Britannica Sources used in this answer

How does the fixed speed of light help explain Einstein’s special relativity?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A horizontal gradient bar displays the visible light spectrum from violet to red, labeled with corresponding frequency in terahertz, wavelength in nanometers, and photon energy in electron volts. — light When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.

To maintain the constant speed of light for all observers, Albert Einstein concluded that space and time are not absolute, as previously believed in classical physics, but are instead relative to an observer's state of motion.1

In classical mechanics, velocities are additive; however, experiments like the Michelson-Morley experiment demonstrated that light does not follow this rule. To reconcile this, Einstein established the constancy of the speed of light as a fundamental postulate. This requirement forces space and time to change for a moving body relative to a fixed observer:21

Length Contraction: A moving object is measured to be shorter along its direction of motion.1
Time Dilation: Time intervals are measured to be longer, meaning time runs more slowly in a moving frame of reference.1

These effects, described by the Lorentz transformations, ensure that all observers measure the same speed of light ( $c$ ), regardless of their relative motion. Consequently, concepts like simultaneity become relative, as events occurring at the same time for one observer may not be simultaneous for another.1