The concept of light having momentum despite being massless can indeed be counterintuitive, but it is a fundamental aspect of the theory of electromagnetism and is supported by experimental evidence. The momentum of light arises from its wave-particle duality and the properties of electromagnetic fields.
In classical electromagnetism, light is described as an oscillating electromagnetic wave. Electromagnetic waves consist of mutually perpendicular electric and magnetic fields that propagate through space. When an electromagnetic wave interacts with matter, it can exert a force on charged particles, causing them to accelerate.
In the early 20th century, with the development of quantum mechanics, it was discovered that light also exhibits particle-like behavior, referred to as photons. Photons are packets or quanta of energy associated with electromagnetic waves. While photons have no rest mass, they do possess energy and momentum.
According to the theory of relativity, the energy (E) and momentum (p) of a particle are related by the equation E² = (pc)² + (mc²)², where m is the rest mass of the particle, c is the speed of light, and p is the momentum. For a massless particle like a photon, the rest mass (m) is zero, but the momentum (p) is not zero. This relationship reduces to E = pc for massless particles.
Experimental observations, such as the Compton scattering of photons and the photoelectric effect, confirm the existence of photon momentum. In these phenomena, the transfer of momentum from photons to other particles or objects is directly observed.
The momentum of light is fundamentally tied to the wave nature of light and the transfer of energy and momentum carried by the electromagnetic field. While it may seem contradictory that a massless particle can have momentum, it is a consequence of the behavior and properties of electromagnetic waves and photons, as described by quantum mechanics and the theory of relativity.