Einstein's principle of light speed invariance, which is a fundamental aspect of his theory of special relativity, finds its roots in James Clerk Maxwell's equations of electromagnetism.
Maxwell's equations, formulated in the 19th century, describe the behavior of electric and magnetic fields and the propagation of electromagnetic waves, including light. These equations unify the laws of electricity and magnetism and provide a mathematical framework for understanding how electromagnetic fields interact with charges and currents.
One of the key insights of Maxwell's equations is that they predict the existence of electromagnetic waves that propagate through space at a constant speed. The speed of these waves is determined by the properties of the medium through which they travel, such as the permittivity and permeability of the medium. In the absence of a medium, the waves propagate through a vacuum, and the speed of these waves is constant.
At the time, the prevailing notion of motion was based on the idea of an "aether," a hypothetical medium that was believed to permeate all of space. It was thought that light waves required this aether to propagate, similar to how sound waves require a medium like air or water. However, experiments conducted in the late 19th century, including the famous Michelson-Morley experiment, failed to detect any evidence of the aether.
Einstein built upon these experimental results and Maxwell's equations to develop his theory of special relativity. In 1905, he postulated that the laws of physics are invariant under Lorentz transformations, which are mathematical transformations that relate the measurements of space and time between different observers in relative motion. One consequence of these transformations is that the speed of light in a vacuum is the same for all observers, regardless of their relative motion.
This principle, known as light speed invariance or the constancy of the speed of light, has been extensively tested and verified through numerous experiments and observations. It implies that the speed of light serves as an absolute cosmic speed limit, preventing any material object with mass from reaching or exceeding this speed.
Einstein's recognition of the constancy of the speed of light in his theory of special relativity revolutionized our understanding of space, time, and motion, and it remains one of the foundational principles of modern physics.