Illustrated Physics
physics, drawn
Concept

Interferometer — where it appears

An instrument that splits a wave, sends the parts along different paths and recombines them. The brightness of the result measures the difference in their paths to a small fraction of a wavelength, which is used to measure lengths, test optics and detect gravitational waves.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

A signal that fades as the fourth power of distance. Received power, in decibels relative to free space at one metre, against distance on a logarithmic scale, for a 900 MHz transmitter 30 m up and a receiver 1.5 m up over flat, perfectly reflecting ground, with the ground's reflection taken as −1; dashed, free space alone, falling 20 dB per decade. Close in, the direct and reflected waves beat as their path difference passes through whole wavelengths. Beyond the break distance 4hₜhᵣ/λ = 540 m, the path difference 2hₜhᵣ/d is less than half a wavelength and shrinking, and the reversed reflection cancels the direct wave more and more completely: the power falls 40 dB per decade, as d⁻⁴, 10 dB below free space at ten times the break and 30 dB below at a hundred times. The ground, which was supposed to add a second path, takes the signal away; raising either antenna, which lengthens the path difference, is the remedy, and is why masts are tall.

The surface that cancels whatever skims it

Glass reflects four per cent of light falling straight on it, and water at radio frequencies two-thirds of the wave's amplitude. Tilt either towards grazing and the difference disappears: every surface, whatever it is made of, reflects a wave that only skims it completely, and turns it upside down. A wave travelling along a surface therefore meets its own reflection reversed and cancels on the surface itself. That one limit explains a dark fringe Humphrey Lloyd saw with a single mirror in 1834, why a mobile signal over flat ground fades with the fourth power of distance rather than the second, why a radar above the sea is blind at the horizon, and how a cliff in Sydney became the first radio interferometer.

optics · Refraction Six ways a wave of gravity could move a ring of free masses. Sixteen free masses on a ring (dashed) and where a passing gravitational wave of each of the six possible polarisations moves them at its crest, drawn at a strain of 0.32, some 10²⁰ times larger than a real wave's. Top row: plus and cross, the two general relativity allows, which stretch one direction across the wave while squeezing the other and change no area; and breathing, which swells the ring evenly. Bottom row, drawn in planes containing the direction of travel (arrow): longitudinal, which stretches along the travel; and the two vector modes, which shear those planes. Any metric theory of gravity permits these six and no others; which of them a theory keeps is decided by the field that carries gravity.

The four shapes a wave of gravity leaves out

A passing gravitational wave squeezes a ring of free masses in one of two patterns, and that is usually presented as the whole menu. It is general relativity's choice from a longer one. The tide a wave carries is a symmetric table of six numbers, so any theory in which gravity is geometry allows six shapes; a theory keeps two only if its graviton has no mass and nothing else rides along with it. Give the graviton the smallest mass imaginable and it gains three more states, and one of them changes how much the Sun bends light — by a quarter, however small the mass.

astrophysics · Gravitational waves

Named alongside it

The objects these essays reach for when they reach for this one.

Brewster's angleFresnel equationsGauge freedomGeneral relativityGravitational waveGravitonGrazing incidenceInterferenceLight deflectionLloyds mirrorPhase reversalPolarisation

All concepts