Special relativity in 1905: redefining time and motion

Einstein’s On the Electrodynamics of Moving Bodies began with relative motion between a magnet and a conductor. The same relative movement produced the same current, yet existing explanations treated a moving magnet and a moving conductor differently. He reconsidered motion, electromagnetic fields and measurement.

Annalen der Physik received the paper on June 30, 1905, and published it later that year. It became the foundational paper of special relativity.

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The paper first specified how time was to be measured. Clocks at separated locations could be synchronised using light signals travelling between them. Deciding whether two events were simultaneous required specifying a clock system and its motion.

Einstein adopted the principle of relativity and a speed of light in vacuum independent of the source’s motion. Together, these determined transformations of spatial coordinates and time between inertial frames.

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From these definitions, the paper derived relations between observers in relative motion. They could disagree on the simultaneity of separated events, while measurements of moving lengths and clocks had to be compared through the transformations.

At everyday speeds, far below the speed of light, the differences are small. Motion near that speed requires the full relativistic relations, including a different rule for combining velocities.

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The latter part applied the transformations to electromagnetic fields, light frequencies and charged particles. Descriptions of electric and magnetic fields depend on the observer’s motion, allowing the initial magnet-and-conductor problem to be treated consistently.

The work changed how physics compares measurements of space and time. Relativistic relations later became necessary in particle accelerators and precise timing, while the study of gravity and curved spacetime developed into general relativity.

References: [1]