|Human beings have not been searching long enough
ability to detect and comprehend intelligent extraterrestrial life has
existed for only a very brief period—from 1937 onwards, if the invention
of the radio telescope is taken as the dividing line—and Homo sapiens
is a geologically recent species.
The whole period of modern
human existence to date (about 200,000 years) is a very brief period on a
cosmological scale, while radio transmissions have only been propagated
Thus it remains possible that human beings have neither
been searching long enough to find other intelligences, nor been in
existence long enough to be found.
One million years ago there
would have been no humans for any extraterrestrial emissaries to meet.
For each further step back in time, there would have been increasingly
fewer indications to such emissaries that intelligent life would develop
In a large and already ancient universe, a
space-faring alien species may well have had many other more promising
worlds to visit and revisit. Even if alien emissaries visited in more
recent times, they may have been interpreted by early human cultures as
This hypothesis is more plausible if alien
civilizations tend to stagnate or die out, rather than expand. In
addition, "the probability of a site never being visited, even [with an]
infinite time limit, is a non-zero value."
Thus, even if intelligent
life expands elsewhere, it remains statistically possible that such
extraterrestrial life might never discover Earth.
Humans are not listening properly
are some assumptions that underlie the SETI search programs that may
cause searchers to miss signals that are present. For example, the radio
searches to date would completely miss highly compressed data streams
(which would be almost indistinguishable from "white noise" to anyone
who did not understand the compression algorithm).
might also use frequencies that scientists have decided are unlikely to
carry signals, or do not penetrate our atmosphere, or use modulation
strategies that are not being looked for.
The signals might be at a
datarate that is too fast for our electronics to handle, or too slow to
be recognised as attempts at communication.
techniques might be employed, but sent from non-main sequence stars
which are searched with lower priority; current programs assume that
most alien life will be orbiting Sun-like stars.
problem is the sheer size of the radio search needed to look for signals
(effectively spanning the entire visible universe), the limited amount
of resources committed to SETI, and the sensitivity of modern
SETI estimates, for instance, that with a radio telescope
as sensitive as the Arecibo Observatory, Earth's television and radio
broadcasts would only be detectable at distances up to 0.3 light years.
detecting an Earth type civilization at great distances is difficult. A
signal is much easier to detect if the signal energy is focused in
either a narrow range of frequencies (Narrowband transmissions), and/or
directed at a specific part of the sky. Such signals can be detected at
ranges of hundreds to tens of thousands of light-years distance.
this means that detectors must be listening to an appropriate range of
frequencies, and be in that region of space to which the beam is being
Many SETI searches, starting with the venerable Project Cyclops,
go so far as to assume that extraterrestrial civilizations will be
broadcasting a deliberate signal (like the Arecibo message), in order to
Thus to detect alien civilizations through their radio
emissions, Earth observers either need more sensitive instruments or
must hope for fortuitous circumstances: that the broadband radio
emissions of alien radio technology are much stronger than our own; that
one of SETI's programs is listening to the correct frequencies from the
right regions of space; or that aliens are sending focused
transmissions such as the Arecibo message in our general direction.
Civilizations only broadcast detectable radio signals
for a brief period of time
may be that alien civilizations are detectable through their radio
emissions for only a short time, reducing the likelihood of spotting
them. There are two possibilities in this regard: civilizations outgrow
radio through technological advance or, conversely, resource depletion
cuts short the time in which a species broadcasts.
idea, that civilizations advance beyond radio, is based in part on the
"fiber optic objection": the use of high power radio with low-to-medium
gain (i.e., non-directional) antennas for long-distance transmission is
wasteful of spectrum, yet this "waste" is precisely what makes these
systems conspicuous at interstellar distances.
Humans are moving
to directional or guided transmission channels such as electrical
cables, optical fibers, narrow-beam microwave and lasers, and
conventional radio with non-directional antennas is increasingly
reserved for low-power, short-range applications such as cell phones and
These signals are far less detectable from
space. Analog television, developed in the mid-twentieth century,
contains strong carriers to aid reception and demodulation. Carriers are
spectral lines that are very easily detected yet do not convey any
information beyond their highly artificial nature.
SETI project is looking for carriers for just this reason, and UHF TV
carriers are the most conspicuous and artificial signals from Earth that
could be detected at interstellar distances.
But advances in technology
are replacing analog TV with digital television which uses spectrum
more efficiently precisely by eliminating or reducing components such as
carriers that make them so conspicuous.
Using our own
experience as an example, we could set the date of radio-visibility for
Earth as December 12th, 1901, when Guglielmo Marconi sent radio signals
from Cornwall, England, to Newfoundland, Canada. Visibility is now
ending, or at least becoming orders of magnitude more difficult, as
analog TV is being phased out.
And so, if our experience is
typical, a civilization remains radio-visible for approximately a
hundred years. So a civilization may have been very visible from 1325 to
1483, but we were just not listening at that time. This is essentially
the solution, "Everyone is listening, no one is sending."
hypothetically, advanced alien civilizations evolve beyond broadcasting
at all in the electromagnetic spectrum and communicate by principles of
physics we don't yet understand.
Some scientists have hypothesized that
advanced civilizations may send neutrino signals. If such signals exist
they could be detectable by neutrino detectors that are now under
If stable wormholes could be created and used for
communications then interstellar broadcasts would become largely
redundant. Thus it may be that other civilizations would only be
detectable for a relatively short period of time between the discovery
of radio and the switch to more efficient technologies.
different argument is that resource depletion will soon result in a
decline in technological capability. Human civilization has been capable
of interstellar radio communication for only a few decades and is
already rapidly depleting fossil fuels and confronting possible problems
such as peak oil.
It may only be a few more decades before
energy becomes too expensive, and the necessary electronics and
computers too difficult to manufacture, for us to continue the search.
If the same conditions regarding energy supplies hold true for other
civilizations, then radio technology may be a short-lived phenomenon.
two civilizations happen to be near each other and develop the ability
to communicate at the same time it would be virtually impossible for any
one civilization to "talk" to another.
Critics of the resource
depletion argument point out that alternate energy sources exist, such
as solar power, which are renewable and have enormous potential relative
to technical barriers.
For depletion of fossil fuels to end the
"technological phase" of a civilization, some form of technological
regression would have to invariably occur, preventing the exploitation
of renewable energy sources.
The evidence is being suppressed
is theoretically possible that SETI groups are not reporting positive
detections, or governments have been blocking extraterrestrial signals
or suppressing publication of detections, perhaps in response to
National Security and Trade Interests from the potential use of advanced
extraterrestrial technology or weapons.
It has been suggested
that the detection of an extraterrestrial radio signal or technology
could well be the most highly classified military information that
Claims that this has already happened are common in the popular
press, but the scientists involved report the opposite experience – the
press becomes informed and interested in a potential detection even
before a signal can be confirmed.
Are we alone in the universe? The dream of answering that question might finally be coming true. For most of this century, astronomers have tried and failed to find evidence of other planets beyond our solar system. The failure of the SETI program to announce an intelligent radio signal after decades of effort has at least partially dimmed the prevailing optimism of the beginning of the space age.
Notwithstanding, the unproven belief in extraterrestrial beings continues to be voiced in pseudoscience, conspiracy theories, and in popular folklore, notably "Area 51" and legends, but has also become a pop culture trope given less-than-serious treatment in popular entertainment.
The SETI program is not the result of a continuous, dedicated search, but instead utilizes what resources and manpower it can, when it can. Furthermore, the SETI program only searches a limited range of frequencies at any one time. In the words of SETI's Frank Drake, "All we know for sure is that the sky is not littered with powerful microwave transmitters".
Drake has also noted that it is entirely possible that advanced technology results in communication being carried out in some way other than conventional radio transmission.
At the same time, the data returned by space probes, and giant strides in detection methods, have allowed science to begin delineating habitability criteria on other worlds, and to confirm that at least other planets are plentiful, though aliens remain a question mark.
The Wow! signal, from SETI, remains a speculative debate. In 2000, geologist and paleontologist Peter Ward and astrobiologist Donald Brownlee published a book entitled Rare Earth: Why Complex Life is Uncommon in the Universe.
In it, they discussed the Rare Earth hypothesis, in which they claim that Earth-like life is rare in the Universe, while microbial life is common. Ward and Brownlee are open to the idea of evolution on other planets which is not based on essential Earth-like characteristics (such as DNA and carbon).
The possible existence of primitive (microbial) life outside of Earth is much less controversial to mainstream scientists, although, at present, no direct evidence of such life has been found.
Indirect evidence has been offered for the current existence of primitive life on Mars. However, the conclusions that should be drawn from such evidence remain in debate.
NOVA: Hunt for Alien Worlds
Suddenly, with improved telescopes and faster computers, we now have the tools to find, for the first time, worlds beyond our own. Follow a new breed of planet hunters as they race to find proof that other planets do exist.
When Will We Discover the Extraterrestrials?
The scientific hunt for extraterrestrial intelligence is now into its fifth decade, and we still haven't uncovered a confirmed peep from any cosmic company. Could this mean that finding aliens, even if they exist, is a project for the ages -- one that might take centuries or longer?
New technologies for use in the Search for Extraterrestrial Intelligence (SETI) suggest that, despite the continued dearth of signals from other societies, there is good reason to expect that success might not be far off -- that we might find evidence of sophisticated civilizations within a few decades.
Why this is so, what contact would tell us, and what such a discovery would mean, are the subject of this talk on the continuing efforts to establish our place in the universe of thinking beings.
This Space Exploration series talk was hosted by Boris Debic.