A Brief History of Particle Physics, Part I

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1. One of the great achievements of nineteenth-century physics was the rejection of Newton's particle model of light in favor of the wave model implied by Maxwell's equations. So you can imagine how upset physicists were when Einstein came along and proposed that light interacted with matter as a quantized, massless particle after all! What phenomenon was Einstein attempting to explain? Brownian motion blackbody radiation photoelectric effect Compton effect

2. In the 19th century, scientists were fascinated by the mystery of cathode rays: put a high voltage across a vacuum in a tube, and a ray of light would extend from end to end. But how? J. J. Thomson set out to solve this problem, and in 1897 he concluded that the rays were electric charge, carried by tiny particles with a high charge to mass ratio. He postulated that these "corpuscles" were constituents of the atom. What do we now call the particle he had discovered? electron neutron proton positron

3. In 1909, Ernest Rutherford assigned an undergraduate student to check whether any alpha particles could be scattered from gold at large angles (basically, whether they could be reflected backward). Everyone was amazed when it was discovered that they were! Rutherford realized what this meant: the positive charge in an atom had to be concentrated in a small volume to generate such a large electric force. What is this central concentration of charge called? orbital quark sea electron shell nucleus

4. Rutherford's discovery led to the idea that each atom consisted of electrons orbiting around protons, positively charged particles. But there was a problem: helium, for example, had twice the nuclear charge of hydrogen (one proton) but four times the weight! And wouldn't electromagnetic repulsion make heavy nuclei extraordinarily unstable? Luckily, in 1932 the English physicist James Chadwick discovered what electrically neutral particle, one of the constituents of the nucleus? neutrino gluon photon neutron

5. At the same time, particle physics was expanding into rather bizarre territory. In 1927, English physicist Paul Dirac (1933 Nobel Prize in Physics) had with his eponymous equation taken the first step toward relativistic quantum mechanics. But his theory allowed negative-energy electrons -- and if electrons had no ground state, they would keep decaying to lower and lower energies, radiating infinite amounts of energy! What strange idea did Dirac propose to explain this problem? Negative-energy states are forbidden by the Dirac exclusion principle. There is another type of electron that "falls up" from negative-energy states, and the negative energy it emits cancels the positive energy radiated by ordinary electrons. Electrons radiate a type of energy that could not be detected by the experimentalists of the day. All the negative-energy states are filled with an infinite "sea" of electrons.

6. Dirac's theory was saved by American physicist Carl Anderson's 1932 discovery of the first known example of antimatter. Examining photographs of a cloud chamber bombarded by cosmic rays, Anderson noticed the track of what antiparticle of an electron? muon positron electron neutrino pi meson

7. Anderson and his group continued examining cosmic rays and in 1935 made another important discovery, with Seth Neddermeyer. This particle had the same charge as the electron and about 200 times its mass; it was initially thought to be the meson transmitting the strong nuclear force, but it interacted too weakly with protons and neutrons. What is this middleweight lepton now called? positron tau muon muonium

8. In 1930, particle physicists noticed yet another problem. A type of radioactivity called beta decay, in which a radioactive nucleus A decays into a lighter nucleus B by emitting an electron, had been studied extensively in the lab. It was discovered that, from experiment to experiment, the energy of the emitted electron varied enormously even in cases where the energies of the parent nucleus A and daughter nucleus B were held constant. Why did this incline physicists to tear out their hair? It violated parity. It violated conservation of mass. It violated gauge symmetry. It violated conservation of energy.

9. As quantum mechanics developed, the photon came to be interpreted as the particle mediating the electromagnetic force: when two charged particles are attracted or repulsed, what's happening is that they're exchanging a photon. The investigations into the nucleus made clear that there must be a "strong force" binding the protons and neutrons together. In 1934, Japanese theorist Hideki Yukawa proposed that the strong force must also be mediated by a particle, which he predicted would be massive (300 times the mass of an electron and 1/6 the mass of a proton) to account for the strong force's short range. What name, meaning "middle-weight", was given to this particle? hadron lepton meson baryon

10. In 1947, particle physicists believed that they had solved almost all of the outstanding problems. Ha! That December, Rochester and Lee discovered a new neutral meson. In 1950, Anderson found a new neutral baryon. These bizarre particles were created in pairs by the strong force but decayed singly by the weak force! This wasn't explained until 1953 when Gell-Mann and Nishijima introduced what new quantum number? strangeness S, conserved by the strong force but not by the weak force weirdness W, conserved by the weak force but not by the strong force strangeness S, conserved by the electromagnetic force but not by the strong or weak forces parity P, conserved for leptons but not for baryons or mesons

11. Throughout the 1930s and 1940s, indirect theoretical evidence for Fermi's neutrino (see Question 8) continued to mount. But it's very difficult to detect a massless, neutral particle! So when American physicists Frederick Reines and Clyde Cowan decided to try to detect it in 1955, they needed an extraordinarily intense source of neutrinos. Where did they set up their experiment? The Savannah River nuclear reactor in South Carolina The Saddle Mountains in Washington State (the higher elevation meant more cosmic rays) The Nevada Test Site during Operation Teapot nuclear testing The Nuclear Physics Division at Los Alamos National Laboratory in New Mexico

12. The 1950s were a time of some despair among particle physicists. One of the most cherished postulates of physics was the parity symmetry, the idea that the mirror image of any physical process is also a valid physical process. Parity had already been confirmed for electromagnetic and strong interactions. But when Chinese-American experimentalist Chien-Shiung Wu turned her attention to parity in weak interactions, she made a shocking discovery. What was it? Most neutrinos are left-handed. All neutrinos are left-handed. Neutrinos don't have any spin at all, right-handed or left-handed. The weak force doesn't really exist.

13. Theoretical physicists were examining antimatter and neutrinos at about the same time. The antiparticles of charged particles have the opposite charge, but the situation for neutral particles is not so clear! The neutron has a distinct antiparticle, but the photon is its own antiparticle. So debate raged over antineutrinos. Are neutrinos their own antiparticles? Yes No

14. The amazing neutrino had not yielded up all of its secrets. In 1959, Bruno Pontecorvo suggested that there were two kinds, or generations, of neutrino, one for each generation of negatively-charged lepton. What prompted this assertion? When positrons decay into electrons, a neutrino and an antineutrino are produced. When muons decay into electrons, a neutrino and an antineutrino are produced. He wanted to save parity. Given the proliferation of "elementary" particles, it didn't make sense that there should be only one neutrino.

15. The next big advance was to impose some kind of order on the particle zoo. Murray Gell-Man was equal to the task with his 1961 Eightfold Way. What shape did this "Periodic Table of Particles" take? three pentagons a rectangle an octagon two hexagons and a triangle