Note 7

Something comparable we encounter in the constitution of physical matter at a very low level :
This matter can occur in two forms, namely ordinary matter and antimatter. In antimatter all electrical charges, where they occur, are reversed with repect to their sign. Hence we can have, instead of protons (positive protons), antiprotons (negative protons), and instead of electrons (negative electrons) positrons (positive electrons).
Atoms, which then consist of antiprotons (plus neutral neutrons) and positrons, form antimatter.

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