[연말 담론 시리즈 #5] Why is the universe so complex and beautiful? – MIT Technology Review | MIT Technology Review Korea

[연말 담론 시리즈(The Biggest Questions)는 우리 인간의 존재에 대한 가장 근본적이면서 심오한 질문을 탐구하는 데 기술이 어떻게 도움이 되는지 알아보는 MIT 테크놀로지 리뷰의 미니 기획 시리즈입니다]

Why isn’t space boring? Of course, it could have been a boring space. The number of subatomic particles (particles that make up atomic structure, such as neutrons, protons, and electrons) in the universe is about 1080, which means a huge number of 1 followed by 80 zeros. Imagine scattering this many particles randomly. If that were the case, the universe would have been nothing more than a monotonous ‘desert of samenesses’ made up of thin vacuums spanning billions of light-years in any direction, devoid of any structure much larger than an atom. However, the universe we live in now is full of stars and planets, canyons and waterfalls, pine trees and people, and is also rich in nature. So why are all these things here?

Cosmologists have answered this question over the past half century using a variety of increasingly complex experimental and observational tools. But as always in science, the answer is incomplete. Now, physicists hope that new experiments using incredibly sensitive equipment will capture an ‘unprecedented phenomenon’ that could explain one of the biggest remaining mysteries. The mystery is why the matter that formed the present complex universe came to exist in the universe in the first place.

This fascinating world that surrounds us feels even more enigmatic when we look at the universe from a macroscopic perspective. The first thing you notice in space are structured chunks. Stars come together to form galaxies, galaxies come together to form galaxy clusters, and galaxy clusters come together to form superclusters, filaments, and barriers surrounding massive voids with little material.

However, if you ‘zoom out’ on the universe and look at the 300 million light-year-wide chunk of space from further away, all of this structure disappears. At this point, the light from all the stars in the universe merges into a blur, making the universe look boringly similar no matter which direction you look at it, with no features or differences to be found anywhere. Cosmologists call this the ‘end of greatness.’

The reason such a boring space landscape exists is because space was once really boring. Immediately after the Big Bang and for hundreds of thousands of years thereafter, the universe was truly dull and boring. All that existed was a haze of red, hot particles that spread almost uniformly across trillions of kilometers and filled every point in the universe. However, there were subtle differences in the density of the material between one point and another.

Since then, as the universe has expanded and cooled, gravity has slowly amplified these subtle differences, and over millions or even billions of years, places in the universe with a little more material have attracted more material. In that way, the universe we know was created. In this way, more and more matter accumulated, and areas that were slightly denser turned into complex places filled with enough matter to form stars, galaxies, and life forms like us, and eventually the universe became overflowing with all kinds of substances as it is today. Looking at the universe from its most macroscopic perspective may still feel as boring as it did when it was born, but here on Earth, in this dust and dirt, diversity abounds.

However, there are still some loopholes in this explanation. First of all, it is not clear where the material came from in the first place. According to particle physics, anything that creates matter must also create an equivalent amount of antimatter, thereby maintaining the balance between matter and antimatter. Therefore, every type of matter particle has an ‘antimatter twin’ that behaves like that matter in almost every way. However, when matter particles come in contact with their own antimatter particles, they annihilate each other and only radiation remains. Antimatter refers to a substance composed of antiparticles (antiprotons, antineutrons, positrons, etc.) of elementary particles (protons, neutrons, electrons, etc.) that make up ordinary matter.

This is exactly what happened right after the Big Bang. As matter and antimatter annihilated each other, radiation remained in the universe, and a small amount of matter slightly exceeding the amount of antimatter initially remained. This tiny discrepancy between matter and antimatter has saved today’s universe from eternal boredom, but we don’t know why this happened. “I don’t know why, but this little imbalance occurred, and that imbalance turned everything into what it is today, including us,” said Lindley Winslow, an experimental particle physicist at the Massachusetts Institute of Technology (MIT). “I am really interested in us,” he said. “There are many questions about the universe and its evolution process. “But it’s also a very basic, kindergarten-level question about why we are here.”

capture the phenomenon

To answer this question, Winslow and other physicists around the world designed several experiments to capture the natural phenomenon that upsets the balance between matter and antimatter. Scientists hope to identify a disruption of this balance in the form of neutrinoless double-beta decay, a type of radioactive decay. As of now, this process exists only in theory, so it may never happen in practice. However, if this phenomenon actually occurs, it will be possible to explain the imbalance between matter and antimatter in the early universe.

At the center of this explanation is the neutrino. Neutrinos, the ghostly oddities of particle physics, are very light particles that float around in space and rarely interact with anything. Trillions of neutrinos are constantly flowing through our bodies and the entire Earth, completely ignoring not only the Earth’s core but also our very existence. To reliably block even one neutrino, a lead plate one light-year thick would be needed.

Neutrinos appear to have other very strange properties. The characteristic is that the neutrino and its antimatter may be the same. In other words, neutrinos may themselves be their own antimatter. This makes neutrinos different from all other known forms of matter and allows them to self-annihilate. “If we capture neutrino-less double beta decay, we could prove that neutrinos are their own antiparticles,” Winslow said. “This could also explain the process by which more matter than antimatter is created.” “It is,” he said.

The radioactive decay process that scientists are trying to capture begins at the core of the atom. When an unstable atomic nucleus collapses, it emits an electron along with an antineutrino, attempting to achieve a balance between one matter particle and one antimatter particle. This is a very common type of radioactive decay known as ‘beta decay’. ‘Double beta decay’ is a much rarer phenomenon, when the nucleus emits two electrons along with two antineutrinos to balance things out.

Winslow explains that double beta decay is “one of the longest processes we have ever measured.” Winslow points out that we would typically have to wait a billion times longer than the current age of the universe to observe a single atom undergoing double beta decay. However, if neutrinos also act as their own antiparticles, even rarer phenomena may be possible. In other words, a double beta decay phenomenon can occur in which two neutrinos immediately annihilate each other, leaving only two electrons and no antimatter to balance. This is ‘neutrinoless double beta decay’.

Capturing such a rare phenomenon is difficult, but not impossible. This is because objects of everyday size contain an enormous number of atoms. An object weighing just a few grams contains a huge number of atoms, almost 1024. “So if you stack a few objects, it’s possible to see something happening on a timeline much longer than the age of the universe,” Winslow explained.

This is the approach taken by CUORE (Cryogenic Underground Observatory for Rare Events, ‘CUORE’ means ‘heart’ in Italian), a detector waiting for evidence of neutrino-less double beta decay events beneath a mountain in Italy. It’s a method. Certain isotopes of tellurium are among the nuclei susceptible to double beta decay. CUORE is waiting for this phenomenon to manifest itself through a set of 988 5 cm tellurium dioxide cubic crystals, each connected to a highly sensitive thermometer. Because the binding energy of the two electrons released in neutrino-less double beta decay is the same every time, when a collapse occurs in one of the crystals, that specific amount of energy is accumulated as heat in the crystal, raising the temperature by one ten-thousandth of a degree Celsius. something to do.

However, compared to all other factors that can change the temperature of a crystal, it is not easy to detect a temperature change this small. This is why CUORE is located at the bottom of the mountain. The rocks on the mountain protect CUORE’s crystals from almost all cosmic rays. Also, for the same reason, CUORE must maintain extremely low temperatures that are only about one thousandth of a degree above absolute zero. Winslow explained, “CUORE may be the coldest place in the known universe.” CUORE’s sensors are so sensitive that they can even pick up the vibrations of waves breaking on a beach 60km away.

CUORE is not alone. There are other experiments attempting to detect neutrinoless double beta decay, including KamLand-Zen, an experiment under a mountain in Japan that uses the gaseous element xenon instead of tellurium crystals. But despite years of waiting, none of these experiments have yet detected such a collapse phenomenon. CUORE plans to upgrade its sensors and increase the number of crystals it uses, while KamLand-Zen also plans to increase its size and sensitivity. But the future of these experiments is uncertain.

“In theory, we could do bigger and better experiments,” said Reina Maruyama, a physicist at Yale University and collaborator on the CUORE experiment. “We could even increase the scale of our current experiments by a factor of 10. “So I think what matters is how many resources humanity wants to put into this experiment.” Winslow estimates that it will take two more refinements to the existing experiment before full exploration is possible. If they do that and find nothing, “then the possibility that neutrinos can also act as their own antiparticles is almost eliminated.”

If that happens, a promising theory may be discarded, but it doesn’t end the search. Physicists also have various theories about how matter and antimatter could become unbalanced. But finding evidence for such a theory is not easy. Some theories may be confirmed if the Large Hadron Collider, the world’s largest particle collider, finds something unexpected in the next few years, but dark matter must be carefully searched for proof. There are also theories that need to be worked out. (Dark matter, a hypothetical invisible substance, is believed to make up more than 80% of the universe, and various evidence discovered over several decades support its existence.)

Some theories also claim that protons, one of the key components of the atomic nucleus, are unstable. According to this theory, proton decay takes much longer than neutrino-less double beta decay, on average about 1024 times longer than the current age of the universe. Japan’s Super-Kamiokande (aka ‘Super-K’) is the largest experiment to observe proton decay, installed around a large underground vat containing 50,220 tons of ultra-pure water. It uses 13,031 optical sensors. With knowledge limited, Super-K waits for a faint flash to appear in the darkness. The proton decay phenomenon has not yet been captured.

But whatever caused the imbalance between matter and antimatter in the early universe, physicists are confident that this phenomenon will eventually end. Over time, as the matter and energy of the universe becomes more and more randomly distributed, all interesting structures will disappear. Billions of years from now, the universe will once again be a completely featureless void, this time with particles much more evenly dispersed than the early fog of the universe. This state, called ‘heat death’, is likely to be the final fate of the universe for all eternity to come.

So, even if we don’t fully understand why, we can say that we are lucky to live in a time when the universe is full of complexity and beauty.

Adam Becker, who wrote this article, is a freelance journalist living in Berkeley, California. <뉴욕타임스(New York Times)>, , <사이언티픽 아메리칸(Scientific American)>, <퀀타(Quanta)>, <뉴사이언티스트(New Scientist)> He has contributed articles to various media such as He is also the author of What Is Real?, a friendly account of the little-known history of quantum physics.