Posted by Two ways of listening have dominated sixty-five years of searching, and they have almost nothing in common except the assumption that somebody out there is deliberately trying to be noticed. One uses dishes tuned to frequencies below a few gigahertz. The other watches for flashes of light lasting a billionth of a second. The radio laser debate has never really been about which physics is superior, because both work; it is about which one a transmitting civilization would pick, and that question has no answer we can check.
How the radio search got its head start
The founding document is Giuseppe Cocconi and Philip Morrison’s 1959 note in Nature, which pointed out that the 1420 MHz hydrogen line would be an obvious meeting frequency for two civilizations with nothing else in common. Frank Drake was already building toward the same idea. In April 1960 he pointed the 85-foot Tatel telescope at Green Bank at Tau Ceti and Epsilon Eridani, listened for about two hundred hours across a single narrow channel, and called it Project Ozma.
The equipment has changed beyond recognition since. Modern backends chop the incoming band into billions of channels at once and hunt for signals that drift in frequency the way a transmitter on a rotating planet would. Project Phoenix, which the SETI Institute ran privately from 1995 to 2004 after Congress killed NASA’s programme, worked through roughly 800 nearby stars using Parkes, Green Bank and Arecibo. SETI@home turned several million home computers into a distributed processor between 1999 and 2020. The Allen Telescope Array in northern California, forty-two small dishes designed specifically for this work, has been listening since 2007. Breakthrough Listen has been buying serious time on Green Bank, Parkes and, since 2021, MeerKAT in South Africa, where it rides along commensally with whatever else the array is observing.
Radio holds its position for practical reasons. Dust that blocks starlight is transparent at these wavelengths. Dishes are comparatively cheap per square metre. Beams are wide, so you cover a lot of sky at once, and the data can be archived and re-searched years later with better algorithms.
The problem radio cannot shake
Interference. Everything humans build radiates, and the sky above every telescope is now crossed by satellites, aircraft, radar and phones.
The Green Bank site sits inside a National Radio Quiet Zone with legal restrictions on transmitters, and it still spends much of its processing budget rejecting terrestrial signals. Satellite constellations in low Earth orbit have made the problem worse in the past few years, and radio astronomers have been negotiating with operators over unintended emissions.
BLC1 is the cautionary tale everyone cites. In 2019 the Parkes telescope recorded a narrowband signal near 982 MHz that appeared to come from Proxima Centauri, survived initial checks, and was announced with considerable excitement in late 2020. Careful follow-up traced it to human equipment, and the team published that conclusion in 2021. The search has produced exactly one thing consistently: false alarms that teach you how to run the next test.
There is also the sheer size of the haystack. A search has to pick a direction, a frequency range, a bandwidth, a polarization and a time window. Get any one wrong and a real beacon passes unnoticed.
What optical searches do differently
Robert Schwartz and Charles Townes made the case for light in 1961, only a year after the first working laser. Their argument was about focus: coherent light can be collimated far more tightly than radio, which means far more of the transmitted energy lands on the target.
The detection strategy that grew out of it is elegant. A laser pulse a nanosecond long can outshine its own star by a large factor during that instant, and nothing in astrophysics varies that fast. A telescope does not need to decode anything or even resolve the source — it just needs fast photodetectors and a rule that a real event must register in two separate detectors simultaneously, which rejects cosmic ray hits and electronic noise.
Paul Horowitz ran optical surveys from Harvard through the late 1990s and later operated a dedicated 1.8-metre telescope built to sweep the northern sky for such flashes. NIROSETI at Lick Observatory extended the idea into the near infrared in 2015. VERITAS, an array of gamma-ray telescopes in Arizona with enormous light-collecting area and nanosecond timing, has been used for optical SETI in partnership with Breakthrough Listen. LaserSETI is taking the opposite approach with cheap wide-field cameras designed to watch large parts of the sky continuously rather than one star at a time.
Where each one breaks down
Any radio laser comparison eventually lands on the same asymmetry. Radio trades sensitivity for coverage; optical trades coverage for a clean signal.
A tight laser beam only helps if it is aimed at Earth during the seconds a telescope happens to be pointed back. Nobody knows whether that coincidence is likely, because it depends entirely on the transmitter’s strategy — a scanning beacon that revisits each target star occasionally would be missed by almost every survey ever run. Interstellar dust also scatters and absorbs visible light over thousands of light-years, so optical searches are effectively limited to the nearer parts of the galaxy. The atmosphere adds its own losses and requires clear nights.
Radio’s wide beams cover more sky and reach further through the disc, at the cost of drowning in the noise our own civilization makes, and requiring a guess about frequency out of an enormous range.
On raw information capacity the optical side wins easily. Higher frequencies carry more bits, which is why NASA’s Deep Space Optical Communications demonstration on the Psyche spacecraft has been returning data rates that radio links to deep space cannot approach. That advantage matters for a conversation, not for an announcement.
Nature’s decoys
Both approaches have to contend with astrophysics that mimics technology. The first pulsar, found by Jocelyn Bell Burnell in 1967, was labelled LGM-1 half in jest before its rotating neutron star explanation emerged. Fast radio bursts, discovered in 2007, delivered a fresh round of speculation before magnetars turned out to be a plausible engine. On the optical side, cosmic rays striking a detector produce exactly the kind of instantaneous flash a search is designed to catch, which is why coincidence requirements exist.
Every one of those decoys was eventually explained. That record cuts both ways: it demonstrates the field’s discipline, and it means any future candidate faces a very high bar.
The answer is that the question is wrong
Nobody in the field seriously argues for abandoning one method. A civilization might use both, and a plausible radio laser combination is easy to imagine — a broad, cheap radio beacon to say here we are, followed by a tight optical link carrying the actual content once someone has noticed.
The practical trend is toward searching for everything at once, cheaply, by riding along with observations taken for other purposes. MeerKAT already works this way, and the Square Kilometre Array is being built with commensal searching in mind. Meanwhile all-sky optical monitors watch continuously instead of star by star.
Sixty-five years of null results is not the same as an answer. It maps how little of the search space has been examined, which is the honest way to read it. SETIworld follows both sides of this work — the surveys, the candidate signals and the arguments about where to point next.
