MIT STS.042J: Einstein and Experiment (L5)

STS.042J / 8.225J — Einstein, Oppenheimer, Feynman: Physics in the 20th Century MIT OpenCourseWare · Fall 2020 · Prof. David Kaiser · Undergraduate · CC BY-NC-SA 4.0 This lecture introduces Albert Einstein and his 1905 work on special relativity, organized in three parts: Einstein's path to 1905, a close reading of his paper "On the Electrodynamics of Moving Bodies," and an examination of why he approached a conventional problem in such an unconventional way. Kaiser first sets the stage with the inheritance Einstein received: the late-nineteenth-century focus on the "electrodynamics of moving bodies," the Michelson-Morley experiment's null result (no detectable motion of the Earth through the ether), and the Lorentz-FitzGerald contraction, which explained that null result through dynamics, by positing that the ether exerts a physical compressive force on moving objects. The lecture then traces Einstein's biography: his birth in 1879 in newly unified Germany, his family's electrical-engineering business, his rebellious dropping out of high school, his initial failure of the ETH Zurich entrance exam, his class-cutting and alienation of professors, and his eventual landing of a patent-clerk job in Bern through a friend's connection. Crucial to his intellectual development was the informal "Olympia Academy" reading group and the influence of Ernst Mach's positivism, which held that only measurable "objects of positive experience" belong in science and which sharply criticized Newton's absolute space and time. This pushed Einstein to begin with kinematics rather than dynamics. The central section walks through the 1905 paper itself. Einstein opens not with an experimental anomaly but with an "asymmetry in the explanation" of magnet-and-coil induction: one physical phenomenon (relative motion producing a current) had been given two different explanations depending on which object was deemed "really" moving. He then introduces two postulates: the principle of relativity (all physical laws hold in any inertial frame, extending Galilean relativity beyond mechanics) and the constancy of the speed of light for all inertial observers. From these, he declares the luminiferous ether "superfluous." Kaiser explains the origin of the second postulate in Einstein's teenage thought experiment about chasing a light wave (which Maxwell's equations forbid as a frozen, static field configuration). He then derives the relativity of simultaneity using the train-and-lanterns thought experiment, showing that length contraction and time dilation (the light-clock derivation) follow purely from kinematics, arriving at the same gamma factor as Lorentz but without any ether or forces. A brief account of the separate 1905 follow-up paper derives E = mc² from a thought experiment about a box emitting radiation in opposite directions. The final section uses Gerald Holton's and Peter Galison's scholarship to reinterpret Einstein's context. Holton argues persuasively that the traditional reading of special relativity as a direct response to Michelson-Morley has little basis, since Einstein may not even have known the result and dismissed only first-order experiments. Galison situates the paper in Einstein's immediate world of train travel, time-zone coordination, and "mother clocks" synchronized by electromagnetic signals (telegraph and radio, including from the Eiffel Tower), all subjects flowing across his patent-office desk. Viewed this way, the paper's operational focus on measuring distance and synchronizing distant clocks, and even its sparse citations (emphasizing priority like a patent), make more sense as the work of an engineer-minded patent clerk than as conventional mathematical physics. Kaiser also notes the historical detail that the paper was likely never peer-reviewed, since journals like Annalen der Physik applied formal refereeing inconsistently and granted established contributors editorial discretion. Source:    • Lecture 5: Einstein and Experiment