Japan is on the verge of discovering the history of the origin of the universe
The interior of the 41-meter-deep Super Kamiokande Observatory during maintenance in 2018 with about 4 meters of water removed.
Koshiba Masatoshi and Kajita Taka Aki are two Nobel Prize-winning physicists who have made significant contributions to the world-class neutrino observatories Kamiokande and Super Kamiokande. After being upgraded in August 2020, the Super Kamiokande Observatory now plays an important role in exploring supernova explosions and space history. We had a lot of fun interviewing Nakahata Masayuki, a professor at the University of Tokyo and director of the Super Kamiokande Observatory in Kamioka, Gifu Prefecture.
Nakahata Masayuki
Director of the Kamioka Observatory operated by the Cosmic Ray Research Institute of the University of Tokyo. Nakahata is also the director in charge of the Nucleolysis Experiment at the Super Kamioka Observatory. Nakahata was born in Matsumoto, Nagano Prefecture, in 1959. He began studying physical sciences at the University of Tokyo in 1978, and obtained a bachelor's degree in 1982 and a master's degree in 1988. He also holds a doctorate in physical sciences and has been a lecturer at the Cosmic Ray Research Institute since 2003. Nakahata took up the position of senior researcher at the Kavli Institute for the Mathematics and Physics of the Universe in 2007 before becoming in 2014 director of the Kamioka Observatory and the Super Kamiokande Observatory Array. Since 2015, Nakahata has served as deputy director of the Institute for Cosmic Ray Research. Among his awards are the Nishina Memorial Prize, and his publications include (Investigation into the Mystery of the Disappearance of Solar Neutrinos) and the third chapter of (Kamiokande and Neutrinos).
From the Philosopher's Stone to the secrets of the universe
John Maynard Keynes, the godfather of macroeconomics, once began his dissertation with an unusual statement: “It is with some skepticism that I attempt to speak to you of Newton as he would have spoken of himself.”
Nakahata takes me on a tour around Super Kamiokande. Beneath his feet is a giant water tank.
The essay “Newton, the Man” was an exposition of the life of Isaac Newton, the founder of modern science. In 1936, a few years before Keynes wrote his essay, a box containing Newton's essays was put up for auction at Sotheby's by a descendant of the Earl of Portsmouth. Keynes bought about half of the articles, including manuscripts on chemistry that were dotted with cryptic references to “green lions” (or Mercurius, an ancient Roman god) and “vile whores” (referring to antimony ore), where alchemists were believed to have At that time, they used symbols and signs in their scientific writings and publications. Keynes then recorded his belief that Newton was “living in body in the Middle Ages, but his mind and soul belong to the age of modern science.”
In fact, in the 17th century Newton was searching for a “philosopher’s stone” that would turn lead and other base metals into gold and grant humanity immortality. Towards the end of his life, Newton invested much of his time and money in alchemy and is said to have gone mad from mercury poisoning.
But what prompted a distinguished scientist like him to immerse himself in occult research? After extensive research, historians of science have come to speculate that after he discovered the laws of the universe governing mass (the laws of universal gravitation), Newton attempted to formulate a theory of matter. These historians believe that Newton believed that alchemy could lead him to answer this fundamental question: “What is matter made of?”
This mine, which passes through Mount Ikinoyama, leads to the Super Kamiokande Observatory.
Newton was ahead of his time by at least 300 years, but we were not able to obtain any satisfactory answers to Newton's question until the twentieth century, thanks to nuclear physics, particle physics, and the Big Bang Theory. Only now do we have sufficient knowledge of neutrino astronomy to explore this mystery.
Leading neutrino research in Japan
Many have heard of the research conducted by Koshiba Masatoshi and Kajita Taka Aki, the famous Japanese Nobel Prize-winning physicists, or are at least familiar with their names. After their success in using the Kamiokande Observatory to observe neutrinos emitted from a supernova explosion in 1987, lead researcher Koshiba was awarded the Nobel Prize in Physics in 2002. This honor had strong repercussions in Japan, and led to the establishment of the Super Kamiokande Observatory (or “Super -K”) which is 25 times larger. Later in 2015, Kajeta was awarded the Nobel Prize for his study of the oscillations that indicate neutrino mass. Thus, Japan has made amazing progress in the field of neutrino research.
Neutrinos are one of the basic units of matter that make up the universe, and they are notoriously difficult to observe. For example, neutrinos that reach Earth are likely to have been created in the Sun. While each of us collides with hundreds of trillions of these neutrinos every second, 99.999999999999999999999％ (that's 23 nines!) of them pass through us without ever touching. On average, only one neutrino every 50 to 100 years collides with any of the atomic nuclei or electrons that make up our bodies. Therefore, any type of interaction involving a neutrino is inherently a miracle.
The entrance to the observatory is located a full kilometer underground.
As Nakahata Masayuki, director of the Super Kamiokande and Kamioka Observatories, explains, the 50,000 metric tons of high-purity water at Super Kamiokande is able to intercept about 20 neutrinos per day from the Sun and about 10 neutrinos from the atmosphere.
A glimpse into the history of the creation of the universe
?So what can trapping neutrinos tell us about space
“First we need to know some information about the history of the universe,” Nakahata says. The universe originated 13.8 billion years ago in conjunction with the Big Bang, which produced mainly hydrogen and helium, and later heavier elements were formed inside stars as a result of nuclear fusion and supernova explosions. “These elements heavier than iron, especially gold and silver, are thought to have formed instantly in the explosions of dying supernovae, stars eight times larger than our Sun, and in high-pressure environments resulting from the merger of neutron stars.”
In other words, the “philosopher’s stone” capable of creating gold did not exist on Earth but in supernova explosions and other cosmic “furnaces.”
Monitoring tank after removing all water during maintenance work. (Courtesy of Kamioka Observatory, University of Tokyo Cosmic Ray Research Institute)
Gravitational forces eventually cause elements scattered across the universe in such explosions to clump together, forming new stars that eventually reach the end of their lives and explode again. In other words, if we consider that lighter elements are high-pitched musical instruments and heavier elements are low-pitched musical instruments, then you can say that every time a star dies and is reborn, it acquires more heavy elements, which enriches the resonance of the instrument. That is, the heart of this universe.
It follows that the solar system, the Earth, and even our bodies are composed of a variety of elements, including necessary amounts of trace elements, and that all of the above benefit from this dramatic cycle of creation and destruction.
?Specifically, when did the formation of these elements occur in the history of the universe
The outside of each photomultiplier tube is made by an experienced glassblower.
“The neutrino is our direct guide to getting a detailed picture of what a supernova explosion looks like,” Nakahata says. “When a supernova explosion occurs, as a huge amount of energy is released, 99 percent of it is dispersed through space as neutrinos.” Because these neutrinos are formed immediately before the explosion that occurs in the core of the star and are ejected without interacting with the surrounding matter, these neutrinos alone are able to reveal to us what is happening inside the supernova explosion.”
But how do we capture a particle that has the ability to penetrate through anything? The answer to this question lies in a neutrino detector buried under the Kamioka area in Hida, Gifu Prefecture.
Gazing into cosmic space from an underground mine
Every year in late October, the mountainside turns orange and red. The surrounding village, with many old houses, has a traditional atmosphere. The lush Mount Ikinoyama sits atop a former zinc and lead mine, and if you walk 1.7 kilometers through the dark mine you'll reach the center of the mountain. In the corner of the dark tunnel surrounded by cold air, there is a door to a very subtle world.
A screen showing the detected neutrinos. The circles at the top and bottom represent the top and bottom of the tank, while the rectangle in the middle indicates data from the vertical part of the tank.
On the other side of the door is a large room with a vaulted ceiling. Beneath our feet, the rock has been deeply excavated to accommodate a cylindrical cistern with a radius of 39.3 meters and a depth of 41.4 meters, large enough to hold three of Kamakura's giant Buddha statues stacked on top of each other, and even large enough to place the Statue of Liberty inside, but only the torch will protrude from it. Despite the cave's massive size, the solid gneiss rock means there is no risk of it collapsing.
The walls of the tank are covered with 11,129 photomultipliers (light sensors) that look like giant bulbs, each 50 cm in diameter. Each photomultiplier is so sensitive that it can detect light from a flashlight on the moon's surface. The glass bulbs were carefully crafted by glassblowers and their surfaces were plated with gold, giving the walls a matte gold appearance. (Other than during maintenance times, the tank is completely dark so the array of golden bulbs cannot be seen.)
The tank is filled with approximately 50,000 metric tons of ultra-pure water. When a neutrino sometimes interacts with water, it gives off a cone of bluish Cherenkov light, and the array of photomultipliers on the walls of the tank is designed to capture this very faint light.
Researchers use this device to inject gadolinium into the cistern.
In August 2020, gadolinium (element number 64) was dissolved in the water in the Super Kamiokande tank, bringing its concentration to 0.01% in the liquid, in order to improve the observatory’s ability to detect neutrinos. This modification means that the observatory can now distinguish between different types of neutrino interactions.
“This enables us to confirm the direction of the supernova explosion,” Nakahata explains. While the fact that supernova explosions emit neutrinos has been proven by SN 1987A, the supernova whose light reached Earth in 1987, the Kamiokande Observatory detected only 11 of these particles, and the American and Russian observatories combined detected only 24 particles. Although adding gadolinium would not increase the number of neutrinos detected, because we now have 25 times as much water and because the Milky Way is closer to the Large Magellanic Cloud where the SN 1987A supernova explosion occurred, if a supernova explosion were to occur in our galaxy, we would be able to detect about “We will also be able to detect the direction of several hundred neutrinos using Super Kamiokande, which will enable us to determine the location of the supernova.”
Improved sensitivity also means that in addition to supernova explosions within the Milky Way, the observatory may, a few times a year, be able to detect background scattered supernova neutrinos resulting from supernova explosions in ancient times.
Researchers from all over the world are able to control the observatory remotely.
“There are hundreds of quadrillions of massive stars in the universe that have had supernova explosions, which means that in the history of the universe, there have been many, many explosions,” Nakahata says. The expansion of space has caused a redshift in the wavelengths of neutrinos emitted by previous explosions. Although these particles are easily dispersed, they are still floating around the universe. “This is the background of the supernova neutrino. If we succeed in capturing these particles, we will be able to know the size of the supernova explosions that caused their emission and the period during which they occurred.”
A field that grabs you
I asked Nakahata why he chose this field.
He replied without thinking, “You only get one chance to choose in this life, and I decided to take a chance with a scientist like Koshiba.”
Nakahata heard about the ambitious Koshiba when he was deciding on a field of graduate research, and Koshiba confirmed that Kamiokande would win the Nobel Prize very soon for his discoveries related to proton decay, which sparked Nakahata's interest and he decided to join the project. However, years passed without a single proton decay event being able to be observed.
“At first I thought I was a fool and had chosen the wrong path,” Nakahata says.
Nakahata has a calm and tactful personality.
But it turns out that Koshiba's real talent was his ability to come up with endless new ideas. Koshiba decided that the team should shift its focus to neutrino capture, and modified the observatory accordingly. Just one year later, when neutrinos from a supernova explosion in the Large Magellanic Cloud, a nebula near our Milky Way Galaxy, reached Earth, our team was the first to successfully detect neutrinos from outer space.
Nakahata felt that his research group was very lucky, as supernova explosions do not occur that frequently. In the Milky Way Galaxy, in which our planet is located, only one such explosion occurs every 30 to 50 years, and if we limit our discussion to events that can be detected with the naked eye, we will find that it is only one event every 400 years. In another remarkable coincidence, Nakahata himself became the first person in the world to obtain raw data from a supernova explosion, followed by a third-year doctoral student who was tasked with analyzing the data.
“When I detected those ten seconds of very strong signals in the data at 16:35:35 on 23 February 1987, my heart started racing.
The group of dots in the middle of the image represents the supernova explosion, and the empty space below indicates a gap in the data stream resulting from a recalibration that took place just minutes before the explosion.
This event occurred just three minutes after a two-minute gap in data that occurs once every two hours while sensors reset, so if the neutrino collisions had occurred during that time gap during which our instruments are unable to detect, the team would not have been able to detect this data. Nakahata says he was greatly relieved when he realized how time was on their side.
“I couldn’t contact Koshiba because he was vacationing in one of the hot springs areas in Hakone,” Nakahata says, laughing. “I think no one imagined that everything that happened would lead to us winning the Nobel Prize.”
When Nakahata reported the results of the data analysis to Koshiba the following week, Koshiba showed no emotion and instead gave him another task: “You only have data for two days. I want you to go back and analyze all the other data recorded by the Kamiokande Observatory and prove that what was observed and analyzed “It was the only event.”
Nakahata and his team install photomultiplier tubes. (Courtesy of Kamioka Observatory, University of Tokyo Cosmic Ray Research Institute)
Every day for the next week, Nakahata worked late into the night and played back hundreds of data tapes while writing his thesis. The results he obtained were evidence of the first detection of neutrinos from a supernova explosion.
Looking back on his career with the Kamiokande and Super Kamiokande observatories, Nakahata says with a smile: “Later we got more unexpected results, and things became very interesting.”
In 2027, neutrino research will take a quantum leap and a huge step forward when construction of the 260,000 metric ton Hyper Kamiokande Observatory, the successor to the Super Kamiokande, is completed. Researchers plan to use this facility as they attempt to elucidate a unified theory of elementary particles and the history of the universe's origins.
“Our quest continues,” Nakahata says. “I hope people appreciate the excitement and spirit of adventure, and I hope more young people get involved in this field of research.” I could have sworn I saw a bright sparkle in his eyes at this moment.
On November 12, 2020, Koshiba Masatoshi, professor emeritus at the University of Tokyo, the father of neutrino astronomy, and a pioneering scientist in the field of neutrino research, passed away. We will always remember him for his immortal scientific achievements. May he rest in peace.
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