When a photon interacts with a solar sail or two opposing plates, it can exert a force on them without reducing its speed. This phenomenon is related to the momentum carried by the photon and the conservation of momentum.
According to classical physics, momentum is defined as the product of an object's mass and its velocity. However, in the realm of quantum mechanics, particles such as photons also have momentum even though they are massless. The momentum of a photon is given by its wavelength or frequency through the equation p = h/λ, where p is the momentum, h is Planck's constant, and λ is the wavelength of the photon.
When a photon interacts with a solar sail or the opposing plates, it transfers its momentum to them. Due to the conservation of momentum, the total momentum before and after the interaction must be the same. Therefore, as the photon imparts its momentum to the sail or plates, it experiences an equal and opposite change in momentum.
Since the photon is massless, its speed remains constant at the speed of light (c) even though its momentum changes. This is because momentum and velocity are related differently for massless particles compared to massive particles. While the speed of light is constant, the photon's frequency or wavelength may change depending on the interaction.
The force exerted by the photon on the solar sail or plates is a result of this momentum transfer. As the photons bounce off the sail or plates, they change their direction and transfer momentum, resulting in a net force on the object.
It's important to note that this explanation is based on quantum mechanics and the wave-particle duality of light. The behavior of photons and their interactions is best described using the principles of quantum physics rather than classical mechanics.