Antimatter

Discussion

Paul Dirac, usually cited by his inititial P.A.M. Dirac (for Paul Adrien Maurice).

Space is filled everywhere with fields. The electromagnetic field is one example. The electron field is another example. A field is any quantity that has a value everywhere in space. The value of the electromagentic field in a region of pure and utter darkness is zero. When a photon passes by, the electromagnetic field stops being zero wherever the photon is located at that particular instant. Likewise, the value of the electron field in empty space is zero. Where there are electrons, the electron field has a positive numerical value. Where there are positrons (antielectrons), the electron field has a negative value. When a positive number is added to a negative number, the sum is zero. When an electron and a positron meet, they both cease to exist or annihilate. The loss of mass excites the electromagnetic field producing two photons that zip away at the speed of light.

We can also visualize fields as a surface whose height at a location corresponds to its value. As far as the electron field is concerned, empty space is flat. Where there are no electrons, there is no height or depth. A bump in the electron field is an electron and a pit to a positron. When an electron and an positron meet, the bump falls into the pit and the electron field flattens out. No height or depth means no electron or positron. The particles cease to exist, but the field continues on for eternity. This transition generates a pair of photons, which are bumps on the normally flat electromagnetic field.

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

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  1. positron (e+)
    1931
    Examining cosmic-ray data, Anderson discovers the positively charged electron later named the positron. He receives the 1936 Nobel Prize.
  2. 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, 1965