How radio astronomy opened up a new perspective on the cosmos

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One can only imagine what Grote Reber’s neighbors thought when, in 1937, the amateur radio enthusiast erected a shallow tin bowl nearly 10 meters wide in his yard, perched on top of a scaffolding. adjustable and surmounted by an open pyramid of gangly towers. His neighbors could not have known that they were witnessing the birth of a new way of looking at the cosmos.

Reber built the world’s first dedicated radio telescope. Unlike traditional telescopes, which use lenses or mirrors to focus visible light, this craft uses metal and circuitry to collect interstellar radio waves, the low-frequency ripples of electromagnetic radiation. With his homemade device, Reber made the first sky map seen with radiosensitive eyes and launched the field of radio astronomy.

“Radio astronomy is as fundamental to our understanding of the universe as… optical astronomy,” says Karen O’Neil, site manager at Green Bank Observatory in West Virginia. “If we’re going to understand the universe, we really need to make sure we have as many different types of eyes as possible on the universe.”

When astronomers talk about radio waves from outer space, they are not (necessarily) referring to extraterrestrial emissions. Most often they’re interested in low-energy light that can emerge when molecules change their rotation, for example, or when electrons swirl in a magnetic field. Tapping into 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 that we had never seen before.

Today, radio astronomy is a global business. Over 100 radio telescopes – from antennas shaped like spiders crouching on the ground to oversized versions of Reber’s satellite dish that stretch for hundreds of meters – dot the globe. These eyes on the sky have changed the game so much that they have been at the center of no less than three Nobel Prize winners.

Not bad for a domain that started by accident.

In the early 1930s, an engineer at Bell Telephone Laboratories, Karl Jansky, looked for sources of radio waves that interfered with wireless communications. He encountered a hiss coming from somewhere in the constellation Sagittarius, heading towards the center of the galaxy.

Karl Jansky, pictured here with his rotating radio antenna, tripped over a radio whistle coming from the direction of the center of the galaxy, marking the beginnings of radio astronomy.NRAO, AUI, NSF, Jeff Hellerman

“The fundamental discovery that there was radio radiation coming from interstellar space confused the theory,” says astronomer Jay Lockman, also of Green Bank. “There was no known way to get this.”

Bell Labs has moved Jansky to other more land-based activities. But Reber, a fan of all things radio, read 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 the principles used to focus visible light in optical oscilloscopes. He upgraded the Jansky antenna – a bunch of metal tubes held together by a swiveling wooden trestle – and fabricated a metal satellite dish to concentrate the incoming radio waves to a point where an amplifier amplified the weak signal. The whole craft rested on a tilting wooden base that allowed it to 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 in part to the Great Depression and World War II – Reber was largely alone. The field only prospered after the war, with a crop of scientists brimming with new radio expertise from radar system design. Since then, surprises have been waiting for you.

black and white image of the first radio telescope
Grote Reber erected the world’s first dedicated radio telescope – shown here – in his backyard in Wheaton, Illinois.GBO, NSF, AUI

“The discovery of interstellar molecules is a big deal,” says Lisa Young, an astronomer at New Mexico Tech in Socorro. Radio telescopes are well suited for observing the dense, cold clouds in which molecules reside and for detecting the radiation emitted when they lose rotational energy. Today, the list of identified interstellar molecules includes many complex organic compounds, some of which are considered precursors of life.

Radio telescopes have also discovered objects unimaginable before. Quasars, the flaming cores of distant galaxies fueled by giant black holes, first appeared on detailed radio maps of the late 1950s. Pulsars, the spinning ultra-dense cores of dead stars, made their appearance. know in 1967 when Jocelyn Bell Burnell noticed that the radio antenna array she had helped build was picking up a continuous beep… beep… beep from deep space every 1.3 seconds. (She was ignored when the 1974 Nobel Prize in Physics honored this discovery – her advisor got recognition. But an accolade came in 2018, when she received a special breakthrough award in fundamental physics.)

Pulsars are “not just interesting for being a discovery in themselves,” Lockman says. They “are now used to do general relativity tests and detect gravitational waves.” This is because whatever pushes a pulsar – say, a transient ripple in space-time – changes when its ultra-precise radio beats arrive on Earth. In the early 1990s, such temporal variations of a pulsar led to the first confirmed discovery of planets outside the solar system.

More recently, brief blasts of radio energy mainly from other galaxies have caught the attention of astronomers. Discovered in 2007, the causes of these “rapid radio bursts” are still unknown. But these are already useful probes of the substance between galaxies. The light from these eruptions encodes the signatures of atoms encountered en route to Earth, allowing astronomers to track down a lot of material they thought might be in the cosmos but had not yet found. “This is what allowed us to weigh the universe and understand where the missing matter is,” says Dan Werthimer, an astronomer at the University of California at Berkeley.

And it was a radio antenna that in 1964 gave the greatest impetus to the nascent Big Bang theory. Bell Labs engineers Arno Penzias and Robert Wilson were blocked by a lingering hiss in the house-sized horn-shaped antenna they were reusing for radio astronomy. The culprit was the radiance that permeates all of space, left over at 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 astronomers have into the very first universe.

Radio telescopes have another superpower. Several satellite dishes linked together across continents can act as a huge observatory, with the ability to see details much finer than any of these antennas acting alone. The construction of a radio eye as wide as the planet – the Event Horizon Telescope – led to the first image of a black hole.

image of a black hole in the center of the galaxy M87
The Event Horizon Telescope, an international network of radio observatories, took this very first image of a black hole in the center of the galaxy M87.Collaboration with the Event Horizon telescope et al

“Not that anyone needs proof of the existence [of black holes]”Young says,” but there is something so wonderful about being able to actually see it. “

The list of discoveries is long: galaxies in the early universe that are completely enveloped in dust and therefore do not emit starlight still glow in radio images. The rings of gas and dust surrounding young stars provide details of the formation of the planets. 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. “The radio is probably the most likely place where we will answer the question, ‘Are we alone?’ Says Werthimer.

disk forming a red planet around HL Tauri
The ALMA radio telescope array in Chile’s Atacama Desert captured this image of what appears to be a disk forming a planet around the young star HL Tauri.ESO, NAOJ, NRAO

This feeling dates back over a century. In 1899, inventor Nikola Tesla picked up radio signals he thought were coming from people on another planet. And for 36 hours in August 1924, the United States ordered all radio transmitters to be silent for five minutes every hour to listen for transmissions from Mars as Earth circled the red planet at a relatively close distance. . The field got a more official kick-off in 1960 when astronomer Frank Drake pointed Green Bank’s original radio telescope at stars Tau Ceti and Epsilon Eridani, just in case someone was broadcasting.

While SETI has had its ups and downs, “there is a kind of rebirth,” says Werthimer. “There are a lot of new young people entering SETI… and there is new money. In 2015, entrepreneur Yuri Milner pledged $ 100 million over 10 years to research other residents of our universe.

Although the collapse of the giant Arecibo observatory in 2020 – at 305 meters in diameter, it was the largest single-antenna radio telescope for most of its life – was tragic and unexpected, radio astronomers have new installations in preparation. The Square Kilometer Array, which will link small antennas and radio antennas across Australia and South Africa when completed in the late 2020s, will probe the accelerating expansion of the universe, will research signs of life and explore the conditions of the cosmic dawn. “We will see the signatures of the first structures in the universe form the first galaxies and stars,” says Werthimer.

rows of radio telescopes at the Square Kilometer Array
The Square Kilometer Array will connect satellite dishes and radio antennas across South Africa and Australia, providing an unprecedented look into the universe.SKA Observatory

But if the history of radio astronomy is any guide, the most remarkable discoveries to come will be those no one thought to seek out. So much on the field is marked by serendipity, notes Werthimer. Even radio astronomy as a field started by accident. “If you just build something to look at a place that no one has ever looked at before,” he says, “you’ll make some interesting discoveries. “


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