You may or may not have heard of neutrinos, but they are one of the most fundamental particles that make up our universe. At face value, they don't seem to have a lot going for them. They have no electrical charge, are nearly massless, and are very hard to detect. Despite this, they are very abundant, along with being really, really small. So small and so abundant in fact, that by the time it took you to read this, approximately 100 billion of them have passed through just the tip of your finger alone (Lang)! To give you an even bigger scale of their abundance, they outnumber protons a billion to one (Heeger and Fellow). They're nearly impossible to get away from too. If you wanted no neutrinos to hit you, you'd have to be in the center of a lead sphere with a radius larger than a lightyear, and that's just to avoid the ones that come from the Sun! Despite all of this, for the longest time scientists weren't sure if they actually existed, and even now they are extremely hard to detect. To explain why this is, its good to have a bit of insight into just what they are, and how they came to be.
Neutrinos are products of nuclear reactions, and many have their origins from things like the Big Bang, explosions from supernovae, and nuclear fusion reactions from stars like the Sun. Neutrinos are also created here on Earth, with many coming from radioactive decay or nuclear reactors. Almost all neutrinos that hit Earth come from the Sun, often referred to as "solar neutrinos." Neutrinos also come in 3 different varieties, or "flavors" (electron, tau, and muon neutrinos), and are able to transform into each of them under certain conditions. As mentioned before, they have nearly no mass. In fact, for a long time, scientists believed that neutrinos had no mass at all, partially due to them moving near the speed of light. It was only until recently that we found out they did indeed have mass, being less than 0.0000059 times that of an electron's. It is this reason, and the fact that they are leagues smaller than even subatomic particles that they are so hard to detect. The reason why so many are able to pass through us is the fact that they are so small, they almost never collide with any other particles (including atoms) in their paths. They are also not motivated by the strong and electromagnetic forces that many other particles are affected by, which means they are only influenced by gravitational and weak forces. This doesn't give scientists a lot to work with, so how do they go about detecting them?
One example of how this is done is through the Super-Kamiokande detector, a large underground facility which was created to detect and observe neutrino interactions. Essentially, it is a large tank of purified water (over 50,000 tons of it) approximately one kilometer underground, surrounded by multiple detectors. Even though billions of neutrinos pass through each square centimeter of the Earth each second, the detectors are only be able to capture a of neutrinos through their detectors. Even this miniscule amount is enough for scientists to study, which lead to many discoveries in their own right.
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| Inside view of the Super-Kamiokande detector. |
This is cool and all, but what does this mean for astronomy? Well, it can give us a lot of insight into how a star's core works, along with aiding in putting together the pieces of the Big Bang. Although scientists don't have an exact number for how many neutrinos there are, it is estimated that they make up 0.3% of the universe (as compared to stars and other luminous bodies, which make up 0.5%, this is a relatively large amount). Some also believe that it may help solve the enigma of dark matter and dark energy, or at least provide some clues for them. Along with this, it might be able to explain why there is more matter than anti-matter present in the universe, despite an equal amount of both being present in the early formation of the universe. Neutrinos also brought about a lot of questions, such as why there were less neutrinos coming from the Sun than there should've been. It was soon found that neutrinos were able to "transform" into different "flavors," known as electron, tau, and muon neutrinos. This meant that one the way to Earth, some of the neutrinos were able to transmute before coming to Earth. There are both many questions and potential answers that these things can provide, and hopefully soon we may have a deeper understanding of them. Neutrinos, at face value, may be somewhat uninteresting, but they can hold several clues into how the universe works.
References:
Haran, Brady. Neutrinos. sixtysymbols.com/videos/neutrinos.htm.
Heeger, Karsten, and Chaimberlain Fellow. Big World of Small Neutrinos. conferences.fnal.gov/lp2003/forthepublic/neutrinos/.
Lang, Kenneth R. Ghostlike Neutrinos. 2010, ase.tufts.edu/cosmos/view_chapter.asp?id=37.
Mann, Adam. What Are Neutrinos? 21 Feb. 2019, www.livescience.com/64827-neutrinos.html.
Observatory, Kamioka. Overview: Super-Kamiokande Official Webiste. www-sk.icrr.u-tokyo.ac.jp/sk/sk/index-e.html.
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