Neutrinos were first proposed in 1930 by Wolfgang Pauli to explain some missing energy in nuclear reactions when neutrons can turn into protons and vice versa by emitting an electron, known as beta decay. At that time, he said: “I have done a terrible thing, I have postulated a particle that cannot be detected.”

But, in fact, we can detect them with large enough detectors. Reines and Cowan detected them for the first time in 1956 in an experiment in a nuclear reactor. Since then, we have detected neutrinos coming from almost everywhere in our universe. We now know that they are the second most abundant particle in our universe. The most abundant are photons. Learn a bit more about neutrinos here.

Neutrinos and Cosmic Rays
While exploration of the universe started using light — photons emitted by stars and other cosmic objects — in 1912 another type of cosmic messenger was discovered: cosmic rays. Visit NASA’s Cosmicopia site and find out what is meant by cosmic rays! Their name could fool us a bit though, since they are not rays of any type. They were first thought to be some sort of electromagnetic radiation, or light, but we learned that they are in fact charged particles, most of them protons.

Cosmic rays reach Earth from everywhere, with energies over a million times those created in the most powerful human-made particle accelerators. Wherever cosmic rays come from, their sources must be the most extreme places in our universe. Watch this TED-Ed lesson to learn more about cosmic rays. Cosmic rays are charged, and they are bent by magnetic fields in the interstellar medium on their way to the Earth. Even though we can detect millions of cosmic rays everyday, they do not carry information about their origins.

The good news is that very high energy neutrinos are also expected to be created in conjunction with those cosmic rays. And neutrinos, which are neutral particles, will directly point to their sources. The problem, as you might guess, is that neutrinos hardly ever interact with anything. So, even if millions of cosmic neutrinos were reaching the Earth every year, most of them would just go through it without leaving a trace. Learn more here about neutrinos, their cosmic origins, and how to detect them.

IceCube
Physicists managed to build a detector that could observe cosmic neutrinos, even though these neutrinos are so hard to detect. Check out this picture titled, “Ice Fishing for Cosmic Neutrinos.” Remember, we cannot see a neutrino directly since it’s a neutral particle that only interacts through the weak force. So we first need some cosmic neutrinos interacting with a medium, and then we have to make sure that we do not miss them.

That’s how the idea of a cubic kilometer detector at the South Pole was born. Listen to the NPR story from Science Friday about this detector! There, the Antarctic ice provides us with an abundant supply of a pure and transparent medium that neutrinos can interact with. When a neutrino interacts with the ice, it will create a shower of charged particles that we can see. These particles go through IceCube at speeds faster than light travels in the ice, creating a burst of bluish light. This is the same blue light you’ve seen in images of nuclear reactors such as this one.

One of the challenges in building such a huge telescope was the location, at the South Pole, where planes can only land during a few months of the year and where outdoor temperatures are always extremely low. This webcast tells the story of IceCube. Listen to the actual scientists who are members of the IceCube team.

But an international collaboration of researchers and technicians, with headquarters in Madison, Wisconsin, managed to execute the project. IceCube was completed in 2010. And the detection of the first very high-energy neutrinos from beyond our solar system was announced in November 2013 in Science.

Now that we have confirmed that cosmic neutrinos exist and can be detected, physicists are getting ready for a new era in the exploration of our universe. Every time we have looked at the universe with a new telescope, we have learned something new. Rest assured that neutrinos will not disappoint us.