Enhetsfarten

Enhetsfarten is a term used in Swedish-language physics and mathematics to refer to the unit of speed employed in certain unit systems, most commonly when the speed of light in vacuum is set to unity. In such natural or relativistic unit systems, the speed of light c is taken to be 1, and velocities are expressed as dimensionless fractions of c. Consequently, a velocity stated as v in enhetsfarter equals v/c in conventional units.

In practical terms, 0.5 enhetsfart corresponds to half the speed of light, while 1 enhetsfart corresponds to the speed of light itself. Velocities greater than 1 enhetsfart are not possible for objects with mass in standard relativity, whereas massless particles like photons travel at 1 enhetsfart. To convert back to SI units, one multiplies the dimensionless value by c, so v_SI = (v_enhets) × c.

The concept is widely used in theoretical physics, cosmology and related fields to simplify equations and enhance clarity. By reducing constants such as c to unity, expressions for energy, momentum and relativistic factors become more compact, facilitating algebraic manipulations and highlighting the structure of physical laws.

While convenient in theory, enhetsfarten requires careful interpretation when communicating results to non-specialists or when converting to conventional units. It is typically confined to formal calculations, simulations, or pedagogical contexts, with explicit conversions provided when presenting numerical values in standard units.

Etymologically, the term translates to “the unit speed” in Swedish, reflecting its role as a conventional reference speed within selected unit systems. See also natural units, speed of light, and relativistic units for related concepts.