What do Neutrinos, Ice Cube, South Pole and Wisconsin have in common? All of the above are components of one of the most interesting current scientific experiments to detect particles from shortly after the Big Bang. From a personal perspective, this experiment can provide incremental clues as to the mind of God who created the beautiful architecture of the Universe we live in.
I am currently taking a Coursera course “From Big Bang to Dark Energy“. It has been a great experience but I am excited to be done with the “final exam” One of the concepts in the course that I was introduced to in more detail was a neutrino, which is described below. Interestingly, last week, NPR’s Science Friday had a segment on the Ice Cube Project at the University of Wisconsin and the South Pole to detect neutrinos. This has double interest for me. First, it is on an interesting subject that relates to the Coursera course I took. Second, it deals with University of Wisconsin, which is virtually in my backyard (at least compared to some of the readers of the blog).
What Are Neutrinos
Neutrinos are invisible, nearly massless subatomic particles that are electrically neutral. They can travel at nearly the speed of light from the edge of the universe without being deflected by magnetic fields or absorbed by matter. They travel in straight lines from their source. This makes them excellent messengers of information about the objects or events in which they originate.
Neutrinos originate in some of the most violent and least understood events in the universe. Events like supernovas and objects like active galactic nuclei and black holes are just a few possible sources of high-energy neutrinos. Other than particles of light, called photons, neutrinos are the most common particle in the universe.
From what we know today, a majority of the neutrinos floating around were born around 13.7 billion years ago, soon after the Big Bang. Since this time, the universe has continuously expanded and cooled, and neutrinos have just kept on going. Theoretically, there are now so many neutrinos that they constitute a cosmic background radiation whose temperature is 1.9 degree Kelvin (-271.2 degree Celsius). Other neutrinos are constantly being produced from nuclear power stations, particle accelerators, nuclear bombs and during the births, collisions, and deaths of stars, particularly the explosions of supernovae.
Neutrinos are very hard to detect because they do not generally pass through ordinary matter. The feeble interaction of neutrinos with matter that makes them uniquely valuable as astronomical messengers. Unlike photons or charged particles, neutrinos can emerge from deep inside their sources and travel across the universe without interference. They are not deflected by interstellar magnetic fields and are not absorbed by intervening matter. However, this same trait makes cosmic neutrinos extremely difficult to detect; immense instruments are required to find them in sufficient numbers to trace their origin.
IceCube is a unique telescope at the South Pole. Most optical telescopes look at photons, but IceCube looks for evidence of a more mysterious neutrinos. Because of this, it is referred to as a neutrino telescope or neutrino detector. Using an optical telescope to look at the universe is like taking a photo, but looking at the universe with a neutrino telescope is similar to taking an X-ray.
Since neutrinos have a very small mass, they are hard to detect as they rarely interact with ordinary matter. IceCube is designed to record these rare interactions of a nearly massless neutrino. In addition, IceCube studies the neutrinos themselves; their energies far exceed those produced by accelerator beams. IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice. In 2012, the IceCube project detected the presence of two neutrinos (dubbed Bert and Ernie after the Sesame Street characters), the first neutrinos detected from outside the solar system since 1987.
IceCube uses a large volume of ice at the South Pole in Antarctica to hold basketball sized detectors called digital optical modules, or DOMs. Altogether, there are over 5,160 DOMs in the ice and an additional 344 on IceTop, a complimentary detector on the surface of the ice. It may seem strange to use the ice at the South Pole, but there are several reasons why it is an excellent location. First, the ice is very clear. IceCube is buried very deep in the ice, about 1,500 to 2,000 meters below the surface. At that depth, pressure has pushed all the bubbles out, which means it is easy for the DOMs to record neutrino interactions.
Second, it is very dark in the ice. This is important because when a neutrino interacts with an atom of ice, a particle called a muon is produced. The muon radiates blue light that is detected by the DOMs, and the DOMs can only detect this light in a very dark environment. The direction and intensity of the light allows us to determine where the neutrino was coming from in the Universe.
Finally, the last great thing about the ice at the South Pole is that there is a lot of it! The IceCube neutrino detector is enormous. It uses a cubic kilometer of ice and is the largest neutrino detector in the world. Currently, the IceCube project involves collaboration of people from 39 institutions in 11 countries.