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Culture, history and society

Culture, history and society

Life beyond the Nobel: how Luis Alvarez deduced the disappearance of the dinosaurs

29 Sep 2021 Laura Hiscott

In the run-up to the announcement of the 2021 Nobel Prize for Physics on 5 October, we’re running a series of blog posts looking at previous recipients and what they did after their Nobel-prize-winning work. In this first instalment, Laura Hiscott explores the wide-ranging research of Luis Walter Alvarez, who won the prize for developing the hydrogen bubble chamber, but also investigated the Egyptian pyramids and dinosaur extinction.

Dinosaurs underneath asteroid in the sky about to impact Earth and cause extinction
Luis Walter Alvarez co-developed the theory that the extinction of the dinosaurs was caused by an asteroid impact (Courtesy: iStock/estt)

I don’t remember the first time I heard the theory that the dinosaurs were wiped out by an asteroid crashing into the Earth. It’s a dramatic story that gets told to wide-eyed children in classrooms and natural history museums at an earlier age than many can remember, so it feels more like absorbed knowledge. What is less commonly known, however, is that one of the originators of this proposal was Luis Walter Alvarez, who won the 1968 Nobel Prize for Physics for his work on the hydrogen bubble chamber.

But it wasn’t just dinosaurs and asteroids that Alvarez got excited about. Throughout his long and varied career, Alvarez was also involved in sending particle detectors into the sky in high-altitude balloons and searching for hidden chambers inside ancient Egyptian pyramids. It appears that his innate curiosity and experimental creativity, which were so vital for winning the Nobel prize, also led him to investigate many more questions both within physics and beyond.

Born in San Francisco in 1911, Alvarez studied at the University of Chicago and gained a PhD there too after building a cosmic ray telescope with Arthur Compton. He then moved to the University of California, Berkeley, working with nuclear scientist Ernest Lawrence to obtain the first observational evidence of K-electron-capture – the process by which a proton in a nucleus can absorb an atomic electron, turning into a neutron and emitting a neutrino. He also developed a method to produce beams of very slow neutrons; and, with Felix Bloch, measured the magnetic moment of the neutron.

His innate curiosity and experimental creativity, which were so vital for winning the Nobel prize, also led him to investigate many more questions both within physics and beyond

After war-time military research, including a stint on the Manhattan nuclear-bomb project, Alvarez returned to Berkeley, becoming an expert on particle accelerators. Most importantly, he led the development of the hydrogen bubble chamber in the 1950s, with which his team then discovered many particles and resonance states.

Luis Walter Alvarez portrait

In the years between developing the hydrogen bubble chamber and winning the Nobel prize for it, however, Alvarez started taking his expertise out of the purpose-built lab and into real-world settings. In 1964 he proposed gathering data on high-energy particle interactions by sending lab equipment to high altitudes in balloons. This might sound like a whimsical idea, but it resulted in the High Altitude Particle Physics Experiment, which paved the way for the Cosmic Background Explorer (COBE) satellite.

Perhaps the 1960s brought Alvarez a flurry of out-of-the-lab inspiration, for it was in 1965 that he suggested a new investigation of the Egyptian pyramids. As unexpected a project as this sounds for a physicist, there was a key connection with his previous work; the idea was to place a particle detector underground beneath the pyramids to measure muons – one component of the cosmic rays constantly showering Earth. This is called muon tomography, and it can indicate hollow spaces in a structure through differences in the energies of muons coming from different directions.

Together with an international team of archaeologists and physicists, Alvarez spent a few years using this technique to search the Pyramid of Khafre – the second largest of the Pyramids of Giza – and the project was in full swing when he was awarded the Nobel prize. His biography published by the Nobel Committee at the time did not mention his archaeological exploits, which was perhaps no bad thing as, when the search concluded the following year, no chambers had been detected despite 19% of the pyramid having been scanned.

This null result might not sound exciting, but it was a meaningful one for archaeologists, and muon tomography has continued to be a useful tool in searching other structures. Speaking to Physics World in 2014, Arturo Menchaca, a physicist who has used muons to study the Pyramid of the Sun in Mexico, recalled once meeting Alvarez and referring to the project at the Pyramid of Khafre as having discovered nothing. “He furiously corrected me,” Menchaca said. “He had demonstrated there was nothing inside the pyramid.”

Since the pyramid project was well under way before Alvarez won the Nobel prize, it can’t have been this prestige that allowed him to pursue such a left-field idea. But perhaps the fact that he had already done such stellar (albeit more conventional) work in physics gave him the freedom and credibility to lead a team on this bold departure from the lab.

So when his son Walter, who was a geologist, told him about the mystery surrounding the dinosaur extinction, it was perhaps no surprise that Alvarez was quick to get involved. He enlisted the help of two nuclear chemists he knew at Berkeley – Frank Asaro and Helen Michel – to study the layer of sediment that represents, within multitudes of geological strata, the point in time when the extinction happened.

Perhaps the fact that he had already done such stellar (albeit more conventional) work in physics gave him the freedom and credibility to lead a team on this bold departure from the lab

The team discovered that the layer is hundreds of times richer in iridium than average, and suggested that an asteroid strike covered the Earth in the element and triggered the mass extinction event. The theory, now known as the Alvarez hypothesis, was hotly debated, and Alvarez defended it ardently right up until his death in 1988.

More evidence has accumulated over the years, not least the discovery of the huge Chicxulub impact crater under the Yucatán Peninsula in Mexico. Although there isn’t a complete consensus among geologists, the “Alvarez hypothesis” is now generally accepted as the most likely explanation for why the dinosaurs disappeared.

It’s hard to compare Alvarez’s eclectic mix of achievements with one another, for the very reason that they are in such disparate areas  – he even looked into the assassination of President John F Kennedy. His bubble-chamber work will naturally be what physicists remember him for, but for the public, it was his research relating to asteroids and dinosaurs that captured the imagination. How remarkable that his Nobel-prize-winning work isn’t even his most famous achievement.

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