"Who Ordered that?": the Extraterrestrials

The Extraterrestrials

A constant shower of cosmic rays, energetic photons, protons, and atomic nuclei, enters the Earth's atmosphere from outer space. The first clue to this was obtained by Father Theodor Wulf, a Jesuit priest, in 1910. He climbed the Eiffel tower with an electrometer and measured more radiation than expected from ground level measurements. Victor Hess followed up on this by going aloft in balloons, in the process showing that the intensity of radiation increased the higher one went -- strong evidence of the extraterrestrial origin of cosmic rays. He was awarded the Nobel Prize in 1936 for this work.

Detecting Aliens

In 1930 Robert Milliken instructed one of his students, Carl Anderson, to build a cloud chamber to study cosmic rays. By placing his chamber in a strong magnetic field he was able to determine that cosmic rays contain positively and negatively charged particles in about equal numbers. Milliken thought that the positive particles must be protons. Anderson thought that the positive tracks were due to electrons coming up through the cloud chamber (having passed through the Earth). He settled the issue by inserting a thin lead plate in the chamber, upon passing through the lead, the particle would lose energy, thereby indicating its direction of motion. Both were wrong. The positive paths were being created by positive particles with the same mass as the electron from above. Anderson had discovered antiparticles, a new class of particles predicted by Paul Dirac in 1928.


In the early 1930's cosmic ray tracks which were more penetrating than electrons but which did not create showers were being detected in greater numbers. By 1936 Anderson and Seth Neddermeyer had convinced themselves that these were due to a new type of particle with a mass intermediate between the electron and the proton. In fact such a particle had been predicted in 1935 by Hideki Yukawa as the particle responsible for binding the nucleus together. Since the new particle had a mass only 20 % removed from his prediction, he claimed it as his own. However he was wrong, the new particle was a muon -- a heavy version of the electron. Why nature required such duplication was, and is, not clear. Upon learning of the discovery, I.I. Rabi's response was, "Who ordered that?".

The pion was discovered by Cecil Powel and Giuseppe Occhialini in 1947 using photographic emulsions at the Pic du Midi, high in the French Pyrrenees.

The following is a false colour streamer chamber image of an antiproton annihilating in the neon gas filling the chamber. The apparatus is in a strong magnetic field, causing the curvature (left for upward moving postitive particles) seen in the photograph. The track near the right cross is due to a positive pion, while the two short tracks to the lower left are a negative and postive pion. A picture with labelled path is here.

Strange Particles

In 1946, George Rochester and Clifford Butler observed something unusual in their cloud chamber at Manchester University. Two tracks appeared from a single point, as if from nowhere. They eventually realized that what they were seeing were the decay products of the decay of a neutral particle with a mass of roughly 1000 times the electron mass. This was the kaon, the first strange particle, so called because of their extraordinarily long lifetimes.

By 1951 physicists had detected anonther, heavier strange particle, called the lambda. This was the first hyperon, the name is Greek for heavy and refers to the mass of the lambda, some 20% greater than that of the proton.

The following photograph is from a liquid hydrogen bubble chamber at Brookhaven National Laboratory. The yellow line at the bottom is an incoming high-energy proton, it collides with a proton at rest in the liquid hydrogen creating a slew of particles. Seven positive pions, a proton, and a positive kaon (shown in red) curve off to the right, while seven negative pions (blue) move to the right. A neutral lambda is also produced which travels upwards undetected and then decays into a proton (yellow) and a negative pion (purple). NB: the green curve at the bottom is due to an electron which has been knocked out of its orbit by the passing proton.

Particle Proliferation

We have described the discovery of many particles, and many more have been discovered! How many are there? Why are there so many? Can we make sense of them in the same way Mendeleev did with the periodic table of elements?

Particles discovered 1898 - 1964: