Natural units are unit systems used in particle physics where the speed of light and the reduced Planck constant are set to one, so mass, energy and momentum share one unit (typically the electronvolt) and formulas lose their factors of c and hbar.Natural units are a way of measuring things in physics that makes equations simpler by setting fundamental constants to 1. In everyday life, we measure length in meters, time in seconds, and mass in kilograms. But in particle physics, the speed of light (c) and the reduced Planck constant (ħ) appear in almost every equation, cluttering them with numbers. Natural units declare that c = 1 and ħ = 1, so these constants vanish from formulas. This means that a velocity is just a number (a fraction of the speed of light), and energy, mass, and momentum all end up measured in the same unit — typically the electronvolt (eV).
The practice emerged organically in particle physics in the mid-20th century, popularized by theorists like Richard Feynman and John Archibald Wheeler, though no single inventor is credited.
To use natural units, you simply drop c and ħ from equations and treat all quantities as having the same dimension. For example, Einstein's famous E = mc² becomes E = m, meaning a particle's energy equals its mass. The Schrödinger equation loses its ħ, and relativistic momentum-energy relations simplify to E² = p² + m². When you need a final answer in conventional units, you put the constants back using dimensional analysis — counting powers of mass, length, and time to figure out which factors of c and ħ to restore.
Natural units are standard in particle physics research, from the Standard Model calculations to quantum chromodynamics, and are used in every paper and textbook on high-energy physics.
In natural units, the electron mass is about 0.511 MeV, and the proton mass is about 938 MeV — so a proton is nearly 2000 times heavier than an electron, a ratio that is hidden in kilograms but obvious in these units.
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