(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.

After years of heated debates among fire officials, scientists and local advocates, California’s Board of Forestry and Fire Protection released new proposed landscaping rules for fire-prone areas Friday that outline what residents can and can’t do within the first 5 feet of their homes.

Many of these proposed rules — designed to reduce the risk of a home burning down amid a wildfire — have wide support (or at least acceptance); however, the most contentious by far has been whether the state would allow healthy plants in the zone.

Many fire officials and safety advocates have essentially argued anything that can burn, will burn and have supported removing virtually anything capable of combustion from this zone within 5 feet of houses, dubbed “Zone Zero.” They point to the string of devastating urban wildfires in recent years as reason to move quickly.

Yet, researchers who study the array of benefits shade and extra foliage can bring to neighborhoods — and local advocates who are worried about the money and labor needed to comply with the regulations — have argued that this approach goes beyond what current science shows is effective. They have, instead, generally been in favor of allowing green, healthy plants within the zone.

The new draft regulations attempt to bridge the gap. They outline more stringent requirements to remove all plants in a new “Safety Zone” within a foot of the house and within a bigger buffer around potential vulnerabilities in a home’s wildfire armor, including windows that can shatter in extreme heat and wooden decks that can easily burst into flames. Everywhere else, the rules would allow residents to maintain some plants, although still with significant restrictions.

The rules generally do not require the removal of healthy trees — instead, they require giving these trees routine haircuts.

Once the state adopts a final version of the rules, homeowners would have three years to get their landscaping in order and up to five years for the bigger asks, including removing all vegetation from the Safety Zone and updating combustible fencing and sheds within 5 feet of the home. New constructions would have to comply immediately.

The rules only apply to areas with notable fire hazard, including urban areas that Cal Fire has determined have “very high” fire hazard and rural wildlands.

Officials with the Board will meet in Calabasas on Thursday from 1 p.m. to 7 p.m. to discuss the new proposal and hear from residents.

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Some L.A. residents are championing a proposed fire-safety rule, referred to as “Zone Zero,” requiring the clearance of flammable material within the first five feet of homes. Others are skeptical of its value.

Where is the Safety Zone?

The proposed Safety Zone with stricter requirements to remove all vegetation would extend 1 foot from the exterior walls of a house.

In a few areas with heightened vulnerabilities to wildfires, it extends further.

The Safety Zone covers any land under the overhang of roofs. If the overhang extends 3 feet, so does the Safety Zone in that area. It also extends 2 feet out from any windows, doors and vents, as well as 5 feet out from attached decks.

What plants would be allowed in the Safety Zone?

Generally, nothing that can burn can sit in the Safety Zone. This includes mulch, green grass, bushes and flowers.

What plants would be allowed in the rest of Zone Zero?

Homeowners can keep grasses (and other ground-covers, like moss) in this area, as long as it’s trimmed down to no taller than 3 inches.

The rules also allow small plants — from begonias to succulents — up to 18 inches tall as long as they are spaced out in groups. Residents can also keep spaced-out potted plants under this height, as long as they’re easily movable.

What about fences, trees and gates?

Any sheds or other outbuildings would need noncombustible exterior walls and roofs in Zone Zero — Safety Zone or not.

Residents would have to replace the first five feet of any combustible fencing or gates attached to their house with something made out of a noncombustible material, such as metal.

Trees generally would be allowed in Zone Zero. Homeowners would need to keep any branches one foot away from the walls, five feet above the roof and 10 feet from chimneys.

Residents would also have to remove any branches from the lower third of the tree (or up to 6 feet, whichever is shorter) to prevent fires on the ground from climbing into the canopy.

Some trees with trunks directly up against a house in this 1-foot buffer or under the roof’s overhang might need to go — since keeping branches away from the home could prove difficult (or impossible).

However, the board stressed it wants to avoid the removal of trees whenever feasible and encouraged homeowners to work with their local fire department’s inspectors to find case-by-case solutions.

What’s new and what’s not

Some of the rules discussed in Zone Zero are not new — they’ve been on the books for years, classified as requirements for Zone One, extending 30 feet from the home with generally less strict rules, and Zone Two, extending 100 feet from the house with the least strict rules.

For example, homeowners are already required to remove any dead or dying grasses, plants and trees. They also have to remove leaves, twigs and needles from gutters, and they already cannot keep exposed firewood in piles next to their house.

Residents are also already required to keep grasses shorter than 4 inches; Zone Zero lowers this by an inch.