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particle: In classical mechanics and quantum mechanics there are objects called particles which are sometimes conceptualized as infinitesimal balls. Some particles are deemed fundamental particles and are considered particles which are not made up of other particles. Particles typically come in two types: fermions and bosons. Fermions are further categorized as leptons and hadrons. Hadrons are further categorized as mesons and baryons. In a field theory one does not consider particles, but instead, oscillations of the field (see the aside).

particle, anti-:

particle, fundamental: Those particles which are not made up of other particles. Just as molecules are combinations of atoms, and atoms are combinations of particles, some particles are also combinations of other particles. Currently, leptons, quarks, and gauge bosons are considered fundamental. (see also the aside on scattering regarding how we know if a particle is fundamental)

phase space:

photoelectric effect: When light of a certain frequency is shone on the cathode, a beam flows (due to a small voltage applied between the plates) from the cathode toward the anode. Einstein used Planck's hypothesis that the energy in light is quantized (see photon and black body radiation). This effect was one of the clues that classical physics was incomplete, heralding the birth of quantum mechanics. It was first discovered by Hertz accidentally while carrying on some investigations into the nature of electromagnetic waves in 1887. (For more information, see the off-site explanation)

photon:

position:

potential energy: (see energy, potential)

proton: