In 1967, a postgraduate student at Cambridge University by the name of Jocelyn Burnell
was surveying the sky with a newly constructed radio telescope.
After a few weeks she discovered something odd.
The telescope had picked up a radio signal that seemed to be pulsating.
The pulses had an interval of exactly 1.33 seconds
and were initially thought to be nothing more than man-made interference.
However, it soon became clear the signal did in fact emanate from deep space
and the unwavering precision of the pulses was unlike anything seen before.
As such, many questioned whether it was a naturally occurring phenomenon or a transmission
from another civilization.
The radio source was even named LGM-1, an acronym for little green men,
and Burnell herself could not help but wonder if she'd actually discovered
the first sign of life beyond the Earth.
As you might expect, it didn't take long for natural explanations to emerge
and we now know these pulsating signals to be produced by rapidly rotating neutron stars,
known as pulsars,
that emit beams of radiation akin to the beams of light emitted by a lighthouse.
If nothing else, it made for a neat album cover.
Ever since, an international effort known as the search for extraterrestrial intelligence
has actively been listening for artificial transmissions
but so far we appear to be the only ones broadcasting into the void.
Humanity has been leaking radio signals into space for the better part of a century
so any eavesdropping aliens within about 100 lightyears could potentially be alerted to our presence.
Beyond this invisible radio bubble, enveloping some 15,000 stars, the Earth is just another silent speck.
Not only that but as technology improves, this radio leakage is dramatically reduced.
Before long, Earth may return to a state of radio silence.
If most civilizations improve and eventually outgrow radio technology at a similar pace,
the radio signature of any one civilization may only be detectable for a very brief period of time.
Furthermore, this radio leakage is extremely faint and only grows more and more diffused
as it expands into the galaxy.
Some of the most powerful signals to leak into space are military radar emissions
and stand a much better chance of detection across interstellar distances
than the average television broadcast.
The Square Kilometer Array, a vast interferometer to be constructed in South Africa and Australia,
could be sensitive enough to detect the faint radio signature of an earthlike civilization
out to a distance of several hundred lightyears.
To detect more distant signals would require more deliberate attempts at communication.
For instance, advanced civilizations may construct powerful beacons
specifically to increase their radio luminosity.
These beacons would be rather expensive to maintain as they would consume vasts amounts of energy
for extended periods of time.
A less expensive alternative would be a focused beam of radiation
as opposed to an omnidirectional broadcast.
In 1974, the Arecibo Observatory in Puerto Rico famously beamed an interstellar message
towards a globular cluster some 25,000 lightyears distant.
The message took less than 3 minutes to transmit so any prospecting aliens in the path of that signal
would have less than 3 minutes to detect it, and they will never get a second chance.
We may have been on the receiving end of such an interstellar message when, in 1977,
a momentary burst of energy swept across the planet.
Repeated attempts at redetection notwithstanding, the famous Wow! Signal
was never detected again and its origin has never been conclusively ascertained.
On the slim chance that it was of artificial origin
a reply was beamed in the general direction whence it came in 2012.
In a galaxy as old and vast as the Milky Way, the probability of two civilizations
stumbling upon one another by briefly screaming in random directions is not great.
To increase our chances, we need to limit our selection of targets by searching for other
technological and biological signatures.
In 1995, the first extrasolar planet orbiting a sunlike star was discovered.
We now know the vast majority of the hundreds of billions of stars in the Milky Way galaxy
are home to at least one planet.
We also know that billions of those planets are of similar size to the Earth
and orbit within the habitable zone of a star similar to the Sun.
This means that liquid water could exist on the surface which is an essential ingredient
for life as we know it.
Furthermore, billions of extrasolar moons may also be capable of supporting life.
With this knowledge in mind, in 2017, a message was beamed towards one of the least distant
and most earthlike exoplanets discovered to date.
The message comprised a number of musical compositions and basic information about humankind
and should the planet be inhabited we could expect a reply as early as 2044.
Here's a quick preview of that future.
However, habitability is no guarantee of habitation.
To determine whether a potentially life-supporting planet is currently supporting life
would require a more careful examination.
For instance, by analyzing the starlight passing though the atmosphere of an exoplanet,
it is possible to deduce its chemical composition.
The spectrum of an oxygen-rich atmosphere will differ from that of, say, a hydrogen-rich atmosphere.
Given that photosynthetic plants and organisms are responsible for the large quantities of oxygen on Earth,
an exoplanetary atmosphere with a similar concentration of oxygen could be taken as a sign of life.
This is known as a biosignature.
But it's far from definitive as oxygen could also be the result of various abiogenic processes.
As such, astrobiologists are more interested in certain chemical combinations
as it would be far less likely for, say, methane and carbon dioxide, to coexist in the absence of life.
Speaking of carbon dioxide, some chemicals could be indicative of industrial pollution
and thus artificially induced climate change could serve as the universal sign of unintelligent life.
In addition to biosignatures, we may be able to detect signs of technology.
For instance, an exoplanet surrounded by a dense orbital belt of artificial satellites or space debris
could be detected during transit of its parent star.
This would be an example of a technosignature.
A more extreme example would be a megastructure.
The initial speculation surrounding the discovery of pulsars has become somewhat of a recurring theme.
Whenever an astronomical discovery initially defies explanation,
vigorous speculation about aliens take center stage.
In more recent years, this discussion has been dominated by a peculiar star
some 1,500 lightyears distant.
The star exhibit erratic light fluctuations and occasionally dim by as much as 22%
and so one hypothesis is that an alien megastructure is blocking the light from the star.
While evidence of astroengineering is still within the realm of possibility,
the dimming is now thought to be caused by nothing more than dust.
The existence of circumstellar megastructures was popularized by theoretical physicist
Freeman Dyson back in 1960 as a potential means for advanced civilizations
to harness the energy of their parent star.
Commonly known as a Dyson sphere or Dyson swarm.
According to the Kardashev scale, a method of measuring a civilization's energy consumption
proposed by astrophysicist Nikolai Kardashev in 1964,
a Dyson sphere would be indicative of a Type II civilization.
A Type I civilization could harness the energy of its home planet.
A Type II civilization could harness the energy of its parent star.
A Type III civilization could harness the total energy output of an entire galaxy.
Extensive surveys of billions of stars notwithstanding, there is no reliable evidence
of a Type II nor Type III civilization in the Milky Way galaxy...
...so far.
What about extragalactic life?
After surveying 100,000 nearby galaxies for signs of a Type III supercivilization,
astronomers found no signs of a galactic empire.
The great silence of the universe can be a bit unnerving.
Humanity is now on the verge of detecting biosignatures on extrasolar planets
but Earth has been radiating detectable biosignatures for billions of years,
yet, as far as we can tell, it's failed to attract the attention of any alien astronomers.
The apparent contradiction between the expectation and lack of evidence for extraterrestrial life
is known as the Fermi Paradox.
Given the sheer size and age of the universe, why does it appear to be so lifeless?
It is possible to concoct a myriad of hypothetical solutions.
Perhaps life is common while intelligent life is exceedingly rare.
After all, it took some 4 billion years of evolution and a number of mass extinctions
before humans emerged on Earth.
Proponents of the rare Earth hypothesis suggest that complex life on Earth
is the result of an improbable chain of events unlikely to occur more than once.
Perhaps there exists a barrier, either improbable or impossible for life to overcome.
If the barrier is behind us, we may be among the fortunate few to have crossed it.
Potential candidates include the emergence of multicellular organisms
and the invention of nuclear weapons.
If the barrier is still to come, we may soon join the cosmological graveyard of fallen civilizations.
Potential candidates include unsustainable climate change
and the invention of nuclear weapons.
Perhaps we severely underestimate how truly alien aliens can be.
All life on Earth is carbon-based and rely on water but life could hypothetically be
silicon-based and thrive in oceans of liquid ammonia.
Just as we search for life as we know it, aliens may be doing the same.
Perhaps an advanced civilization did in fact pay us a visit in the distant past.
Unless some sort of evidence of that visitation survived for millions or billions of years,
we'd never know.
Even a visitation in the recent past could have been misconstrued as gods descending from the heavens.
Well, I mean, that would have been a fairly accurate interpretation.
Perhaps all civilizations inevitably develop technology that transcend physical reality.
For instance, reality could be rendered obsolete by hyperrealistic simulations.
By transferring ones consciousness into these virtual wonderlands, one could achieve digital immortality.
The happenstance offerings of nature would struggle to compete with the promise of utopia.
Throughout the galaxy, pockets of advanced civilizations may occupy no more than a few star systems
as they explore inner space in place of outer space.
In his transcension hypothesis, futurist John Smart takes it a step further and suppose this
introverted evolution will progressively miniaturize computers until they are so intensely compressed
that they generate an environment analogous to black holes.
These postbiological civilizations would thus transcend the spacetime continuum
and vanish from the visible universe.
Hypothetically speaking.
Apart from artificial transmissions and distant signs of astroengineering,
there's also a far less remote form of technosignature.
The Voyager spacecraft was launched in 1977 and, as of the making of this video,
it is the only probe to have reached interstellar space.
It famously carries a golden record with information about mankind
but it will take another 40,000 years before the probe encounters another star.
In an effort to more expeditiously explore the galaxy, an advanced civilization may launch
sophisticated interstellar probes capable of self-replication.
These probes would be intelligent enough to mine the available resources
in any given planetary system to create copies of themselves.
These copies would then travel to neighboring systems and create additional copies.
If these probes could travel at just 10% of the speed of light,
a speed attainable by modern methods of propulsion,
every corner of the galaxy could be exhaustively explored in just a few million years.
A relatively short amount of time on cosmological timescales.
Mankind is now fast approaching the technological sophistication to launch such a probe.
This begs the question...
Why is the Solar System so probeless?
If human progress is any indication, the galaxy should be completely overrun
by self-replicating automata, yet we see no evidence of that.
An ongoing hunt for alien artifacts has yet to locate any covert probes lurking in the Solar System
but space is vast.
An object of extrasolar origin could easily be zipping around our cosmic backyard
without our knowledge.
In fact, that is exactly what happened in 2017.
An extrasolar object, named 'Oumuamua, passed through the inner Solar System
and went completely unnoticed until it was moving away from the Sun.
Even then, its discovery was pure luck and thousands of extrasolar visitors
could go undetected each year.
The size and shape of 'Oumuamua can only be inferred by the amount of light it reflects
but it appears to be highly elongated or extremely flat.
Initially, it was thought to be a comet but the absence of a cometary coma,
a trailing cloud of dust and gas typically formed when comets approach the Sun,
lent credence to it being an asteroid.
But then the object began to accelerate.
This acceleration would be consistent with the outgassing of a comet but, as mentioned,
'Oumuamua does display the expected characteristics of a comet.
One possibility is that this outgassing is simply too faint for us to detect but there is that remote possibility
of it being an interstellar probe.
More specifically, a probe using a light sail as a means of propulsion.
On the other hand, 'Oumuamua is perfectly radio silent.
Perhaps it's a defunct probe, destined to aimlessly roam the galaxy, much like Voyager.
It's important to remember that with a sample-size of one, that being Earth,
we cannot possibly know how common or rare life truly is.
There could be hundreds of planets or millions of galaxies between us and them.
If life was discovered elsewhere in the Solar System, perhaps beneath the frozen mantle of Europa,
we'd be able to compare and contrast two distinct sources of life
and gain crucial insight into its commonality or rarity.
Many reject the anthropocentric view of Earth as a cosmological oddity
due to the sheer number of earthlike planets coupled with the resilience of life on Earth
but the truth is, we don't know.
We don't know if billions of planets is sufficient for intelligence to arise more than once.
We don't know the prerequisites for abiogenesis, the transition from non-life to life.
We don't know the probability of cosmic solitude.
But to conclude that we are alone in the face of all this ignorance is more than a bit presumptions.
The extent of our search for extraterrestrial intelligence has been compared to searching
a glass of water for evidence of fish in all of Earth's oceans.
Space is unimaginably vast and we have barely begun to scratch the surface.
There are so many untapped avenues for detecting signs of life that if we just turned that telescope
a bit to the left, made it a bit larger, and listened to a slightly different range of frequencies
perhaps the universe wouldn't be so silent.