Neutrinos are one of the most interesting particles in the Universe – if you can catch them.
Next Generation Science Standards:
- HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
- HS-PS4-5. Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.*
Every second of every day, 65 billion of the strangest particles in existence, traveling from the furthest reaches of the Universe, pass through every square centimeter on earth. Even as you read this article, these particles are flying through you as if you weren’t even there. These fascinating bits of matter are called neutrinos, and if Dr. Sílvia Bravo Gallart and the rest of the team at the IceCube Neutrino Observatory can catch them, they could unlock the secrets of the black holes, pulsars and other bizarre objects in the cosmos. If they can catch them.
Neutrinos are as elusive as they are interesting – they hardly ever interact with mass and so are very challenging to observe. Which is why the ambitious $280 million experiment IceCube, located at the Amundsen–Scott South Pole Station in Antarctica, was built. Buried in the ice nearly a mile below the surface are 60 detectors designed to look for neutrinos and explore the highest-energy phenomenon in our Universe.
Dr. Bravo Gallart is currently a communication and outreach specialist with the IceCube. Hailing from a small town in Northeast Spain, Dr. Gallart grew up with little exposure to scientists or research. But she fell in love with physics at a young age and went on to complete her Ph.D. working on an experiment at the LEP particle accelerator at CERN. Upon finishing her doctoral work, she switched gears to communicate the findings of the physics community. I sat down with Dr. Gallart to discuss what are neutrinos, how do you catch one, and what makes them so special.
Chris Anderson: What is a neutrino and what is so special about them?
Silvia Bravo Gallart: A neutrino is a tiny, tiny particle, much smaller than an electron. It’s similar to light in that it can travel long distances from very far away in the universe, but if you put a piece of paper in front of a beam of light, you can stop the photons. That’s because what makes neutrinos so special is that unlike particles such as electrons or protons or photons, it almost never interacts with mass.
CA: So if neutrinos have no charge, almost no mass, and barely interact with other particles, what do they do?
SBG: That’s what we want to find out. There are researchers who want to learn about the properties of neutrinos, which we also do at IceCube. But the main reason why IceCube was build was to learn something about the Universe. Because they don’t interact with mass, neutrinos can bring us information from far away places that no other particle can do. Light can travel very long distances, but it interacts with almost everything. Neutrinos can bring us information about places in the Universe from which light cannot escape.
But what makes the particle so interesting is also what makes it so hard to detect, because it only interacts weakly with mass means that it’s very, very difficult to detect. IceCube is a cubic kilometer and for every million neutrinos that cross the detector, we see on average only one. Most of them just sail right through.
CA: How do you observe a neutrino?
SBG: It’s just a matter of luck.
Matter, even though it appears solid to us, is mostly empty space. If an atom was the size of a football stadium, its nucleus would be the size of a small marble sitting on the 50-yard line with the electrons in the far corner of the stands. This means most of the Universe is empty – when a particle passes through matter, it’s basically crossing empty space. And since neutrinos interact so weakly with mass, we can only detect them when they hit a nucleus head-on.
But if you have enough matter and enough time, eventually a neutrino smashes into the nucleus of an atom and produce something we can see. We never actually observe the neutrino directly; when a neutrino has interacted with mass it produces another set of particles we can detect.
CA: Let’s say a neutrino hits a nucleus dead on a gives off these observable particles. What kind of particles does it give off?
SBG: They end up producing charged particles. Physicists aren’t very good at detecting neutral particles, except for photons, but we are very good at detecting neutral particles.
The type of particles produced depends on the type of neutrino. There are three different types of neutrinos – the electron neutrino, tau neutrino, and the muon neutrino. For example, if you have an electron neutrino it will produce an electron, which will interact with the mass around it. When those charged particles are produced, they are superluminous, meaning they travel faster than the speed of light.
CA: But I thought nothing could go faster than the speed of light?
SBG: They can go faster than the speed of light in the ice. When light goes through a media, it moves slower than it does in empty space. When it travels through the ice, it goes around 75% of the speed of light.
When the charged particles are produced, they are superluminous, traveling faster than the speed of light in the ice. When they start traveling at that speed, they produce blue light called Cherenkov light. If you’ve ever seen the water around the rods in a nuclear reactor, the rods and water glow blue. It’s the same kind of light that we see in the ice.
Our light sensors in IceCube are so sensitive they can detect one photon at a time so we can see a tiny, tiny, tiny bit of light. By having many sensors, we detect how the particle is moving and where it was coming from.
CA: The highest energy neutrinos come from very deep space. As we get more information about them, what could they tell us about the Universe?
SBG: That’s exactly what we are hoping to learn. We’ve known about black holes for years, but we know very little about what’s going on there. Quasars too, any supermassive object spitting out particles at very high energies. These are the most powerful objects the universe has ever created. We know they are there because we have detected very high energy particles from the universe, but because they are so far away and so dim, light can’t tell us what’s going on. We don’t know much about what is happening there.
With neutrinos and gravitational waves, we are trying to explore what is happening with these types of objects. We’ll never to be able to travel there, but we could get a much better understanding these humongous objects in the sky. Wouldn’t it be cool if we could detect a neutrino and a gravitational wave from the same object?
CA: That would be cool!
SBG: They would probably make a holiday for that, right?
CA: For sure! What advice would you give to a student who wanted to be a particle physicist?
SBG: It’s a really cool job! You need to study math and you need to study physics and enjoy it. Know that being a particle physicist doesn’t mean only one thing – you have people who work on the theoretical side, living in the abstract world of ideas and math. But we also have people who are working on the experiments, they know how to build the detector, design a detector. Explore the whole branch of opportunities.
You also get to travel! We work in big teams, sometimes hundreds or thousands of people from all across the world, which means you get to travel for meetings and conferences. And our detectors are located in exciting places! IceCube is located at the South Pole and there are other detectors in Switzerland and Japan. Or you could go to a telescope on top of a mountain in Mexico. All in all, it’s a really interesting job.