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The Eccentric Seer of Supernovas

Fritz Zwicky decoded how exploding stars fill space with cosmic rays.


There was little expectation that anything important would occur at the 1933 meeting of the American Physical Society, which began on December 15 in the main lecture room of the physics department at Stanford University. Forty papers were presented to an audience of only 60, an indication of how modest the session was expected to be. The subject that produced the most research was cosmic radiation, now called cosmic rays. It was one of the deeper mysteries to trouble the scientific world at the time. The fact that invisible forces shape our world was still something of a novel concept, so the idea of cosmic radiation flitting around Earth, pushing its way into buildings and penetrating bodies, seemed simply weird.

The radiation—which we now know as the emission of energy from subatomic particles, mostly free-range protons—first appeared in electroscopes, one of the first scientific instruments to chart the effects of electricity. An electroscope consists of a metal rod from which are suspended two gold leaves; when the rod is exposed to a source of electricity, the two leaves repel each other because both leaves have the same charge. But after a time, an unaccountable thing happens: The leaves spontaneously discharge. Each leaf falls back to its original position. Scientists realized some form of radiation was penetrating the chamber and robbing the leaves of their charges. But what was it, and where did it come from?

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MEET THE NEW STAR: In May, 1934, Zwicky published the co-authored paper that made him famous, and that brought “supernova” into popular language.

Some believed the strange radiation came from somewhere far off in space. Others thought there had to be a local source, maybe under the sea, maybe in the still uncharted wilderness of the solar system. Caltech’s Paul Epstein, a Russian-born immigrant who made important contributions to quantum theory, presented a paper at the conference coming down squarely on the side of locality. He argued that the rays “can travel only a finite distance before completely losing their energy.” so they had to be local. As support for this idea, he cited a theory by a young colleague named Fritz Zwicky.

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As it happened, Zwicky himself was in attendance at the 1933 Stanford meeting. With his colleague Walter Baade, he was about to propose a set of theories that would rival Einstein’s in their imagination and presumption, transforming the humble Stanford meeting into an unexpected crossroads of scientific history. Zwicky’s journey to these theories may not have been as dangerous as the voyage of the Beagle, but by revealing the great life and death struggles in the heavens, the result was as important to understanding the evolution of the universe as Darwin’s journey was to understanding the progress of life on Earth.

Baade and Zwicky were an odd pairing. Baade, who was on the staff of the Mount Wilson Observatory north of Los Angeles, was polite, respectful, and walked with a pronounced limp from a congenital deformity. Exacting in everything he did, no one was better at manipulating the new astronomical tools to tease out secrets from the cosmos. Zwicky, on the other hand, was bold, imaginative, and instinctual. A roughneck mountain climber who never shied from a good battle, whether it was with nature or a superior at Caltech, he had formed an unlikely but productive alliance with Baade.

Zwicky, always a witty and relaxed public speaker, even if his Swiss- German accent was still very much in evidence, presented their joint paper, titled “Supernovae and cosmic rays.” Supernovae were a special class of exploding stars. They were not unknown at the time, but they were a curiosity—circus freaks among the stars—rather than essential to understanding the great forces at work in the universe.

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The idea of cosmic radiation flitting around Earth, pushing its way into buildings and penetrating bodies, seemed simply weird.

There is debate about who introduced the term. Many credit it to Zwicky, but the basic idea appears to have come from the Swedish astronomer Knut Lundmark, who in 1920 referred to a special class of novae—or ordinary exploding stars—that he called “giant novae.” Nevertheless, it would be Zwicky who made the term as familiar and exciting to the interested public as black holes would be to later generations.Zwicky and Baade’s idea that supernovae could produce cosmic rays was remarkable in that only a handful of giant exploding stars had been observed, and none by either Zwicky or Baade. The most famous supernova was Tycho’s Star, named for the Danish astronomer Tycho Brahe. A nobleman who is often remembered for having lost a chunk of his nose in a duel with a fellow student over who was the better mathematician, Brahe was out walking on the evening of Nov. 11, 1572, when he spotted a “strange star” in the constellation Cassiopeia. Not trusting his own eyes, he quizzed his servants, as well as the occupants of a passing carriage. Only when they said they saw it, too, was Brahe satisfied that he had indeed witnessed a startling event, the explosion of a star in our own neighborhood of space. At its brightest, Tycho’s Star rivaled Venus and remained visible to the naked eye for two years before fading away. Such events were at the time seen as evil omens, evidence that the heavens were disturbed.

According to notes from the Stanford session, preserved in the archives of the Physical Society, Zwicky said such “temporary stars,” as they were known at the time, flared up in every stellar system once every few centuries. How he knew this he didn’t say. Producing a light show equal to 100 million normal stars, a floodlight among lighted matches, a supernova would outshine the galaxy it was in. The star’s explosion, according to Zwicky, would yield hellish temperatures of 2.5 million degrees centigrade. Relying on his and Baade’s calculations and his own suppositions, Zwicky suggested that these temporary stars were in the process of self-annihilation, eviscerating themselves down to the atomic level. In “the supernova process (a star’s) mass in bulk is annihilated,” he said.

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All this was just a warm-up for his bold theory about cosmic radiation. Given certain behaviors of cosmic rays, there were three possibilities for their origin, according to Zwicky. Possibly the rays came from empty, intergalactic space, or they were “survivors from a time when physical conditions in the universe were entirely different from what they are now.” Both of those ideas were unsatisfactory, he said. “If, however, the production of cosmic rays is related to some sporadic process, such as the flare-up of a super-nova, the above-mentioned difficulties disappear,” Zwicky and Baade wrote in a paper published several months later.

Zwicky went on to show that the intensities of the rays were in “surprisingly good agreement” with what should be expected if they came from a giant exploding star. Based on the stupendous amount of light such a flare-up produced, each proton of energy must be equal to at least 100 million volts. That was why the radiation was so strong when it reached Earth, even after traveling enormous distances. The hypothesis also seemed to answer the question of why no cosmic rays had been detected from the Milky Way. “The reason is simply that no super-nova eruption (had recently) occurred in our galaxy.”

This made the prediction inspired and breathtakingly cheeky, as if he had described the age of the dinosaurs from a wishbone.

Provocative as these ideas were, Zwicky was still not done. “With all reserve,” Zwicky said, with in fact very little reserve, “we advance the view that a super-nova represents the transition of an ordinary star into a neutron star, consisting mainly of neutrons.” Such a star would have shrunk far below the size of the original star, with a corresponding increase in density and gravity. The “‘gravitational packing’ … may become very large, and … far exceed the ordinary nuclear packing fractions.”

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According to their theory, after exploding, a supernova would undergo a catastrophic collapse, blowing off most of its electrons and protons while crunching others into a nugget more densely packed than anything ever contemplated. At the time, it was beginning to be understood that there was a lot of empty space in an atom. We now know that more than 99 percent of it is empty; if a hydrogen atom were the size of Earth, the proton at its center would be only around 200 yards wide. But no one had dared imagine a process that could smash through the atom’s borders and go on crunching until the last breath was sucked out of it. A star 500,000 miles across, according to Zwicky, would be reduced to a searing marble only 19 miles wide. A teaspoonful of the stuff would weigh 10 million pounds.

It’s important to remember that the term “black hole” had not yet been invented. In fact, the neutron particle had been discovered only the year before Zwicky unveiled his new theory. This made the prediction inspired and breathtakingly cheeky, as if he had described the age of the dinosaurs from a wishbone.

The scientific revolutionaries admitted that their suggestion invoked processes and energies that had never been observed in nature. “We are fully aware that our suggestion carries with it grave implications regarding the ordinary views about the constitution of stars,” Zwicky and Baade concluded.

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The Physical Society did not preserve the reaction from the scientists at Stanford. But in a letter to a Swiss friend, Zwicky described it in amused terms. “With a German astronomer I have cooked up a new theory for cosmic radiation,” he said. The prediction of neutron stars was far more important, but no one could anticipate how much. “And if the indignation of the foreign physicists takes on the same dimensions as those of the local ones, then we will have to fear for our lives. When I presented the new theory here in a seminar there was such an uproar that some of the more conservative gentlemen nearly died of heart attacks.”

Zwicky’s account revealed more than his confidence in his new theories. It showed his preternatural ability to welcome opposition as proof that he was on the right track. It was a characteristic that would underpin all the accomplishments of his working life, one that would bring him both honor and calumny. It lay behind his prediction that the universe is full of unseen dark matter, the scaffolding on which the stars and galaxies are arranged like ornaments on a Christmas tree. It also contributed to his reputation as a difficult, enigmatic man. Feuding with many of the important scientists of his day, he inspired so much resentment that after his death his critics did all they could to forget or disparage what he had done. Like the great forces he chronicled, Fritz Zwicky distorted the orbits of everyone who came in contact with him, attracting many, driving just as many away.

STARRY CORPSE: The “new” star seen by astronomer Tycho Brahe in 1572 exploded and left behind this hot, expanding gas cloud—a source of cosmic rays. NASA / CXC/RIKEN & T. Sato et al.; DSS

Zwicky and Baade made several errors in their presentation. At one point Zwicky suggested that before exploding, supernovae could be “quite ordinary stars” like our sun. In fact, only giant stars, those with at least eight times the mass of our sun, are candidates for Zwicky’s type of supernova, which occurs when the star begins to run out of fuel and its internal furnace can no longer support the star’s enormous bulk and gravity. Within one second the giant star collapses, and then the core begins to form a neutron star, just as Zwicky theorized. At that point, when the core reaches temperatures thousands of times hotter than the surface of the sun, a shock wave rebounds outward, causing a powerful explosion and ejecting the outer layers of the star at nearly a tenth of the speed of light.

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Zwicky could perhaps be forgiven his mistakes because he depended on old and unreliable data for his research; the most recent supernova for which there were good records occurred in 1885. It was long gone by 1934. No remains could be found, not even with the 100-inch Hooker telescope, the world’s most powerful, located atop Mount Wilson north of Los Angeles. By the time they formalized their proposal, Zwicky and Baade assumed that a supernova occurs in a given nebula, or galaxy, once every thousand years. This also would be proven incorrect as time went on. It’s now known that, despite their apparent rarity, supernovae occur every 50 years or so in a galaxy such as our own.

Explosions in space billions of years ago created carbon, the basis of all organic life, and oxygen, which sustains life on Earth.

Observers might not have to wait centuries to see an eruption, the two wrote. Because there were around a thousand galaxies in our region of the universe, odds suggested that “one super-nova per year should be expected in this ‘immediate’ neighborhood of ours.” This suggestion triggered a worldwide hunt for exploding stars, led, appropriately, by Zwicky himself.

In time, the presentation at Stanford and the brief, five-page paper that followed would prove to be among the most influential works in 20th-century astrophysics. The Caltech physicist Kip Thorne calls it “one of the most prescient documents in the history of physics and astronomy.”

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Many years later, looking back on his career, Zwicky himself called it “in all modesty … one of the most concise triple predictions ever made in science,” referring to supernovas, neutron stars, and cosmic rays. He went on: “I think even David Hilbert”—the influential mathematician whom Albert Einstein consulted while formulating his general theory of relativity—“would have been pleased since, in his will … he had left us with the admonition to be brief in all writings and to try to present our life’s work in ten minutes.”

Zwicky and Baade’s theories were essentially the birth of high-energy astrophysics. They were relevant to everything from pulsars and quasars to supermassive black holes—all the awesome processes in the universe that made life here on Earth seem even more trivial and inconsequential than before.

Zwicky’s bold ideas shocked the more conservative members of his fraternity, who were still trying to figure out the basic building blocks of atoms. Here was this upstart, who suggested that temporary stars are not just interesting nighttime entertainment and fodder for the fantasies of court astrologers. Instead, Zwicky asserted, they play an important role in phenomena on Earth.

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It was many years before it became clear just how important supernovae were to life on Earth. Explosions in space billions of years ago created iron, found everywhere from the ore in the Iron Range in northern Minnesota to our bloodstream; carbon, the basis of all organic life, and oxygen, which sustains life on Earth. These discoveries were years off, but that didn’t prevent the two theorists from making sweeping claims— many of which would hold up over the coming decades.

An article in Time magazine, “Star Suicide,” explained Zwicky and Baade’s new theory. “If they are right, the old concept of the end of the world,” of life “freezing to death under a cooling sun—must give way to the prospect of [being] scorched to death by a sun having a final fling before joining the stellar ghosts in the cosmic graveyard.” The Los Angeles Times, on Dec. 8, 1933, called the prediction, a week before the APS meeting, “probably the most daring theory on the origin of cosmic radiation.”

Only in recent years has it finally been considered settled that supernovae do in fact produce highly accelerated cosmic rays. It also took decades to confirm the existence of neutron stars. Although the mathematics of it were shown to be correct earlier, it was the discovery of pulsars in 1967 that proved they existed. Pulsars are neutron stars that spin faster than the blades in a kitchen blender.

Even without final proof, Zwicky’s ideas were an immediate hit with the public. At the time, when a transcontinental commercial flight aboard the new Boeing 247 consumed 20 hours, readers of newspapers and magazines were fascinated by the new scientific theories emerging around the world. The problem was that most people couldn’t understand them. Einstein’s relativity? Niels Bohr’s quantum mechanics? Interesting, but could you give that to me one more time? The idea of a supernova, an exploding star putting on a deadly galactic show, on the other hand, captured the fancy and anxieties of the public precisely because anyone could understand it.

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Zwicky became the darling of reporters everywhere. Quotable, accessible, if occasionally truculent when some writer called him a Bulgarian (he was born there but considered Switzerland his true homeland), he became the hot new cultural commodity.

Popular magazines noted his birthday along with those of movie stars. He even made it onto the comics page, where he was portrayed as a character known as Doc Dabble—a small, mustachioed man who looked a bit like Dagwood Bumstead’s boss, Mr. Dithers—in the popular comic strip Blondie. In a January 1934 panel, Doc Dabble was shown bedding down in his observatory while his assistant sat at the telescope. “Call me when something explodes,” Doc says.

“Now I understand Americans,” Zwicky said. “I am in the funny papers.”

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John Johnson Jr. spent 22 years at the Los Angeles Times, where he covered space and physics. He was a member of two Pulitzer Prize-winning teams and shared an IRE Medal from Investigative Reporters and Editors, Inc.

Excerpted from Zwicky: The Outcast Genius Who Unmasked the Universe by John Johnson Jr., published by Harvard University Press. Copyright © 2019 by John Johnson, Jr. Used by permission. All rights reserved.

Lead image: NASA

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