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We’ve been looking for messages from the stars ever since Frank Drake pointed the Green Bank
radio telescope at Tau Ceti and Epsilon Eridany 65 years ago. He saw nothing that couldn’t be
explained by natural causes. Nor have the much more extensive SETI surveys conducted since. So,
maybe there are no alien signals to see. Or maybe we need to update how we search for
them. And maybe when we do that we’ll find that the aliens have been talking to us all along,
and the messages are already sitting in our astronomical data archives,
or will be glaringly obvious is the new generation of giant surveys that are just now starting.
it lights up after sun exposure—even in the depths of space. Now on to the episode.
Although it’s not exclusively the case, most major SETI programs have been based on ideas of what WE,
humanity could transmit—if not right now, then in the relatively near future. Well,
we’re now IN the relatively near future compared to the first SETI search of 1960.
So how do we shift our thinking based on what we can now do, and what we now know?
As a launching point, let’s look at a new paper by Ben Zuckerman. Dr. Zuckerman has
been contributing to and commenting on SETI research for many decades. The new paper pulls
together his thoughts on how that search needs to evolve given some updated thinking. Zuckerman’s
proposed strategy updates are centered around making better guesses about what a
technological civilization might do, or at least rejecting some previous unfounded assumptions.
There are three main, interconnected points to consider: transmission technology, how to target
those transmissions; and tying it all together, the energy available for those transmissions.
Starting with transmission technology itself. In the 60s we were just getting
really good at building giant radio antennae. And there were advantages compared to, say,
visible or infrared light transmission. The tech is simpler—just currents running along chunky
wires—versus advanced materials and micro-scale engineering needed for visible and infrared light
detectors and transmitters. Radio frequencies also travel relatively unimpeded through the dustier
regions of interstellar space. So yeah, obviously aliens would be doing interstellar ham radio.
But the big challenge with radio is that it’s hard to send a tight beam. For an EM wave,
the tightest possible beam has a spread that’s equal to the wavelength times distance traveled
divided by the size of the transmitting aperture. An alien 100 light years away who wanted to flood
earth’s entire orbit with a radio signal would need a radio array with a 1000 km baseline.
We can and have built the 1000km-scale baseline radio arrays by now. But even
with this level of collimation, the power of the signal is still enormously spread
out—over an area 140 billion times the surface area of the Earth. It would take
an enormous amount of power to make such a diffuse signal stand out above the galactic
radio background. Early SETI folk guessed that an energy-limited civilization might
solve this issue by channeling their limited power into a very narrow radio frequency band.
That spike in the radio spectrum would then stand out above the galactic background.
Frank Drake even guessed what frequency aliens might use—the so-called water hole.
It’s a 300 Megahertz span where the galactic radio background is weakest,
and right between an oxygen and hydrogen emission lines that Drake guessed formed
a natural framing of a galactic narrow-band ham network. This sort of narrow-band radio
search strategy drove many of the biggest past and present SETI programs. And it
seems a reasonable to try this. Except this strategy hasn’t yielded anything yet. If the
aliens are trying to talk to us, then they’re not doing it how we think they’re doing it.
According to Zuckerman, one of the keys to rethinking our strategy is to take seriously
the fact that any technological alien civilization we’re likely to encounter will have been around
for much longer than we have. That shifts our thinking on each of our three points:
the transmission tech, the targeting, and the energy limits. I’ll come back
to how we may need to shift those. But first, let’s see if we can convince ourselves that
any aliens that we can plausibly notice are almost certainly more advanced than we are.
Now it comes down to the average longevity of a technological civilization. Let’s say that
they tend to last for, I dunno, 10,000 years after becoming able to send noticeable signals
or probes or whatever. We, humanity, are in our first century of this period. That’s the youngest
1% of galactic civilizations. So, 99% would be more advanced than us, and 90% are around 1000
years into their technological phase. So also much more advanced. Even if the civilization
survival time is only 1000 years, most will still be ahead of us. But if that survival time is much
less than 1000 years then a good fraction of civilizations are of comparable tech level to
ours. But in that case then the window of their noticeability is tiny. In order to notice them,
our own 100 years of existence would have to occur within their 100 or so years plus
the travel time for their signals. And that overlap window is a vanishingly
small 1-in-100-million fraction of the 10 billion years of the Milky Way’s lifespan.
Of course, the Milky Way may have many of these razor thin expanding bubbles containing
the desperate hails from newly emergent but soon to be doomed intelligent species. But the hails
themselves are doomed because no species lasts long enough to exist during the passage of such
a bubble. And that could be true, but if so the case is hopeless and we should focus on finding
the signals of species that last longer. Or at least, so also advises Ben Zuckerman.
So, what will an advanced civilization do differently? As a relatively more advanced
civilization ourselves—compared to the 60s and only in very specific
respects—what can we advise regarding our key factors—tech, targets, energy.
Regarding transmission technology: we know how to do other things besides radio. With
modern laser technology we can transmit highly collimated visible and infrared beams of light.
We haven’t built ones big enough to be space telephones, but in principle we know that it’s
possible and can track a path towards this in a way that we couldn’t in 1960.
And that helps with… Targeting. Remember that the spread of a collimated beam is proportional
to wavelength. Visible-light is several tens of thousands of times shorter wavelength than radio,
with proportionally higher collimation. The beam collimation we could achieve with our
1000km radio array can be achieved with a 1meter laser aperture or array. Or, if we
say to hell with it and build a 1000-km laser array—probably an interferometer in space—then
we can focus it to the scale of a single planet rather than the entire planet’s orbit.
But is it even possible to target a single planet 100s of light years away. Well maybe,
yes. In the 60s we had no idea whether there were planets around other stars at all. Now
we know that essentially all stars have them, and that Earth-sized planets around Sun-like
stars are common. The Kepler mission told us that this is a statistical necessity, even though we
haven’t actually found exact Earth-analogs. But we know how to find them in principle.
Most importantly, as emphasized by Zuckerman, we will ultimately be
able to take pictures of exo-Earths. Using space-based coronographs and star-shades
and optical or infrared interferometry, one day we’ll be resolving the surfaces
of Earth-like planets within hundreds of light years to actually see continents and
oceans and alien forests even city lights. Spectral information gathered this way,
especially in the infrared, can even reveal the chemical signatures of an active biosphere.
Even back in the 1980s were gearing up to build the Terrestrial Planet Finder,
which would have been able to image exo-Earths within 50 light years.
It was canned due to politics and JWST costing too much, but now the Habitable
Worlds Observatory will pick up the mission and get us our first pics in the 2030s. So,
if we’re this close to finding out where the local habitable and even inhabited worlds are,
surely a civilization far more advanced than us will have already done so.
And this is one of Zuckerman’s key points. An advanced species in the local region
that cares at all about locating neighbors will already know that we are here. And by
“we” I mean life. Our technological signals are only 100 or so light years out, so only
those within that range know that some primates figured out how to move electrons around wires.
So now, not only does a technological neighbor have the ability to send highly collimated
signals, but they also know where to send them. To the life-bearing worlds in their vicinity.
Which brings us back to the energy issue. An advanced civilization probably isn’t even
energy limited in the way that the original SETI programs assumed. We may be able to assume that
signalers will be pumping more energy into more efficient beams, enabling them to do
various things—like transmitting messages across many frequencies at once, rather than in single,
narrow bands. And they could also get their message to us from much further away.
OK, let’s bring together what we’ve learned. If we follow Zuckerman’s
reasoning, the most likely signals are not narrow-band radio with wide angular spread,
but rather highly targeted beams that could appear at any or all frequencies,
more likely within optical or infrared wavebands. And these could come potentially
from much further away than is possible with energy-limited radio broadcasts.
He suggests a goal of observing all Sun-like stars within around 650 Light years,
with the range being chosen somewhat arbitrarily, but feels like an OK definition of “local” in
our 100,000 light year radius galaxy. In the 650 light year range, there are approximately 60,000
Earth-like planets orbiting Sun-like stars based on our Kepler projections. Zuckerman whittles
this down to a very crude and maybe conservative estimate of 600 that resemble Earth in various
other important factors. Of course we don’t know how close to Earth-like a civilization-spawning
planet needs to be, but erring on the side of conservative is the right way to start.
OK, so let’s say we locate our target planets to monitor. What are we really looking for?
Beyond the manner of transmission, there are also many questions about
what would flag a signal as technological in origin. Many people have thought about this,
and maybe we cover it in another episode. In general, we have a good idea of what natural
sources of electromagnetic radiation look like. They tend to consist of
a combination of a fairly well understood family of spectra—from hot gas and plasma,
from charged particles moving in magnetic fields, from electrons hopping between atomic energy
levels. Frequency spikes in unexpected parts of the spectrum and other unexpected intensity
patterns can signal a non-natural origin. Or we might expect a strange time-dependence—patterns
can also be encoded in how the signal changes over time. Sudden and-or periodic changes
in signal strength that may reveal encoded information not possible for natural sources.
If the signal really does come from another planet, and if the signal has a distinct
frequency structure, then that should at least be clear. It’ll move back and forth
slightly in frequency due to Doppler shift as the planet orbits its star.
The truth is we just don’t know what a technological transmission might look
like and we can’t rely on aliens thinking how we think they should think. We’ve been
teased by weird stuff in the past. Like with the regular pulsation of the first pulsar or radio
spike in the Wow signal—but so far all have been explained as combinations of natural processes.
The hope is that if our astronomical observations get smart enough and broad enough, we’ll know a
technological signal when we see it. But for now, what we need to do is look for anomalies. And
modern SETI is becoming more and mor about anomaly detection rather than targeted searches for the
digits of pi encoded in a doppler-varying water hole spike or whatever we imagine “they” will do.
While it’s good to keep an open mind, the problem with looking for generic anomalies is that the
possibility space is enormous. Back when Frank Drake did his first SETI search it wasn’t possible
to just look for anything weird coming from anywhere. Now though? Now it’s not just possible,
we’re kind of already doing it. For example, the European Space Organization’s HARPS program
uses a telescope in the Chilean Andes to look for exoplanets around nearly 3000 stars by measuring
the tiny Doppler shift in a star’s spectrum caused by the similarly tiny wobble in the
star’s motion induced by orbiting planets. But in a study published last year, Benjamin Fields and
Jason Goodman showed that this same data could be sensitive to laser communication from these
orbiting worlds. They didn’t find anything … but it’s a good proof of concept that technosignatures
may be hidden in data we’re already taking. And maybe in the data we’ve already taken.
This whole idea of piggybacking SETI on existing astronomy surveys is being called commensal SETI,
and it’s something that Zuckerman highlights also. The sorts of signals that he expects,
and many others besides, may be observable in our current and very near future surveys.
Because those surveys are insane. Our ability to put extremely sensitive, gigantic cameras
on ever-larger telescopes, and to put some of those telescopes in space has opened up a new
era in astronomy in general—an era of big data, of all-sky, time domain surveys.
For example, the Rubin Observatory, also in the Chilean Andes, is about to start its 10-year
survey in which it images the entire southern sky every 3 days. The Euclid and Grace Roman
telescopes bring this wide-field capability to the crystal clarity of space, the SKA is
old-fashioned radio, but will be so powerful that it feels like it’s getting close to the “advanced
civilization” radio telescopes that people like Frank Drake were imagining back in the 60s.
These facilities are just beginning or about to begin their work of downloading the universe with
a breadth, depth, and resolution both in space and time that has never been seen before, and that few
imagined back in the 60’s. And while the main science drivers are to characterize the natural
universe, they’re also going to be better SETI programs than any dedicated SETI program to date.
In a paper published in July last year, Eleanor Gallay and team outlined what the possibilities
might be with the Rubin Observatory. The data flow from this survey is gigantic,
and there aren’t enough graduate students in the observable universe to look at every
pixel for every anomaly. But systems are now in place to automatically monitor the
20 terabytes that come from the telescope every night to spot interesting changes.
This is done by teams known as data brokers, who gain quick access to the data and run their clever
algorithms—many of them leaning heavily into machine learning—to look for different types of
“events”. They’re really focused on natural stuff, like supernovae and asteroids. But unnatural
variation can also be picked up, especially with some light patching of the algorithms.
The Gallay team show how the existing alert structure might also be co-opted
to find a variety of technosignatures, and that Rubin-LSST is going to enable a scale
of search that may allow us to spot even the most excruciatingly rare technosignatures.
Commensal SETI isn’t restricted to the optical-IR-focused surveys that Zuckerman’s
paper emphasizes. For example, it really could be that the aliens use radio. The square kilometer
array is currently being assembled in South Africa and Australia and will ramp up its
sensitivity towards its completion date in the early 2030s. The SKA will take as much data every
second as Rubin does every night. That means the challenges are of the same family as for Rubin,
but even crazier. How do you pick out potential signals without having to store every byte?
Several research teams have been working hard towards building the right alert algorithms. If
aliens do send radio signals, the SKA is our best change yet to catch them.
Our eyes on the universe are clearer and wider than ever before. They’ll
show us the beginning and end of the universe and the lives and
deaths of stars and galaxies and strange natural phenomena we’ve never imagined.
But a few of us will be watching that feed from the universe for something
else. Someone else. Signals that we’re not the only intelligence in this corner of space time.
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