photon is the smallest quantity (quantum) of energy that can be transported and it was the realization that light traveled in discrete quanta that were the origins of Quantum Theory. A photon is massless, has no electric charge, and is a stable particle with two possible polarization states.

The concept of the photon was introduced in quantum physics to explain the contradictions that emerged between classical electromagnetism and experiments carried out at the turn of the late nineteenth century and the twentieth century. According to the classical theory developed by Maxwell, light, radio waves and UV rays are all electromagnetic radiation, that is, electric and magnetic fields that propagate in the matter and the vacuum following wave dynamics. The photon was introduced as an elementary constituent of these radiations by Max Planck and Albert Einstein between 1900 and 1905, as an entity that cannot be further divided.

In classical physics, each wave, according to the superposition principle, can always be decomposed as the sum or the contribution of two or more other waves. In contrast, the quantum mechanics postulates for electromagnetic waves, in agreement with the experiments, the existence of a “quantum” of indivisible fundamental energy, which therefore has both wavelike and particle properties (a phenomenon known as wave-particle duality).

From the standpoint of the particle, the photon has zero mass and does not carry any electric charge. Its intrinsic angular momentum, the spin, can assume only two values of ±1 which correspond to the different classical polarization states. In the void, photons always propagate at the speed of light (there is no observer against which they are stationary) and their range of action is unlimited. This means that a photon can continue to travel in space-time indefinitely without any limit until it is absorbed by another particle. For this reason, it is still possible to detect the photons emitted in the early life of the universe, which form the cosmic background radiation.

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