The Giant's Shoulders is a monthly blog carnival devoted to classic scientific papers. Sitting in boxes downstairs in the basement are a fair number that might qualify, raw material for the book. It's moments like this that I'd like to take advantage of to tell some of the great stories and introduce some of the memorable personalities from my Carbon Age research, but who didn't make it into the book, or whose material was cut short.

Edwin Salpeter is a Cornell astrophysicist, who in the early 1950s first posited how carbon might be created in stars. How stars released energy, and built heavier atomic nuclei in the process through nuclear fusion, was a major battleground in the war of the day over the origin of the Universe. The Big Bang eventually prevailed over the so-called Steady State hypothesis, which put forth that the Universe has always existed; galaxies fly away from each other because new matter is always being regenerated in the center of things. Here's the split: The Big Bang team wanted to show that all of the elements were created in the expansion of the Universe at the beginning of time. The Steady-Staters needed to show that stars produce the elements. Salpeter belonged to the latter camp.


Edwin Salpeter

Salpeter's story might be that of the giant standing on his shoulders. Fred Hoyle, the cantankerous genius and eventual lunatic, is one of the 20th century's greatest gifts to science historians. Perhaps his most celebrated insight occurred in 1953, when he saw a major breakthrough hiding in an observation Salpeter had made in a paper some months earlier.

The Salpeter paradox is this: If Fred Hoyle was the genius who figured out how stars make carbon, why is that process called the "Salpeter" or "Triple-alpha process"? Answer to follow.

Salpeter's story is something of a nail-biter. As an Austrian teenager, he was pulled from public school and shuttled off to a segregated facility for Jewish students shortly after Hitler annexed Austria. That lasted until official-looking men began showing up at school inquiring about him. Things were getting scary. Salpeter’s father sold their house in Vienna to a Nazi party member who worked as a hotel porter. The two ended up living in hiding in the hotel for several months until they could safely exit the country. “He was a rather nice Nazi,” Salpeter recalled, speaking honestly no doubt, but infused with irony and a dry wit. “There were some nice Nazis. He let my father and me stay in his hotel.”

Otherwise a dutifully oblivious teenager, Salpeter was forced by circumstance into a mature contemplation of mortality that only war can provide. “It was curious that during my hiding period, when I assessed that my probability of surviving to adulthood was small, I started thinking seriously what I might do if I became an adult after all,” he wrote in a 2002 autobiographical essay. “Sons of professional sword swallowers often become sword swallowers, and my father was a physicist.” In an interview he added: “I think I would have enjoyed being an anthropologist or an opera singer more than a scientist, but I had no talent.” After three months living in the hotel, the family acquired exit papers and, denied refugee status in the United States, left for a new home in Australia, where he earned his doctorate, eventually landing a post-doc at Cornell University.  There he worked with Hans Bethe, the first scientist to crack the basics of energy production in stars.

In 1951, Salpeter spent a summer at Caltech, working with Willy Fowler, the amiable head of the Kellogg Radiation Laboratory. Fowler handed off some unprocessed 1949 nuclear physics experimental data to Salpeter to see if he could solve a tough problem: Physicsts couldn't figure out how stars make carbon.

Salpeter saw that two helium-4 nuclei burn into a beryllium-8 nucleus and stick around for a scant 9.68 femtoseconds each. He calculated that that span is long enough that one Be-8 nucleus should always exist for every 10 billion or so other particles. And that's enough for a third helium-4 to seek out beryllium-8 and fuse into a carbon-12 nucleus. Stars hotter and denser than the sun generate heat and radiation this way. Conceiving a carbon nucleus with three alpha particles became known as the "triple-alpha process" or "Salpeter processs." Salpeter eventually published his results in an annual journal:



Fred Hoyle recognized that Salpeter's analysis in this work was incomplete. He pounced. If Salpeter's scenario were accurate, Hoyle supposed, stars would not produce enough carbon to match known cosmic abundances. Without known carbon abundances, human life—Fred Hoyle's life in particular—could not exist. Salpeter wrote:

I calculated the rate for this indirect conversion of helium into carbon... in the summer of 1951 and published it in the following year. I noted in that paper that my calculated rate could easily be too low by a factor of 1000, say... but I did not have the chutzpah (or guts) to do anything about it: My energy production rate for red giant stars required a central temperature that was within the rather uncertain range given by stellar evolution theory at the time; my calculation would lead to most of the helium being converted to oxygen and neon instead of carbon, but I just did not have the guts to think of resonance levels that had not been found yet! A short while later Fred Hoyle demonstrated both chutzpah and insight... to show that there JUST HAD to be an appropriate resonance level in C[arbon], and he was able to predict its energy. Willy Fowler and his colleagues soon looked for Hoyle’s predicted resonance level and found it just where it should be.