medical imaging technology used millions of times a year in hospitals around the world.

Not bad for something every previous scientist wrote off as fluorescence.

The best discoveries in science often start the same way. Not with a grand announcement. Not with a eureka moment. Not with the immediate recognition of their importance.

Just a careful person, in a quiet lab, looking at something everyone else has already looked at — and thinking:

Huh. That’s weird.