What is the velocity of a photon?

The velocity of a photon is defined as the speed of light in a vacuum, which is approximately 299,792 kilometers per second, or about 186,282 miles per second. This constant value is fundamental in physics and is represented by the letter 'c'. Photons, being massless particles of light, travel at this speed regardless of the observer's frame of reference.

The speed of light is crucial in various physical theories, including Einstein’s theory of relativity, where it plays a significant role in the relationship between space and time. This concept underpins many technological and scientific applications, from telecommunications to understanding the behavior of stars and galaxies.

The other options do not represent the speed of a photon. The speed of sound, for example, is significantly slower than the speed of light and varies depending on the medium through which it travels. The speed of gravity, while a nuanced concept, refers to the speed at which gravitational changes propagate and is also equivalent to the speed of light, but it is not specifically the speed of individual photons. Thus, the clear and scientifically established answer is that photons move at the speed of light.