by STEPHEN M. MAURER IDEA MAN Sixty years ago,

Fritz Zwicky

was the only

astronomer who

thought that dark

matter, neutron

stars, and gravita-

tional lenses

were worth

worrying about. Astronomical Society of the Pacific

HEN FRITZ ZWICKY DIED IN 1974, he was remembered as a gifted observational astronomer who had discovered more supernovae than everyone Welse in human history combined. Today, Zwicky’s reputation is bigger than ever, except that now astronomers think of him as a theorist. When researchers talk about neutron stars, , and gravitational lenses, they all start the same way: “Zwicky noticed this problem in the 1930s. Back then, nobody listened . . .”

BEAM LINE 21 Previous page: Fritz Zwicky invented a technique called morphology in order to think more efficiently. He is pictured here in the 1950s doing just that. Opposite: Fritz Zwicky at the Mt. Wilson 24-in. telescope in approximately the late 1930s. (Courtesy of the Archives, Institute of Technology)

Fritz Zwicky was born in Bulgaria were immensely distant. In order to discovered the neutron in 1932. Sup- in 1898 but went to live with his be seen at all, Baade and Zwicky pose that the star’s electrons and pro- grandparents in Switzerland at age realized that these “super-novae” had tons could be turned into neutrons? six. In 1916, he enrolled in ’s to be enormously bright (100 million “With all reserve,” Baade and Swiss Federal Institute of Technology times brighter than the Sun). They Zwicky wrote in March 1934, to study mathematics, engineering, announced their discovery at an “. . .we advance the view that a and physics. With World War I rag- American Physical Society meeting super-nova represents the transi- ing, Switzerland provided a haven for in late 1933. tion of an ordinary star into a many of Europe’s greatest minds. Not content with this empirical , consisting mainly Zwicky met , Wolf- result, Baade and Zwicky added two of neutrons. Such a star may pos- gang Pauli, and Vladimir Lenin. After key theoretical insights. First, they sess a very small radius and an graduation, Zwicky stayed on to pur- connected supernovae to the myste- extremely high density. As neu- sue a degree in theoretical physics. rious high altitude particles known trons can be packed much more closely than ordinary nuclei and His thesis applied the new science of as cosmic rays. Unfortunately, their electrons, the ‘gravitational pack- quantum mechanics to crystals. He evidence—which was limited to the ing’ energy in a cold neutron star received his doctorate in 1922. “surprisingly good agreement” in may become very large, and, In 1925, the Rockefeller energy between the two phenomena— under certain circumstances, Foundation—eager to bring quantum remained circumstantial. Although may far exceed the ordinary mechanics to the — Zwicky spent much of the 1930s try- nuclear packing fractions. A gave Zwicky a fellowship. A lifelong ing to explain how particles heated neutron star would therefore represent the most stable skier and mountain climber, Zwicky inside an exploding could configuration of matter as such.” asked the Foundation to send him escape into space, he admitted that “where there are mountains.” Try- his results fell “well short” of what Zwicky spent the next forty years ing to comply, the Foundation sent was needed. The correct answer was pointing out that this comment had him to the California Institute of found in 1949, when Enrico Fermi “checked out in all essential aspects.” Technology. Zwicky grumbled that realized that shock waves hitting However, it was largely ignored until Pasadena only had “foothills.” interstellar gas could produce cosmic Robert Oppenheimer and George Nevertheless, Caltech was a happy rays outside the supernova itself. Volkoff worked out the detailed choice. At nearby Mt. Wilson, Edwin Second, Baade and Zwicky tried physics of stellar collapse in 1939 Hubble was working on his famous to explain how such titanic explo- without mentioning Zwicky’s re- relation. Zwicky began sions could occur at all. Then as now, search. Even then, final confirmation thinking about . When his any reasonable theory had to involve had to wait until radio astronomers fellowship ended, Caltech hired gravitational collapse. However, a discovered the extraordinarily dense Zwicky as a professor. simple calculation showed that the objects called “” in 1967. collapsing progenitor star had to free- WICKY BEGAN collabo- fall over enormous distances in order N ORDER TO LEARN more, rating with a fellow German- to liberate enough energy. Unless the Baade and Zwicky had to discover Zspeaker named was unbelievably Inew supernovae. However, the in 1931. Astronomers knew that cer- small (and dense) the process would chance of finding a supernova in any tain stars flared abruptly from time stop too soon. By the early 1930s, given was small, and con- to time. Although most of these quantum calculations had shown ventional telescopes could only “novae” were close to the Earth, that the required densities could not examine a few at a time. Baade noticed that a few old records be met by any form of matter that Fortunately, Baade had heard of a spe- described novae inside galaxies. contained electrons. cial telescope that could capture huge During the 1920s, astronomers at The breakthrough came when numbers of galaxies in a single, wide- Mt. Wilson had shown that galaxies English physicist James Chadwick angle photograph. Zwicky persuaded

22 WINTER 2001 Caltech to build an 18-inch “Schmidt nearby galaxies were part of larger estimated from the velocity of its camera” in 1936. Zwicky used this clusters. Now, Zwicky used the components. This is because, all else instrument for the rest of his life, Schmidt to find new and more dis- being equal, bodies in more massive finding 129 supernovae in all. Beyond tant examples. These observations systems must travel faster in order their intrinsic interest, Zwicky proved that clusters were the rule to resist the increased pull of .) believed that supernovae would and not the exception. This led to a paradox. Based on data “eventually allow us to survey the from the Milky Way, Zwicky should to distances of billions of STRONOMERS HAD have been able to guess each cluster’s light years.” That dream is only now already measured velocities mass from its observed brightness. coming true. (See “The Fate of the Afor most of the galaxies This estimate turned out to be 500 Universe” by Gerson Goldhaber and inside Zwicky’s clusters. In 1937, times too small. Had Nature hidden Judith Goldhaber in the Fall 1997 Zwicky used astronomy’s “virial the extra mass in a second, unseen Beam Line, Vol. 27, No. 3.) theorem” to infer the clusters’ component? Zwicky dubbed the sub- Zwicky’s supernova search had an masses from these data. (The virial stance “dark matter.” unexpected spinoff. Conventional theorem says that the total mass of Most astronomers ignored Zwicky’s telescopes had shown that a few a group of orbiting bodies can be result, much as Zwicky himself had

BEAM LINE 23 Zwicky believed that

supernovae would

“eventually allow us

ignored an earlier paper by Johannes to survey the Universe Naturally—to hear Zwicky tell Kapetyn and Sir James Jeans on the it—genius was persecuted. Friends ground that stellar motions inside say that Zwicky could explain his the Milky Way were too complicat- to distances of billions ideas patiently and was given to bois- ed to interpret. Undaunted, Zwicky terous laughter. This did not stop spent most of the next two decades him from railing against astronomy’s conducting searches for previously of light years.” That “self-interested cliques” and “high overlooked gas and dust. In 1950, this priests.” One of his favorite jokes was dim object search uncovered starry to call enemies “spherical bastards” “bridges” between galaxies, which dream is only now (“spherical,” he explained, because Zwicky correctly attributed to near- they still looked like “bastards” from collisions. Later searches found every possible angle). Zwicky even “pygmy stars” within the Milky Way coming true. convinced himself that Baade had and “faint blue stars” that were ac- stolen his ideas. The famously gen- tually distant galaxies. Today we tle Baade was afraid that Zwicky know that the latter objects are close- might try to kill him. ly related to quasars. Beginning in 1974, cosmologists that lensed galaxies would not only URING WORLD WAR II, began to rediscover the arguments be amplified, but also bent into a dis- the U.S. Army needed rock- for dark matter. Today, it is one of tinctive ring. For the rest of his life, Dets to get heavily-loaded astronomy’s hottest research topics. Zwicky begged astronomers to bombers airborne. Caltech aerody- search for lensed galaxies. Finding namicist Theodore von Karman HE IDEA that massive bod- even one lens, he insisted, could test founded a corporation called Aerojet ies can act as lenses goes ; deliver light from and won the contract. In 1943, Aero- Tback to the eighteenth cen- unprecedented distances; and give jet asked Zwicky to run its research tury and received its most famous astronomers a new way to detect department. At War’s end, Zwicky formulation in Albert Einstein’s 1916 dark matter. These are the same rea- toured secret weapons programs in General . In 1935, sons that astronomers give for study- Germany and Japan. Later, he helped a Czechoslovakian engineer named ing lensed galaxies today. Aerojet develop many of the high- R. W. Mandl wrote to Einstein sug- Zwicky was so confident of his energy fuels used in today’s solid gesting that nearby stars could act as prediction that he sometimes won- rocket boosters. gravitational lenses by bending light dered why lensing hadn’t been Corporate life did not improve from more distant objects. (The noticed already. In fact, the first grav- Zwicky’s people skills. Once, Zwicky physicist Oliver Lodge had made a itational lens was found five years met a visiting delegation (including similar suggestion in 1919.) Initially after Zwicky’s death. two admirals) at the plant gate and intrigued, Einstein soon became dis- ordered them to leave. To hear couraged after calculating that the WICKY WAS NEVER shy Zwicky tell it, they were a bunch of lensed image would almost certainly about describing himself as a unqualified civilians who had some- be overwhelmed by glare from the Z“romantic figure out of the how wangled commissions. Aerojet foreground star. Renaissance” or “lone wolf” genius. old-timers also claim that Zwicky Meanwhile, word of Mandl’s ideas In fact, he invented a technique liked to win arguments with his fists. had reached Zwicky. Zwicky realized (“morphology”) which supposedly al- “But he was a small guy,” they usu- that foreground glare could be over- lowed practically anyone to think ally add, “so it wasn’t hard to pull come if searchers used galaxies one hundred times more efficiently. him off.” instead of stars. In 1937, Zwicky Zwicky credited the technique for During the Second World War, the published three articles predicting most of his insights. government gave Zwicky a security

24 WINTER 2001 clearance even though he had never taken out American citizenship. In 1955, the exception was revoked. Zwicky, who was intensely proud of being Swiss, decided to leave Aero- jet rather than change his nationality. Besides, he told people, the U.S. Con- stitution does not allow naturalized Americans to become president. Why accept “second-class citizenship?”

HORTLY AFTER THE War, Zwicky used his Aerojet Sconnections to place an exper- iment aboard a captured V-2 rocket. Zwicky wanted to use shaped charges similar to the explosives used in Army bazookas to generate jets of liquid metal traveling at up to seven miles per second. Earthbound obser- vatories could then photograph these “artificial meteors” to learn about natural meteors, atmospheric struc- ture, and orbital reentry physics. Lat- er experiments would have been more ambitious. Zwicky wanted to study flash and dust on the Moon and even return samples to Earth. Data from artificial meteor experiments also would have helped the U.S. to track shock waves from airplanes fly- ing over Siberia. In 1999, the U.S. revived Zwicky’s idea by crashing its Clementine satellite into the Moon. Earthbound observatories searched for water vapor, but saw none.

The Aerobee was America’s first rocket to probe outer space. Here, a launch in the mid-1950s is shown, similar to the one that detonated Fritz Zwicky’s explo- sives above the New Mexican desert. (Courtesy Aerojet, Sacramento, California)

BEAM LINE 25 In 1957, Fritz Zwicky used high explo- sives atop an Aerobee rocket to blast jets of metal into space. Tracking cam- eras proved that at least one jet escaped Earth (arrow) and became the first man-made object to achieve its own independent orbit around the Sun. (Courtesy AP/Wide World Photos)

Zwicky’s V-2 flew on December 18, 1946. The New York Times called the experiment “a symbolic milestone in man’s exploration of the universe” which might “open the secrets of travel between the planets.” But while the rocket behaved flawlessly, observatories and amateur astrono- mers saw nothing. Zwicky believed that the charges had fizzled and asked for a second chance. The Army said “no” and went on saying “no” until Sputnik was launched in 1957. Twelve days later, an Aerojet Aerobee rocket detonated three shaped charges 54 miles above the New Mexican desert. (This time, Zwicky installed the explosives him- self.) Three jets were observed as far away as Mt. Palomar. Tracking cam- eras confirmed that at least one centimeter-sized projectile had be- come the first man-made object to escape the Earth and enter a separate orbit around the Sun.

WICKY HAD criticized President Harry Truman for Zdropping the atomic bomb on Japan. Now he brooded “that scien- tists’ inventions have gotten com- pletely out of hand.” “I myself,” Zwicky said, “can think of a dozen ways to annihilate all living persons within one hour.” Zwicky even claimed (mistakenly) that future weapons or misguided fusion exper- iments could reduce the Earth to a miniature neutron star. Zwicky’s dreams were usually less gloomy. During the war, Zwicky had patented an air-breathing “pulse jet” similar to Germany’s V-1 buzz bomb. Why stop there? By the late 1940s, Aerojet was testing a

26 WINTER 2001 “hydropulse” engine that “burned” light redshifted? Zwicky offered three ELIEVING that educated water (using sodium fuel) to make answers. First, Einstein’s laws im- people owe a debt to society, steam. And if water could burn, why plied that photons passing a star BZwicky organized a program not rock? Zwicky dreamed of the day would first gain and then lose after World War II that collected and when “terrajets” would burn min- energy. Zwicky argued that the first shipped 15 tons of scientific journals erals (using fluorine) in order to spit effect was slightly larger, producing to war-damaged libraries around the out jets of lava and gas. Zwicky “gravitational drag.” Second, small world. He also directed an organi- claimed that the technology could overlooked terms in Maxwell’s zation that supported orphanages. dig tunnels and colonize the planets. equations and quantum mechanics Charity reinforced Zwicky’s con- Military versions could attack might allow light to become “tired” tempt for the “inhuman and idiotic straight through the earth. after billions of years. Finally, the treatment” of native peoples under Terrajets were only the start. physical laws themselves might colonialism. “We are not likely to Eventually, atomic power could hol- have changed: Why shouldn’t light succeed in unifying the world,” he low out the moon, give it an atmos- emitted billions of years ago be red- warned, “as long as the Americans phere, or even move it elsewhere. der than it is today? and the British, or for that matter And if mankind needed living Strangely, the man who had any other people, feel and act as if space—for example, to separate its shown that most galaxies reside in they are better and superior to all warring ideologies—the giant plan- clusters refused to admit that “super others.” ets could be shattered into conve- clusters” (clusters of clusters) nient, earth-sized pieces. Finally, existed. This led to a series of acri- WICKY BECAME profes- future engineers could fire particle monious debates with astronomer sor emeritus in 1968. There- beams at the Sun to create a fusion- George Abell during the 1950s. Zafter, he traveled extensively powered hot spot. This would gen- Zwicky even claimed that the ab- and opened a second home near erate enough thrust to move the sence of superclusters showed that Berne, Switzerland. Friends thought whole solar system to Alpha Cen- gravity stopped working beyond 60 that he wanted to become a mem- tauri in just 2,500 years. million light years or so thereby ber of the Swiss Parliament. He died invalidating General Relativity and six days before his seventy-sixth RITZ ZWICKY HATED the . According to Zwicky, birthday, on February 8, 1974. He the idea that galaxies were a “graviton” weighing 10−64 grams was buried in Switzerland. Frushing apart because it im- “explained” the effect quite nicely. Zwicky often joked that he plied a starting point, that is, that the Finally, Zwicky concocted a wanted to live to be 102, since hardly Universe was young. He therefore wild theory in which chunks of anyone gets to live in three cen- called Hubble’s “indicative” neutron matter (“goblins”) orbited turies. He would have enjoyed his or “symbolic.”* But if the Universe deep inside massive stars. Goblins current reputation. In the final was not expanding, why was distant were ordinarily invulnerable, since analysis, though, Zwicky probably a 10-meter (1029 ton) object would didn’t care whether people believed *In 1939, Zwicky “proved” that the Uni- cut through the star like tissue his ideas or not. Zwicky knew. That verse had to be much older than Hubble’s paper. Occasionally, however, a was enough. law implied. Many clusters, he argued, close encounter between two gob- were beautifully spherical. But the laws lins would boost one of them to a of physics said that an initially chaotic group of galaxies could only become sym- higher orbit. No longer stabilized metrical through a series of close encoun- by the star’s high pressure core, the ters among three or more galaxies. Since unlucky goblin would explode pro- such encounters were fantastically rare, the Universe had to be at least 1018 years ducing flare stars and gamma-ray old. bursts.

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