Why can photons travel at the speed of light?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

In the framework of special relativity, the reason photons can travel at the speed of light is that they possess zero rest mass.12

According to relativistic mechanics, as a material object with mass approaches the speed of light, its mass effectively increases, requiring an infinite amount of energy to continue accelerating. Consequently, it is impossible for any object with mass to reach or exceed this cosmic speed limit.13

Photons, however, are massless particles. Because they have no rest mass, they are not subject to the same energy constraints as material bodies and are required by the laws of electromagnetism to propagate at the speed of light in a vacuum.145