Physicists are famous for building  some big honking experiments.

Take for example the Large Hadron Collider. That thing is so big, they had to  bury it under two separate countries!

All to study some of the smallest  particles in the universe! But the LHC is nothing compared to  an experiment called the KM3NeT. Because that thing spans the length  of the entire Mediterranean Sea.

And even though it’s still under construction, it’s already making some  record-breaking discoveries. [intro music] Over the millennia, humanity has figured  out a lot about how reality works. But despite physicists having multiple theories that do a really good job  of explaining most things … those theories are also incomplete. Which is actually kind of great for physicists, because if not, they’d be out of a job!

And how boring would life be if we  didn’t have questions left to answer? The theory we’re going to focus on today is called the Standard Model of Particle Physics. It tells us about electrons, quarks,  and other subatomic particles … what their properties are, and how  they interact with one another.

It’s by far the best model we  have for this area of knowledge, but there are several major  phenomena that it just can’t explain. Like dark energy and dark matter, and  the nature of nature’s least massive, most elusive particles: neutrinos. For example, the Standard Model  assumes neutrinos don’t have any mass.

But we know from experiments that they do. We don’t know how much mass  each flavor of neutrino has, but we know that it’s not zero. And that gap in knowledge comes from the fact that it’s really hard to study neutrinos.

They’re everywhere, streaming  through the whole cosmos. Including your body right now But they absolutely hate  interacting with other particles … be they inside a science  experiment, or inside of a mountain Or a dog or…. A hat. about 100 trillion neutrinos  pass through you every second.

So scientists would love to  capture a neutrino directly and just ask it what its deal is, Bring it in the station, put  it in the interrogation room Good cop bad cop, we’re gonna  figure out what’s going on. I mean as much as you can ask  a subatomic particle anything. And they’ve realized that the  best chance of doing that, Is to go big Humongous.

Gargantuan. Titanic. Well, maybe not titanic, even though  giant blocks of ice can be involved.

Sorry The bigger your detector,  the more chances a neutrino will interact with an atom inside the detector. So over the same time span, you’ll have more datapoints to analyze. For a sort of baseline, let’s  look at Super-K in Japan.

It houses 20 Olympic swimming pools of water  to help its detectors capture neutrinos. Basically, a massive vat of liquid  greatly increases the chance that a neutrino will collide with a  particle within the observation zone. And using water specifically helps us  visualize the aftermath of that collision.

We’ll get into that in a minute. But even with all that water, Super-K might  capture 30 neutrinos in a typical day. That sounds pretty pitiful  compared to the number of neutrinos moving through a much smaller human body, but that’s how ghostly these particles are.

So to get even bigger, some organizations  have swapped fluid-filled detectors for burying their detectors in  pre-existing bodies of water. Like, Lake Baikal home to the Baikal  Deep Underwater Neutrino Telescope. Or IceCube near the South Pole.

Which is a block of solid water. And then there’s ANTARES, in the  Mediterranean off the coast of France. It was a 14-year pilot  project, decommissioned in 2022 to make way for something even bigger.

That’s right. The KM3NeT. Which by the way, gets its  name from its sheer size.

Its thousands of individual sensors will be spread over many cubic kilometers of seawater. KM3NeT stands for Kilometre cube  and then net is Neutrino Telescope. Each of these sensors is a  0.44 meter-wide glass orb full of delicate, photon-detecting equipment.

Chains of them are arranged in a 3D pattern, and anchored along the bottom of the sea at depths between 2.5 and 3.5 kilometers. The largest cluster is off the coast of Sicily, and spans a full cubic kilometer of sea water. That’s equivalent to 400,000 Olympic pools.

Meanwhile, the cluster off the  shore of Toulon, France, is smaller, only 2800 Olympic pools. But inside that volume it has  a denser array of detectors, which allows for more detailed measurements. And finally, there are several test detectors located off the coast of Peloponnese, Greece.

The research collaborations plan to install  a full cluster there in the coming years, assuming they can get the funding. So yes, this thing will indeed  span most of the Mediterranean Sea. As for deploying the detectors into that Sea, let’s just say it has been challenging.

They have to navigate the detectors  to the exact right position, secure them to the seabed, and  ensure they don't get tangled up. The team even had to design a special  launch vehicle just for this job! And that job begins with the  construction of a string of 18 orbs, wound up around a big, spherical frame.

Then, an anchor sinks the  whole apparatus to the seabed, so the frame can slowly unwind  to deploy each orb on the string. Each string is connected to others via cables, and the entire cluster is linked  to shore via a 100-kilometer cable that transmits data and provides power. Once the orbs are online, they  can start their passive search for flashes of a special blue  light called Cherenkov radiation.

See, light travels at different speeds depending on what it’s travelling through. It’s fastest when it’s traveling  through the vacuum of outer space, and has to slow down when it  stumbles upon a different medium. Like a big body of water.

The exact speed depends on  the medium’s properties, as well as the wavelength of  the light doing the traveling. In water, it’s about 25% slower than in a vacuum. But subatomic particles,  like our precious neutrinos, don’t have to abide by this new speed limit.

And when a charged particle passes  through a medium like water, if it’s traveling faster than  light can traveling water, something weird happens: it generates a cone of eerie, bluish light. Basically, the particle jostles  the atoms in the medium, giving those atoms some of its energy. That energy then gets rereleased  as a wave front of visible light.

As a particle races through the medium, it creates a track of Cherenkov light. And over the years, scientists have gotten really  good at looking at those tracks and reconstructing the event that made them. Thanks to the KM3NeT’s 3D configuration, all those strings of orbs floating in a grid, physicists can trace a  particle’s exact trajectory.

And by combining the trajectory’s shape and the amount of energy released as light, they can reverse engineer the type of particle. “But wait a minute”, I hear you  scream through the internet. “You said Cherenkov radiation is created when charged particles travel faster than light. But you also said neutrinos are neutral.” That’s why they call them neutrinos! Yes, I did say that.

The KM3NeT’s detectors cannot  directly spot neutrinos. But in certain situations, neutrinos can react with other kinds of particles to create new, charged particles. Like the famous electron, or its nowhere  near as famous larger sibling, the muon.

And those particles can not only  produce Cherenkov radiation. They produce tracks that are easily  distinguishable from both one another, and any other random kinds of blue light. Like, for example, light made by  bioluminescent Mediterranean organisms that might wander through the detector.

Didn’t you worry about that?  Did you think about that? Turns out they did too They’re smart, these scientists. And this giant experiment has already discovered some things And I'm going to tell you about that right after we do a real quick ad This SciShow video is supported by Brilliant: an online learning platform that  helps you build real knowledge hrough lessons you’ll actually remember.

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You’ll also get 20% off an  annual premium subscription. Even though the KM3NeT hasn’t  yet reached its final form, it’s been collecting data since 2017. And on February 13, 2023 it witnessed  a truly extraordinary event, which scientists filed under  the catalog name KM3-230213A.

I don’t love that, we’re  just gonna call it The Event from here on out. The Event manifested as a nearly  horizontal streak of Cherenkov radiation, cutting across the Italian cluster, and lighting up more than  a third of its detectors. The research team concluded that the  light had been created by a muon.

Specifically, a muon created by a neutrino. The most energetic neutrino ever detected. Again, they only had the muon data to go off of.

But the team estimates that the neutrino’s energy had been around 220 Petaelectron Volts. That unit doesn’t mean a whole lot to  people who aren’t particle physicists, but from what I’ve heard, it’s enormous. Roughly 20 times more energy  than the last record holder.

After 2 years of data analysis, the discovery was finally  published in the journal Nature. Which sure, sounds like an obnoxiously long wait, but if you found an event this shocking, You would want to not just double, but septuple check your work The team also spent time running the  stats for an event this energetic, cross-checked catalogs of  other high energy events, and searched for potential origin  sources of the muon’s parent neutrino. To determine how much energy  the neutrino must have had, they ran a bunch of simulations.

But because they didn’t know exactly where the neutrino created the muon, the simulations had to cover  a ton of different scenarios. This is all work you want  to be really careful about. Plus, with huge collaborations like the KM3NeT, it’s super important for everyone  on the team to review the results.

And like, imagine coordinating  the opinions of 286 paper authors? I think it’s very stressful! But it was a necessary hurdle to overcome, because The Event might have been  the result of a class of neutrinos that, until now, only existed in theory.

There’s a lot we still don’t know about  how neutrinos are born out in the universe. It’s a very active area of  astroparticle physics research. But we have some theories that could describe most of the high energy cosmic neutrinos that we observe experimentally.

Physicists think that high energy neutrinos might be produced in cosmic accelerators, like supermassive black holes, supernovae, or neutron stars emitting gamma ray bursts. The idea is that these powerful  objects accelerate protons to incredibly high speeds, creating cosmic rays with  massive amounts of energy. Then, when a cosmic ray inevitably bumps into some other particle floating around in space, it can trigger interactions that  produce high energy neutrinos.

It’s a solid idea. But unfortunately, it can’t explain The Event. Because if you run the numbers, based on all our existing theories  about neutrino generation, observing a neutrino with that much  energy is really, really unlikely.

So unlikely “it’d take 70 years  of continuously running the KM3NeT to maybe spot an event like this”. But the KM3NeT did see this event, and it happened after only  335 days of data collection. This led physicists to propose The Event could be part of a class of neutrinos  that’s never been detected before.

If a cosmic ray were to interact with a photon from the cosmic microwave background, it could produce what’s  called a cosmogenic neutrino. The cosmic microwave background, or CMB, is made of photons that have been bouncing around since the universe was a mere 380,000 years old. In other words, there are  around 13.8 billion years old.

These CMB photons are literally  everywhere, and relatively low energy, hence the word “background”. In fact, right before  scientists discovered the thing, the signal was briefly thought to be static  caused by bird poop on a satellite dish. Anyway, if an ultra-high-energy cosmic ray were to perfectly pinball off one of  these ancient CMB photons floating around, the resulting particles, including cosmogenic neutrinos, would carry an enormous amount of energy.

If The Event really is our first  “photo” of a cosmogenic neutrino, then the KM3NeT has unlocked an entirely  new sector of astronomy and cosmology for us to explore. Regardless of The Event’s exact origins, this is an incredibly exciting detection. The KM3NeT is already observing the most  energetic neutrinos in the Universe, and it’s still under construction!

And of course it’s not the only  neutrino detector out there. It’s part of a collaboration  called the Global Neutrino Network, an alliance of neutrino detectors across the globe that are coordinating their research efforts. Who knows?

Maybe one day scientists will  turn an even bigger body of water into a particle physics experiment. I’m looking at you, Caribbean! We’re coming, and than we’re gonna  have a margarita on the beach. [ outro music ]