When people think of the scientific detection of of evidence of alien life, a lot of people
think of radio telescopes picking up signals from huge beacons that pulse out prime numbers
or some other unambiguous broadcast.
And for a very long time, looking for that was essentially the only game in town regarding
the search for extraterrestrial intelligence.
But recent improvements in technology have opened up multiple new avenues for detecting
alien life and civilizations independent of radio, greatly improving our chances of actually
picking something up.
So here are ten ways we might potentially detect alien life other than radio signals.
Number 10.
Altered Atmospheres
It’s no secret that life dramatically affects earth as a planet, from its geology to its
atmospheric composition.
But perhaps most markedly from a distance is the oxygenation of the atmosphere through
photosynthesis.
Without the plants, our atmosphere would look radically different, and we wouldn’t be
here.
But other gases being actively created by life on this world also include methane, and,
of course, carbon dioxide which you are exhaling as you watch this video.
This makes Earth’s atmosphere unusual.
As we begin the detailed study of exoplanet atmospheres as our instrumentation improves,
it could be that we see a planet with a similarly odd mix of gases.
While natural processes can produce these gases without life, when you see the right
mix so to speak, such as with Earth, then biology being the origin for that mix becomes
the simplest explanation.
But knowing what that life is really like would be very difficult to determine other
than perhaps being able to reasonably conclude certain things, such as exoplant photosynthesis
creating oxygen.
But, there is also a chance that we might see gases that are not likely to come from
nature, but instead from technology.
One of these are the CFC’s and if we detected those in the spectrum of an exoplanet atmosphere,
we would know there was a technological civilization there, perhaps in the process of destroying
their ozone layer.
Also here is the possibility of an atmospheric curve ball.
We might also see exoplanets with very unlikely atmospheric compositions suggesting that someone
had terraformed those worlds and assembled a custom atmosphere for whatever purpose they
had in mind.
Perhaps a machine civilization might get rid of their old, reactive biological oxygen atmosphere
entirely and replace it with an unnaturally pure inert gas.
Number Nine.
City Lights
Radio emissions aren’t the only type of electromagnetic radiation that human civilization
produces that alien scientists might see if they were studying this planet looking for
unambiguous signs of a civilization.
Another would be our artificial lighting on the night side of our world.
With sufficiently sensitive equipment, our city lights would stick out like a sore thumb
in a spectrum as purely technological in nature.
Likewise, it might be reasonable to expect that aliens might also light up the dark sides
of their world with similarly technological lighting.
This is not yet a feasible technosignature for SETI to look for, our current instruments
wouldn’t pick up earth’s lighting much past our own Oort cloud, but future instruments
might be able to perform such a search.
One speculative variant however might be significantly easier to spot.
If an alien civilization decided to use the light of their star to create daylight conditions
on the dark side of their world using orbital mirrors, that arrangement might currently
be visible in a light curve as the planet transits its star.
Number 8.
Alien Waste Heat
One thing that’s thought to be likely for any advancing civilization is that they will
generate increasing amounts of heat as they consume increasing amounts of energy.
This is true of all alien technologies that we can envision, whether it’s a Dyson swarm
or an ecumenopolis planet.
Waste heat thusly would be detectable to us as infrared radiation.
The problem with detecting these sorts of emissions are that nature also produces lots
and lots of infrared radiation.
For example, young stars still surrounded by protoplanetary disks radiate brightly in
infrared as the material in orbit absorbs solar energy and then reradiates it as heat.
However, there are ways to distinguish natural from artificially produced infrared, for example
if one found a galaxy that emits lots of infrared, but little or no visible light, then this
would be a good indicator that a Kardashev type III civilization has encased the galaxy’s
stars inside Dyson spheres, if such a thing is possible, and in fact searches for this
have been made for this, though so far nothing has been found.
Number 7.
Laser Emissions
In addition to radio, SETI began some time ago to look also for laser emissions emanating
from star systems, known typically as optical SETI.
This makes sense, laser light is very recognizable as technological in nature, and laser also
happens to be really useful in communications.
It’s also useful in things like space-based propulsion, scientific measurement and may
other areas.
Once again, if we find this sort of thing useful, a technological alien civilization
might also, so it’s at least worthwhile to search for evidence of alien laser communications.
The problem with laser SETI however is that it’s very directional meaning that the laser
would have to be pointed at us, either intentionally as a beacon, or accidentally as in a detection
of alien internal communications, or leakage from a light sail, or some other purpose that
happens to be pulsed or emitted in such a way that it can be detected and verified.
You’d be right if you were to say that it’s a long shot that we would detect intentional
alien laser emissions.
In order to point a laser at earth and start sending pulsed communications the alien civilization
would have to know that we’re here.
They might do this if they suspected that earth might be inhabited, and our atmospheric
composition has given away life’s existence here for some time, but it’s unlikely that
anyone could know about our civilization much further out than about 80 light-years, and
even then, our own emissions to date would be very hard to detect.
In short, if they don’t know about us, they probably wouldn’t put the energy into a
laser beacon to contact us.
But say they did.
Just such a detection would be far less ambiguous than what’s possible with radio.
The reason for this is that radio is plagued by two issues, one is the abundance of natural
radio that lots of things in the universe emit, if you turn a radio telescope towards
space you will hear bizarre, difficult to explain signals.
But you can’t say they are of alien origin unless they are absolutely unambiguous.
All you can say is that you saw a weird signal, as is the case for the Wow!
signal.
The other issue is our own radio interference which complicates things further.
We produce a ton of radio emissions and it’s not always easy telling difference between
a signal coming from afar, or one originating from earth.
With laser, these concerns are less of a problem.
Number 6.
Alien Artifacts
This one is an outright trope within science fiction.
Over the decades many stories have been written about humans finding some kind of alien artifact.
These include Clarke and Kubrick’s monolith, the various Prothean artifacts from Mass Effect,
and my own personal favorite, the 20 million year old ship frozen in the arctic ice from
John W. Campbell’s novella “Who Goes There?”
the film adaptation of which was John Carpenter’s “The Thing”.
But in reality, it actually is plausible that at some point we may run into some kind of
alien artifact in our solar system.
This could take many forms, and in fact if we did find one, it may not be what we expect.
Instead of a monolith or Von Neumann probe, it could simply come in the form of an asteroid
that, at some point in the distant past, had been mined by aliens passing through leaving
it mysteriously exhausted of certain materials.
Another might be an accidental artifact, perhaps a dead probe sent out by aliens for scientific
exploration that just happened to enter the solar system just as some of our probes, such
as Voyager II, might some day, millions of years from now, pass through someone else’s
star system long after they ceased their original mission and stopped functioning.
Another possibility is seeing artifacts from a distance, perhaps the remains of a space-faring
civilization might be detected through infra-red emissions, such as a dyson swarm, but with
no evidence that the civilization is still active.
And, of course, there’s always the possibility of an intentional artifact, a kind of self-replicating
or repairing Rosetta stone sent out by a civilization wishing to announce its presence to others
by planting physical messages in all star systems in the galaxy that seem suitable to
eventually develop life.
Number Five.
Satellites and Orbital Rainbow Telescopes
These two concepts are relative newcomers to the field of technosignatures.
The first is that extensive geostationary alien satellite networks might be visible
in light-curves as they transit across the face of their star.
Satellites should be distinguishable from natural ring systems by their overall geometry
as satellites would form more of a belt rather than a disc as natural ring systems typically
do.
The other possibility here is an orbital telescope.
This one has not yet been well fleshed out but the idea of such a thing was put forth
by Marco Quadrelli and colleagues at JPL, link below.
Known as orbiting rainbows, the idea was to create an extremely large telescope in space
by using the optical properties of particulate matter spread out over a large distance, much
like droplets of water bend light to form rainbows.
While this is in itself an interesting idea for us to do here on earth, a way to build
true mega-telescopes straightforwardly, alien telescopes could also be feasibly built and
,if so, such a structure might be visible in the light-curves of stars with exoplanets,
though it might ultimately prove to be difficult to distinguish from ordinary dust.
Number Four.
Arnold Louvers
Perhaps the biggest problem with announcing your presence to the rest of the universe
is that while there are many of the methods for doing so, particularly radio, most would
be very energy intensive, which is made particularly unattractive when you have no idea if you’ll
ever get a response to your signal.
But what of more passive ways to announce your existence?
What of methods that are useful in more ways than just saying hello?
In 2005, astronomer Luc Arnold suggested in a paper, link below, a way of sending a message
simply by having an unnaturally shaped object transit in front of a star.
Any civilizations studying stellar astronomy might then see the weird geometric dip in
light caused the transit of a shaped solid object, such as a set of louvers, that would
not likely exist in nature.
Then that civilization would know that what they were seeing in the light curve was of
alien origin.
The beauty of this approach is that such a megastructure in orbit of a star could be
dual use, such as being both a visual beacon to other civilizations but also a massive
orbital habitat or energy collector.
The aliens then could have their exocake and eat it too.
Number 3.
Unlikely Movement
One thing that can virtually be said for sure is that the universe changes and presents
situations where it might be advantageous, or even critical, for a civilization to move
their own planet, or even their own star.
Hypothetical technologies to do this range from gravitationally migrating planets using
asteroid passes over very long periods of time, something we may well do with earth
long term to avoid the effects of the eventual swelling of the sun into a red giant.
Eventually, through this method very unnatural looking star systems could be designed, or
over millions of years of astronomical observation, a planet might be spotted that is being migrated
to a different orbit unnaturally.
Or even a star might be seen moving.
Enormous shkadov thrusters might be spotted in light curves, where partial dyson spheres
are built nearby to stars to allow them to thrust themselves into different areas of
the galaxy.
Number Two.
Star Tickling
One of the biggest issues with radio SETI is that if you really want to tell the universe
in general that you exist you must essentially beam out very powerful omnidirectional, or
nearly so, signals for a very long period of time to have any hope of success.
In other words, it would be among the most expensive things an alien civilization could
do.
But there is an alternative.
There are objects in this universe that already do all of the above and if you could tweak
them into somehow behaving unnaturally, then you’ve got yourself a more economical beacon.
Of particular interest here are the variable stars.
The idea is that if you could bombard a cepheid variable star with neutrinos, you might be
able to make it pulse unnaturally, perhaps pulsing out prime numbers or some other mathematical
message that shouldn’t occur in nature.
The beauty of this approach is that, while fairly cheap to set up, it’s also fairly
easy to spot.
A lot of data has been taken by astronomers on variable stars over the years, and much
more will come, meaning that to search for such signals one would merely need to look
through data already being taken anyway for variable star study.
Number One.
The Unknown Signal
Ultimately, nothing says that we currently possess the technology to even intercept an
alien communication at all.
Maybe they use none of the above methods, and, simply put, we don’t know everything
about the universe and essentially nothing about advanced alien civilizations.
We do not yet have a full understanding of our universe and can only guess about the
technologies it allows.
Because of this, within SETI there are hypothetical ideas of signals that highly advanced alien
civilizations might produce that we can’t intercept yet, such as using neutrinos or
gravity to communicate, if that’s even possible.
As a result, the galaxy could be teeming alien communications that we simply do not have
the science and tech yet to be able to receive, and some day if we do come upon technologies
like that we might just find that the universe is indeed full of intelligent alien life,
we merely just didn’t know how to see them.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently reminding the interested
that I’ll be on Open Space with Fraser Cain over on his channel later today at 5 pm Pacific,
8 PM Eastern where we’ll take questions and talk about space, science fiction, or
whatever you want.
Literally.
I’ll talk about Exomusks, toast, or octopus intelligence, or just about anything if prodded.
See you there and be sure to check out my books at your favorite online book retailer
and subscribe to my channel for regular, in-depth explorations into the interesting, weird and
unknown aspects of this amazing universe in which we live.