The rest mass energy of a photon is zero.
Understanding Rest Mass and Energy
The concept of rest mass is crucial here. Rest mass, also known as invariant mass, refers to the mass of an object when it's at rest relative to an observer. Einstein's famous equation, E=mc², relates energy (E) to mass (m) and the speed of light (c). This equation specifically refers to rest mass energy—the energy an object possesses solely due to its mass when stationary.
Photons, the fundamental particles of light, are massless particles. This means they have zero rest mass. Therefore, their rest mass energy, calculated using E=mc², is also zero (0 * c² = 0).
Photon Energy and Momentum
While a photon has zero rest mass, it does possess energy and momentum. Its energy is given by the equation E = hf, where h is Planck's constant and f is the frequency of the light. This energy is entirely kinetic energy—energy of motion—as the photon is always traveling at the speed of light. The photon's momentum is related to its energy and wavelength.
Some sources mention a photon having a "non-zero rest mass," which is a point of ongoing discussion and interpretation in certain theoretical frameworks. However, within the standard model of particle physics, the accepted value for the rest mass of a photon remains zero. This means its rest energy is also zero.
Several sources support this conclusion:
- Wikipedia: Invariant Mass: Clearly states that massless particles, such as photons, have zero rest mass.
- Wikipedia: Mass–energy equivalence: Reinforces the concept that rest mass is a fundamental property independent of velocity, and massless particles have zero rest mass.
- ScienceDirect article: While mentioning potential interpretations of non-zero rest mass in specific contexts, it acknowledges the standard electromagnetic theory's assertion of zero rest mass for photons in free space.
Conclusion
The discrepancy between some sources claiming a non-zero rest mass for photons and the standard model’s assertion of zero rest mass arises from different theoretical frameworks and interpretations. However, within the generally accepted standard model of physics, the rest mass energy of a photon is undeniably zero.
