Light waves and other forms of electromagnetic waves travel through free space at the speed c = 3.0 x 108 m/s. The speed of light sets an upper limit for the speeds of particles, waves and the transmission of information. In our everyday experiences, we mostly deal with objects that move at speeds much less than that of light. Classical mechanics (Newton mechanics) and initial ideas on space and time were devised to describe the motion of such objects and this formalism is very effective in describing a wide range of phenomena. While classical mechanics works well at low speeds, it fails when applied to particles whose speeds approach that of light.
Experimentally, we can test the predictions of Newtonian theory at high speeds by accelerating an electron through a large electric potential difference, for instance, it is possible to accelerate an electron to a speed of 0.99c by using a potential difference of more than a few million volts. As per Newton mechanics, if the potential difference as well as the corresponding energy is increased by a factor of 4, then the speed of electron should be doubled to 1.98c. However, experiments show that the speed of the electron, in addition to speeds of all other particles in the universe always remains less than the speed of light, regardless of the size of the accelerating voltage; partly because it places no upper limit on the speed that a particle can attain. Classical/Newton mechanics is contrary to modern experimental results and is thus a limited theory.
Albert Einstein special theory of relativity tries to remedy the limitations of classical mechanics. With this theory, one can correctly predict experimental observations over the range of speeds from rest to speeds approaching the speed of light.
Related: Einstein’s Special Theory of Relativity
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