Final answer:
Photon momentum is a well-established fact in physics, evidenced by the photoelectric effect. It is significant on a quantum-mechanical scale and can have an effect on a large scale, as seen with proposals for space sails.
Step-by-step explanation:
Photon momentum is a well-established fact in physics. Despite photons having no mass, they carry momentum. This is evidenced by the photoelectric effect, where photons knock electrons out of a substance. On a quantum-mechanical scale, photon momentum is significant, especially when high-energy photons interact with small masses. Even on a large scale, photon momentum can have an effect, as seen with proposals to build space sails that use low-mass mirrors to reflect sunlight and gradually propel spacecraft.