In quantum physics, the relationship between light and mass is described by the theory of quantum electrodynamics (QED). According to QED, light is composed of particles called photons, which are considered massless particles.
In classical physics, mass is a fundamental property of matter, and any object with mass cannot travel at the speed of light. However, in the realm of special relativity and quantum physics, the behavior of light is described by a different set of rules. According to the theory of relativity, the speed of light in a vacuum is constant and is the fastest possible speed in the universe.
While light does not have mass in the traditional sense, it does have energy and momentum. In the famous equation E=mc², where E represents energy, m represents mass, and c represents the speed of light, it can be observed that mass and energy are interchangeable. Photons possess energy and momentum, and their energy is directly proportional to their frequency or inversely proportional to their wavelength.
When light interacts with matter, it can exert a pressure or a force, known as radiation pressure, even though it does not have mass. This effect can be observed in situations such as solar sails, where the momentum of photons is transferred to the sail, propelling the spacecraft.
Regarding the concept of something being both massive and massless at the same time, it's important to note that light, being composed of photons, is considered massless. However, in certain situations, particles that do have mass, such as electrons, can exhibit wave-particle duality, meaning they can exhibit both wave-like and particle-like properties. This duality is a fundamental aspect of quantum physics, and it allows for phenomena such as interference and diffraction to occur. The wave-particle duality does not imply that a particle has both mass and is massless simultaneously, but rather that its behavior can exhibit characteristics of both waves and particles depending on the experimental setup and observation.