High-energy neutrinos from a supernova move almost at the speed of light. I read that they were discovered on February 23, 1987 in the Northern Hemisphere, while the Large Magellanic Cloud containing the supernova is only visible in the Southern Hemisphere. So the neutrinos traveled right through the earth. Were some of the neutrinos stopped by the particles that make up the Earth’s interior or could they fly right through? Particles have a wave function. What happened to it because of the neutrino wave?
Answer
The existence of a ‘neutrino’ was already suggested by physicists in the 1930s, including during the well-known Solvay meetings here in Brussels! The first experimental clue to the existence of the neutrino came in 1955 with the experiment of Cowan and Reines in the United States. Since then, several ‘neutrino observatories’ have been built around the world (in Europe, Japan, USA, etc…). So the neutrino was not discovered by the supernova in 1987. The neutrino was already known then and had already been observed.
Now, could the neutrinos fly through the Earth? Absolute !
The big problem with the neutrino is that it hardly interacts with matter. That is why the neutrino is often referred to as the ‘ghost particle’. It is there, but we very rarely see it. It takes an enormous amount of neutrinos to ‘see’ one. There is also no danger associated with the neutrino, because it very rarely ‘collides’ or interacts with matter (and therefore not with humans). If it does collide, then the effect is so small that very sensitive and large detectors are needed to determine that there has been a collision …
Neutrinos are mainly produced during nuclear processes (such as radioactive decay, fission, etc…). Large sources of neutrinos are therefore nuclear reactors and supernovas. That is why during the supernova in 1987 a lot more neutrinos suddenly reached the Earth and why just a little more neutrinos were suddenly observed in the sensitive neutrino observatories, spread over the different continents.
Many of the ‘neutrino observatories’ are located deep under the Earth’s crust, to keep the background of natural radioactivity in the detectors as low as possible. When a supernova suddenly appeared in the sky on February 23, 1987, the neutrino observatories had unmistakably observed a “lot” of neutrinos a few hours earlier. The neutrino observatory “Kamiokande II” detector in Japan observed a total of 11 (eleven!) neutrinos. That’s a lot in terms of neutrino observation 🙂 The neutrinos arrived on Earth a few hours before visible light. This is due to the specific course of a supernova. The supernova in 1987 was by the way the first supernova that could be ‘seen’ on Earth with modern detectors and through which a lot of information could be obtained about the course of such an explosion in the universe.
The 11 neutrinos observed in Japan give a good idea of how rarely a neutrino interacts with matter. With our limited physical detectors (our hands, eyes, etc.) there is therefore no possibility to ‘see’ neutrinos. We need to design special detectors that aim to detect a sequence of signals that are unmistakably a fingerprint that has ‘passed’ a neutrino.
The neutrino remains a particle that is intensively researched and continues to provide surprises. For example, it was not until 2002 that a problem that had dragged on since the early days of neutrino observatories (since the 1960s) was resolved. This was about the number of neutrinos that were observed from the sun (also a very large source of neutrinos, since the sun is in principle 1 large thermo-nuclear power station). They detected barely 30% of the expected neutrinos from the sun (based on models of the sun and how it creates neutrinos).
Only in 2002 a solution was found with the ‘neutrino oscillations’. That is, neutrinos change their ‘character’ on their way from the sun to Earth. The simple fact that this happened was proof that neutrinos have mass! That was a huge breakthrough and a huge problem, because the standard model predicted that neutrinos have no mass, just like light particles. Therefore, they had no choice but to move at the speed of light. Now it is known that neutrinos have a gigantic minuscule small mass. So small that they move almost at the speed of light. So small that we as humans on Earth have to go to great lengths to determine the mass of the neutrinos. A work of many years that is still going on … With the regularity of the clock there are publications that put the limits on the neutrino mass more precisely.
The neutrino is and remains a fantastic particle that could one day revolutionize physics!
Regards
Jarno Van de Walle
ISOL physicist
Answered by
SCK CEN Academy for Nuclear Science and Technology
Boeretang 200, 2400 Mol
http://www.sckcen.be
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