The speed of light, denoted by the symbol "c," is a fundamental constant of nature. According to Einstein's theory of relativity, the speed of light in a vacuum is the maximum attainable speed in the universe. This limit applies to all particles with mass, as well as to massless particles like photons.
The theory of relativity introduces the concept of spacetime, which combines the three dimensions of space with the dimension of time into a unified framework. It postulates that the speed of light in a vacuum is the same for all observers, regardless of their relative motion. This principle is known as the constancy of the speed of light.
As an object with mass accelerates, it requires more and more energy to continue increasing its speed. According to the theory of relativity, as an object with mass approaches the speed of light, its relativistic mass increases, and the amount of energy required to accelerate it further becomes infinite. This implies that it is impossible to accelerate a massive object to or beyond the speed of light.
On the other hand, photons are massless particles. Since they have no rest mass, they always travel at the speed of light in a vacuum. Photons are governed by the laws of special relativity, and they possess energy and momentum, even though they lack rest mass.
In summary, the constancy of the speed of light is a fundamental postulate of relativity. It sets a universal speed limit in the universe and applies to both massive and massless particles. While massless particles like photons naturally travel at the speed of light, objects with mass cannot reach or exceed this speed due to the infinite energy requirement to do so.