What is the phenomenon called when simultaneous events for one observer are not simultaneous for another observer?

The phenomenon where simultaneous events for one observer are not simultaneous for another observer is known as simultaneity. This concept is a crucial aspect of Einstein's theory of relativity, specifically the special theory of relativity. It highlights how the perception of time and events can differ depending on the observer's frame of reference and relative motion.

In a classical sense, two events that happen at the same time in one frame of reference may not appear simultaneous in another frame that is moving relative to the first. This effect is most pronounced at high speeds, close to the speed of light, where time dilation and length contraction come into play.

Understanding simultaneity is fundamental in the study of relativistic effects, as it challenges our intuitive notions of time and how events are ordered in different frames of reference. The concept has far-reaching implications for physics, particularly in understanding the nature of time and space.

The other terms listed, such as distance, acceleration, and velocity, relate to the measures of motion and physical quantities but do not address the concept of time perception and the relativity of events.