One can only imagine what Grote Reber’s neighbors thought when, in 1937, the amateur radio enthusiast erected in his yard a nearly 10-meter-wide shallow bowl of sheet metal, perched atop an adjustable scaffold and topped by an open pyramid of gangly towers. Little could his neighbors have known that they were witnessing the birth of a new way of looking at the cosmos.
Reber was building the world’s first dedicated radio telescope. Unlike traditional telescopes, which use lenses or mirrors to focus visible light, this contraption used metal and circuitry to collect interstellar radio waves, low frequency ripples of electromagnetic radiation. With his homemade device, Reber made the first map of the sky as seen with radio-sensitive eyes and kicked off the field of radio astronomy.
“Radio astronomy is as fundamental to our understanding of the universe as … optical astronomy,” says Karen O’Neil, site director at Green Bank Observatory in West Virginia. “If we want to understand the universe, we really need to make sure we have as many different types of eyes on the universe as we possibly can.”
When astronomers talk about radio waves from space, they aren’t (necessarily) referring to alien broadcasts. More often, they are interested in low-energy light that can emerge when molecules change up their rotation, for example, or when electrons twirl within a magnetic field. Tuning in to interstellar radio waves for the first time was akin to Galileo pointing a modified spyglass at the stars centuries earlier — we could see things in the sky we’d never seen before.
Today, radio astronomy is a global enterprise. More than 100 radio telescopes — from spidery antennas hunkered low to the ground to supersized versions of Reber’s dish that span hundreds of meters — dot the globe. These eyes on the sky have been so game-changing that they’ve been at the center of no fewer than three Nobel Prizes.
Not bad for a field that got started by accident.
In the early 1930s, an engineer at Bell Telephone Laboratories named Karl Jansky was tracking down sources of radio waves that interfered with wireless communication. He stumbled upon a hiss coming from somewhere in the constellation Sagittarius, in the direction of the center of the galaxy.
“The basic discovery that there was radio radiation coming from interstellar space confounded theory,” says astronomer Jay Lockman, also of Green Bank. “There was no known way of getting that.”
Bell Labs moved Jansky on to other, more Earthly pursuits. But Reber, a fan of all things radio, read about Jansky’s discovery and wanted to know more. No one had ever built a radio telescope before, so Reber figured it out himself, basing his design on principles used to focus visible light in optical scopes. He improved upon Jansky’s antenna — a bunch of metal tubes held up by a pivoting wooden trestle — and fashioned a parabolic metal dish for focusing incoming radio waves to a point, where an amplifier boosted the feeble signal. The whole contraption sat atop a tilting wooden base that let him scan the sky by swinging the telescope up and down. The same basic design is used today for radio telescopes around the world.
For nearly a decade — thanks partly to the Great Depression and World War II — Reber was largely alone. The field didn’t flourish until after the war, with a crop of scientists brimming with new radio expertise from designing radar systems. Surprises have been coming ever since.
“The discovery of interstellar molecules, that’s a big one,” says Lisa Young, an astronomer at New Mexico Tech in Socorro. Radio telescopes are well suited to peering into the dense, cold clouds where molecules reside and sensing radiation emitted when they lose rotational energy. Today, the list of identified interstellar molecules includes many complex organics, including some thought to be precursors for life.
Radio telescopes also turned up objects previously unimagined. Quasars, the blazing cores of remote galaxies powered by behemoth black holes, first showed up in detailed radio maps from the late 1950s. Pulsars, the ultradense spinning cores of dead stars, made themselves known in 1967 when Jocelyn Bell Burnell noticed that the radio antenna array she helped build was picking up a steady beep … beep … beep from deep space every 1.3 seconds. (She was passed over when the 1974 Nobel Prize in physics honored this discovery — her adviser got the recognition. But an accolade came in 2018, when she was awarded a Special Breakthrough Prize in Fundamental Physics.)
Pulsars are “not only interesting for being a discovery in themselves,” Lockman says. They “are being used now to make tests of general relativity and detect gravitational waves.” That’s because anything that nudges a pulsar — say, a passing ripple in spacetime — alters when its ultraprecise radio beats arrive at Earth. In the early 1990s, such timing variations from one pulsar led to the first confirmed discovery of planets outside the solar system.
More recently, brief blasts of radio energy primarily from other galaxies have captured astronomers’ attention. Discovered in 2007, the causes of these “fast radio bursts” are still unknown. But they are already useful probes of the stuff between galaxies. The light from these eruptions encodes signatures of the atoms encountered while en route to Earth, allowing astronomers to track down lots of matter they thought should be out in the cosmos but hadn’t found yet. “That was the thing that allowed us to weigh the universe and understand where the missing matter is,” says Dan Werthimer, an astronomer at the University of California, Berkeley.
And it was a radio antenna that, in 1964, gave the biggest boost to the then-fledgling Big Bang theory. Arno Penzias and Robert Wilson, engineers at Bell Labs, were stymied by a persistent hiss in the house-sized, horn-like antenna they were repurposing for radio astronomy. The culprit was radiation that permeates all of space, left behind from a time when the universe was much hotter and denser than it is today. This “cosmic microwave background,” named for the relatively high frequencies at which it is strongest, is still the clearest window that astronomers have into the very early universe.
Radio telescopes have another superpower. Multiple radio dishes linked together across continents can act as one enormous observatory, with the ability to see details much finer than any of those dishes acting alone. Building a radio eye as wide as the planet — the Event Horizon Telescope — led to the first picture of a black hole.
“Not that anybody needed proof of the existence [of black holes],” Young says, “but there’s something so marvelous about actually being able to see it.”
The list of discoveries goes on: Galaxies from the early universe that are completely shrouded in dust and so emit no starlight still glow bright in radio images. Rings of gas and dust encircling young stars are providing details about planet formation. Intel on asteroids and planets in our solar system can be gleaned by bouncing radio waves off their surfaces.
And, of course, there’s the search for extraterrestrial intelligence, or SETI. “Radio is probably the most likely place where we will answer the question: ‘Are we alone?’” Werthimer says.
That sentiment goes back more than a century. In 1899, inventor Nikola Tesla picked up radio signals that he thought were coming from folks on another planet. And for 36 hours in August 1924, the United States ordered all radio transmitters silent for five minutes every hour to listen for transmissions from Mars as Earth lapped the Red Planet at a relatively close distance. The field got a more official kickoff in 1960 when astronomer Frank Drake pointed Green Bank’s original radio telescope at the stars Tau Ceti and Epsilon Eridani, just in case anyone there was broadcasting.
While SETI has had its ups and downs, “there’s kind of a renaissance,” Werthimer says. “There’s a lot of new, young people going into SETI … and there’s new money.” In 2015, entrepreneur Yuri Milner pledged $100 million over 10 years to the search for other residents of our universe.
Though the collapse of the giant Arecibo Observatory in 2020 — at 305 meters across, it was the largest single dish radio telescope for most of its lifetime — was tragic and unexpected, radio astronomers have new facilities in the works. The Square Kilometer Array, which will link up small radio dishes and antennas across Australia and South Africa when complete in the late 2020s, will probe the acceleration of the universe’s expansion, seek out signs of life and explore conditions from cosmic dawn. “We’ll see the signatures of the first structures in the universe forming the first galaxies and stars,” Werthimer says.
But if the history of radio astronomy is any guide, the most remarkable discoveries yet to come will be the things no one has thought to look for. So much about the field is marked by serendipity, Werthimer notes. Even radio astronomy as a field started serendipitously. “If you just build something to look at some place that nobody’s looked before,” he says, “you’ll make interesting discoveries.”