Press "Enter" to skip to content

How the hidden complexity of matter unleashed the power of nuclear physics

The chair is a lush tapestry, woven from a complex assortment of yarns. Various subatomic particles weave together to make the universe we inhabit. But a century ago, people believed that matter was so simple that it could be built with just two types of subatomic fibers: electrons and protons. That view of matter was a meaningless picture instead of an ornate brocade.

Physicists in the 1920s thought they had a solid understanding of what matter made up. They knew that atoms contained electrons surrounding a positively charged nucleus. And they knew that each nucleus contained a series of protons, positively charged particles identified in 1919. The combinations of those two particles made up all the matter in the universe, it was thought. That applied to everything that was or could have been, through the vast and unexplored cosmos and into the house of the Earth.

The scheme was appealing, but it swept under the rug a variety of clues that all was not well in physics. Two discoveries in a revolutionary year, 1932, forced physicists to peek under the rug. First, the discovery of the neutron revealed new ways of peeking into the hearts of atoms and even splitting them in two. Then came the positron news, identical to the electron but with the opposite charge. His discovery foreshadowed many surprises to come. Other particle discoveries introduced a new framework for the fundamental pieces of matter, now known as the standard model.

That “annus mirabilis” – miraculous year – also firmly fixed the view of physicists on the functioning of the heart of atoms, how they decay, transform, and react. The discoveries there would send scientists to a more devastating technology: nuclear weapons. The atomic bomb has cemented the importance of science and science journalism in the public eye, says nuclear historian Alex Wellerstein of the Stevens Institute of Technology in Hoboken, New Jersey. "The atomic bomb becomes the definitive proof that … in fact, this is a world-changing issue."

After bombing Japan in 1945, the U.S. military continued testing nuclear weapons, shown in 1946 at Bikini Atoll.Library of Congress

Appeal of two particles

Physicists of the 1920s embraced a particular type of conservatism. Embedded deep in his psyche there was a reluctance to declare the existence of new particles. Researchers remained in the status quo of matter composed exclusively of electrons and protons, an idea called the "two-particle paradigm" that lasted until about 1930. In that period, says science historian Helge Kragh of the University of Copenhagen, "I'm pretty surely no general physicist had the idea that there could be more than two particles. "The sheer simplicity of two particles explaining everything that was the reward of nature proved so appealing to the sensibilities of the physicists who found the idea. hard to let go.

The paradigm stopped the theoretical descriptions of the neutron and the positron. “Proposing the existence of other particles was considered reckless and contrary to the spirit of Occam’s razor,” Graham Farmelo wrote in 2010 Physics Contemporary Biograph.

Still, in the early twentieth century, physicists were investigating some enigmas of matter that, after some hesitation, would inevitably lead to new particles. They included unanswered questions about the identities and origins of energy particles called cosmic rays, and why chemical elements occur in different varieties called isotopes, which have similar chemical properties but varying masses.

Headlines and summaries of the latest Science News articles, delivered in your inbox

The neutron arrives

New Zealand-born British physicist Ernest Rutherford stopped introducing a fundamentally new particle in 1920. He realized that neutral particles in the nucleus could explain the existence of isotopes. These particles became known as "neutrons". But instead of proposing that neutrons were fundamentally new, he thought they were composed of protons combined very closely with electrons to form neutral particles. He was right in the role of the neutron, but he was wrong in his identity.

Rutherford's idea was compelling, British physicist James Chadwick reported in a 1969 interview: "The only question was how the devil could get proof of that." The lack of electric charge of the neutron made it a particularly cunning target. Among other projects, Chadwick began hunting for particles in the Cavendish Laboratory at Cambridge University, then run by Rutherford.

Chadwick found his evidence in 1932. He reported that the mysterious radiation emitted when beryllium was bombarded with the nuclei of helium atoms could be explained by an uncharged particle with a mass similar to that of the proton. In other words, a neutron. Chadwick did not foresee the important role his discovery would play. "I'm afraid neutrons will be of no use to anyone," he told the New York Times shortly after his discovery.

image of James Chadwick next to an image of his devicePhysicist James Chadwick used this device to discover the neutron in 1932, based on radiation emitted after beryllium was struck with alpha particles, the nuclei of helium atoms.From left: Los Alamos National Lab; SSPL / Getty Images

Physicists faced neutron identity for years to come before accepting it as a completely new particle, rather than the fusion Rutherford had suggested. On the one hand, a mash-up of protons and electrons came into conflict with the new theory of quantum mechanics, which characterizes physics on a small scale. Heisenberg's uncertainty principle – which states that if the location of an object is well known, its momentum cannot be – suggests that an electron confined within a nucleus would have excessively large energy.

And the rotations of certain nuclei, a quantum mechanical measure of angular momentum, also suggested that the neutron was a full-fledged particle, as were improved particle mass measurements.

Positron perplexity

Physicists also resisted the positron, until it became difficult to ignore it.

The detection of the 1932 positron had been foreshadowed by the work of British theoretical physicist Paul Dirac. But he took some mockery before physicists realized the meaning of his work. In 1928, Dirac formulated an equation that combined quantum mechanics with Albert Einstein's theory of special relativity in 1905, which describes physics close to the speed of light. Now known simply as the Dirac equation, the expression explained the behavior of electrons in a way that satisfied both theories.

But the equation suggested something strange: the existence of another type of particle, one with opposite electric charge. At first, Dirac and other physicists clung to the idea that this charged particle could be the proton. But this other particle should have the same mass as the electron and the protons are almost 2,000 times heavier than the electrons. In 1931, Dirac proposed a new particle, with the same mass as the electron but with opposite charge.

Meanwhile, American physicist Carl Anderson of Caltech, regardless of Dirac's work, used a device called a cloud camera to study cosmic rays, energy particles originating in space. Cosmic rays, discovered in 1912, fascinated scientists, who did not fully understand what particles were or how they were produced.

Inside Anderson’s chamber, liquid droplets condensed along the paths of the energetically charged particles, the result of the particles ionizing the gas molecules as they rotated. In 1932, experiments revealed positively charged particles with masses equal to those of an electron. Soon, the connection with Dirac's theory became clear.

black and white image of a particle track cloud cameraA particle track in a cloud chamber in the early 1930s was the first evidence of a positron, a positively charged particle with the mass of an electron. The track curves due to a magnetic field and the curvature increases as the positron loses energy after crossing the center plate from below.C.D. Anderson / Wikimedia Commons

Science News Letter, the predecessor of Science News, had a hand in the name of the new particle. Editor Watson Davis proposed "positron" in a telegram to Anderson, who would consider the surname independently, according to a 1933 Science News Letter article (SN: 25/02/33, p. 115). In a 1966 interview, Anderson recounted the consideration of Davis' idea during a bridge game and eventually accompanied it. He later lamented the choice and said in the interview, "I think it's a very poor name."

The discovery of the positron, the electron’s antimatter companion, marked the arrival of antimatter research. The existence of antimatter still seems baffling today. Every object we can see and touch is made of matter, making antimatter seem downright strange. The lack of relevance of antimatter to everyday life – and the liberal use of the term in Star Trek – means that many non-scientists still imagine it as a science fiction thing. But even a banana sitting at a counter emits antimatter, periodically spitting positrons into radioactive decays of potassium.

Physicists would discover many other antiparticles, all identical to their material pairs, except for an opposite electric charge, including the antiproton in 1955. The subject still keeps physicists awake at night. The Big Bang should produce equal amounts of matter and antimatter, so today researchers are studying how rare antimatter has become.

In the 1930s, antimatter was such a leap that Dirac's hesitation in proposing the positron was understandable. The positron would not only break the paradigm of the two particles, but would also suggest that electrons had mirror images with no apparent role in the composition of atoms. When asked, decades later, why he had not predicted the positron after formulating his equation, Dirac replied, "pure cowardice."

But by the mid-1930s, the two-particle paradigm was already out. The understanding of physicists had advanced and his austere view of matter had to be dispelled.

Unleashing the power of the atom

Radioactive decay suggests that atoms keep energy accumulations locked up, ripe for capture. Although radioactivity was discovered in 1896, that energy remained for a long time as an untapped resource. The discovery of the neutron in the 1930s would be key to unlocking that energy, for better and for worse.

The discovery of the neutron opened the understanding of the nucleus by scientists, giving them new abilities to split atoms in two or transform them into other elements. Developing that nuclear knowledge has led to useful technologies, such as nuclear power, but also devastating nuclear weapons.

Just a year after the neutron was found, Hungarian-born physicist Leo Szilard thought of using neutrons to split atoms and create a bomb. "It suddenly occurred to me that if we could find an element that is divided by neutrons and emit two neutrons when it absorbs a neutron, such an element, if mounted on a large enough mass, could withstand a nuclear chain reaction, release energy on an industrial scale and build atomic bombs, ”he later recalled. It was an incipient but beautiful idea.

Because neutrons lack electric charge, they can penetrate the heart of atoms. In 1934, the Italian physicist Enrico Fermi and his colleagues began bombarding dozens of different elements with neutrons, producing a variety of new radioactive isotopes. Each isotope of a particular element contains a different number of neutrons in its nucleus, with the result that some isotopes may be radioactive while others are stable. Fermi was inspired by another surprising discovery of the time. In 1934, French chemists Frédéric and Irène Joliot-Curie reported the first artificially created radioactive isotopes, produced by bombarding elements with helium nuclei, called alpha particles. Now, Fermi was doing something similar, but with a more penetrating probe.

There have been some scientific advances along the way to understand the results of these experiments. One of the main goals was to produce new elements, beyond the last known element of the periodic table at that time: uranium. After exploding uranium with neutrons, Fermi and colleagues reported successful tests. But that conclusion would be incorrect.

German chemist Ida Noddack had an idea that all was not well with Fermi's interpretation. She approached the correct explanation of her experiments in a 1934 journal by writing, "When heavy nuclei are bombarded by neutrons, it is conceivable that the nucleus splits into several large fragments." But Noddack did not follow the idea. “It didn’t provide any kind of supporting calculation and no one took it very seriously,” says physicist Bruce Cameron Reed of Alma College Michigan.

In Germany, physicist Lise Meitner and chemist Otto Hahn had also begun bombarding uranium with neutrons. But Meitner, an Austrian of Jewish heritage in increasingly hostile Nazi Germany, was forced to flee in July 1938. She had an hour and a half to pack her bags. Hahn and a third team member, chemist Fritz Strassmann, continued the work, corresponding from afar with Meitner, who had landed in Sweden. The results of the experiments were puzzling at first, but when Hahn and Strassmann informed Meitner that barium, a much lighter element than uranium, was the product of the reaction, it became clear what was happening. The core was splitting.

Lise Meitner is next to a lab table and Otto Hahn holds a clipboardLise Meitner (left) and Otto Hahn (shown in their laboratory in Germany in 1913) established that atoms could split, or fissure, when bombarded with neutrons. The two worked together before Nazi policies forced Meitner to flee to Sweden.Smithsonian Institution / Science Photo Library

Meitner and his nephew, physicist Otto Frisch, collaborated to explain the phenomenon, a process the couple would call "fission." Hahn received the 1944 Nobel Prize in Chemistry for the discovery of fission, but Meitner never won a Nobel, in a decision that is now considered unfair. Meitner was nominated for the award, sometimes in physics, others in chemistry, 48 times, most after the discovery of fission.

“His physics peers recognized that it was part of the discovery,” says chemist Ruth Lewin Sime of Sacramento City College, California, who has written extensively about Meitner. "That included almost anyone who was anyone."

News of the discovery soon spread, and on January 26, 1939, renowned Danish physicist Niels Bohr publicly announced at a scientific meeting that fission had been achieved. The possible implications were immediately obvious: fission could release energy stored in atomic nuclei, which could produce a bomb. A Science News Letter story describing the ad tried to dispel concerns the discovery may raise. The article, titled "Atomic Energy Released," reported that scientists "fear that the public will not worry about a" revolution "in civilization as a result of their research," as "the suggested possibility that atomic energy may be used as some super-explosive or as a military weapon. ”(SN: 2/11/39, p. 86) But minimizing the catastrophic implications did not prevent them from being fulfilled.

A ball of fire

The question of whether a bomb could be created rested, once again, on neutrons. For the fission to ignite an explosion, it would be necessary to trigger a chain reaction. This means that each fission would release additional neutrons, which could induce more fissions, and so on. The experiments quickly revealed that enough neutrons were released to make that chain reaction feasible.

In October 1939, shortly after Germany invaded Poland at the beginning of World War II, an ominous letter from Albert Einstein reached President Franklin Roosevelt. Composed at the behest of Szilard, then at Columbia University, the letter warned: "It is conceivable … that extremely powerful bombs of a new kind can be built in this way." American researchers were not alone in their interest in the subject: German scientists, the letter noted, were also in the case.

Roosevelt responded by creating a committee to investigate. That step would be the first step toward the U.S. effort to build an atomic bomb, the Manhattan Project.

On December 2, 1942, Fermi, who had then emigrated to the United States, and 48 colleagues achieved the first self-sustaining nuclear chain reaction controlled in an experiment with a uranium and graphite stack at the University of Chicago. Science News Letter would later call him "a ranking of events with the first prehistory of man lighting a fire." As physicists celebrated its success, the possibility of an atomic bomb was closer than ever. “I thought this day would be a black day in human history,” Szilard reminded Fermi.

illustration of scientists observing the first nuclear chain reaction experimentThe first controlled, self-sustaining nuclear chain reaction took place on a uranium and graphite stack (right in this illustration) at the University of Chicago in 1942.National administration of archives and records

The experiment was a key step in the Manhattan project. And on July 16, 1945, about 5:30 a.m., scientists led by J. Robert Oppenheimer detonated the first atomic bomb in the New Mexico desert: the Trinity test.

It was an amazing sight, as physicist Isidor Isaac Rabi recalled in his 1970 book, Science: The Center of Culture. "Suddenly, there was a huge flash of light, the brightest light I've ever seen or I think anyone has ever seen. It exploded; "I wish it had stopped; though it lasted about two seconds. It finally ended, diminishing, and we looked at where the bomb had been; there was a huge ball of fire that grew and grew and rolled as it grew; it rose into the air, in flashes." yellow and in scarlet and green. It seemed threatening. It seemed to come to one. A new thing had just been born; a new control; a new understanding of man, which man had acquired about nature. "

Physicist Kenneth Bainbridge put it more succinctly: "Now we're all bitch kids," he told Oppenheimer in the moments after the test.

The construction of the bomb was motivated by the fear that Germany would get it first. But the Germans were not even close to producing a bomb when they surrendered in May 1945. Instead, U.S. bombs would be used in Japan. On August 6, 1945, the United States dropped an atomic bomb on Hiroshima, followed by another on August 9 in Nagasaki. In response, Japan surrendered. More than 100,000 people died as a result of the two attacks, and perhaps as many as 210,000.

“I saw a bluish white blind blindfold from the window. I remember feeling floating in the air, "survivor Setsuko Thurlow recalled in a keynote speech on the 2017 Nobel Peace Prize to the International Campaign to Abolish Nuclear Weapons. She was 13 when the bomb hit Hiroshima. bomb destroyed my beloved city. Most of its residents were civilians who were cremated, vaporized, charred. "

Nuclear anxieties

Humanity has entered a new era, with new dangers to the survival of civilization. “With nuclear physics you have something that within 10 years … goes from being this arcane area of ​​academic research … to something that bursts onto the world stage and completely changes the relationship between science and society,” Reed says.

In 1949, the Soviet Union fired its first nuclear weapon, initiating decades of nuclear rivalry with the United States that would define the Cold War. And then came a bigger and more dangerous weapon: the hydrogen bomb. While atomic bombs are based on nuclear fission, H-bombs take advantage of nuclear fusion, the fusion of atomic nuclei, along with fission, resulting in much larger explosions. The first H-bomb, detonated by the United States in 1952, was 1,000 times more powerful than the bomb dropped on Hiroshima. In less than a year, the Soviet Union also tested an H-bomb. Scientists who were part of an advisory committee of the U.S. Atomic Energy Commission, which had previously recommended not developing the technology, called the H-bomb a "weapon of genocide."

Fear of the devastation that would result from a full-scale nuclear war fueled repeated attempts to curb nuclear weapons accumulations and tests. Since the signing of the Comprehensive Nuclear-Test-Ban Treaty in 1996, the United States, Russia, and many other countries have maintained a moratorium on testing. However, North Korea tested a nuclear weapon as recently as 2017.

Still, the dangers of nuclear weapons were accompanied by a promising new technology: nuclear power.

In 1948, scientists first demonstrated that a nuclear reactor could harness fission to produce electricity. The X-10 graphite reactor at Oak Ridge National Laboratory in Tennessee generated steam that powered an engine that lit a small Christmas light bulb. In 1951, Experimental Breeder Reactor-I at the Idaho National Laboratory near Idaho Falls produced the first usable amount of electricity from a nuclear reactor. The world’s first commercial nuclear power plants began to ignite in the mid-to-late 1950s.

But nuclear disasters dampened enthusiasm for technology, including the 1979 Three Mile Island accident in Pennsylvania and the Chernobyl disaster in Ukraine, which was then part of the Soviet Union. In 2011, the disaster at the Fukushima Daiichi power plant in Japan revived society’s burning nuclear cravings. But today, at a time when the effects of climate change are alarming, nuclear power is attractive because it does not emit greenhouse gases directly.

workers measuring radiation in a Chernobyl forestWorkers take radiation measurements after the 1986 Chernobyl disaster, which raised awareness about the dangers of nuclear power.UPI / Alamy Stock Photo

And humanity’s mastery over matter is not yet complete. For decades, scientists have dreamed of another type of nuclear energy, based on fusion, the process that feeds the sun. Unlike fission, fusion energy would not produce long-lasting nuclear waste. But progress has been slow. The ITER experiment has been in planning since the 1980s. Once built in the south of France, ITER aims, for the first time, to produce more energy from fusion than is introduced. If you are successful it can help determine the energy prospects of future centuries.

From the current perspective, the dizzying pace of progress in nuclear and particle physics in less than a century may seem incredible. The neutron and positron were found in small laboratories compared to the current ones, and each discovery was attributed to a single physicist, relatively shortly after the particles were proposed. Those discoveries sparked frantic events that seemed to roll one after another.

Now, finding a new element, discovering a new elementary particle, or creating a new type of nuclear reactor can take decades, international collaborations from thousands of scientists, and huge, costly experiments.

As physicists discover the tricks for understanding and controlling nature, apparently, the next level of secrets becomes increasingly difficult to expose.

Source link