Photons are elementary particles that serve as the force carriers of the electromagnetic force, one of the four fundamental forces in nature. They are massless, meaning they have zero rest mass, which allows them to travel at the speed of light in a vacuum (approximately 299,792 km/s).
Photons are responsible for electromagnetic interactions between charged particles. Every time electrically charged particles interact—whether it’s light hitting your eye or an electron repelling another electron—photons are involved. In this role, they mediate forces between particles without themselves being affected by electric or magnetic fields.
Photons also exhibit wave-particle duality: they behave like particles in some experiments (e.g., the photoelectric effect) and like waves in others (e.g., interference patterns in the double-slit experiment). As quantum packets of electromagnetic energy, photons are the fundamental units of light, ranging from gamma rays to radio waves, depending on their energy.
Their massless nature is essential for the long-range behavior of electromagnetism and ensures that electromagnetic radiation can travel vast cosmic distances without losing speed.