The speed of light in vacuum is constant and independent of the motion of the source or the observer.

Experiments had shown that light travels at roughly 300,000 kilometers per second. Einstein took this as a universal truth: no matter how fast you move toward or away from a light beam, its speed remains the same. This idea contradicts everyday experiences with sound or throwing balls, where speeds add up or subtract. This constant speed leads to surprising effects like time dilation and length contraction.