Science past and present

Tom Siegfried



Top 10 subatomic surprises

Nobel Prize–winning neutrinos rank among science’s most unexpected discoveries

subatomic particles captured in Brookhaven collider

The 20th century brought many surprising discoveries about the subatomic realm, including that there are many, many types of subatomic particles, some of which are seen above in the aftermath of gold ion collisions at Brookhaven National Laboratory's Relativistic Heavy Ion Collider.

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Neutrinos are popular among the people who award the Nobel prizes.

In 1995 Fred Reines won the physics Nobel for detecting neutrinos, bizarre subatomic particles that some experts said could never be detected. In 2002, Ray Davis and Masatoshi Koshiba won for measuring how many neutrinos the sun sends to the Earth. In 1988, the physics prize honored the discovery of the muon neutrino, one of three “flavors” in the neutrino family. And this year, Takaaki Kajita and Arthur McDonald shared the prize for demonstrating that neutrinos can change themselves from one flavor into another.

Wolfgang Pauli, the Austrian physicist who predicted the neutrino’s existence, also won a Nobel, but not for the neutrino (he did a lot of other very important stuff). He might have won for the neutrino except that his prediction came in the form of a letter to physicists attending a conference that Pauli decided to skip so he could go to a dance.

Pauli’s idea that a previously unknown and uncharged particle could explain a perplexing physics mystery came as a surprise. That particle’s eventual detection was perhaps even more surprising still. But neutrinos are just one of many surprises from the subatomic realm that physicists encountered during the 20th century. I can think of at least 10 more, thereby qualifying this topic for a top 10 list. Starting with:

10. There is, in fact, such a thing as subatomic.

Throughout the 19th century, the existence of atoms was a hot topic, thanks largely to the success of the atomic theory in chemistry articulated by English schoolteacher John Dalton. Before then atoms had been primarily a philosophical concept, showing up in arguments about the ultimate nature of matter but typically regarded as beyond the reach of experimental investigation. Many physicists believed atoms were fictions, perhaps convenient for explaining experimental results although not physically real. But hints accumulated that atoms not only existed, but also weren’t really atoms, if you applied the etymological definition of atom as something indivisible. The periodic repetition of atomic properties identified by Russian chemist Dmitri Mendeleyev suggested some sort of internal atomic structure. By the late 19th century more clues had accumulated, and J.J. Thomson’s discovery of the electron in 1897 — the first subatomic particle to be identified — pretty much clinched the case that atoms had parts.

9. Atomic nucleus

Once physicists agreed that atoms had parts, the next task was figuring out how those parts arranged themselves. Thomson suggested that his negatively charged electrons dispersed themselves like plums in a positively charged pudding. But when Ernest Rutherford had his assistants shoot alpha particles at a thin sheet of gold, some of the alpha particles bounced backward. To express his surprise, Rutherford commented that it was like shooting an artillery shell at a piece of tissue paper and seeing it bounce back at you. He soon figured out that almost all of the atom’s mass was crunched into a tiny ball in the middle. Rutherford called that ball the kern; today it’s known as the nucleus.

8. Neutrons

Before the 1930s, physicists knew about two subatomic particles: the proton and the electron, which seemed to explain everything about matter. But in 1920 Rutherford had suggested the existence of another particle in the nucleus, a neutral particle about the same mass as the proton. In 1932, Rutherford’s protégé James Chadwick found the neutron. The discovery of the neutron was a big surprise, the late Hans Bethe, one of the last century’s most prominent nuclear physicists, once told me. But how could it be a surprise? I asked. Hadn’t Rutherford already predicted its existence? “Yes,” Bethe said, “but nobody would believe him except Chadwick.” (Historical footnote: Pauli called his new particle a “neutron” because it had no charge. But it was not the same as Chadwick’s neutron, which was much more massive. So Enrico Fermi called Pauli’s particle the neutrino, Italian for “little neutral one.”)

7. Subatomic particles are actually waves (sometimes)

Thomson won a Nobel Prize in 1906 for the experiments establishing the existence of the first known subatomic particle, the electron. So he must have been surprised when, in 1937, his son George also won a Nobel Prize — for demonstrating that electrons (at least in some experiments) were actually waves. This wave-particle duality is at the heart of quantum physics, which is of course full of so many surprises that it will get its own top 10 list someday.

6. Neutrinos can be detected

In 1934, Bethe and Rudolf Peierls calculated that neutrinos would interact so weakly with matter that you’d be stupid to try to detect one. You would need a tank of solid matter (perhaps liquid hydrogen) roughly 1,000 light-years across. “There is no practically possible way of observing the neutrino,” they concluded. But if the odds are a gazillion to one against detecting a neutrino, all you need to do is make a gazillion neutrinos and your chances will be much better. There was no way to do that in 1934, but after nuclear fission was discovered a few years later, and then nuclear reactors were invented, physicists all of a sudden had a prolific source of neutrinos. So Fred Reines and his collaborator Clyde Cowan set up detectors outside a reactor (after rethinking their first idea, which was to set off an atomic bomb) and recorded clear-cut evidence of neutrinos’ existence in 1956. “So why did we want to detect the free neutrino?” Reines said later. “Because everybody said you couldn’t do it.”

5. There are a gazillion subatomic particles

By the 1950s, physicists had built atom smashers powerful enough not only to reveal the particles within the atom, but to create new ones, subatomic in size but not ordinarily atomic constituents. Dozens of new particles appeared in atom smasher debris, their names outnumbering the available letters in the Greek alphabet. Leon Lederman (one of the 1988 Nobel winners for the muon neutrino discovery) once told me he was waiting in a lunch line with Fermi and asked the great physicist what he thought of the newly discovered V-zero-2 particle. “My boy,” Fermi replied, “if I could remember the names of these particles I would have become a botanist.”

4. Quarks

In the 1950s, physicists were surprised to learn about all the subatomic particles that did not actually reside in atoms. Then in the 1960s came the surprise that the subatomic particles residing in the atomic nucleus comprised a set of three still smaller particles — and that those particles carried electric charges a fraction of the supposedly unitary charge of protons or electrons. Murray Gell-Mann, who called those particles quarks, almost gave up on his idea because of the fractional charges, which most people considered impossible. But once he realized fractional charges were OK because the quarks couldn’t escape from the nucleus, he published his Nobel Prize–winning idea. A few years later, when experiments confirmed the existence of his quarks, many physicists were surprised. But Gell-Mann wasn’t.

3. Parity violation

Long before the explosion of subatomic particle discoveries, the esteemed mathematician Hermann Weyl noted that nature knew nothing about handedness (or parity). “There can be no doubt,” he wrote, “that all natural laws are invariant with respect to an interchange of right and left.” But then in 1956, Chen Ning Yang and Tsung-Dao Lee raised some doubts that left-right symmetry was obeyed in certain cases where subatomic particles decayed. Soon thereafter experiments confirmed Yang and Lee’s suspicion. “It was so surprising,” Lederman told me. “It was socko!”

2. Protons don’t decay

Outside the atomic nucleus, neutrons are notoriously unstable, decaying within a few minutes to form a proton, an electron, and (of course) a neutrino (well, actually an antineutrino, but that’s not important right now). The leftover proton, though, is supposedly stable and should last forever. But in the 1970s, theorists began to believe that protons should also decay (although in trillions of trillions of trillions of years, rather than a few minutes). But that surprise never happened — prodigious efforts to detect proton decay have turned up no sign of it. And that’s an even bigger surprise.

1. Positrons

In 1932, the neutron wasn’t the only surprising subatomic particle to be discovered. That same year Carl Anderson analyzed the tracks of cosmic rays in a cloud chamber and found one that looked like it belonged to an electron, except that it curved the wrong direction. It turned out to be the positron, the antiparticle of the electron (Anderson called it a “positive electron”). The discovery of an antimatter particle was in some senses a big surprise, but maybe it shouldn’t have been, since Paul Dirac had deduced its existence by analyzing his equation describing electrons. So maybe the surprise was that somebody could infer the existence of something so strange just by playing around with equations.

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