interferometer

History

The play of light on soap bubbles attracted the attention of Robert Hooke in the 17th century — these were the first observations of interference. But the instrument was built in 1881 by American physicist Albert Michelson, hoping to use it to detect the mysterious 'ether' in which light was supposedly propagating. The Michelson-Morley experiment found no ether, but proved the constancy of the speed of light. Today, laser interferometers LIGO and Virgo detect the tremors of spacetime from the merger of black holes.

How it works

Imagine two boats on a calm lake, rowing synchronously and creating circles from the oars. If the boats move strictly side by side, the circles meet and form calm and rough areas. The same thing happens in the interferometer: light waves from one source travel along two arms and then combine. If the distance traveled is slightly different, some waves arrive 'peak to peak' and amplify each other, while others arrive 'peak to trough' and cancel. By measuring how much the fringes have shifted, we learn the microscopic difference in paths.

History

The play of light on soap bubbles attracted the attention of Robert Hooke in the 17th century — these were the first observations of interference. But the instrument was built in 1881 by American physicist Albert Michelson, hoping to use it to detect the mysterious 'ether' in which light was supposedly propagating. The Michelson-Morley experiment found no ether, but proved the constancy of the speed of light. Today, laser interferometers LIGO and Virgo detect the tremors of spacetime from the merger of black holes.

How it works

Imagine two boats on a calm lake, rowing synchronously and creating circles from the oars. If the boats move strictly side by side, the circles meet and form calm and rough areas. The same thing happens in the interferometer: light waves from one source travel along two arms and then combine. If the distance traveled is slightly different, some waves arrive 'peak to peak' and amplify each other, while others arrive 'peak to trough' and cancel. By measuring how much the fringes have shifted, we learn the microscopic difference in paths.

History

Interference phenomena were observed by Newton ('Newton's rings'), but systematic study began with Thomas Young in 1801 (double-slit experiment). The first interferometer was built by Albert Michelson in 1881 to test the hypothesis of the luminiferous ether. The famous Michelson-Morley experiment (1887) showed with the highest accuracy the absence of ether wind, which became a decisive argument for the constancy of the speed of light. In the mid-20th century, Joseph Weber built resonant gravitational antennas, but did not achieve decisive success. Modern laser interferometers (LIGO, Virgo) were proposed in the 1970s-80s and in 2015 for the first time directly detected gravitational waves (from the merger of black holes). Robert Hooke in 1665 described the colors of thin films, which was a manifestation of interference. Alain Aspect used interferometers in pioneering experiments on photon entanglement.

How it works

In the Michelson interferometer, light from a laser hits a beam splitter, where half the intensity is reflected into a perpendicular arm, and half passes straight through. At the end of each arm, a mirror reflects the beam back. After returning, both beams partially pass through the beam splitter and reach a photodetector. The phase difference Δφ = 2πΔL/λ determines whether interference is constructive (light) or destructive (dark). The intensity of the detected light is described by the formula I = I₁+I₂+2√(I₁I₂)cos(Δφ). Any slightest change in ΔL (due to gravitational waves, thermal expansion, or vibrations) leads to a change in the signal. To enhance sensitivity, Fabry-Perot cavities are installed in the arms, greatly increasing the effective light path. Maxwell's equations guarantee the constancy of the speed of light, allowing interferometry to be used as a 'ruler' of exceptional precision.

Open problems & fun facts

Overcoming the quantum limit of sensitivity due to photon shot noise.; Development of compact and stable interferometers for navigation systems (atomic gyroscopes).; Suppression of seismic and thermal noise in ground-based gravitational observatories.

💡 The interferometer suggested to Einstein that the speed of light is always the same for any observer, which later became the basis of special relativity.