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Massless particles
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Photons have E = pc.
Photons (and gluons) have no rest mass; they always travel at c with E = pc and zero rest frame.
A photon of 1 eV has p = 5.3×10⁻²⁸ kg·m/s.
They have no rest frame to transform into.
In more detail
Massless particles, such as photons, always travel at the speed of light and have energy equal to pc. Their momentum is set entirely by their energy.
Real-world uses
- Photons carry momentum in radiation pressure and optical tweezers.
- Neutrinos are nearly massless and travel close to light speed.
- Physicists note that photons and gluons travel at the speed of light because they have no rest mass.
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Frequently asked questions
Can massless particles stop?
No. They must always move at c in vacuum.
Are neutrinos massless?
They have very small masses that were discovered from oscillation experiments.
Can a massless particle be at rest?
No. A massless particle must always move at the speed of light.
Related topics in Relativity
Rest energy · Energy–momentum relation · Relativistic kinetic energy · Relativistic Doppler effect · Aberration of light · Four-vectors