Because it is a physical constant that exists independently of our knowledge and understanding. But such a simple answer does not suit, it is too much like the banal «God’s will for everything.» Curiosity torments people and often leads to paradoxical conclusions. A striking example of such a conclusion is the special theory of relativity, developed by Albert Einstein. What she predicts is contrary to intuition, but the Earth is also a ball, and we see it relatively flat.
It is necessary to figure out where this question came from at all and why the correct answer is so important. After all, its truth causes continuous passionate debate, despite the available experimental evidence. The answer requires a foreword.
Until the middle of the 19th century, physics was dominated by the theory of the luminiferous ether, formulated by Rene Descartes and confirmed by Maxwell in the framework of his electromagnetic theory. There were other hypotheses of the ether, but all of them, in one way or another, described a three-dimensional space filled with a certain substance that carries light and other electromagnetic waves. They tried to explain their behavior by Newton’s laws, and when changing the coordinate system, Galileo’s transformations were applied.
It was assumed that the speed of light obeys the same rules, albeit very high. Everything changed in 1881 when Michelson and Morley tried to measure it. It turned out that the speed of light does not depend on the direction of motion of the Earth. This result undermined the theory of ether at the root, and the experimenters concluded that it was inconsistent.
They did not believe them and repeated the experiment many times, clarifying the data obtained. In 1964, for this purpose, helium-neon lasers were used as light sources, then optical cryogenic resonators with a measurement accuracy of 1 to -16 degrees. The result again confirmed the constancy of the speed of light under any external influences. So Michelson deservedly received the Nobel Prize in Physics in 1907.
Outstanding Dutch physicist Hendrik Anton Lorenz has been studying the theory of ether for many years and in the course of his research came to the need to introduce the concept of «local time» for a system moving in motionless ether. He himself considered the derived formula intermediate, but received new equations for the transformation of velocities, different from the Galilean one.
It was assumed that this theory applies only to electromagnetic waves. Lorenz did not dare to take the last step and extend his postulates to objects with mass. Despite this, he received the Nobel Prize in Physics in 1902.
So, it has been proven that light travels in a vacuum with a speed of 299 792 458 m / s and it does not depend on the speed of the source or the frame of reference. Albert Einstein in 1905 made this invariance one of the postulates of his theory, excluding the luminiferous ether from it. Which entailed revolutionary consequences.
This formula is known even by people who are far from physics. But it is true only at speeds much lower than light. As soon as the speed of the object becomes commensurate with it, the so-called Lorentz factor ƴ comes into its own.
Where v is the speed of the object, c is the speed of light. At low speeds, which Newtonian mechanics deals with, the coefficient approaches 1 so much that it can be neglected without loss of accuracy. But as V approaches C, its value grows exponentially, which is clearly demonstrated by the graph below.
The formula now looks like this:
We see that to reach the speed of light for a material body with an inert mass, an infinite amount of energy is required. This is the reason why the speed of light cannot be exceeded.
Even the scientists working at the Large Hadron Collider at CERN were unable to accelerate protons to light speed, although they came close to it. The result was the discovery of the «God particle», the Higgs boson, which is responsible for the presence of the mass of bodies.