Is Modern Physics Based on a Photon Being Massless and Having Momentum?

Carl, my dude, you’ve asked this question over and over and over. It’s been explained to you with way more patience than you deserve countless times. Momentum doesn’t require mass.

Understanding the Mass of Photons

We think the photon’s mass is zero or very close to zero. However, we cannot rule out the possibility that photons may have a tiny mass that we can never measure. This is a fascinating and ongoing area of research in modern physics.

The Relativistic Perspective

The formula p mv (momentum mass times velocity) is a fundamental principle in classical physics. However, in the realm of relativistic speeds, this formula becomes inaccurate. The correct relativistic formula is p γmv, where γ is the factor used in Special Relativity. γ is close to 1 at low speeds but approaches infinity as the speed approaches the speed of light.

Are Photons Massless?

Yes, theoretically, relativity and the Standard Model of Quantum Field Theory both assume photons are massless. Experimentally, we can’t prove they are massless; only that if they do have mass, it is extremely tiny, below 10-24 MeV.

Photon Momentum and E2 P2c4 M2c2

Photons have momentum, which is a fundamental property of any particle. According to Einstein, this momentum is not zero. You might be confused because of the classical notion p mv. However, in the context of Special Relativity, this approximation is not valid for massless particles moving at light speed. Instead, we use the relation E2 P2c4 M2c2. When M 0, the momentum just is the kinetic energy divided by a unit conversion factor of c.

From quantum physics, we know E hf, which tells us that the momentum of a photon is simply its frequency multiplied by a unit conversion factor h/c. Alternatively, it can be described as its inverse wavelength. Red photons carry less momentum than blue photons, which is precisely what we observe in experiments involving the measurement of radiation pressure on sails or the prediction of the Sun’s fusion reactor’s radiation pressure on its outer layers.

Closing Thoughts on Photons and Relativity

Technically, Special Relativity doesn’t have a problem with particles having zero momentum. However, such particles would carry no energy and thus cannot interact with anything else in a measurable way. A photon with zero momentum would have an infinite wavelength, which is practically impossible in a finite observable universe.

For a massive particle at rest, p 0, the equation simplifies to E mc2. When there is no kinetic energy and only mass-energy, the energy is equivalent to the mass times a unit conversion factor of c2. In experiments involving the LHC, both mass-energy and kinetic energy come into play, necessitating the use of the slightly more complicated energy-momentum relation.