How invisible particles can help us see the Milky Way
Every second, there are 100 trillion tiny particles, tinier than an atom, proton, or even an electron, called neutrinos, passing through your body. Don’t worry, though, it’s unlikely even one neutrino will interact with any particles in your body. You might have to wait over a hundred years to have even a single one interact with any of the particles in your body, and when one does, it would simply bounce off a single electron, or maybe turn one neutron into a proton and an electron. On the scale of all the protons and neutrons in your body, where there are about 7 x 10^27 atoms all with their own number of protons and neutrons, a neutrino won’t do much at all.
Neutrinos are a bit antisocial, but we can learn so much from them! They can help us discover new types of neutrinos (called sterile neutrinos). They might help us solve what the mysterious dark matter is made up of (see our post about dark matter, discovered by Dr. Vera Rubin). They can even teach us new things about the Milky Way. Professor Naoko Kurahashi Neilson, a professor at Drexel University, has made an incredible breakthrough, figuring out how we can use these tiny particles to take new pictures of our galaxy.
Icy Cool Neutrinos
A neutrino, pronounced new-trino, is one of the fundamental building blocks of our Universe. The building blocks, better known as particles like electrons, photons, and quarks, can’t be broken down into any smaller particles, and the rules of how they interact with each other is known as the Standard Model (learn about all the building blocks of the Standard Model here!). They don’t have any charge, so they won't shock you, and they are so tiny that scientists have still not been able to measure their actual size. Neutrinos are produced in a number of ways, from the natural radioactivity in bananas to nuclear fusion in the sun. The sun is producing the majority of those 100 trillion neutrinos per second passing through your body, but there are much higher energy neutrinos coming from outside our solar system.
So how can we study these interesting particles if they hardly interact with matter? We know a detector that is human sized won't cut it because the likelihood of an interaction is so low. What if we make the detector HUGE? This is exactly what scientists have done! There is a detector made of a cubic kilometer of Antarctic ice, called IceCube. Scientists drilled holes 2450 meters deep into the ice, and dropped a chain of sensors into the holes. The chains are as long as three Eiffel towers stacked on top of each other! When a high energy neutrino passes through the ice and interacts around or in the detector, it produces charged particles which then produce a type of radiation called Cerenkov radiation.
Light, made up of the fundamental particles called photons, travels incredibly fast. In a vacuum, where there are no other particles, it travels almost 3x10^9 m/s! However, in some mediums like water, light can slow down. When a particle moves faster than light in a medium, charged particles in the medium give off a blue light called Cerenkov radiation. The ice surrounding IceCube makes it a perfect medium for neutrinos crashing through and hitting charged particles to produce this characteristic signal.
This radiation is what is collected by the sensors in IceCube. This can be visualized like this diagram. Each circle is a different sensor, the size represents the amount of energy seen by the sensor, and the color shows the arrival time, with redder circles arriving first. IceCube sees almost 300 neutrinos a day, but most of these are from our atmosphere, and scientists are most interested in the higher energy neutrinos coming from space which are much rarer.
Since we need to see Cerenkov radiation to understand how the neutrino interacted, scientists needed a detector whose material was transparent, and could be large and dense enough to enclose entire neutrino interactions. This made Antarctic ice the perfect candidate for the detector material, since it is very dense, clean, and vast. Keeping the light sensors deep under the ice also helps protect from lots of other particles coming from the Earth’s atmosphere that make our signal unnecessarily messy. IceCube can even use the entire planet to block these particles, and only look at the highest energy neutrinos that can travel all the way through the Earth and up through the detector!
Picturing Our Universe
When you think of a picture, you might think of a typical photograph, maybe of you and your family enjoying the sunshine. These kinds of pictures are made with visible light, but a picture can be made with more than just visible light. Light is made up of fundamental particles called photons, photons can be both a particle and a wave. Depending on their energy, they can have different wavelengths, like the visible, radio, gamma ray, and x-ray. There are different kinds of cameras, telescopes, and microscopes that can make images with different wavelengths of light and even some that use particles other than photons (like an electron microscope). For example, many different telescopes, specializing in different wavelengths (or energy) of light, have given us a wide range of pictures of our Milky Way Galaxy. So with telescopes like the Very Large Array that uses radio we can see black holes, planetary disks, and young stars, and with the Fermi Gamma-Ray telescope we can see neutron stars and supernovae. Each wavelength tells us something unique about our Universe and the galaxy we live in.
IceCube can take a very different type of picture of our Milky Way, one made entirely with neutrinos! This is incredible, because we have never taken a picture of the Milky Way with anything other than light before. This can tell us where neutrinos are produced in our galaxy, which we can then compare with objects we see in that area from our other pictures of the Milky Way using light, giving us new insight into how our galaxy works. The neutrino picture has already been compared with the gamma-ray picture, since many of the objects in space that create gamma-rays should create neutrinos as well, and scientists are excited to study and learn even more.
However, unlike optical telescopes, we can’t just look through a neutrino telescope to see neutrinos, it takes additional steps to turn the Cerenkov radiation detected by IceCube into an image or map of neutrinos in the Milky Way. When IceCube detects a neutrino, we see an image like before, where each detector in the chain collects some Cerenkov radiation and measures the time of the signal. Scientists can then determine whether the particle in a given event was a neutrino, based on how much light was produced. Once we know it’s a neutrino, then scientists like Professor Kurahashi Nielson can reconstruct where that detected neutrino came from in our galaxy.
This requires recognizing patterns, starting from the electrical signal seen by the sensors, to creating a visual of the energy and time of the signal the sensors saw, and finally figuring out which direction that neutrino came from in the sky. This is an excellent job for machine learning! (Check out our post on Dr. Yasaman Bahri to learn more about machine learning.) With this powerful tool, we can turn the image of one neutrino interaction in IceCube that might just look like a fuzzy blob to us, into a position in our galaxy where the neutrino came from. With many many of these interactions, we can then create an image of our Milky Way, with only these neutrinos.
Some of the neutrino events in IceCube are much easier to determine the direction of than others, and Professor Kurahashi Neilson’s group worked on a machine learning method for the most difficult events.
Physics for the Future
Naoko was first drawn to physics from her love of problem-solving and art of getting things approximately right. She did her undergraduate at UC Berkley, and went on to get her PhD at Stanford. Now as a professor at Drexel University, she leads a research team focusing on IceCube, and is passionate about mentoring the next generation of scientists. She is excited to see what Astrophysicists can learn about gamma-ray sources using this new tool of picturing the Milky Way, and to work on innovative ways to discover more about our Universe using neutrinos.
Written by Taylor Contreras
Edited by Katie Fraser and Manasvi Verma
Illustrations by Helena Almazan
Discover more about neutrinos and IceCube!
Follow Along (1-2 hours): Print out the adventures of Rosie and Gibbs Comics and follow along as they discover the fascinating science happening at the South Pole.
Create (1 hour): Print out and create your own model of IceCube.
Challenge (1-2 hours): How can you experiment on an object that you can’t see? Put a mystery object in a box and devise a series of tests that will help you figure out what it is. Use this as your guide.
Simulate Radioactive Decay (45-1 hour): Follow along the Radioactive Decay of M&M’s activity and learn about the rate that different isotopes decay.