# Physics Glossary – M

Mach’s Principle

The inertia of any system is the result of the interaction of that system and the rest of the universe. In other words, every particle in the universe ultimately has an effect on every other particle. Named after Ernst Mach in 1893.

Magnetopause

The boundary of the magnetosphere, separating plasma attached to Earth from the one flowing with the solar wind.

Magnetic Resonance Imaging (MRI)

An imaging technique based on the principles of Nuclear Magnetic Resonance. Uses positrons to trace the structure of the brain using radioactive dyes injected into the patient.

Maxwell’s Equations

Maxwell’s equations are the set of four equations by James Clerk Maxwell that describe the behavior of both the electric and magnetic fields. Maxwell’s equations provided the basis for the unification of electric field and magnetic field, the electromagnetic description of light, and ultimately, Albert Einstein’s theory of relativity.

Meson

A hadron made from an even number of quark-antiquark constituents. The basic structure of most meson is one quark and one antiquark. These particles are not fundamental. These are made up of quarks and anti-quarks. Mesons are a sub-category of hadrons; that is to say, all mesons are hadrons, but not all hadrons are mesons. A meson is made up of a constituent quark and anti-quark pair. These need not be the same flavor. However, they must have opposite color charge, leaving the meson uncharged with respect to the strong interaction. It could still be electrically or otherwise charged.

Muon

Is a collective name for two semistable fundamental particles with positive and negative charge. Muons have a mass that is 207 times greater than the electron (105.6 MeV) and a spin of 1/2. Both electrons and muons belong to to the same family of fermions (i.e., fundamental particles) called the leptons. Because of this, a negatively-charged muon can be thought of as an extremely heavy electron. Muons are denoted by µ- and µ+ depending on their charge.

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