Why Radio Telescopes Are So Huge and What They Hear That Light Cannot Show

Visible light is a narrow slice. Of all the radiation streaming through the universe, the band human eyes can detect covers less than a single octave of the full electromagnetic spectrum. Everything else, radio waves, microwaves, X-rays, gamma rays, passes through or around us without making an impression on the retina. For most of astronomy's history, that narrow slice was all we had.

Radio waves sit at the opposite end of the spectrum from visible light. Their wavelengths range from about a millimetre to several metres, compared to the few hundred nanometres that define visible light. That difference in scale is not cosmetic. It is the reason radio telescopes have to be built the way they are, and the reason they find things no optical telescope can.

A telescope's resolving power, its ability to distinguish two closely spaced objects, depends on the ratio of the wavelength it is collecting to the diameter of its aperture. Visible light has short wavelengths, so a mirror a few metres across can resolve fine detail. Radio waves have wavelengths millions of times longer. To achieve the same resolving power at radio frequencies, the collecting dish would need to be millions of times wider.

The Parkes Observatory dish in New South Wales is 64 metres across. The Five-hundred-metre Aperture Spherical Telescope in China, FAST, the largest single-dish radio telescope on Earth, is exactly what its name says: half a kilometre of dish built into a natural karst depression in Guizhou province. Even at that scale, a single dish cannot match the angular resolution of a modest optical telescope.

The solution engineers arrived at is interferometry. Rather than building one impossibly large dish, you build many smaller ones and connect them electronically, treating the entire array as a single instrument whose effective aperture is the distance between its outermost antennas. The Very Large Array in New Mexico links 27 dishes spread across 36 kilometres. The Event Horizon Telescope, which produced the first image of a black hole's shadow in 2019, linked observatories across four continents, Hawaii, Chile, Spain, Greenland, the South Pole, creating an effective aperture roughly the diameter of Earth. The angular resolution that produced was finer than any optical telescope has ever achieved.

Dust is opaque to visible light. The dense molecular clouds where stars condense from gas are essentially walls to an optical telescope, all you see is the surface of the cloud. Radio waves and millimetre waves travel straight through. The Atacama Large Millimeter Array, ALMA, observes at wavelengths just above the radio band and has imaged protoplanetary discs around young stars in detail that reveals the gaps where planets are already forming. Those discs are invisible to any optical instrument pointed at the same patch of sky.

The centre of the Milky Way is another case. From Earth, the galactic core sits behind roughly 25 magnitudes of optical extinction, dust so thick that visible light is attenuated by a factor of ten trillion. Radio waves arrive unimpeded. The supermassive black hole Sagittarius A*, four million solar masses sitting at the centre of the galaxy, was identified through its radio emission decades before any other wavelength confirmed it.

Hydrogen, the most abundant element in the universe, emits a characteristic radio signal at 21 centimetres, a wavelength produced when the electron in a hydrogen atom flips its spin. No optical counterpart exists. By mapping 21-centimetre emission across the sky, radio astronomers charted the spiral structure of the Milky Way from inside it, something geometrically impossible in visible light because the disc of the galaxy obscures its own shape.

Pulsars, rapidly rotating neutron stars that sweep a beam of radio emission across Earth like a lighthouse, were discovered in 1967 by Jocelyn Bell Burnell at Cambridge, using a radio array she helped build by hand. The regularity of the pulses was so precise that the team briefly called the signal LGM-1, for Little Green Men, before the astrophysical explanation arrived. Pulsar timing arrays now use networks of pulsars as a galaxy-scale detector for gravitational waves at frequencies too low for LIGO to hear.

The cosmic microwave background, the thermal afterglow of the early universe, emitted roughly 380,000 years after the Big Bang, is a radio signal. It was discovered accidentally in 1965 by Arno Penzias and Robert Wilson at Bell Labs, who initially blamed pigeon droppings in their antenna for the persistent noise. That noise turned out to be the oldest light in the universe, now stretched by 13 billion years of cosmic expansion into the microwave band.

Fast Radio Bursts, or FRBs, are millisecond flashes of radio energy from extragalactic distances, first detected in 2007 and still not fully explained. Some repeat. Some do not. FAST has detected hundreds. Their dispersion, the way higher frequencies arrive fractionally earlier than lower ones after crossing intergalactic space, is being used to measure the density of matter between galaxies, a quantity that was previously almost impossible to constrain.

The Giant Metrewave Radio Telescope near Pune, operated by the National Centre for Radio Astrophysics, is one of the most sensitive low-frequency radio arrays in the world. Its 30 dishes, each 45 metres across, spread across a 25-kilometre baseline. GMRT has contributed to studies of pulsars, hydrogen in distant galaxies, and the large-scale structure of the universe. At the frequencies it covers, roughly 50 to 1500 MHz, it operates in a range that complements higher-frequency instruments like ALMA and FAST rather than duplicating them.

The Square Kilometre Array, currently under construction in South Africa and Australia, will eventually have a collecting area of one square kilometre distributed across thousands of antennas. NCRA is among the institutions contributing to its design. When SKA reaches full operation, its sensitivity will be sufficient to detect an airport radar on a planet 50 light-years away, not as a search strategy, but as a benchmark for what the instrument can do.

Radio astronomy did not begin as a plan to study the universe. Karl Jansky, an engineer at Bell Telephone Laboratories, was tasked in 1931 with identifying sources of interference on transatlantic radio calls. One source rotated with the sky. It took him a year to confirm it was coming from the direction of the galactic centre. The universe had been broadcasting for 13 billion years. It took a telephone engineer trying to reduce static to notice.