The search for life: what SETI is doing

What it looks for

The core target is narrowband signals. Natural radio sources (pulsars, hydrogen clouds, galactic emission) spread their energy across broad frequency ranges. Concentrating power into a very narrow band is something transmitters do and physics generally does not.

So the search targets a carrier instead of a message: a signal whose structure indicates technology, regardless of content.

Certain frequencies receive extra attention, particularly the water hole: the quiet region between the hydrogen line at 1420 MHz and hydroxyl emission. The reasoning is that this band is relatively free of natural noise and that the components spell out water, which any water-based chemistry might notice. It's an assumption about alien reasoning, and its weakness is acknowledged.

The Wow! signal

On 15 August 1977 a radio telescope at Ohio State recorded a narrowband signal lasting 72 seconds. Exactly the time the target would remain in the beam as Earth rotated, which is what a genuine sky source would produce.

It was strong, narrowband, near the hydrogen line, and appeared to come from a fixed point in Sagittarius. The astronomer reviewing the printout wrote 'Wow!' in the margin, which is how it got its name.

It has never been detected again despite many searches. Explanations proposed include a comet's hydrogen cloud, a reflected terrestrial transmission, and an interstellar cloud of hydrogen stimulated by a passing source; none is established, and objections exist to each.

The fair position: it remains unexplained, and unexplained is not evidence of anything. A signal that occurs once and never repeats can't be verified, so it's a famous anomaly rather than a discovery.

SETI is the most disciplined search for the least likely thing, and I'd fund it before another Mars rover.

Why nothing has been found

The two searches

SETI looks for technosignatures: evidence of technology. The other approach looks for biosignatures: evidence of biology, through atmospheric composition, as attempted with exoplanet spectroscopy and in the subsurface oceans of icy moons.

Biosignature searches are more likely to succeed sooner, because microbial life is presumably far more common than technological civilisation. They're also far harder to interpret: any candidate molecule has abiotic production routes, so the K2-18b claim remains contested.

Both fields have adopted deliberately high standards, precisely because the claim is so consequential and the history of premature announcements is so poor.

Why the discipline is admirable

SETI operates under conditions that would tempt anyone toward overclaiming: an enormous prize, a public appetite for the answer, and decades of null results.

Its response has been the opposite. Candidate signals are published as candidates, follow-up is demanded before announcement, and the field has repeatedly identified its own apparent detections as interference: including signals that turned out to originate from a microwave oven at the observatory.

That's the standard to hold any extraordinary claim to, and it's the direct contrast with the astrology content this journal spends most of its time examining. The difference isn't the subject matter. It is that one field publishes what would falsify it.

What does SETI search for?

Primarily narrowband radio signals: transmissions concentrated into a very narrow frequency range, which natural astrophysical sources don't produce. It searches for evidence of a transmitter rather than for a decodable message.

What was the Wow! signal?

A narrowband radio signal detected in 1977 lasting 72 seconds, exactly matching what a fixed sky source would produce as Earth rotated. It has never been detected again despite repeated searches, so it remains unexplained but can't be verified.

Why has SETI found nothing?

Possibly because the searched fraction of sky positions, frequencies and times remains vanishingly small; because radio may be a brief technological phase; because incidental leakage is far harder to detect than a deliberate beacon; or because nothing is there. These can't currently be distinguished.

What is the difference between a technosignature and a biosignature?

A technosignature is evidence of technology, such as an artificial radio signal. A biosignature is evidence of biology, such as an atmospheric composition difficult to produce without life. Biosignature searches are more likely to succeed sooner but far harder to interpret.