It is very misleading that we are taught light travels at the speed of light, which is a constant value, and that it takes about 8 minutes for light from the Sun to reach the Earth. We are also told that photons, the particles of light, are massless. So how can a massless entity travel? It’s greatly bending our intuition.

The more you learn and apply logic and physics, the more it becomes clear that our daily common sense or intuition is often misguided. The irony is that textbooks strive to teach these counterintuitive truths through intuitive analogies, which ultimately cloud understanding and make it more challenging for most people.

The massless photo travels at speed of light is a typical example!

In relativity, the fundamental structure is not space + time separately.

It is spacetime.

The invariant quantity is:$$ds^2 = -c^2 dt^2 + dx^2 + dy^2 + dz^2$$

There are three possibilities:

• Timelike: $ds^2 < 0$ → massive particles
• Null (lightlike): $ds^2 = 0$ → massless excitations
• Spacelike: $ds^2 > 0$ → forbidden for causal propagation

Massless fields follow:$$ds^2 = 0$$

so it’s not light is traveling at the speed of light, it’s a constant value under that condition, i.e. worldline lies on the light cone. It’s the conversion factor between space and time inside the metric. So asking how fast it moves is almost the wrong question. It does not experience time.

In field theory, Propagation is not a particle flying. It is: The geometry of spacetime restricting how field correlations extend.