Michelson Morley Experiment

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Prior to 1905, classical relativity—originally formulated by Galileo and Newton—described physical events strictly from an observer's local frame of reference. For instance, a stationary car parked beside you appears at rest only because you are moving alongside it at the exact same velocity relative to the ground.

However, in 1905, Albert Einstein published his Special Theory of Relativity, fundamentally altering our understanding of space and time. By postulating that the speed of light in a vacuum is invariant (constant for all observers, regardless of their motion), Einstein demonstrated that classical Galilean relativity was incomplete.

The following lessons explore the historical developments, experimental evidence, and mathematical consequences behind Einstein's Special Theory of Relativity.

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Michelson and Morley

The Michelson-Morley Experiment

In 1865, James Clerk Maxwell unified electricity and magnetism into a single theory, predicting that light is an electromagnetic wave traveling at a fixed speed $c \approx 3 \times 10^8\text{ m/s}$. However, classical wave physics dictated that all waves require a medium to propagate (such as sound through air or ripple waves through water).

This raised a crucial question: What frame of reference is the speed of light measured relative to? Physicists hypothesised the existence of an all-pervading, invisible medium called the luminiferous aether, which served as the absolute stationary reference frame for light propagation.

Detecting the "Aether Wind"

In 1887, Albert Michelson and Edward Morley designed an exceptionally sensitive interferometer to detect the Earth's motion through this hypothetical aether:

  • Beam Splitting: A single light beam was split into two perpendicular paths of equal length (L), reflected off mirrors, and recombined at a detector to form an interference pattern.
  • Expected Shift: As Earth orbited the Sun through the aether, the speed of light along the parallel path (c - v and c + v) was predicted to differ from the perpendicular path, producing a measurable shift in interference fringes when the apparatus was rotated by 90°.
  • The Null Result: Despite extraordinary precision, no shift in interference fringes was observed. The experiment was repeated across different seasons and elevations—consistently yielding a null result.

Significance to Modern Physics

The null result demonstrated that the speed of light is completely independent of the motion of the source or observer. This paved the way for Einstein's 1905 **Special Theory of Relativity**, which discarded the concept of an absolute aether altogether and established that the speed of light in a vacuum (c) is invariant for all inertial observers.

Watch the videos below for a qualitative walkthrough of the interferometer setup, followed by a detailed mathematical analysis of the trip times along both light paths.

If you are wondering about the mathematics involved, watch this video
 
Conclusion
Its important to note that this experiment did not conclusive disprove the idea of an aether. It was simply a null result - an experiment whose result simply provided no evidence of this aether.
In fact the experiment was repeated numerous times, as late as 1930, by Joos,  to see if aether could be detected. Again,  null results were recorded.
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