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About these units
Meter (m)
The meter is the foundational unit of length in the International System of Units (SI) and forms the backbone of virtually all modern scientific and engineering measurements. Originally defined in the late 18th century as one ten-millionth of the distance from the equator to the North Pole along a meridian, the meter's definition has evolved alongside advances in physics and measurement technology. For much of the 19th and 20th centuries, it was tied to a physical bar stored in Paris—an artifact vulnerable to temperature fluctuations and damage. Today, the meter is defined using a universal constant rather than a physical object: the distance light travels in vacuum in 1/299,792,458 of a second. This definition is elegant because it anchors the meter to an invariant physical constant, ensuring precision across laboratories and nations. It allows modern metrology labs to realize the meter through laser interferometry, achieving extraordinary accuracy. The meter's universality and reliability make it the most important single unit of length ever devised.
Electron Radius (re)
The classical electron radius, approximately 2.818 × 10⁻¹⁵ meters, is a theoretical value derived from classical electromagnetic theory rather than an actual measured size. It represents the radius a charged sphere would need to have in order for its electrostatic self-energy to equal the electron's rest energy. Although electrons are now understood to be point-like or extremely small compared to this radius, the classical electron radius remains useful in scattering theory, especially in calculations involving Thomson scattering — the elastic scattering of electromagnetic radiation by free electrons. Thus, while not a physical dimension of the electron, the classical radius serves as a meaningful parameter in specific areas of physics and retains importance in radiation modeling and plasma physics.