Antimatter was first discovered in the form of the positron, the positively charged electron. In 1932, American physicist Carl Anderson discovered the positron in cosmic ray studies. Through a cloud chamber modified to curve the trajectories of cosmic rays, Anderson noticed that the curves of some particles followed that of a negative charge, while others curved like a positive charge. However, the positive curve showed not the mass of a proton but the mass of an electron. This outcome is shown below and suggests the existence of a positively charged version of the electron, created by the destruction of solar photons.
The image above is from the Fermilab 15 foot bubble chamber and shows the production of an electron and positron (or
antielectron) from an incident photon. This event is titled pair production and provides evidence of antimatter, as the two repel each other.
Antimatter is considered the opposite of matter. For most antiparticles, this means that they share the same properties as their original particles with the exception of their charge. This is why the positron can be considered a positive electron while the antiproton is considered a negative proton. The idea of an opposite charge for neutral particles (like the neutron) can be confusing, but it makes sense when considered from the quark perspective. Just as the neutron is composed of one up quark and two down quarks (of charge +2/3 and -1/3, respectively), the antineutron is composed of one anti-up quark and two anti-down quarks (of charge -2/3 and , +1/3respectively). While the overall charge of the neutron remains the same, its constituent particles do not.
A word about antiparticles: Like regular particles, antiparticles could function just fine on their own. In fact, a universe made up of antimatter may operate just as our own matter-based universe does. However, we do not know fully whether this is the case. The reason for this is annihilation. Annihilation is the process of destruction that occurs when a particle and its antiparticle interact. As soon as two particles (like a positron and an electron) coincide, they convert their masses to energy through the equation. This mass-to-energy conversion, which typically results in photon release, happens instantaneously and makes it very difficult for scientists to study antimatter. That said, scientists have had success creating antimatter through high energy particle collisions. Both antineutrons and antiprotons were created through accelerator experiments in 1956, and an anti-hydrogen atom was even created at CERN in 1995. As referenced in, the annihilation of antiparticles is currently used in medical studies to determine the location of radioisotopes.
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