Fermi, Oppenheimer, Szilárd, Lawrence, Rabi – these scientists are all famous in the history of America’s nuclear program, but hardly ever is the name Luis Alvarez listed among them in popular culture. Possibly, this is because his work, and fame, expanded into so many other fields – even into the realm of what killed the dinosaurs.
In 1968, Alvarez received the Nobel Prize in Physics for discovering numerous subatomic particles through what physics calls “resonance states” and for developing a device and a method to make the discoveries possible. His work facilitated America’s budding nuclear technology in World War II and also changed the way that physicists viewed matter and the universe. He created technology to help aircraft approach and land at airports more safely and applied the physics of cosmic rays to help archaeologists study Egyptian pyramids. All of this happened over a career lasting half a century, and yet Alvarez is probably best remembered for answering the dinosaur question right at the end of his career.
Born into a family of medicine
Luis Walter Alvarez was born in San Francisco, California, on June 13, 1911. Both his father and paternal grandfather were physicians, which made science a frequent topic in the Alvarez home. Though medicine beckoned as a logical career choice, Alvarez was more interested in pure science. Thus, in 1928, he enrolled at the University of Chicago with a major in chemistry. He was only a B student, until he switched his major to physics, which interested him much more. After receiving his BS in 1932, he continued at the University of Chicago as a graduate student.
His graduate school mentor was none other than Arthur Compton, a Nobel Prize-winning physicist. Compton is also remembered as a leader in work on “Chicago Pile One (CP-1)”, the first nuclear reactor that was a cornerstone of the atomic weapons research program (Figure 2). The weapons research peaked during the 1940s, when Alvarez also was part of the team. Both men were destined to work on nuclear development, because that’s what the most talented physicists of the time would be summoned to during the war.
This included not just Alvarez and Compton, but various scientists from the University of Chicago, which had one of the best physics departments. During the early 1930s, however, Compton’s focus was on cosmic rays, and so this is the phenomenon that Alvarez studied as a graduate student (Figure 2). Working in Compton’s lab, he built an array of Geiger counters to study cosmic rays. In 1933, this work led to a joint publication in the journal Physical Review in which Alvarez and Compton showed that cosmic rays were positively charged particles.
Back to the Bay Area to start his career
Alvarez received his Ph.D. in 1936 and returned to the San Francisco Bay area to work as an experimental physicist in the Radiation Laboratory at the University of California Berkeley. Along with his alma mater, Berkeley was a focal point of America’s budding nuclear research that would come to be called the Manhattan Project. During his first few years at the Berkeley lab, Alvarez devised instruments and experiments aimed at understanding the phenomenon of K-electron capture. Through this mechanism, an electron captured by an atom combines with one of the protons in the atom’s nucleus. This transforms the proton into a neutron, thereby transmuting the atom into a different chemical element. Theoretical physics predicted K-electron capture, but Alvarez’s instruments and experimental design allowed the Berkeley scientists to observe the process directly in 1937. Around the same time, Alvarez also used a device called a cyclotron to prove the stability of helium-3. This and his K-electron capture research contributed to a growing understanding of atomic physics just as it was becoming clear that Europe was on the pathway to war.
Having taken up flying as a graduate student, Alvarez had an interest in applying his physics knowledge to aircraft safety. By the early 1940s, the US was entering World War II. As one of the nation’s top young physicists, Alvarez was assigned to a new laboratory set up at the Massachusetts Institute of Technology. Alvarez’s role at the new MIT lab was to work on microwave radar systems. This led him to invent a new radar system that gave ground controllers extreme precision for monitoring positions of aircraft and for directing them to approach and land on runways. This allowed aircraft to land safely in foggy conditions and more easily at night. Along with civilian aviation authorities, the military took interest in this research. Given Alvarez’s talent in numerous areas of physics, however, the government was particularly interested in recruiting him for the Manhattan Project, so his stay in the Boston area would be cut short.
Top-secret nuclear science
In 1943, Alvarez was invited to work secretly on nuclear issues and like most other scientists on the team was not given the details of the overall project. All he knew at first was that he would be working on ways to detect whether Germany was developing its own atomic bomb. Because research to develop nuclear weapons releases xenon-133 and other radioactive gases, Alvarez was confident that he could develop a detection strategy, so he agreed to work with the government. In 1944, he packed his bags and set out for Los Alamos, New Mexico.
Using special airborne equipment and techniques whose development Alvarez spearheaded, US flight crews eventually determined that German scientists were not on a pathway to a nuclear bomb, or at least they were not very close. On June 6, 1944, the invasion of Normandy set the Allies on course for ending the war in Europe within a year, but the Manhattan Project continued, based on the idea that the bomb might be needed to defeat Japan. It was under this scenario that Alvarez’s focus shifted from the problem of nuclear detection to detonation of an actual weapon.
Two explosive nuclear fuels were being created at an excruciatingly slow rate: uranium and plutonium. Each fuel required a different kind of detonation system and the US military wanted both kinds of bombs in order to have more bombs sooner. Alvarez was a key researcher in the development of electrical detonation systems for the plutonium fuel, which was used in two explosions during the war. The first, called the Trinity Test, took place July 16, 1945, at the White Sands Proving Ground in New Mexico. In this test, a plutonium bomb was exploded at the top of a 30-meter tower to see whether it would function but also to simulate a bomb dropped from an aircraft exploding prior to hitting the ground. As expected, the test showed that explosion above the ground maximized the amount of destructive energy yet minimized the amount of nuclear fallout (Figure 3).
The second plutonium explosion, which also depended on the Alvarez detonation equipment, was the bomb codenamed “Fat Man” – the weapon that destroyed the city of Nagasaki on August 9, 1945. Three days earlier, the first uranium bomb, called “Little Boy” was dropped on Hiroshima. During all three explosions, Alvarez was part of a team of scientists that flew in a specially shielded observation aircraft to observe the explosion and take numerous measurements. As for the consequences of the bomb, Alvarez had an attitude similar to that of many other Manhattan Project contributors. In a letter to his son on the same day as one of the bombings, he wrote:
What regrets I have about being a party to killing and maiming thousands of Japanese civilians this morning are tempered with the hope that the terrible weapon we have created may bring the countries of the world together and prevent further wars.
- Luis Alvarez in a letter to his son, August 1945 [source: Indiana University, Reactions to Hiroshima]
Essentially, Alvarez was not happy, but he could live with situation knowing that things might have been worse without the bomb. Like all Manhattan Project scientists, near the end of the war, he knew of plans for a US invasion of Japan set for 1946-47 with predictions of massive numbers of deaths that could have dwarfed the Hiroshima and Nagasaki bombings. And so, rather than sink into a state of depression, like many of his colleagues, he put the war behind him and moved onto other areas of physics.
The hydrogen bubble chamber
The two atomic bombings of Japan ended the war, and so Alvarez returned to Berkeley, where he was appointed as a full professor in experimental physics. No longer was he applying physics to weapons development, but he was still applying his instrumentation skills. As before the war, Alvarez built new instruments and applied them to experiments aimed at understanding the subatomic world. Since his college graduation in 1932 when only the proton and electron had been known, physicists had expanded their knowledge of subatomic particles. By the early 1950s, that knowledge-base extended to neutrons, positrons, K mesons, and various other “-ons”, but there was still a plethora of particles to be discovered.
Then, in 1953, Alvarez met a young physicist named Donald Glaser with an invention called a bubble chamber. It used ether, kept at a very cool temperature to remain a liquid, in order to track the pathways of otherwise invisible subatomic particles. The ether bubble chamber enabled novel particle studies, leading Glaser to win the Nobel Prize in 1960. But long before that happened, Alvarez thought of a way to make the bubble chamber function better, and it was very simple. Instead of ether, he tried using liquid hydrogen, which could be kept at a much lower temperature than ether.
By 1956, Alvarez had a large hydrogen bubble chamber assembled and the results of the experiments were phenomenal. A whole new collection of subatomic particles were discovered by Alvarez and others, particles too short-lived to be seen by any other method. In the hydrogen bubble chamber, the particles etched webs of straight and twisting pathways called resonance states, essentially shadows of particles’ movement (Figure 4). This revealed a wealth of fundamental information about the atom, leading Alvarez to receive his own Nobel Prize on December 11, 1968. In his acceptance speech, he made a point of how protons and electrons had been the only subatomic particles known during his college years.
Applying physics to multiple questions of science
In 1967, a year before receiving the Nobel Prize, Alvarez came up with a remarkable idea for imaging the inside of Egyptian pyramids. Why not X-ray the structures? he thought. As a child of two generations of physicians, it did make a certain kind of sense, although the sheer size of the pyramids and their thick stone walls meant that literally shooting an X-ray beam through them would not be practical. But Alvarez was not thinking of X-rays in the literal sense. Instead, thinking back to his first year of graduate school under the mentorship of Arthur Compton, he was cognizant of the fact that cosmic rays were constantly penetrating our planet, along with everything on its surface. In other words, the radiation projector was already in place and turned on. Thus, somebody need only place a cosmic ray detector near an area of a pyramid and the inner contents could then be imaged.
Alvarez tried the technique at the Pyramid of Chephren, in Giza. While the images revealed no unknown chambers, the technique enabled Alvarez to help archaeologists characterize roughly 20 percent of the pyramid’s volume. It also served as a prelude for Alvarez to apply particle physics to another question about the past that was equally as intriguing as ancient Egypt.
What happened to the dinosaurs?
By the 1970s, Alvarez’s son, Walter, had also grown up to be a scientist, although instead of physics he studied geology, with a particular interest in paleontology. Today, dinosaurs and fossils account for a large segment of the toy industry and children’s books, but this was not the case in the 1950s and 60s. In those days, geology hardly ever came up at the dinner table, even in the homes of notable scientists. Being at the forefront of particle physics, Alvarez thought that geology was boring. He could not fathom why his son would be interested in digging up rocks and cutting into them. But the boy grew up and made a career of it.
In 1977, he mentioned to his father a problem in the rock layers known as the K/T boundary. The K/T boundary is a layer of clay, a visible line within the rock layers, that exists all over the planet at a depth corresponding to the end of the Mesozoic Era, 65 million years ago. It is also a boundary in time, marking the time span between the Cretaceous period (the "K") and the Tertiary period (the "T"). The K/T boundary had been discovered early in the 19th century and by Alvarez’s time geologists knew that it corresponded to the demise of the dinosaurs, and for a very simple reason. Below the boundary, there are dinosaur fossils, while none exist above it. In other words, the dinosaurs went extinct rather abruptly, but nobody knew why.
Iridium and the K/T boundary
Even though Alvarez lacked an interest in geology, he loved the idea of applying physics to help his son solve the mystery (Figure 5). It was not a simple matter. It required knowledge not just of particle physics, but astrophysics too, because Alvarez had a strong suspicion that the answer must involve outer space. Thus, as with the pyramids, Alvarez tapped into his background from graduate school research on cosmic rays. He decided that the answer might have to do with the element iridium. Iridium is a very rare element in the Earth’s crust, but it is more common in certain celestial objects, especially comets, asteroids, and meteorites. After making several calculations regarding the expected iridium concentrations in and around the clay layer, Alvarez turned to his colleagues Frank Asaro and Helen Vaughn Michel of the radiation lab at Berkeley (by this time called Lawrence Berkeley National Laboratory or “LBL”) to make precise measurements of the iridium level in the K/T boundary clay.
Seeing their mentor with rock samples in the lab, graduate students and postdoctoral fellows would often ask Alvarez what he was doing, since it didn’t seem to correspond with the usual physics research. Occasionally, he asked if they’d like to get involved and usually they politely declined. One Alvarez protégé, Richard Muller, now a senior scientist at the Lawrence Berkeley Lab, recalls declining the offer with a sigh, thinking the K/T project had little potential; the respected scientist simply wanted to work with his son. Regarding Alvarez’s motivations, Muller was 100 percent correct. He really was doing it just to work with his son. Indeed, Alvarez himself didn’t expect the project would go anywhere scientifically. But when the team members looked at the results, they were startled.
The iridium concentration at the precise level of the dinosaurs’ extinction was higher than anyone would have believed. In K/T samples taken from different sites on the planet, the iridium concentration was found to be at least 20 times as high as before and after the K/T extinction event, and in some parts of Earth, the K/T iridium level was found to be as high as 160 times the normal level.
Astrophysics and the demise of the dinosaurs
Based on these results, in 1980 Luis and Walter together with colleagues Frank Asaro and Helen Michel published a paper in the journal Science, hypothesizing that the K/T extinction, which involved numerous life forms including the dinosaurs, had an extraterrestrial cause. Based on Alvarez’s calculations, the team proposed that a comet or asteroid 10-kilometers wide had stricken the world of the dinosaurs at 25 kilometers per second, leaving a layer of iridium and ejecting dust in the atmosphere, blocking light from the sun (Figure 6). This, in turn, had interfered with photosynthesis and triggered a climate change, causing disruption of the web of life.
Given the apparent arrival of a large amount of iridium at the time of extinction, the hypothesis was very strong. Nevertheless, scientists were not all convinced, and paleontologists in particular were divided. Some found the hypothesis intriguing, but others dismissed it on the grounds that the mere presence of iridium did not prove that an object large enough to cause such widespread extinction had struck the Earth at the end of the Mesozoic era. At the time, there was an emerging theory, based on limited evidence but a strong rationale, that the K/T extinction period corresponded to an age of massive volcanic events.
To sway the paleontology community away from the volcanic explanation and to accept their idea, the Alvarez team needed an additional piece of evidence, namely a crater left over from the impacting asteroid or comet. People looked around for such an impact crater and ultimately named a few candidates, one in particular in the Arizona desert, but it was far too small. An impact consistent with Alvarez’s calculations, made by an object 10 kilometers wide striking the ground at 25 kilometers per second, would have released energy on an enormous scale. To get an idea of how enormous, one must consider that the nuclear arms race that began when Alvarez worked on the first bombs in the 1940s reached a peak in the 1980s when the combined arsenal of the United States and USSR numbered more than 70,000 nuclear bombs. Were all those bombs detonated together over a single spot, the explosion would release only a fraction of the energy of the impact that Alvarez calculated as the cause of the demise of the dinosaurs. That kind of impact would leave a crater more than 100 km wide and several km deep, and no craters that anyone could find by the mid 1980s were anywhere near that big.
Alvarez died in 1988, just two years before a discovery that would change the tide regarding his hypothesis. In the 1990s, using satellite imaging over the Gulf of Mexico, a team of scientists that included Adriana Ocampo took notice of the Chicxulub crater, on the seafloor off the coast of the Yucatan Peninsula in Mexico. For years, there had been hints of the crater, but new advances in spacecraft imaging now allowed for an accurate assessment of the crater’s size. It was found to be huge, 180 km in diameter and 20 km deep. Examination of the details showed it to be precisely the kind of crater that would be formed from an impact of Alvarez’s calculations.
Science is full of surprises, not just involving discoveries about nature, but also connected with how society remembers the discoverers. The story of Luis Walter Alvarez is such a case. His work has touched so many areas of science, not all of which are even mentioned here, yet uncovering the demise of the dinosaurs may end up as his greatest legacy. This may seem odd, since he won the Nobel Prize for something completely different, but such unpredictable outcomes make the history of science all the more interesting.
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