One of the greatest open questions in all of the Universe today is just how abundant intelligent alien life is right at this very moment. We can imagine that extraterrestrial intelligence is common, and that the Milky Way is swarming with civilizations — thousands or even millions of them — just as intellectually advanced or even more technologically advanced than we are. Alternatively, we could be uncommon, where only a scarce few civilizations ever come into existence within the galaxy, and where finding anyone else to communicate with is exceedingly difficult at the moment. At an even greater extreme, they could be very rare, so rare that perhaps we’re entirely alone in the galaxy, and that we’d have to search at extragalactic distances to find anyone else to communicate with, or perhaps in the most extreme scenario of all, we’re completely unique in all the Universe.
Until we find a second example of life of any type somewhere else, we shall remain completely ignorant of what else, or who else, may be out there. But that doesn’t mean we shouldn’t be listening, and on the flip side of that, that we shouldn’t be broadcasting with the intention of being found. While much effort has been focused on SETI, or the search for extraterrestrial intelligence, we’ve put far fewer resources into METI, or messaging extraterrestrial intelligence. If we got serious about that, what would it look like? That’s what our Patreon supporter Jeff Bonwick wants to know, writing in to ask:
“If the governments of the world united and named you Transmitter In Chief, what message and what medium would you choose? Personally I would choose primes as the message because any civilization that discovers math will discover primes, and there is no natural process that produces them. The medium is less obvious — would you go electromagnetic, gravitational, neutrino, or something else? I would be partial to neutrinos because the Universe is transparent to them regardless of redshift, but then again they’re difficult to receive for the same reason. What would [your] “Hello world” look like?”
It’s a great question, and it gives us so much to consider. Here’s what my top priorities would be.
Let’s start by considering the current limitations of what we could actually detect, since that informs what types of signals we should actually consider sending, noting that all signals can travel no faster than the speed of light in a vacuum.
Electromagnetic signals — all the various forms of light — have a huge benefit to them: they’re easy to detect. Photons are easy to generate, and with large enough, sensitive enough telescopes, they’re easy to detect. In fact, any signal that stands out against the background of cosmic noise in any particular wavelength can be seen. The catch is that it takes energy to generate that light, and the shorter-wavelength the light is, the more energy it takes. In addition, light spreads out like a sphere, and so if something is twice as far away, we only get one-quarter of the energy deposited in our telescope. Those are the pros and cons.
While we can consider “particles” in a general sense as an alternative, neutrinos are the only particle signal that it makes sense to send, since all other particles interact with whatever’s in the interstellar (or intergalactic) medium, meaning that such signals inevitably degrade. Neutrinos have the huge advantage (and disadvantage) that they almost never interact at all, meaning that although they’re very hard to detect, a neutrino signal can travel through almost anything, even a star or stellar remnant, and still make it to the intended target.
And gravitational waves, or ripples in the fabric of spacetime itself, might be the most difficult signal of all to detect, but can only have their energy absorbed by very specific configurations, making them the ideal long-distance signal. In fact, if we would build an upgraded LIGO-like detector with ten times the current length baseline, we would be able to detect merging black holes of stellar-like masses from any location within the observable Universe.
Of course, it isn’t just how far a transmitted signal can go that makes it interesting as far as detection goes, but also criteria like:
- How easy is such a signal to produce?
- How much energy is required to produce it?
- How energetic of a received signal is required to be detected?
- And how does the signal strength degrade with distance?
For light, or electromagnetic radiation, these are actually very easy signals to produce, but it’s a very big challenge to create a large-magnitude, sustained signal that goes out in all directions. We normally detect light signals based on the amount of energy that gets deposited in a detector, and the energy of any light signal is directly dependent on that light’s wavelength, and spreads out like a sphere: as the inverse of the distance squared.
Those latter two properties are true of particles (like neutrinos) and gravitational waves, too. The shorter the (de Broglie) wavelength of a neutrino or the shorter-wavelength a gravitational wave is, the more energy it carries, with energy being inversely proportional to wavelength. The energy of any of these signals spreads out like a sphere, degrading as the inverse of the distance squared. For photons and neutrinos, individual particles spread out like a sphere, meaning that fewer of them get intercepted by the same size telescope at greater distances, and for gravitational waves, the wave energy does the same thing.
Now neutrinos have a big advantage at very high energies: their cross-section, or the probability for their interaction, is dependent on their energy. A higher-energy neutrino is much, much easier to detect than a lower-energy neutrino. This is why we’ve never directly detected any of the (very low-energy) neutrinos from the Big Bang; we’ve only seen them indirectly. But thanks to the power of the Nobel-winning IceCube neutrino detector, we’ve been able to detect ultra-energetic neutrinos from sources as far as billions of light-years away. However, because there’s no “obvious” signal that neutrinos emitted from Earth would be identifiable by any other means (like cross-correlating with an electromagnetic signal), a neutrino signal that were intelligently created would not be identifiable as such; we don’t know how to generate a large enough neutrino flux to make that viable.
On the other hand, gravitational waves have an advantage that neutrinos don’t have. Whereas neutrinos are only detectable by their flux, which spreads out as the inverse of the distance squared, gravitational waves are detectable by the wave amplitude, which only falls off as the inverse of the distance, rather than the inverse of the distance squared. This is because gravitational wave detectors are based on the resonance principle, where the energy is proportional to the amplitude squared. If you go ten times as far away, even though the energy signal is just one-hundredth as strong, the wave amplitude only falls off by a factor of ten, giving you a huge advantage at greater distances.
Credit: NASA/JPL-Caltech/NASAEA/ESA/CXC/STScl/GSFCSVS/S.Barke (CC BY 4.0)
Unfortunately, gravitational waves suffer from an enormous problem: they are colossally difficult to detect, requiring multiple masses greater than that of the Sun at distances comparable to the size of a black hole’s event horizon to even be detectable with current technology. We have no way of generating such a large-amplitude gravitational wave signal with anything other than a black hole or neutron star, and those are far beyond our ability to manipulate at the present time. If we could do so, vibrating a few of them back and forth in the appropriate pattern could generate the type of signal that would deposit itself in a device we’re capable of building in a laboratory — such as a Weber bar — but again, that’s far, far beyond the means of current technology, and all Weber bar searches have only turned up null (or non-reproducible) results.
That leaves electromagnetic radiation, which still remains the optimal choice on many metrics. Just like gravitational waves operate on the antenna principle, we can use antennae to detect electromagnetic waves, too. When we talk about creating “pulses” in a signal, which is the way we generally encode information in them, that’s tantamount to varying the amplitude. This is very difficult for high-energy signals, such as gamma rays and X-rays, but it’s very easy for low-energy signals, which is why we’ve traditionally transmitted media, like radio and television, through low-energy and long-wavelength electromagnetic waves. It’s also why it’s no coincidence that we’ve used amplitude-based detectors, like antennae, to receive them.
(Credits: Andreas -horn- Hornig/Gbleem of Wikimedia Commons)
Ideally, then, that’s the lowest-hanging fruit as far as both observing a signal of intelligent extraterrestrial origin and creating a signal fit for observation by an intelligent extraterrestrial observer: the longest-wavelength, highest-amplitude radio waves we can conceivably generate and look for.
On the observational front, this is why SETI so greatly craves new facilities, such as:
- a rebuilt Arecibo observatory, to have a large single-dish antenna that’s ideal for searching for long-wavelength signals and transmitting high-power, long-wavelength ones,
- and the next-generation Very Large Array (ngVLA), which was dubbed a highest priority mission among ground-based facilities by the Astro2020 decadal survey, which would be the ideal upgrade to our ancient, outdated facilities in the search for intelligently-generated radio signals, as well as for pulsar timing in the search for gravitational wave signals and in direct imaging of radio sources.
It might be the oldest, most “classical” way of looking for signals, but there’s a reason for that: it’s the place where the lowest-hanging fruit should be located. Any civilization that reaches the point of communicability — to listen and to send messages — will reach that point in the long-wavelength range of the electromagnetic spectrum first. Because of that very reason, that’s where we should be looking, and that’s where we should seek to send messages if we hope to be heard.
Unfortunately, if we were located pretty much anywhere in the galaxy, with our current technological capabilities, we would not have been able to detect our own planet, even with all the signals we’ve ever generated, at all.
While the movie Contact famously depicted an alien civilization receiving and re-sending Earth’s broadcast from the opening ceremonies at the 1936 Olympics — the highest-power radio signal ever sent at the time — the truth is that no signals sent prior to the late 1950s could even travel to the nearest star and stand out against the background of our radio-loud Sun. The earliest signals capable of exiting Earth’s atmosphere and not being overwhelmed by our Sun’s radio emissions, even temporarily, came from Cold War-era radar transmissions: approximately from 65-70 years ago. Only about 300 total star systems, comprising about one-billionth of the stars in the Milky Way, reside within the radius of the round-trip (32-35 light-years) light-travel time to those signals.
But even at that, those signals weren’t continuous, but were brief, intermittent, and directional. They weren’t repeating, weren’t regular, and wouldn’t necessarily be identifiable as an intelligent signal unless an extraterrestrial civilization happened to understand how to decode the signals we were transmitting. However, we did, at one point, send a signal of:
- high power,
- a constant intensity,
- with a simple, information-rich, constant message,
- at a single, narrowband frequency,
- along a directed, collimated broadcast beam.
That was the Arecibo message: our first intentional broadcast with the intent of being seen by intelligent aliens.
As shown above, even if decoded incorrectly, the Arecibo message, emitted for a period of three minutes in November of 1974, was the first signal that could be capable of actually traveling across the Milky Way: for tens of thousands of light-years. If received by an alien civilization, they would indeed, and rather easily, be able to tell that this was a signal generated intentionally by someone who meant to be found. But even at that, it took the world’s most powerful radio dish, to do it, and the signal reached much less than one square-degree (which itself is just 1/40,000th of the sky) of area, and would reach just a total of perhaps a few hundred thousand stars: mostly in one single globular cluster, which is a disfavored location for potentially habitable planets.
It turns out that the information content of the signal is not particularly important. Any mathematical choice of pulses that doesn’t also occur in pulse-producing objects in nature, including:
- prime numbers,
- a mathematically interesting series of any type (even 1, 2, 3, 4…),
- a musical pattern,
- a Fibonacci sequence (often used in laser pulses),
- something that encoded information about our star, planetary system, or galaxy,
would easily be identifiable as a signal of intelligent origin. Again, the problem is not with “what type of signal should we send to indicate that we’re intelligent?” The fact that we’re intentionally sending an identifiable signal at all is proof of that. The key is to send a signal that persists to the right location.
What you see, above, is a map of the stars in the galaxy. It’s color-coded by what astronomers call metallicity, or the abundance of heavy elements within the parent star. It is that factor, moreso than any others, that determines whether a stellar system might indeed have the potential to possess a habitable planet, at least as far as we understand the concept of “habitability” today. Because the stars with sufficient metallicity are primarily found within a few degrees of the galactic plane, and preferentially towards the galactic center, it makes sense to choose this as the region over which we should target our broadcasts.
Beyond that, what we should take care to do is to cover this region with a message that is all the following:
- Long wavelength and low frequency, in order to to maximize the signal-to-noise ratio for the smallest amount of emitted power.
- Consistent and uninterrupted, to ensure that someone who scans the skies — like we do and have done up until this point — doesn’t happen to scan us while we’re not broadcasting, missing us.
- Over a long period of time, like centuries or more, so that our signal arrives and keeps arriving, not just in one brief burst where, if they aren’t looking at that moment, they’ll miss us.
- And, yes, with a sequence of amplitude-modulated pulses, as well as whatever type of information-rich message we wish to encode (including instructions for where we are and how to communicate back with us) as a more sensitive signal superimposed atop the main, high-power, obvious signal.
Credit: Danielle Futselaar
The disadvantage of this approach is that it requires a sustained investment of resources where a positive outcome is not guaranteed: we will likely have to transmit for a long time, like hundreds of years, before we get our first “hit” or response even in the most optimistic scenario, where intelligent life is common throughout the galaxy. If intelligent life is uncommon, and there are only perhaps a few (or a few dozen) civilizations in the galaxy, the time to wait for a response will likely be thousands or tens of thousands of years, requiring a far longer sustained, substantial investment. And if intelligent life is rare, then the only response we’d ever get would be extragalactic, requiring typical wait times of millions of years or more.
Still, most people talk about SETI as though it’s an enterprise that’s simply failed to detect anything at all, whereas the truth is that based on what we ourselves have been broadcasting, we couldn’t even detect ourselves from almost anywhere within the galaxy. We can only rule out perhaps the most over-optimistic case imaginable: where there would be perhaps a billion intelligent civilizations in the Milky Way right now, and they’d all be looking at us continuously, scrutinizing our world for any sort of signal that could indicate intelligence.
As far as both looking for other civilizations goes and announcing our own presence, the problem is not with the message or the means. The problem is with the effort and the resources we’ve put into the enterprise. If we want to have an actual chance at success, we have an awfully long way still to go.
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This article Ask Ethan: What should humanity’s message to the Universe be? is featured on Big Think.