(That is the ultimate a part of a sequence on Cherenkov radiation — the “gentle growth.” Learn Half 1, Half 2, and Half 3 first.)
So we all know what Cherenkov radiation is. We all know the way it works. We all know that Pavel Cherenkov spent three years poking a glowing bottle of water earlier than anybody believed him.
Now: what’s it good for?
The reply, it seems, is quite a bit. Cherenkov radiation reveals up in a few of the most dramatic, excessive, and essential contexts in trendy physics. And likewise, splendidly, in hospitals.
Let’s begin with probably the most visceral picture in all of nuclear physics.
You’ve got most likely seen pictures of nuclear reactors — those the place the gas rods are submerged in a deep pool of water, and the water glows. That electrical, otherworldly blue. It appears virtually supernatural. Like one thing from a science fiction movie. Just like the reactor has a soul, and it is blue.
That glow is Cherenkov radiation.
The reactor’s gas rods are always releasing high-energy electrons and different decay merchandise that journey by way of the encompassing water quicker than gentle strikes in water. And every of these particles drags a cone of blue gentle behind it. Billions of them, always, all producing that regular chilly impossible-looking glow.
What makes this significantly placing is that it is one of many only a few locations in all of physics the place a genuinely relativistic phenomenon is immediately seen to the bare eye. A lot of the deep outcomes of recent physics are invisible to human notion. You may’t see an electron. You may’t watch a quark change taste. You may’t immediately understand spacetime curving round an enormous object. You need to belief your devices, belief your colleagues, belief the maths.
However the Cherenkov glow in a reactor pool? You simply have a look at it. That is the sunshine wake of particles outracing gentle. That is a consequence of Maxwell’s equations and particular relativity, seen and blue, proper in entrance of you. No mediation required.
That is Brad Bradington, sprinting by way of water, leaving gentle in his wake. The reactor’s heartbeat, made seen.
Here is one thing humbling: we did not invent Cherenkov radiation. The universe has been doing this always, in every single place, for billions of years, fully with out our enter or appreciation.
The higher ambiance of Earth is repeatedly bombarded by cosmic rays — high-energy particles streaming in from supernovae, neutron stars, black gap jets, and different excessive corners of the universe. When these particles slam into the ambiance, they create cascades of secondary particles, lots of that are transferring quicker than gentle strikes in air.
The consequence: temporary, faint, downward-pointing cones of blue and ultraviolet Cherenkov gentle, flashing always within the higher ambiance, all around the planet, day and evening, proper now. You may’t see them from the bottom — they’re too faint, and the sky is simply too vivid. However they’re there. They have been there since lengthy earlier than there was anybody to note them, or care, or construct experiments round them.
As soon as we knew the universe was doing this, we determined to observe.
A category of instrument referred to as an Imaging Atmospheric Cherenkov Telescope — IACT — does precisely what the identify suggests. These are massive mirror arrays constructed at high-altitude, dark-sky websites, pointed upward. They don’t seem to be searching for gentle from stars or galaxies. They’re looking ahead to the faint Cherenkov flashes produced when very-high-energy gamma rays from area hit the higher ambiance.
When an extreme-energy gamma ray enters the ambiance, it creates a slim, intense cascade of secondary particles — all of them transferring quicker than gentle in air — all producing Cherenkov radiation in a good downward cone. The flash lasts just a few nanoseconds. The telescope has to catch it immediately and reconstruct the path and power of the unique gamma ray from the form of the flash.
The key devices are MAGIC on La Palma within the Canary Islands, H.E.S.S. in Namibia, and VERITAS in Arizona. Between them, they’ve mapped the gamma ray sky in extraordinary element — discovering the remnants of supernovae, the jets of energetic galactic nuclei, the neighborhoods of pulsars — as a result of the ambiance itself is the detector, and the Cherenkov flash is the sign. We took a phenomenon we did not create and turned it into some of the highly effective instruments in high-energy astrophysics.
Probably the most audacious utility of Cherenkov radiation is not a telescope pointed on the sky. It is buried within the ice beneath the South Pole.
IceCube is a neutrino detector. Neutrinos are terribly tough to detect — they haven’t any cost, virtually no mass, and work together with matter so hardly ever that trillions of them move by way of your physique each second with out leaving a hint. Catching one requires both huge endurance, huge volumes of fabric, or each.
IceCube selected huge volumes. It comprises over 5,000 optical sensors embedded in a full cubic kilometer of Antarctic ice, monitoring the everlasting darkness for flashes of blue gentle.
Here is the way it works. Often — very sometimes — a high-energy neutrino passing by way of the ice will work together with an atomic nucleus and produce a charged particle, normally a muon. That muon, if it is energetic sufficient, travels quicker than gentle strikes in ice. And when it does, it produces Cherenkov radiation: a faint cone of blue gentle, spreading outward by way of the ice because the muon strikes.
The sensors catch these photons. The timing and sample of hits throughout 1000’s of sensors permits physicists to reconstruct the path the muon was touring — and subsequently the path the neutrino got here from — and subsequently the situation within the universe the place one thing violent sufficient to supply such an lively neutrino should have occurred.
Probably the most elusive particles within the universe, detected not by catching them however by the sunshine wake they go away after they’re not fairly elusive sufficient. Brad Bradington, transferring by way of a cubic kilometer of Antarctic ice, leaving footprints made of sunshine.
After which there are hospitals.
PET scanning — positron emission tomography — works by injecting a affected person with a radioactive tracer that emits positrons because it decays. A positron is the antimatter associate of an electron. When a positron meets an electron contained in the affected person’s physique — which occurs virtually instantly, as a result of electrons are in every single place — the 2 annihilate and produce a pair of high-energy gamma ray photons flying off in precisely reverse instructions.
These gamma rays journey quicker than gentle strikes by way of human tissue.
They produce Cherenkov radiation. The path and timing of these faint flashes can be utilized to reconstruct precisely the place contained in the affected person the annihilation occurred — which tells docs the place the radioactive tracer accrued — which reveals the place the metabolically energetic tissue is — which might establish tumors, measure blood circulate, and map neurological exercise.
Brad Bradington, in a really actual and non-metaphorical sense, helps diagnose most cancers.
Pavel Cherenkov’s glow in a bottle of water in 1934 has develop into: the seen heartbeat of a nuclear reactor. The fixed invisible gentle present in our higher ambiance. The muse of gamma ray astronomy throughout three continents. A cubic kilometer of Antarctic ice bristling with sensors looking the universe’s most elusive particles. A medical imaging expertise used tens of millions of instances a yr in hospitals around the globe.
Not unhealthy for one thing each earlier scientist wrote off as fluorescence.
The most effective discoveries in science usually begin the identical means. Not with a grand announcement. Not with a eureka second. Not with the instant recognition of their significance.
Only a cautious particular person, in a quiet lab, one thing everybody else has already checked out — and considering:
Huh. That is bizarre.
