Time Dilation

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Lore & Background

Until the turn of the 20th century, the assumption had been that the three-dimensional geometry of the universe was distinct from time. However, space and time took on new meanings with the Lorentz transformation and special theory of relativity. In 1908, Hermann Minkowski presented a geometric interpretation of special relativity that fused time and the three spatial dimensions into a single four-dimensional continuum now known as Minkowski space. This interpretation proved vital to the general theory of relativity, wherein spacetime is curved by mass and energy. Non-relativistic classical mechanics treats time as a universal quantity of measurement that is uniform throughout, is separate from space, and is agreed on by all observers. Classical mechanics assumes that time has a constant rate of passage, independent of the observer's state of motion, or anything external. In the context of special relativity, time cannot be separated from the three dimensions of space, because the observed rate at which time passes for an object depends on the object's velocity relative to the observer.

In Their Own Story

In 1905, Albert Einstein analyzed special relativity in terms of kinematics rather than dynamics. His results were mathematically equivalent to those of Lorentz and Poincaré. He obtained them by recognizing that the entire theory can be built upon two postulates: the principle of relativity and the principle of the constancy of light speed. His work was filled with vivid imagery involving the exchange of light signals between clocks in motion, careful measurements of the lengths of moving rods, and other such examples. The Michelson–Morley experiment of 1887 showed no differential influence of Earth's motions through the hypothetical aether on the speed of light. George Francis FitzGerald in 1889, and Hendrik Lorentz in 1892, independently proposed that material bodies traveling through the fixed aether were physically affected by their passage, contracting in the direction of motion by an amount that was exactly what was necessary to explain the negative results of the Michelson–Morley experiment.

Reader's Guide

In special relativity, an observer will, in most cases, mean a frame of reference from which a set of objects or events is being measured. Reference frames are inherently nonlocal constructs. In Fig. 1-1, imagine that the frame under consideration is equipped with a dense lattice of clocks, synchronized within this reference frame, that extends indefinitely throughout the three dimensions of space. The latticework of clocks is used to determine the time and position of events taking place within the whole frame. The term observer refers to the whole ensemble of clocks associated with one inertial frame of reference. Physicists distinguish between what one measures or observes, after one has factored out signal propagation delays, versus what one visually sees without such corrections. Failing to understand the difference between what one measures and what one sees is the source of much confusion among students of relativity.

Did You Know?

• Henri Poincaré was the first to combine space and time into spacetime, arguing in 1898 that the simultaneity of two events is a matter of convention.
• The Michelson–Morley experiment of 1887 showed no differential influence of Earth's motions through the hypothetical aether on the speed of light.
• A scale factor, c (conventionally called the speed-of-light), relates distances measured in space to distances measured in time, with nearly 300,000 kilometres or 190,000 miles in space being equivalent to one second in time.
• In 1905/1906, Henri Poincaré mathematically perfected Lorentz's theory of electrons in order to bring it into accordance with the postulate of relativity.
• George Francis FitzGerald in 1889, and Hendrik Lorentz in 1892, independently proposed that material bodies traveling through the fixed aether were physically affected by their passage, contracting in the direction of motion.