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Energy–momentum relation
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Links energy, momentum and mass.
Key relation
E² = p²c² + m²c⁴E² = (pc)² + (mc²)² applies to all particles; for photons m = 0 and E = pc.
Worked example
An electron with p = 1 MeV/c: E = 1.12 MeV.
Common mistake
Valid at any speed.
In more detail
The energy-momentum relation E² = (pc)² + (mc²)² links energy, momentum and rest mass. It holds for all particles and reduces to classical forms at low speeds.
Real-world uses
- Particle physicists use it to compute masses from measured energies and momenta.
- It describes massless particles such as photons.
- Particle physicists use the energy–momentum relation to find the mass of a particle from its measured energy and momentum.
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Frequently asked questions
What happens for a photon?
Its mass is zero, so E equals pc.
Why is it important?
It gives a simple way to relate measured quantities to particle masses.
What happens for a massless particle?
The relation reduces to E equals pc, which is the case for photons.
Related topics in Relativity
Relativistic momentum · Relativistic energy · Rest energy · Relativistic kinetic energy · Massless particles · Relativistic Doppler effect