interferometry

# tag · @ author · ! scientist

Interferometry is a technique that uses the interference of coherent waves for precision measurements. By comparing the phases of superimposed waves, it is possible to detect changes on the order of a wavelength - in the optical range, this is fractions of a micron. It is widely used in metrology, astronomy (VLBI, astrometric interferometer), and fundamental physics.

History

Invented and applied by Albert Michelson in the late 19th century. With his interferometer, he proved that light does not need a special substance (ether) to propagate, and accurately measured the speed of light.

How it works

In an interferometer, a laser beam is split into two. One reflects off a moving mirror, the other off a stationary one. Together they create fringes. The shift of fringes indicates microscopic movement of the mirror, enabling measurement of length, vibrations, and even gravity.

💡 The LIGO interferometer detects mirror displacements of 10⁻¹⁸ meter - that's a thousand times smaller than a proton, which is like measuring the distance to the nearest star with hair's breadth accuracy.

Interferometry is a set of methods in which waves (light, radio waves, atoms) are superimposed, creating an interference pattern sensitive to phase differences. It enables measurement of distance, refractive index, velocity, deformations, and gravitational waves with accuracy unattainable by direct methods. In optical astronomy, using aperture synthesis (e.g., VLTI), angular resolution equivalent to a telescope hundreds of meters in diameter is achieved.

History

Albert Michelson first used an interferometer in 1881 in an experiment to detect the ether; later (1887) with Edward Morley he conducted the famous experiment proving the constancy of the speed of light. In 1920, Michelson and Francis Pease measured the diameter of the star Betelgeuse using a stellar interferometer. The development of lasers in the 1960s made interferometry ubiquitous, and in 2015 LIGO discovered gravitational waves, which earned the Nobel Prize.

How it works

Two or more coherent beams are split, travel different optical paths, and recombine. The output intensity depends on the phase difference: I = I₁ + I₂ + 2√(I₁I₂) cos Δφ. The change in this pattern under controlled displacement allows calculation of the measured quantity. In heterodyne interferometry, the frequency of one beam is slightly shifted, and the signal is read out at the beat frequency. Sensitivity is limited by photon shot noise, and in gravitational-wave antennas additionally by quantum light noise and thermal noise. It is based on the principle of wave superposition and the constancy of the speed of light in a vacuum.

I = I_1 + I_2 + 2\sqrt{I_1 I_2} \cos(\Delta\phi)

I is the resulting intensity, I₁ and I₂ are the intensities of the two interfering beams, Δφ is their phase difference

\theta = \frac{\lambda}{B}

θ is the minimum resolvable angle, λ is the wavelength, B is the baseline (distance between apertures)

Open problems & fun facts

Suppression of thermal and seismic noise in gravitational-wave detectors; Increasing sensitivity without increasing size; Developing broadband interferometers for astrophysics

💡 The LIGO interferometer is so sensitive that it can detect a change in the length of its 4-kilometer arm caused by a gravitational wave, equivalent to an ant walking across the entire Earth, scaled down to the size of an atomic nucleus.

No articles yet

interferometry

History

How it works

History

How it works

History

How it works

Open problems & fun facts

Related articles