By Boris Sorokin, Radio Spectrum Engineer, SKA Observatory, United Kingdom

Somewhere in the vast depths of space, a photon leaves a distant source, threads across thousands of light-years of empty space, and arrives – faint and ancient – at a radio dish on Earth.

But what if that emission source is not of a natural origin?

The search for extraterrestrial intelligence (SETI) exists because we have decided to take that seriously. For more than sixty years, radio astronomers have pointed their dishes at distant stars to ask the oldest question: Are we alone in the Universe?

SETI might be entering its most ambitious chapter yet — and that chapter depends, entirely, on an increasingly scarce commodity: A quiet radio sky.

Where to look

In 1971, engineer Bernard Oliver suggested a patch of spectrum bounded by natural hydrogen emissions at 1420 megahertz (MHz) and hydroxyl near 1660 MHz — the ingredients of water. He called it the water hole: A natural meeting place, a cosmic echo of the watering holes where species gather. Yet our biology is just one possibility.

Life elsewhere may exceed our boldest imaginings – Stanisław Lem’s sentient ocean in Solaris reminds us that the Universe can be stranger than we are – and even host non-water-based life.

While the radio astronomy service (RAS) operates in bands defined by natural processes, SETI signals may avoid them and instead resemble terrestrial active transmissions. Both natural and SETI signals may also be redshifted, causing notable frequency shifts.

The Breakthrough Listen initiative, launched in 2015 and now headquartered at the University of Oxford, has turned SETI into a global, industrial-scale effort, already scanning a million nearby stars across billions of channels for any signal nature alone cannot explain.

A new era with the square kilometre array

On remote plains in South Africa and Western Australia, two of the most sensitive radio telescopes ever conceived are taking shape.

The SKA Observatory, an intergovernmental organization headquartered at Jodrell Bank in the United Kingdom, is building them. Tens of thousands of antennas working in concert will make the SKA up to ten times more sensitive than any existing arrays, and able to survey the sky hundreds of times faster.

For SETI, those numbers redraw the map of the possible. Today’s instruments can spot powerful transmitters only in our immediate neighbourhood; the SKA will reach a vast stretch of the Milky Way within listening range for the first time.

SETI sits within the SKA’s “Cradle of Life” scientific working group, alongside planet formation and prebiotic chemistry. Precursor telescopes MeerKAT in South Africa and ASKAP in Australia are already searching in parallel with other observations, pioneering techniques the full SKA will inherit.

Challenges ahead

The signals SETI seeks are faint — many orders of magnitude weaker than a mobile phone ping. That is why radio astronomers speak of the “quiet spectrum”. A single poorly filtered transmitter, in orbit or on the ground, can render thousands of observing hours unusable.

Tens of thousands of satellites already circle the Earth, with plans on the books for hundreds of thousands more. Beyond their intentional transmissions, many radiate unintended electromagnetic leakage from onboard electronics. These can outshine real cosmic signals across the sky.

For SETI, this is a double blow: every artificial burst from Earth orbit must be ruled out before a distant candidate can be examined.

In 2019, a narrowband signal from the direction of Proxima Centauri — our nearest stellar neighbour — looked almost too good to be true. Nearly a year of analysis by Breakthrough Listen identified the candidate, dubbed BLC-1, as terrestrial interference.

Without a quiet sky, the likelihood of false positives will inevitably increase.

Decisions that will shape the search: WRC-27

The upcoming World Radiocommunication Conference (WRC-27) being organized by the International Telecommunication Union (ITU) in Shanghai, China, will shape the quiet sky for decades.

Four WRC-27 agenda items matter especially for radio astronomy:

• Agenda item 1.16 considers protecting radio quiet zones around flagship observatories, and globally allocated radio astronomy bands, from aggregate interference from large non-geostationary satellite constellations.
• Agenda item 1.13 addresses direct satellite-to-phone services, with transmitters powerful enough to blind a radio telescope if directly in view.
• Agenda item 1.15 deals with radiocommunications in the lunar region, including the shielded zone of the Moon: A uniquely pristine environment for SETI in bands used on Earth.
• Agenda item 1.18 considers protecting radio astronomy and passive Earth-observation sensors above 76 gigahertz (GHz) from unwanted emissions of active services.

These debates are technical; their stakes are civilizational. Getting the rules right means future generations can still ask the oldest question “Are we alone?”.

Getting them wrong means surrendering our windows to the universe to noise of our own making.

The astronomical community – including the International Astronomical Union’s Centre for the Protection of the Dark and Quiet Sky –ꟷ is working with regulators, operators and space agencies to find compromises in which both connectivity and discovery can flourish.

Are we ready to listen?

A confirmed detection would change everything.

Even without one, SETI is a test of the reach of our instruments, the soundness of our regulations, and our willingness to protect something we cannot yet see.

Somewhere tonight, a photon is already on its way. The next few years – and WRC-27 – will decide whether humanity is listening carefully enough to hear an answer.

Header image credit: SKAO/Max Alexander (SKA-Mid)