According to modern physics, light does not possess rest mass. Rest mass, also known as invariant mass or simply mass, is a property of particles that determines their resistance to changes in motion. It is a fundamental property of matter particles such as electrons, protons, and neutrons.
In contrast, light, which consists of particles called photons, is considered to be massless. Photons are particles of electromagnetic radiation and travel at the speed of light in a vacuum. According to the theory of special relativity proposed by Albert Einstein, massless particles must always travel at the speed of light and have zero rest mass.
The absence of rest mass in photons is one of the reasons why they can travel at the speed of light and have unique properties, such as being able to travel long distances without experiencing significant decay or slowing down. The energy of a photon is related to its frequency and wavelength through the equation E = hf, where E is the energy, h is Planck's constant, and f is the frequency.