Antimatter

The Physics Hypertextbook™
© 1998-2008 by Glenn Elert -- A Work in Progress
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Discussion

Every particle has its own antiparticle. Two major discoveries helped physicists to establish this fundamental principle:

positron (e⁺)
1931
Examining cosmic-ray data, Anderson discovers the positively charged electron later named the positron. He receives the 1936 Nobel Prize.
antiproton (p⁻)
1955
Using an accelerator at Berkeley University, Segre and Chamberlain discover the antiproton. They receive the 1959 Nobel Prize. (Later, physicists learn that a proton contains quarks and an antiproton consists of antiquarks.)

pair production

Paraphrase needed …
Carl David Anderson (1905-91) received Nobel prize in 1936 for the discovery of the positron. In 1936, with Seth Neddermeyer, Anderson also discovered the positive and negative "mesotron", now called the muon. Thus he added three new particles to physics and pointed the way to the existence of antimatter. To obtain more intense, higher energy cosmic rays, the pair transported their magnet cloud chamber to the summit of Pike's Peak, colorado. Analyzing the cloud chamber photos after the summer at the Peak, they found positive and negative tracks that were different from electrons and protons and appeared to have intermediate mass.

annihilation

interesting (but wrong) idea

As a by-product of this same view, I received a telephone call one day at the graduate college at Princeton from Professor Wheeler, in which he said, "Feynman, I know why all electrons have the same charge and the same mass" "Why?" "Because, they are all the same electron!" And, then he explained on the telephone, "suppose that the world lines which we were ordinarily considering before in time and space - instead of only going up in time were a tremendous knot, and then, when we cut through the knot, by the plane corresponding to a fixed time, we would see many, many world lines and that would represent many electrons, except for one thing. If in one section this is an ordinary electron world line, in the section in which it reversed itself and is coming back from the future we have the wrong sign to the proper time - to the proper four velocities - and that's equivalent to changing the sign of the charge, and, therefore, that part of a path would act like a positron." "But, Professor", I said, "there aren't as many positrons as electrons." "Well, maybe they are hidden in the protons or something", he said. I did not take the idea that all the electrons were the same one from him as seriously as I took the observation that positrons could simply be represented as electrons going from the future to the past in a back section of their world lines. That, I stole!
Richard Feynman, "The Development of the Space-Time View of Quantum Electrodynamics," Nobel Lecture, 11 December 1965

Summary

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Problems

practice

Write something.
- Answer it.
Write something.
- Answer it.
Write something.
- Answer it.
Write something completely different.
- Answer it.

numerical

Determine the characteristics of the photons capable of forming different particle-antiparticle pairs. Compile your results in a table like the one below …

pair	energy (eV)	frequency (Hz)	wavelength (fm)

electron-positron
proton-antiproton
neutron-antineutron

Resources

propulsion
- Bold Enterprise: Antimatter could carry us to the edges of the Solar System, New Scientist, 13 October 2000

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