1. Special Relativity:
* Energy-Momentum Relation: Einstein's special relativity established the famous equation E² = (mc²)² + (pc)², where E is energy, m is mass, c is the speed of light, and p is momentum.
* Photons Always Travel at the Speed of Light: Photons are massless particles, which means they always travel at the speed of light. This is a fundamental property of photons, not something derived from quantum mechanics.
* Zero Rest Mass: If a photon had rest mass (m ≠ 0), its energy would be infinite because its speed is always c. Since we observe photons with finite energy, they must have zero rest mass.
2. Quantum Field Theory:
* Quantum Fields: In quantum field theory, photons are described as excitations of the electromagnetic field. This field is quantized, meaning its energy can only come in discrete packets called photons.
* Massless Excitations: The electromagnetic field, as described in quantum field theory, is a massless field. Therefore, its excitations (photons) are also massless.
In Summary:
The zero rest mass of photons is not a consequence of quantum mechanics alone, but a fundamental property that arises from the combination of special relativity and quantum field theory. It's a result of their unique nature as massless particles that always travel at the speed of light and are excitations of a massless field.
Important Note: While the rest mass of a photon is zero, it still has energy and momentum, which are related to its frequency and wavelength. This is because photons are not stationary particles; they are always moving at the speed of light.
