While there are a number of ways currently on the table to search for evidence of extraterrestrial
civilizations, known as technosignatures, one stands out as the focus of a majority
of SETI experiments both past and present.
It’s the radio part of the electromagnetic spectrum.
But why?
What makes that method of communication so attractive over interstellar distances?
Firstly, electromagnetic radiation is easy to produce, we do it all the time with our
own signals in everything from our cell phones to satellite communications to radio stations.
They also travel as fast as you can in the universe, in a vacuum traveling at the speed
of light, because it is light.
Some have said that radio communication isn’t likely to be used by aliens as it’s an old
technology, one we’ve had for about a century.
It’s been pointed out that alien civilizations might not use radio as a method of communication,
rather they may do something far more advanced, such as the use of gravitational waves or
neutrinos for communication.
Or even some method we have no knowledge of yet.
It’s very difficult to look for something for which you have no knowledge of, since
you have no idea what you’re looking fo,r so that’s out, at least for now.
As to gravitational waves and neutrinos, with gravitational waves they would have obvious
advantages as everyone with sensitive enough detectors would see such messages if announcing
your existence is the goal.
But, you need a whole lot of mass and energy for this, you are manipulating gravity after
all.
A lot of it.
Short of moving planets or neutron stars around, this method probably just isn’t worth it
when you simply want to communicate something.
And then there is the problem of receiving such communications, it’s far harder to
pick up gravitational waves than it is picking up a radio signal.
Likewise, neutrinos are also difficult to detect.
Not impossible, but clarity of communication is important and radio wins out here as well.
So as to the unknown or conceptual forms of communication we know about, it becomes a
situation similar to that of fire.
Humans have been using fire since the beginning, it’s a very, very old idea.
Yet we still use it today because, for its applications, it’s still quite useful in
comparison to its alternatives.
Another advantage to radio is that one section of the radio spectrum is naturally very quiet,
whether you’re here or on an alien planet.
It’s an area where you don’t get much interference from the galaxy that you do with
lower frequencies, but you also don’t get the noise from your own equipment that comes
with higher frequencies.
Yet another advantage that aliens would know about is that the dust of the galaxy is fairly
transparent at these wavelengths, unlike in the optical area of the spectrum where it
blocks light quite well.
This all will be the case for advanced aliens anywhere in the galaxy and why they might
indefinitely use radio.
Just because an idea is old, doesn’t automatically make it bad so it still seems like aliens
would understand and use the radio spectrum indefinitely.
This is one of the reasons why the focus has been on radio in SETI.
It simply works.
If some anomaly is ever detected in the world of gravitational waves or neutrinos, then
we’ll see that as well, but it doesn’t seem likely.
The idea of searching for alien signals in the radio spectrum goes back to a paper published
in 1959 by Guiseppe Cocconi and Philip Morrison where they envisioned that alien civilizations
might build beacons to announce their presence to the galaxy.
They proposed that scientists search the nearest sunlike stars for signals, just in case someone
might be out there.
Most importantly, they proposed a very special frequency in which to look.
1420 Megahertz, or the 21 centimeter line.
This frequency is that which hydrogen emits radio waves in nature.
Alien scientists will know this frequency as well as they study radio emissions from
hydrogen clouds in the galaxy.
Hydrogen is the most common element in the universe, it’s everywhere and any species
with science is going to know what it is and how it behaves.
This makes the 1420 mhz frequency an attractive place to locate a signal, since if you just
sent out a signal at any random frequency anyone that you would wish to contact would
need to just happen to tune into that frequency to hear you.
That’s not likely to happen, so a galactic benchmark is needed.
1420 mhz seems like an obvious choice, though modern SETI experiments look at far more frequencies
than just that one.
And there are other choices.
Another example would be the frequency at which hydroxyl emits radio at around 1660
mhz, hydroxyl and hydrogen together make water, the most likely solvent for life in the universe.
And there are many other possible places to locate a signal like that.
But the hydrogen line seems the most likely place.
Now, if we look at the hydrogen line, won’t we see a bunch of interference from hydrogen?
Yes and no.
While hydrogen emits narrowband at the hydrogen line, it only stays there if it’s absolutely
stationary in a lab.
Hydrogen clouds in the galaxy constantly move around causing blue and red shift, as does
earth and the sun, so the end result is a broadband interference around the hydrogen
line.
When you break that down into millions of narrow channels, then comparatively little
interference gets in the way.
One issue with radio however is the earth’s atmosphere.
The various gases in the atmosphere absorb some wavelengths of radio.
This could help or hurt our chances of detecting alien signals.
If the aliens have a completely different atmosphere, or no atmosphere at all as may
be the case for a machine civilization, they may not think to transmit at frequencies that
will pass through an oxygen, nitrogen, water vapor atmosphere like our own.
And, there is some terrestrial radio interference to contend with as well.
Only eventual SETI experiments in space, such as a radio telescope on the far side of the
moon where the moon will block earth interference, and there is no atmosphere to contend with,
will be able to get past these issues and open up other potential areas of the spectrum
for searching.
One interesting thing however is that by international agreement, the 1420 mhz hydrogen line is not
intentionally broadcast on by humans, or at least it isn’t supposed to be, leaving it
free for radio astronomy.
That’s not to say that it’s completely clean, spy satellites often do things they
aren’t supposed to, and you can also get interference due to harmonics of other frequencies
invading the line as well.
But, there is an interesting aspect here, the idea of narrowband signals as signs of
technology.
In general, and only in very specific cases, does nature generate truly narrowband signals.
But technology almost always does.
There are several reasons for this, number one it saves energy.
Broadcasting broadband takes more energy than broadcasting narrowband.
It’s assumed the aliens would know this and be sensible and economical with their
signals.
The second is that narrowband signals don’t bleed all over the radio spectrum, useful
if you want things like radio and television.
Third, in respect to SETI, if you want everyone to know that you’re there, narrowband is
an indicator of a technologically produced signal in and of itself.
It seems logical for an alien civilization to employ this type of signal if they want
to be seen and verified.
The problem though is that narrowband is narrowband, and the more narrowband you get, the more
channels you need to pick it up to the point of millions or more, though signals travelling
through interstellar space and being affected by it do place a limit on what’s practical
here.
The plus to looking at such narrow slices of the spectrum is that any signal found that
narrow would make for an excellent candidate for an extraterrestrial signal.
It’s worth nothing here, though not overstating, that such signals have been found.
The most famous example is the Wow!
Signal of 1977.
The receiver that detected it had only 50 channels, but wow was seen in only one of
them.
It was very narrowband.
Secondly it was very close to the hydrogen line, just slightly above it.
And, no the wow signal was not due to comets, that hypothesis was shot down by the scientific
community fairly quickly after the paper came out.
If comets emitted that strongly at 1420 mhz, radio astronomers would have seen that by
now.
It’s often forgotten how powerful the Wow!
Signal actually was, it was 30 times greater than the background, and was the strongest
candidate signal the telescope that detected it ever found over decades of searching.
The Wow signal stands as the best candidate signal ever detected thus far for a signal
from an alien civilization.
While the design of the telescope essentially eliminated the possibility of it having been
interference from here, there is one thing about the Wow signal that renders it useless.
It never repeated, and unless it ever does, it will forever be a huge question mark.
And, even if it does repeat, it takes us into rather mysterious territory.
There was no modulation in the signal that could be detected, just a raw radio signal
similar to radar.
There was no message in the main signal itself.
But there was an interesting pattern in the other numbers that are circled on the printout.
What those are is unknown, nor is it known if they are related to the main signal, or
are just random other signals.
But unless it repeats, we won’t ever know what those were either.
Wow!
Does illustrate that even narrowband signals in SETI can be ambiguous.
And you’d think aliens trying to announce themselves would try to do it unambiguously.
But it’s not the only candidate signal.
A similar experiment called META in the 1990s found 37 events that were strong signals that
looked good with no immediately obvious natural explanation, but not a single one has ever
repeated.
Modern receivers would have told us much more about these signals.
But in the end, the receiver is only one part of the equation.
To receive anything, someone must be transmitting, and that transmission must cross the vastness
of interstellar space.
The further from the transmitter, the weaker a signal gets, but you don’t need that much
power to transmit if you do it directionally, the power of a radio station for example put
into a directional beam could be seen over a hundred light years away with a modest dish.
But the signal would have to be aimed right at us.
So, how would an alien civilization seeking contact know to do that?
This is where things get interesting.
While human civilization has only been detectable for a short time, about a century,
Earth’s biosphere has been detectable from a distance for millions upon millions of years.
These types of signatures are biosignatures, and in the case of earth include our atmosphere
which is profoundly affected by life.
The alien civilization would see a world that has very odd levels of methane and oxygen.
They might take a closer look in infrared and detect our vegetation, known as the vegetative
red edge where plants become highly reflective in that area of the spectrum.
Essentially, anyone with a big enough telescope in the galaxy may know about this planet,
and know that it has life.
They may send a signal just in case there is someone intelligent here.
But a civilization within 100 light years would be able to detect our civilization directly,
providing they were looking right at us.
If they were watching long term, they would notice changes to the atmosphere that can’t
be natural, such as the addition of CFCs.
They might also pick up our radar, or possibly even our television or radio signals.
Contrary to popular belief however, they would not pick up the 1938 Berlin Olympics broadcast.
That was far too weak.
Some have suggested in the past that civilizations might be so interested in communications that
they might broadcast their signals omnidirectionally.
That would be a daunting task.
Firstly it requires enormous amounts of energy to run a beacon transmitting in all directions.
Sure, it would make it obvious to anyone in the galaxy looking, the broadcast does not
need to be aimed directly at you here rather it’s going out to everyone, the amount of
energy and challenges inherent to broadcasting such a signal make it seem somewhat unlikely.
So what happens when a signal is discovered?
Verification.
One of the first things to do is to determine if it’s earth interference.
This is relatively easy, if you move the telescope slightly away from the source of the signal
you should still see the signal if it was interference from earth.
With something distant, you won’t.
Then you track the motion of the signal itself, if it moves with the earth’s rotation, as
the Wow!
Signal seemed to do, then that eliminates any real likelihood of satellites and other
sources here on earth or nearby it.
And there are other indicators that the signal is far away that can be used.
Once that’s all accomplished and a signal seems to pass muster, the scientists would
simply call other scientists and have them point their radio telescopes at the source
and see if they pick it up as well.
If they do, then such a signal becomes very interesting indeed.
But what happens if and when they do detect a signal?
One comment I often see on this channel is that the government would never let that information
out.
There are many problems with that thinking.
Firstly, which government?
There are nearly two hundred countries on earth, all of which have governments.
And SETI experiments are not limited to any one country.
They are done in Russia, the US, China, and others.
You’d need at least several, if not tens of governments that don’t really get along
that well to all agree to keep the secret.
And then you have to keep the scientists quiet.
Internationally.
SETI in the US is privately funded, contrary to popular belief NASA stopped doing SETI
decades ago when a particular congress person went on a rampage throughout the government
and did everything he possibly could to get funding for it canceled.
It hasn’t been government funded in any meaningful way since.
And you don’t really even need scientists to do SETI searches, there are many private
amateur radio astronomers also building dishes and taking a look.
Point is, this would not be a very keepable secret.
And, in the end, an alien civilization light years away very likely does not pose an immediate
threat to national security.
If they’re close enough to know to send a radio signal, they already know about earth
at least, and probably would have been here with the invasion fleet by now if they wanted
this planet.
And, it’s worth noting that such a hostile civilization wouldn't seem likely to give
up the element of surprise and send out a signal saying they’re on their way.
And, to be honest, there really isn’t anything here for an alien civilization to expend all
the energy to get here to take.
The earth is made of the same materials as the rest of the universe, things like gold
are everywhere.
It would make more sense to mine a local asteroid for that sort of thing rather than spend years
crossing space at great energy expense just to take earth’s gold.
Same with the water, the universe is so loaded with ice that you don’t need to leave home
to find it.
It’s also worth remembering that Earth does not even have the largest liquid water ocean
in the solar system, that distinction belongs to Ganymede.
And even Europa has about twice the amount of liquid water that earth has.
So any water seeking aliens would probably go there rather than here, earth does have
a rather large gravity well that you need to defeat to take the water.
Less so with the Jovian moons.
That said, scientists within SETI have thought out a system of protocols for what should
ideally happen after a detection.
They are common sense, and very extensive.
Essentially once it’s known that the signal is of likely alien origin, and not a false
alarm, but a verified unambiguous signal where alien origin is the best explanation then
the public gets told.
Openly, from the Secretary General of the UN to you and I.
This would be one of the greatest, most profound discoveries in human history, it should and
would be out there.
What else is in the protocols is that no response should be given to such a signal until it’s
internationally agreed on just what to say, if we even think it’s prudent to respond.
And that’s where we get into what may be a huge problem with this whole thing.
We may never be able to decipher the message, if there is one at all.
It may be something creative, like a description of hydrogen similar to our own pioneer plaque
which depicted just that since all civilizations with the science to send interstellar signals
or find wandering probes would know of hydrogen.
Or it may be much more disappointing.
Imagine waking up one day to an announcement of the unambiguous discovery of an alien civilization
in the radio spectrum.
Imagine the scientists proudly stating that we are not alone.
And then would come the questions from the press.
What are the aliens like?
We don’t know.
“What did the message say?”
… “There wasn’t one, it’s just a very powerful raw signal.”
… “Well, what’s that tell us?
The reporters would ask.
The answer might simply be “The aliens have radar.”
And that may be all we ever know about that particular alien civilization.
And that would be poetic, since some of the strongest signals we have ever emitted came
from the Arecibo radio telescope.
There were two types.
One was a radar signal to map some asteroids that never repeated, and the second was an
intentional message to alien civilizations, and it never repeated.
If anyone ever picks either of those up, we will be their Wow!
Signal.
What does that tell us?
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently
addressing the commenters regarding the existential crisis crab rangoon concept from last week.
I defeated the takeout food, and consumed it.
Decisively.
It no longer exists.
But the next day there may have been regrets and be sure to check out my books at your
favorite online book retailer and subscribe to my channels for regular, in depth explorations
into the interesting, weird and unknown aspects of this amazing universe in which we live.