In the framework of quantum mechanics, the wave-particle duality refers to the concept that particles, including photons, can exhibit both wave-like and particle-like properties depending on how they are observed or measured. However, it is important to clarify the distinction between mass and wave-particle duality.
In the case of photons, which are elementary particles of light, they are indeed considered to be massless according to our current understanding of physics. The assumption of photon masslessness is supported by various experimental observations and theoretical considerations.
One of the fundamental aspects of a particle's mass is its resistance to acceleration. Massive particles require energy to be accelerated, and as a result, their motion is affected by inertia. On the other hand, massless particles, such as photons, do not possess inertia and can travel at the speed of light in a vacuum.
The masslessness of photons is also supported by the theory of electromagnetism, specifically Maxwell's equations, which describe the behavior of electromagnetic waves (including light). These equations predict the speed of light to be constant and independent of the observer's reference frame, which has been experimentally verified numerous times.
Additionally, the concept of superposition, which you mentioned, refers to the ability of quantum particles to exist in multiple states simultaneously. While it is true that photons can exhibit wave-like properties, such as interference and diffraction, this does not imply that they have mass. The wave-particle duality is a description of their behavior and characteristics, but it does not imply the presence of mass.
In summary, the assumption that photons are massless is based on experimental evidence, theoretical consistency, and the fundamental principles of electromagnetism and quantum mechanics. While they exhibit wave-like behavior, the wave-particle duality does not imply the presence of mass for photons.