Final answer:
Light and all electromagnetic radiation are considered massless and always travel at the speed of light in a vacuum, 2.998 × 10^8 m/s. If light had mass, it would defy the principles of Einstein's theory of relativity, making it impossible to achieve the speed of light.
Step-by-step explanation:
In the realm of physics, particularly relating to the speed of light and its fundamental constants, it is critical to note that by our current understanding, light, and all other forms of electromagnetic radiation, are massless and travel at c in a vacuum, which is 2.998 × 108 m/s. If light had mass, it would not be able to reach this speed due to the principles outlined in Einstein's theory of relativity. Specifically, as an object with mass approaches the speed of light, its relativistic mass increases, requiring an infinite amount of energy to reach the speed of light itself, which is impossible with finite energy. Therefore, it is not meaningful to talk about a mass-bearing photon slowing down because such a particle wouldn't be light as we understand it. Instead, we use formulas for mass-bearing particles, such as E = mc2, to describe the energy of a particle at rest rather than its speed in motion, which is why the information about E = mc2 does not apply here as a method to calculate the speed of a photon if it had mass.