The neutrino was known from the start to be an extraordinarily light particle, perhaps massless, since its rest mass could never be detected.
Any object with a non-zero rest mass (like a dust mote) can never reach light speed.
A photon has zero rest mass, but it carries energy and momentum.
The concept of the rest mass derives from special relativity.
Wolfgang Pauli suggested in 1930 that beta decay must involve a third particle that would be neutral, have negligible rest mass and a spin of one-half.
Definition of rest mass in US English:
rest mass
noun
Physics
The mass of a body when at rest.
〔物理〕静（止）质量
Example sentencesExamples
A photon has zero rest mass, but it carries energy and momentum.
Any object with a non-zero rest mass (like a dust mote) can never reach light speed.
The neutrino was known from the start to be an extraordinarily light particle, perhaps massless, since its rest mass could never be detected.
The concept of the rest mass derives from special relativity.
Wolfgang Pauli suggested in 1930 that beta decay must involve a third particle that would be neutral, have negligible rest mass and a spin of one-half.