25. Michelson-Morley experiment. Einstein’s postulates of special relativity - explanation of their meaning. (3p.) Flashcards
Michelson-Morley experiment
backstory) The dominant theory of how light worked was that it was a wave of electromagnetic energy; however, there had to be some medium through which a wave moved. Physicists hypothesized that there was a substance, which filled the entire universe – the luminous ether. Michelson and Morley came up with the idea that it should be possible to measure the motion of the Earth through the ether. It was believed to be unmoving and static, whereas the Earth was moving quickly. As long as there was some sort of motion between the ether and the Earth, it should have created an effective “ether wind” that would have either pushed or hindered the motion of the light wave.
(experiment) To test this hypothesis, Michelson and Morley designed a device that split a beam of light and bounced it off mirrors so that it moved in different directions and finally hit the same target (detector). The assumption was that if two beams travelled the same distance along different paths through the ether, they should move at different speeds, and therefore when they hit the final target screen those light beams would be slightly out of phase with each other, which would create a recognizable interference pattern.
The thesis was that either the ether doesn’t exist or the speed of light doesn’t depend on it, which meant that it didn’t matter whether it existed or not. Different teams repeated the experiment, but it always ended up showing no movement with respect to the ether.
RYSUNEK
Einstein’s postulates relativity - explanation of their meaning.
• Einstein’s postulates of special relativity (both postulates are restricted to inertial frames – accelerated frames were later included in the general theory of relativity)
- The principle of relativity: The laws of physics are the same in all inertial frames of reference.
The first postulate declares that the laws of physics are absolute, universal and the same for all inertial observers.
- The principle of the constancy of the speed of light: The speed of light in vacuum has the same value c in all inertial reference frames and is independent of the motion of the light source.
The second postulate is a consequence of the first, since if the speed of light depended on the motion of its source the laws of electrodynamics (which determine the speed of freely propagating electromagnetic waves) would depend on the inertial frame of the source, which contradicts the first postulate. (trzeba napisać całość, żeby dał za to punkt – albo wystarczy napisać, że the second postulate is based on the results of the Michelson experiment).
