by Seth D. Baum,
Jacob D. Haqq-Misra and Shawn D. Domagal-Goldman
- Seth D.
Baum - Department of
Geography, Pennsylvania State University. E-mail: email@example.com
- Jacob D.
Haqq-Misra - Department of
Meteorology, Pennsylvania State University
- Shawn D.
Domagal-Goldman - NASA Planetary
2011, 68(11-12): 2114-2129
This file version:
22 April 2011
While humanity has
not yet observed any extraterrestrial intelligence (ETI), contact
with ETI remains possible.
Contact could occur through a broad range
of scenarios that have varying consequences for humanity.
many discussions of this question assume that contact will follow a
particular scenario that derives from the hopes and fears of the
In this paper, we analyze a broad range of contact scenarios
in terms of whether contact with ETI would benefit or harm humanity.
This type of broad analysis can help us prepare for actual contact
with ETI even if the details of contact do not fully resemble any
1 - Introduction
not yet encountered or even detected any form of
extraterrestrial intelligence (ETI), but our efforts to search
for ETI (SETI) and to send messages to ETI (METI) remain in
At this time we cannot rule out the possibility
that one or more ETI exist in the Milky Way, nor can we dismiss
the possibility that we may detect, communicate, or in other
ways have contact with them in the future. 1
Throughout this paper we define the term "contact" broadly to
include any way in which ETI has some impact on humanity. This
includes human-ETI interactions that only involve remote
detection or communication without any physical contact.
Contact with ETI would be one of
the most important events in the history of humanity, so the
possibility of contact merits our ongoing attention, even if we
believe the probability of contact to be low.
concern regarding possible contact with ETI is whether the
contact would be beneficial, neutral, or harmful to humanity.
This concern will help us decide, among other things, whether or
not we should intentionally message ETI and what we should say
if we do. The short answer is that we do not know how contact
would proceed because we have no knowledge of ETI in the galaxy.
Indeed, we cannot know for sure until after contact with ETI
actually occurs. Nevertheless, we do have some information that
can help us at least make educated guesses about the nature of
contact with ETI.
Developing and analyzing this information may
help prepare us for contact and increase the probability of an
outcome that we consider favorable.
There have been
many previous analyses of and commentaries on how contact with
ETI would proceed. Unfortunately, this previous work tends to be
quite narrow in the sense of only considering one or a small
number of possible contact outcomes. There appears to be a
tendency to jump to conclusions on a matter which remains highly
uncertain and for which a broad range of outcomes are within the
realm of possibility.
Such narrow and hasty thought ill prepares
us for actual contact. Instead, given the extremely broad range
of possible contact outcomes, we would be much better prepared
by identifying and thinking through a broad range of possible
presents a broad synthesis of available information regarding
the possible outcomes of contact with ETI.
Our work is in the
form of a scenario analysis: we analyze many possible ETI
contact scenarios in terms of whether and how they would harm or
benefit humanity. In the process, we draw upon numerous prior
discussions of contact with ETI that cover a broad range of
possible outcomes, but tend to do so narrowly.
with ETI has been discussed in the scientific literature for
over fifty years  and in science fiction at least since
The War of the Worlds by H.G. Wells in 1898, there has been
relatively little effort to cumulatively analyze the possible
outcomes compared to the synthesis presented here.
To the best
of our knowledge, the only previous broad synthesis is in the
excellent work of Michaud .
The present paper has some
similarities to Michaud's work but also includes several new
scenarios, a different organizational structure that explicitly
organizes scenarios in terms of harms and benefits to humanity,
and new discussion of scenario analysis as a contribution to our
understandings of and recommendations for possible ETI
analysis of ETI contact serves several purposes.
scenario analysis is of strong intellectual interest to the SETI
and METI community and others, given the nuances and challenges
involved in imagining an ETI we have never observed. But this
scenario analysis is of practical value as well. An individual
scenario is a narrative of a possible outcome of, in this case,
contact between humanity and ETI.
Such scenarios can help us
train our minds to recognize patterns in actual outcomes. By
"training our minds" we mean simply that our minds grow
accustomed to thinking about, identifying, and analyzing
specific scenarios and variations of them.
The training process
is thus simply reading and reflecting on the scenarios and the
encounter patterns found in them. The patterns of an actual
encounter may resemble the analyzed scenarios even if the
specifics differ from the scenario details. By training our
minds in this way, we build our capacity to analyze and respond
to actual contact with ETI.
The scenario analysis presented here
thus holds practical value in addition to the noteworthy
intellectual insights that come from considering how contact
with ETI might proceed. Additionally, by considering a broad
range of possible contact scenarios, including some that might
seem unlikely, we improve both the range of patterns our minds
are trained for and the breadth of intellectual insight
This sort of broad scenario analysis can thus be an
especially fruitful process.
We organize ETI
contact scenarios into three basic categories based on whether
the consequences would be beneficial, neutral, or harmful to us.
Although the possibilities surely fall along a spectrum along
these lines, we believe these three bins represent a useful
As defined here, beneficial contact would
be desirable for humanity; neutral contact would cause
indifference for humanity; and harmful contact would be
undesirable for humanity.
A relatively large number of the
scenarios we consider fall within the harmful-to- humanity
category. We thus further divide these scenarios into two
sections in which ETI are either intentionally or
Note that the large number of
harmful-to- humanity scenarios does not imply that contact with ETI is likely to harm humanity.
Quantitative estimates of the
probabilities of specific scenarios or categories of scenarios
are beyond the scope of this paper. Here we focus instead on the
breadth and form of the possible modes of contact with ETI.
Before developing these scenarios, we present some background
information of relevance to the discussion that follows.
2 - Relevant background
information is relevant to many of the ETI contact scenarios
discussed in the rest of the paper and is thus worth considering
separately and in advance of the scenarios.
concerns why we have not yet detected ETI (i.e. the
Fermi paradox), the challenge of interstellar communication, why ETI
are likely to be more technologically advanced than humanity,
what we can learn about the ethics held by ETI from the study of
ethics held by humans, and the possibility of heterogeneity
within an ETI population.
2.1 The Fermi
So far, no
extraterrestrial civilization has been unequivocally
observed by humans.
Nearly 50 years of listening for ETI
transmissions has found no artificial signals in space
[3-4], and the search for ETI artifacts in the Solar System
has also produced null results [5-7].
However, a simple
back-of-the-envelope calculation initially performed by
physicist Enrico Fermi suggests that ETI should be
widespread throughout the galaxy .
advanced ETI civilization could
easily colonize the galaxy to form a Galactic Club among
intelligent societies, a concept popular in science fiction
(such as the "United Federation of Planets" of Star Trek
fame) that in the nonfiction literature dates back at
least to Ronald Bracewell .
This conspicuous absence of
extraterrestrials is often referred to as the Fermi paradox
 or the Great Silence  and raises the question:
if ETI should be widespread, then
where are they...?
number of resolutions to the Fermi paradox have been
proposed and explored [11-12], and three paradox resolutions
are worthy of consideration in our discussion.
resolution to the Fermi paradox is that life, or at least
intelligence, is rare and thus sparsely distributed
throughout the galaxy. This rarity could be because few
intelligent civilizations form  or because intelligent
civilizations tend to have short lifetimes, perhaps because
they quickly destroy themselves [14-15].
If intelligence is
rare, then it is quite unlikely that humanity would have
In the extreme case, humanity is the only
intelligent civilization in the galaxy or even in the
universe. Along the same lines, other intelligent
civilizations may be beyond the physical limits of contact
even if they do exist [15-17].
These scenarios are of
limited value to this paper because they imply that contact
with ETI is impossible.
possible resolution to the Fermi paradox derives from the
challenges of expanding rapidly throughout the galaxy.
Perhaps rapid expansion is unsustainable at the galactic
scale, just as rapid expansion is often unsustainable here
on Earth. This suggests that the absence of
extraterrestrials might be explained by the fact that
exponential growth is an unsustainable development pattern
for intelligent civilizations , a response to the Fermi
paradox known as the Sustainability Solution .
to the Sustainability Solution, rapidly expanding
civilizations may face ecological collapse after colonizing
the galaxy, analogous to the fate of Easter Island .
the other hand, the galaxy could be teeming with ETI that
expand too slowly to have reached Earth yet .
slowly expanding ETI civilizations could still be detected
by us or send us messages, and their nature as slow
expanders has some implications for contact scenarios.
response to the Fermi paradox suggests that ETI are actually
already widespread throughout the galaxy but are somehow
invisible to us. The ETI could be unintentionally invisible,
if it just happens to take some form that is undetectable to
or otherwise undetected by humans.
Alternatively, the ETI
could be intentionally invisible. The intentional form of
this solution is sometimes known as the
Zoo Hypothesis 
because it implies that ETI are treating Earth like a
wildlife preserve to be observed but not fully incorporated
into the Galactic Club.
This idea has been popularized
through the Star Trek series as the "prime directive"
for non- interference with a primitive culture.
Hypothesis thus implies that ETI could make contact with
humans at any time. Perhaps such stealthy ETI will reveal
themselves once Earth civilization has reached certain
They may be waiting until we have reached a
sufficient level of sophistication as a society such as the
start of a METI program or the discovery
of light speed travel [22-23], or they could be applying a
societal benchmark such as sustainable development or
The possibility that the Zoo Hypothesis
explains the Fermi paradox has several important
implications for contact scenarios.
Even if ETI
exist in the nearby galactic vicinity, this does not
necessarily imply that communication with them will be
possible or straightforward. One major challenge is
selecting the frequency at which to broadcast and listen
The electromagnetic spectrum consists of a continuum
of wavelengths for communication that includes radio,
microwave, infrared, visible, ultraviolet, and x-ray bands.
Searching this entire range is a monumental and nearly
impossible task, so we choose particular wavelengths that
seem more probable for interstellar communication.
example, the 21 cm hyperfine transition of neutral hydrogen
was the first suggestion for a communication wavelength .
The water hole
at a wavelength of 18 cm is another
popular choice for SETI , and recent analysis has
suggested that we shift our focus toward higher frequencies
However, because there is an infinite number of
wavelengths for interstellar communication, we must
acknowledge the possibility that ETI may be transmitting or
listening at wavelength ranges that we have not yet
The possibility also remains that ETI do not use
electromagnetic radiation for communication but instead have
discovered some other method (possibly something more
efficient or effective) for exchanging information across
Communication via electromagnetic radiation is limited by
the time required for a signal to reach its destination,
i.e., the speed of light. On Earth, electromagnetic
communication is nearly instantaneous because of the short
However, galactic communication occurs
over astronomical distances so that even a message traveling
at light speed will take a long time to reach its
For example, communication with ETI on a planet
just 50 light years away - which is relatively close by
galactic standards - will still take place on a timescale of
Sagan  notes, this makes communication
with ETI an intergenerational project:
communication across astronomical distances will require
unprecedented cooperation that spans several human
This difficulty in communicating across such vast
distances also might limit the ability for ETI to engage in
interstellar warfare for the simple reason that the
communications problem renders such warfare too logistically
difficult to coordinate ; peaceful endeavors such as the
formation of a Galactic Club may face similar logistical
Such physical limits on interstellar
communication by ETI are in turn limits as to how ETI could
more generally come into contact with and affect humanity.
implication of these long communication times across the
galaxy is that ETI might become alerted to our presence
without us realizing it.
Communication with electromagnetic
waves on Earth has been used for nearly one hundred years,
during which time our radio shows, television
programs, and mobile phone conversations have isotropically
leaked into space. If ETI search for us just as we search
for them, i.e. by scanning the sky at radio and optical
wavelengths for any type of interstellar communication ,
then they might detect our leakage signals.
Advanced ETI within 100 light years could receive our earliest radio
transmissions; those less than 50 light years away could
watch our television shows ; and those less than 10
light years away could receive our earliest intentional METI
Thus, the radiation that has been
unintentionally leaking and intentionally transmitted from
Earth may have already alerted any nearby ETI to our
presence and may eventually alert more distant ETI.
become alerted to our presence, it will take at least as
many years for us to realize that they know we are here.
During the intervening time, ETI can respond to our presence
or prepare for contact in ways that we would have no
knowledge of or influence on.
humanity can successfully exchange signals with ETI, there
is no guarantee that the information will be successfully
In order for information to be exchanged, it
is also necessary that humans and ETI understand the
contents of each others' messages. It will likely be
difficult at first to communicate anything subjective about
human experience, emotions, and expressions, so mathematical
conversation may comprise our first few exchanges with ETI
It may eventually be prudent to develop a framework
for METI so as to increase the probability of successful
communication anytime a transmission is sent from Earth
Perhaps such schemes will succeed in effectively
communicating with ETI.
However, our extreme ignorance about
the nature of any ETI means that we cannot rule out the
possibility that we will fail or at least severely struggle
to exchange information with them.
advanced nature of extraterrestrials
between humans and ETI is possible, then it is important to
consider the capability of ETI to cause us benefit or harm.
This information is important across nearly the full breadth
of contact scenarios. Although we cannot know the level of
technological sophistication achieved by ETI, we do have a
compelling reason to believe that ETI would be significantly
stronger than us and therefore highly capable of causing our
This point has been raised repeatedly
throughout the literature [1,4,14-16,31-33].
to believe that ETI would be more advanced is because humans
and human technology are relatively recent phenomena in the
history of Earth. We have only had radio communication for
about a century, or just a few generations, which suggests
that advanced technology can develop quickly compared to
Following this reasoning, it is
likely that any extant ETI has been around much longer than
us and would have developed far greater technological
abilities than we could imagine for ourselves.
Even if an ETI is younger than us, the very ability to contact us would
likely imply progress beyond that which our society has
We have not yet figured out how to achieve
interstellar communication or travel; a
society that has these capabilities is almost certainly more
technologically advanced than we are.
communications are directed toward a general audience and
not only intended for humans or Earth, then they may also be
more advanced in their ability to communicate across
This is reminiscent of
Arthur C. Clarke's
"any sufficiently advanced technology is
indistinguishable from magic".
If ETI are indeed more
advanced, then any form of contact will likely proceed
according to the ETI's desires, whatever those might be
For example, we are almost guaranteed to lose in a
fight between us and them, and there is a strong likelihood
that such a loss would be so severe that we would cease to
survive as a civilization.
On the other hand, if ETI decide
to use their superior abilities to help us, then they may be
able to help solve many of our problems.
Extraterrestrial ethics - Selfishness and universalism
noted above, if ETI are significantly more advanced than
humanity, then the outcome of contact may depend primarily on
However, this leaves open speculation as to the specific desires
of ETI and raises the question of what ethical framework they
follow. Much can be said about ETI ethics. Here we focus on one
key aspect: selfishness vs. universalism.
In rough terms, a selfish ETI is one that desires to maximize
its own self-interest, whereas a universalist ETI is one that
desires to maximize the interests of everyone, regardless of
which civilization they are part of. But this is a crude
explanation of selfishness and universalism; more precision is
needed for our purposes in this paper.
As a starting point, it is helpful to think of ETI as trying to
maximize some sort of value function. 2
2 - The
discussion here is derived from the more detailed discussion
found in the work of Baum. 
Specifically, they are trying to maximize intrinsic value,
which is something that is valuable for its own sake. Intrinsic value contrasts with extrinsic value, in
particular instrumental value, which is valuable because it
causes additional value.
One can place intrinsic value on
many different things, such as life, ecosystems, happiness,
knowledge, or beauty. Human ethics is often anthropocentric
in the sense that it places intrinsic value only on human
phenomena, such as human life, human happiness, or other
Such anthropocentrism is selfish on a civilizational scale because it involves humans only placing
intrinsic value on the interests of their own civilization.
In contrast, a universalist ethical framework would place
equal intrinsic value on certain phenomena regardless of
which civilizations possessed these phenomena.
a universalist civilization that places intrinsic value on
life will place equal intrinsic value on all life,
regardless of which civilization (or non-civilization) the
life is part of. In this case, the civilization will try to
maximize the total amount of life, regardless of whose life
it is maximizing.
If instead it places intrinsic value on
some phenomenon other than life, then it will try to
maximize that phenomenon wherever it occurs.
between humans are often, though not necessarily always,
rooted in selfishness. These conflicts include struggles for
power, land, resources, prestige, and many other instruments
Even when human conflicts have overtones
of being for some higher purpose, such as for liberty or
against oppression, the basic desire for the survival and
flourishing of the self often remains a core motivation.
Likewise other conflicts we see throughout the sentient
animal kingdom appear to be motivated by the desire for
instruments of self-interest such as survival, food, or
While non-sentient species (animal or
otherwise) may also appear to act in their own
self-interest, it is inappropriate to attribute intent to
them because intent is presumably a property of sentience.
It is worth
noting that the analysis in this paper is in a sense selfish
in that it focuses on benefits and harms to humanity.
Throughout the paper, we do not consider how contact with
humanity could benefit or harm either the ETI or any other
entities affected, including other entities on Earth and
elsewhere in the galaxy.
By focusing on benefits and harms
to humanity, we do not intend to advocate for a selfish
ethics. Instead, this focus is simply an expository tactic
aimed at keeping this article reasonably concise.
view, consideration of impacts of contact to nonhumans is
important and would be well worth considering in future
scenario analysis presented throughout this paper assumes
that any given encounter will follow one general trajectory.
The encounter might benefit, be neutral to, or harm humanity
for a certain reason, but the encounter would only have one
of these outcomes and follow one general trajectory to reach
this outcome. This follows from the idea of a homogenous ETI, i.e. an ETI with one defining attribute or combination
of attributes that dominates the encounter.
could be the ETI's strength, ethics, politics, or something
If it is the case that the ETI has one defining
attribute or combination of attributes, then it is
reasonable to expect one general trajectory for the
encounter. However, this requires a homogenous ETI
possible that an ETI would have a heterogeneous population
instead of a homogenous one. Evidence for this can be found
in the human population, which features a highly diverse mix
of technological abilities, ethical views, national
identities, and other attributes.
For example, in the event
of an ETI encounter, humanity may be fiercely divided on
whether to respond peacefully or with protective aggression.
ETI may be similarly divided.
At a minimum, humanity's
diversity provides proof of the principle that intelligent
civilizations can be heterogeneous.
possibility of ETI heterogeneity suggests that an encounter
might not follow one general trajectory but instead could
have multiple trajectories in series or perhaps even in
For example, an encounter could rapidly change
form if a shift in power occurred within the ETI leadership.
Or, we might receive mixed signals from the ETI if it lacks
a single unified leadership
structure; perhaps several ETI factions or nations that
originate from the same home world will make contact with
us, each in pursuit of different objectives.
possibilities of ETI heterogeneity and multiple trajectories
are worth keeping in mind when considering the specific
encounter scenarios that could occur.
considered these points of background information, we can
now proceed to specific scenarios of contact between
humanity and ETI.
An overview of these scenarios is provided
in Fig. 1.
Overview of the contact scenarios
presented in this paper.
3 - Beneficial to
optimistic scenarios assume that contact with ETI would somehow
benefit humanity (Figure 1, left column).
These scenarios are
broadly popular: survey results have shown that many people
across the world anticipate that contact with ETI will benefit
humanity in some way [36-37; see
The nature of this benefit could range from simple
remote detection of intelligent life elsewhere to more extensive
contact with cooperative ETI. There is also at least one set of
scenarios in which we benefit from contact with uncooperative ETI.
While we cannot know whether an ETI would be cooperative,
we present some reasons to suspect that they would be
cooperative by developing in some length an argument based on
the Sustainability Solution to the
detection of ETI refers to scenarios in which the entirety
of contact is limited to the discovery that ETI exist.
other words, we detect the presence of ETI and thus can
confirm their existence but have no further contact. This
means no communication, direct contact, or any other
possible contact mode.
Here we argue that mere detection
would provide a nontrivial benefit to humanity.
If ETI do
exist within the galaxy, then confirmation of their presence
would have profound implications for human science,
philosophy, religion, and society. This point has been noted
repeatedly throughout the literature [15,33,39-41].
ongoing SETI activities are based to a large degree on the
premise that humanity wants to learn about ETI. One reason
for this is that the discovery of ETI would answer the deep
and longstanding philosophical question of whether we are
alone in the universe.
This in turn relates to the question
of our role in the universe as intelligent beings. Humanity
has a strong interest in obtaining answers to these major
questions and thus would benefit tremendously from the mere
detection of ETI.
might consider mere detection to be harmful to humanity.
These people include those with religious perspectives and
other worldviews that depend on the idea of humanity (and
Earth-life more generally) playing a unique and privileged
role in the universe (e.g., [42-43]).
The detection of ETI
could challenge these worldviews and therefore be perceived
as harmful by those who hold such beliefs.
perception of harm depends on a philosophical mistake. The
existence of ETI in the universe is independent of whether
or not they have been detected by humanity. It is the
existence of ETI that challenges such worldviews and not the
act of detection.
If ETI do in fact exist, then the harm has
already been done in the sense that such worldviews are
Detection simply alerts us to this
invalidity. This alert itself might be classified as a
benefit or harm, because of its affects on the wellbeing of
those whose worldviews are challenged with the discovery of ETI, but this is seemingly a lesser matter than the broader
benefits of mere detection.
troubling is the possibility that detection could initiate
or exacerbate conflicts in our society.
The conflict could
be over how to interpret or reply to such a discovery. There
are already disagreements over how to message to ETI,
whether or not we should, and who should speak for
humanity; such disagreements would become much fiercer if
ETI were detected.
Meanwhile, the groups whose worldviews
would be challenged could respond in harmful ways if they
feel threatened, nullified, or otherwise worsened by the
discovery or the intent to reply.
While we hope that
detection would unify humanity towards positive outcomes,
the opposite result remains entirely possible.
detection of ETI would be beneficial for the insight it
offers, these benefits could be limited. That is, mere
detection would leave much of humanity's situation intact.
Perhaps mere detection would be on par with the Copernican
revolution in that it would change human thought but not
radically alter our geopolitics .
So while mere
detection may offer net benefits, these benefits are likely
not very large, especially relative to the benefits and
harms found in many other contact scenarios.
of their magnitude, the impacts of mere detection serve as a
baseline set of impacts for almost all other contact
scenarios. This is because nearly all other contact
scenarios involve detection along with other forms of
The exceptions here are contact scenarios that do
not involve detection, which include scenarios in which ETI
manipulate our world (in good ways or bad) while hiding and
scenarios in which ETI destroy us without our having the
opportunity to notice the ETI.
These scenarios are discussed
Even if we
receive no more than a simple greeting or passive artifact
from a distant ETI civilization, it will at least tell us
that life has developed more than once in the galaxy and
that human-like technology to broadcast across space has
been invented elsewhere.
Advanced ETI may have little to no
interest in a society as primitive as Earth, but if they do
acknowledge our presence and initiate communication, then
even this knowledge will benefit humanity.
with ETI involves more than mere detection, then it is
possible for humanity to receive additional benefits by
cooperating with the ETI.
The nature of these benefits
depends on the degree of ETI cooperation - that is, it is
unlikely that uncooperative ETI would benefit humanity. This
is because ETI are likely to be much more advanced than
humanity and would therefore be capable of dictating the
terms of contact.
Thus cooperative ETI would have the
ability to bring benefits to humanity, just as uncooperative
ETI would likely harm humanity.
scenario of cooperative ETI involves friendly and
informative communication between our respective
Assuming ETI are sufficiently interested in
humanity (which is not guaranteed, given that they would
likely be much more advanced), they may choose to maintain
communication at length to discuss mathematics, physics, and
chemistry  and to learn more about Earth life.
reasonable to assume that the general principles of physics
and chemistry apply
everywhere in the galaxy, even if mathematical descriptions
of these physical phenomenon differ among intelligent
This type of dialog with ETI may require that
we first develop a common mathematical language using
physical observables that are known by both civilizations
(such as properties of neutral hydrogen).
In a more
remarkable and unlikely case, we may learn that ETI occupy
some region of space where different or unknown physical
principles apply, which would certainly be a unique
discovery for humanity.
Thus through such a conversation we
may come to acquire a deeper understanding of mathematics or
science, and we may also discover specifics about the ETI
home world or ETI biology.
As with mere detection, such
contact would have considerable intellectual benefits,
though here the benefits would be larger - potentially much
on the nature of information shared through communication
with ETI, there could also be more in the way of practical,
non-intellectual benefits. An advanced ETI may be capable of
solving a great many of humanity's problems, such as world
hunger, poverty, or disease.
Benevolent ETI may even design
their first message to contain information on how to avoid
technological catastrophe in order to help less developed
civilizations succeed .
From humanity's perspective,
this is the best-case scenario for ETI contact. However,
while we suspect that the basic principles of physics and
chemistry apply across the universe, it is somewhat less
likely that ETI knowledge would be useful in addressing
social issues on Earth.
The usefulness of ETI knowledge,
combined with the willingness of ETI to employ it on our
behalf, plays an important role in the benefits that a
cooperative ETI would bring to humanity.
We do not
know if ETI would be cooperative, but we have several
reasons to suspect that they would be. Noncooperation can be
a risky and harmful strategy, and non-cooperative
civilizations may tend to have shorter lifetimes as their
noncooperation eventually leads to their demise.
reason, a long-lived civilization that explores the galaxy
may have transcended any aggressive patterns out of the need
to maintain long-term survival [36,46].
It is also possible
that intelligent civilizations may inevitably develop
cooperative tendencies as part of their evolutionary process
However, there are also reasons to suspect that
evolution would proceed along different, less desirable
reason to suspect that ETI would be cooperative follows from
the Sustainability Solution to the Fermi paradox. A
corollary of the Sustainability Solution is that extant ETI
civilizations in the galaxy may be less prone to violence
and destruction in the event of contact.
follows from the tendencies of sustainable human
sustainable human populations tend to be more protective of
their ecosystems. This protectiveness can be for either of
First, humans can protect ecosystems for their
own benefit. This protection is known as conservationism and
involves humans placing intrinsic value on themselves.
Second, humans can protect ecosystems for the ecosystems'
benefit. This protection
is known as
preservationism and involves humans placing
intrinsic value on the ecosystems. (See  for a similar
approach to environmental ethics in the context of
In either case, human populations that
follow a sustainable mode of development are less likely to
expand for lack of resources, although they may choose to
explore out of sheer curiosity.
ETI populations may be
similar in this regard .
Thus, if exponential growth is
in fact unsustainable on the galactic scale as Haqq-Misra
and Baum  suggest, then we are much more likely to
encounter a long-lived ETI civilization that follows a
sustainable development pattern.
Such a civilization may
have no need to consume Earth systems (or humans) because
they will have already found a way to effectively manage
their resources over long timescales.
possible unsustainability of long-term rapid expansion
decreases the probability that ETI will destroy us. However,
there is a scenario in which sustainable ETI would destroy
us - specifically if the ETI is expanding at the maximum
rate possible given its sustainability constraints.
This "maximally expansive" scenario is one of the
humanity" scenarios discussed below.
ETI are likely much more advanced than human civilization,
contact with uncooperative ETI seems likely be harmful to
Harm from uncooperative ETI is discussed in detail
in Section 5.
However, there are certain scenarios in which
contact with uncooperative ETI would benefit humanity. These
are scenarios in which ETI attempts to harm us but fails.
Perhaps the ETI, no matter how strong or powerful, just
happen to be vulnerable to something humanity has.
illustrated in the conclusion to The War of the Worlds,
where the invading Martians are rendered helpless by
infection by Earth microbes. Or perhaps humanity somehow
goes against the odds and defeats the ETI.
scenario is widespread throughout science fiction, including
in major Hollywood films such as Independence Day (1996).
In these scenarios, humanity benefits not only from
the major moral victory of having defeated a daunting rival
but also from the opportunity to reverse engineer ETI
A final scenario involves a second ETI learning
of our situation and coming to our rescue, again leaving us
better off than we were to begin with. Scenarios such as
these might make for quality entertainment, but they also
appear rather unlikely.
Still, such contact scenarios are
possible and thus worth including in this analysis.
4 - Neutral to us
Another set of
scenarios involves contact with ETI that are neutral toward us
(Figure 1 above, center column).
Neutral here means that humanity is
indifferent to contact with ETI: we are just as well-off with it
as we are without it. There are two fundamental ways in which
ETI could be neutral.
The most straightforward way is that ETI
have no impact on us at all.
Here it is important to
recognize that ETI would have a profound impact on humanity if
we simply become aware of its presence - that is, through mere
detection, as discussed above. Indeed, the discovery of ETI
could well be the most profound and important discovery that
humanity has ever made.
Thus, for ETI to have zero impact on us,
they must go undetected. In other words, ETI will have no impact
on us only if they remain invisible to us.
The other way
in which ETI could be neutral is if they have an impact on
humanity, but the cumulative effect of that impact is neutral.
In this case, humanity becomes aware of the presence of ETI. As
discussed above, detecting ETI is generally considered
beneficial for humanity.
Therefore, if we detect ETI and are
neutral toward them, then there will have to be some harm in
order to offset the benefit of contact.
It is unlikely that this
harm would precisely offset the benefit of detecting ETI (and
any other benefits that might come with contact), so here we
consider scenarios in which the offset is of approximately the
same magnitude, which results in a net impact that is roughly
several scenarios in which ETI could be invisible to us in
the sense that we do not detect the presence of any ETI.
of these scenarios assume that ETI do in fact exist, but we
do not detect their presence, perhaps because we are
physically unable to do so.
As far as humanity is concerned,
invisible extraterrestrials could be no different than
non-existent extraterrestrials if they both have no impact
on us. This scenario would be completely neutral to us.
However, it is not necessarily the case that an invisible ETI would have no impact on us.
invisibility scenario involves ETI that intentionally hide
from us. This corresponds to the Zoo Hypothesis of the Fermi
paradox. ETI could have the capability of hiding from us
given the likelihood of their superior technology, and there
are many ways that ETI could remain undetected by us if it
chooses to do so.
The simplest approach would be to hide
among the asteroids and observe us at a distance [51-54].
this case, such ETI will cease to be invisible to us when we
have searched enough of the asteroid belt to detect signs of
their presence, such as mining on asteroids [55-57], excess
infrared radiation from spacecraft [7,58], or intelligent
conversational space probes .
A more sophisticated
approach would eliminate all outgoing electromagnetic
signals by to hide any signatures of its presence, and ETI
with even greater technological prowess could engineer a
virtual planetarium surrounding Earth so that we are forced
to observe an empty universe .
on the form of the intentional hiding, the scenario need not
be strictly neutral.
Deardorff  argues that hidden ETI
may actually be beneficial because they know we are here and
presumably check up on us from time to time. Perhaps they do
have our best interests in mind and will initiate friendly
contact when we begin a long-term METI program  or when we start
conversing with an intelligent space probe .
scenario may even have some implications for human behavior
that are somewhat parallel to scenarios in which humanity is
actually the manifestation of a computer simulation .
sustainable ETI may be hiding from us to see if we can turn
into a sustainable society on our own before we gain the
ability to travel between stars. Such a society would be
temporarily neutral to us, but potentially harmful or
beneficial to us in the long term.
possibility is that ETI would unintentionally escape our
Even if they took no extraordinary measures to
remain concealed, ETI that pass by Earth may draw as much
attention from humans as a passing-by scuba diver would
alert a sea anemone by taking a photograph.
This could be
because ETI take a different physical form than Earth life -
a form that we are unable to recognize - or because their
technology is unobtrusive enough that we fail to take
Although it is common to assume that
extraterrestrial life will most likely be carbon-based and
require liquid water, there are a number of suggestions for
more exotic configurations of life.
alternative biochemistries based on alcohol solvents or
silicon [62-63], a shadow biosphere that invisibly coexists
with the life we know [33,64], pure energy beings that lack
a physical form, and even residence between multiple
In the same way, we may fail to notice ETI messages that are transmitted at a different wavelength
range than we typically listen to. In such a scenario, ETI
are actively attempting to communicate with us, but we miss
the message because our search efforts are less than
ETI may be interested in observing the Earth
system for scientific purposes or may simply be galactic
tourists passing through the Solar System. But as long as
they refrain from significantly interfering with humans or
our environment, these ETI provide no threat or benefit to
It is also
plausible that nearby ETI simply have no desire to
communicate with us.
Non-expansive ETI that pursue a
sustainable development pattern may also find all the
contentment and meaning they need on their own planet so
that they have no desire for interstellar communication
They may have taken up transcendental spiritual
practices that focus their efforts inward rather than
outward , or they might limit their space exploration to
passive interstellar probes [31,67-70].
Perhaps ETI actually
do inhabit nearby star systems and detect our radio leakage
but have no plans to send a response until we send them a
more intentional message [23,39].
They may be unimpressed
with the quality of our broadcasts, or they may choose to
conserve their resources and decide that interstellar
communication is too expensive.
For our purposes here, these
non-communicative ETI are invisible all the same.
must acknowledge the possibility of scenarios in which ETI
are too far away for communication. It may be that ETI have
no desire to maintain long-term communication with us, but
they also may be too physically distant from Earth to
consider communication [15-17].
An ETI broadcast from
another galaxy, for example, may not have yet reached Earth
and would probably be
too faint to detect with modern technology. Likewise, ETI
that live beyond 100 light years from Earth would not have
detected our radio leakage and may not yet know of our
Even if intelligent life is common in the
universe, it may still only arise once or twice within a
galaxy  so that the likelihood of interstellar
communication is diminished.
Then again, the galaxy may be
full of non-expansive ETI that may still embark upon
interstellar radio communication but are too far away for us
to have yet received their messages.
Human expansion in
space may lead to eventual contact with non-expansive ETI,
but aside from this possibility, non-expansive ETI will
remain invisible to us and have little influence on
but indifferent to us
possible that humanity could succeed in identifying ETI in
the galaxy, only to find that we are indifferent to the
This may seem unlikely, given that
the discovery that extraterrestrials exist elsewhere in the
galaxy would have wide-reaching implications.
there are several scenarios in which our evaluation of the
encounter could be one of indifference.
initial scenario, suppose that planet-finding missions
successfully identify an extra-solar terrestrial planet
orbiting a Sun-like star with an atmospheric composition
similar to Earth .
Follow-up observations with radio
telescopes reveal unintentional electromagnetic leakage
coming from the planet, which suggests the presence of
intelligent life. However, suppose further that we decode
this leakage to find no more than the ETI equivalent of old
television shows and obscure military transmissions.
broadcasts may contain next to nothing in terms of
information usable by humans, and the public may quickly
lose interest in non- responsive ETI with uninteresting
Even active ETI broadcasts that are targeted
toward Earth may contain information that we find useless or
esoteric. Remote observation of an ETI planet may also
reveal strikingly different chemical compositions between
their world and ours.
ETI that originate from a gas giant
planet, for example, may have followed a completely
different evolutionary trajectory that leaves little room
for biological similarity between us and them.
with such ETI may provide little useful information for
After all, an ETI society that eats only hydrogen
might not have any practical information relating to
development issues on Earth, and the vast difference in
biology might render them unable to communicate with us at
If the search for life finds that the galaxy is in fact
teeming with ETI, then uninteresting ETI planets such as
these will likely fall to low priorities for making contact.
scenario involves us finding that contact with ETI creates a
mild nuisance or requires more effort than we would like to
District 9 (2009) highlights a
contact scenario where we discover a helpless ETI crew that
requires human assistance in order to survive .
in a temporary refugee camp, the ETI in District 9 display a wide range of temperaments, but their
overall presence annoys the humans because the ETI refugee
camp seems to be a permanent fixture. Similarly, we may
discover through remote messaging that ETI need our help but
provide little in return, so that contact with ETI
eventually begins to drain human resources.
circumstances, perhaps ETI make contact with Earth to
welcome us into the Galactic Club but only after we complete
a set of required bureaucratic tasks.
ETI that make contact
with Earth will certainly be more technologically advanced
than humans today, so it is entirely plausible that the
requirements to enter a Galactic Club will be beyond our
In these scenarios, contact with ETI benefits
humanity by confirming the presence of life elsewhere, but
the consequences of contact are sufficiently disruptive,
annoying, or complicated that human civilization remains
scenario involves disagreement within human civilization
regarding whether or not contact has occurred. The simplest
conditions for this to occur would be if we received a
message from ETI that cannot be unambiguously decoded.
No SETI signal has yet been identified as extraterrestrial in
origin, and if we do ever stumble upon an actual ETI
broadcast then there could be a long and tedious process to
demonstrate its authenticity.
Less probable modes for this
form of contact have been explored in films such as Contact
(1997) and K-PAX (2001) in which the
nature of the ETI is only realized by a handful of humans
and dismissed by the rest.
If our detection of ETI lacks an
obvious and unambiguous signal, then different humans - even
including different ETI researchers - could reach different
conclusions on the question of detection. Any benefits of
mere detection could be offset by the turmoil of the
A scenario involving more than mere detection
could also still create conflict and disorder, but this
outcome seems less likely.
5 - Intentional
harm to us
scenarios we consider are those in which contact with ETI is
harmful to humanity (Figure 1 far above, right column).
This is a
particularly important set of scenarios because of the strong
caution they impose on our SETI and METI endeavors. These
scenarios have also received extensive consideration in both
fictional and non-fictional realms.
Here we explore one main
type of scenario in which an ETI could be harmful: intentional
The possibility of ETI causing unintentional harm is
discussed in the following section. In the intentional harm
scenarios, ETI decide that they wish to cause us harm and then
follow through on this wish.
In the unintentional harm
scenarios, ETI do not wish us any harm but inadvertently harm us
We see two
types of scenarios in which ETI might intentionally harm us.
first scenario involves hostile, selfish ETI that attack us so
as to maximize their own success. This scenario suggests a
standard fight-to-win conflict: a war of the worlds.
scenario involves ETI that are in no
way selfish but instead follow some sort of universalist ethical
framework. ETI might attack us not out of selfishness but
instead out of a universalist desire to make the galaxy a better
ETI is one that places intrinsic value only on properties of
its lives, its welfare, etc.
The idea of a selfish ETI is quite prominent in discussions of ETI.
geographer Jared Diamond , drawing from his expertise in
encounters between different intelligent populations on
Earth, argues that astronomers are often overly optimistic
about ETI encounters:
astronomers and others hope that the extraterrestrials,
delighted to discover fellow intelligent beings, will sit
down for a friendly chat. Perhaps the astronomers are right;
that's the best-case scenario.
A less pleasant prospect is
that the extraterrestrials might behave the way we
intelligent beings have behaved whenever we have discovered
other previously unknown intelligent beings on earth, like
unfamiliar humans or chimpanzees and gorillas.
Just as we
did to those beings, the extraterrestrials might proceed to
kill, infect, dissect, conquer, displace or enslave us,
stuff us as specimens for their museums or pickle our skulls
and use us for medical research.
My own view is that those
astronomers now preparing again to beam radio signals out to
hoped-for extraterrestrials are naïve, even dangerous.
Diamond is correct in noting that many astronomers neglect
the potential perils of an ETI encounter, it would be a
mistake to assume that astronomers are uniformly naïve in
For example, Nobel Laureate astronomer Sir
Martin Ryle opposes active efforts to communicate with ETI
due to concern that humans would be attacked [36,74-75].
Similar concerns have been raised by several others
Carl Sagan, who is usually quite
optimistic about ETI encounters, has expressed concern
regarding ETI risks . A common theme underlying the
pessimism of these various commentators is the likelihood
that ETI would be more advanced than humanity.
concern is that ETI will learn of our presence and quickly
travel to Earth to eat or enslave us.
Predation is common
among life forms on Earth because it can be more efficient
to prey upon other biota than it is to independently utilize
autotrophy for energy, carbon fixation, and other nutrients
for cellular material .
This may be less of a concern if
chirality of organics on Earth is poorly suited as a
universal food source .
Additionally, an advanced
society capable of interstellar travel may be less likely to
turn to humans as a source of food or labor because they
should have already solved these problems through some
combination of machine labor, artificial synthesis, and
Nevertheless, other selfish motives may
cause ETI to harm us, such as their drive to spread their
beliefs through evangelism (akin to the spread of
Christianity or Islam) or their desire to use humans for
and Sagan  discuss:
human beings have some relatively uncommon talent, of which
they are themselves entirely unaware.
J.B.S. Haldane once
pointed out to me that sea lions and seals have a remarkable
ability to balance a rubber ball on their noses, which is
part of the reason we maintain them in captivity.
Thus in one
ETI contact scenario, the ETI use humanity for entertainment
purposes just as we use sea lions and seals for this.
Shklovskii and Sagan  continue to point out that ETI may
desire to be the sole galactic power and will eliminate
other life forms when they start to get in the way.
Similarly, an ETI may simply be interested in using us as a
means for growth of their economy.
On an individual level
they may not be interested in killing us, but may be
interested in incorporating us into their civilization so
they can sell us their products, keep us as pets, or have us
mine raw materials for them.
Such a scenario could be
harmful or beneficial to us, depending on the methods they
use to bring us into their society.
conditions might ETI be self-interested?
Here it is again
useful to consider possible resolutions to the Fermi
paradox, in particular the Sustainability Solution. It is
unlikely that humanity will encounter an exponentially
expansive civilization [18-19] because we likely would have
already detected ETI if exponential expansion could be
maintained on galactic scales.
Thus exponentially expanding ETI probably do not exist or otherwise do not have the
capacity to expand throughout the galaxy.
This is fortunate
for humanity, since exponentially expansive ETI would likely
be quite harmful, just as exponentially expansive
populations on Earth (including at least some portions of
humanity) can be harmful for other members of their
An exception to this is a civilization that has
exponentially grown and collapsed in the past but did not
succumb to complete ecological collapse.
Such a society may
recover and choose once again to embark upon a development
pattern of exponential expansion.
If such an ETI
civilization exists today, then they could be extremely
harmful, even if they are only moderately more advanced than
we are, because if they continue upon their developmental
trajectory to rapidly colonize the galaxy, then they will
likely consume our resources before their collapse occurs.
discussed above, we have reason to believe that a
sustainable ETI is less likely to be harmful than an
unsustainable, exponentially expansive ETI.
remains entirely possible for an ETI to be both sustainable
Such an ETI could be expanding as fast as
happens to be sustainably possible, along a colonization
wave-front as in the simulations by Newman and Sagan .
Unlike the sustainable civilization described above, this
maximally expansive civilization would be sustainable
but still eager to consume whatever resources it could. This
type of ETI civilization would likely consume all the
resources of Earth and destroy humanity if we got in its
In the analysis of ETI expansion, a key question is
thus whether or not the expansion
is occurring at or near the maximal possible rate.
seem unlikely that a universalist ETI would intentionally
This is because universalist ETI place inherent
value on whatever traits that it values (lives, ecosystems,
etc.) regardless of whether it relates to our civilization
or theirs. In other words, a universalist ETI civilization
would be in no way biased against us.
universalism is commonly associated with peace and
cooperation, not with harm and destruction. But this is
because human populations are all generally similar.
example, we seek to maximize total happiness, then we will
succeed by avoiding conflict within humanity, because
conflict generally reduces happiness for nearly all humans.
not be the case for ETI. Just because an ETI civilization
holds universalist ethics does not mean that it would never
seek our harm. This is because ETI may be quite different
from us and could conclude that harming us would help
maximize whatever they value intrinsically .
example, if ETI place intrinsic value on lives, then perhaps
they could bring about more lives by destroying us and using
our resources more efficiently for other lives.
of intrinsic value may cause universalist ETI to seek our
harm or destruction as long as more value is produced
without us than with us.
Novelist Douglas Adams captures
this scenario vividly in
The Hitchhiker's Guide to the
Galaxy, where ETI place intrinsic value on civic
infrastructure (or, more likely, on some consequence of its
use) and destroy Earth to make way for a hyperspace bypass.
At the heart of these scenarios is the possibility that
intrinsic value may be more efficiently produced in our
interesting and important case of universalist ethics in
this context is when civilization itself holds intrinsic
ETI that support this ethical framework would seek to
maximize the total number of civilizations, the diversity of
civilizations, or some other property of civilizations. All
else equal, such ETI would specifically wish for our
civilization to remain intact.
But all else may not be
It is plausible that such ETI might try to harm or
even destroy us in order to maximize the
number/diversity/etc. of civilizations. This could occur if
our resources could be used to more efficiently to generate
or retain other civilizations, though this possibility seems
highly remote given how efficiently tuned humanity is to its
Alternatively, such ETI could seek our harm if
they believe that we are a threat to other civilizations.
of humanity being a threat to other civilizations may seem
implausible given the likelihood of our technological
inferiority relative to other civilizations. However, this
inferiority may be a temporary phenomenon.
observe our rapid and destructive expansion on Earth and
become concerned of our civilizational trajectory. In light
of the Sustainability Solution to the Fermi paradox, perhaps
ETI believe that rapid expansion is threatening
on a galactic scale.
Rapidly (maximally) expansive
civilizations may have a tendency to destroy other
civilizations in the process, just as humanity has already
destroyed many species on Earth. ETI that place intrinsic
value on civilizations may ideally wish that our
civilization changes its ways, so we can survive along with
all the other civilizations.
But if ETI doubt that our
course can be changed, then they may seek to preemptively
destroy our civilization in order to protect other
civilizations from us.
A preemptive strike would be
particularly likely in the early phases of our expansion
because a civilization may become increasingly difficult to
destroy as it continues to expand.
Humanity may just now be
entering the period in which its rapid civilizational
expansion could be detected by an ETI because our expansion
is changing the composition of Earth's atmosphere (e.g. via
greenhouse gas emissions), which therefore changes the
spectral signature of Earth.
While it is difficult to
estimate the likelihood of this scenario, it should at a
minimum give us pause as we evaluate our expansive
It is worth
noting that there is some precedent for harmful universalism
This precedent is most apparent within
universalist ethics that place intrinsic value on
ecosystems. Human civilization affects ecosystems so
strongly that some ecologists now often refer to this epoch
of Earth's history as
the anthropocene .
If one's goal
is to maximize ecosystem flourishing, then perhaps it would
be better if humanity did not exist, or at least if it
existed in significantly reduced form. Indeed, there are
some humans who have advanced precisely this argument
If it is possible for at least some humans to
advocate harm to their own civilization by drawing upon
universalist ethical principles, then it is at a minimum
plausible that ETI could advocate harm to humanity following
possibility of harmful contact with ETI suggests that we may
use some caution for METI. Given that we have already
altered our environment in ways that may viewed as unethical
by universalist ETI, it may be prudent to avoid sending any
message that shows evidence of our negative environmental
The chemical composition of Earth's atmosphere over
recent time may be a poor choice for a message because it
would show a rapid accumulation of carbon dioxide from human
Likewise, any message that indicates of widespread
loss of biodiversity or rapid rates of expansion may be
dangerous if received by such universalist ETI.
On the other
hand, advanced ETI may already know about our rapid
environmental impact by listening to leaked electromagnetic
signals or observing changes in Earth's spectral signature.
In this case, it might be prudent for any message we send to
avoid denying our environmental impact so as to avoid the ETI catching us in a lie.
6 - Unintentional
harm to us
scenarios considered thus far have all involved ETI that intend
to cause us harm, but it is not the only
type of scenario in which ETI actually do cause us harm.
Specifically, it is possible for ETI to cause us harm despite
them not wishing to do so.
Here the desires of ETI may even be
irrelevant: such ETI could hold any value system from selfish to
universalist while still causing unintentional harm in several
ways. In one set of scenarios, ETI could inadvertently bring us
some sort of physical hazard, such as a disease or an invasive
In another set of considerations, ETI could
inadvertently bring an information hazard, such as technological
damage or a presence that demoralizes or destabilizes human
comes into direct physical contact with either ETI
themselves or some ETI artifact, then it may be possible for
humanity to be unintentionally harmed.
One of the most
prominent scenarios of this kind is the transmission of
disease to humanity.
This scenario is inspired by the many
instances in which humans and other species on Earth have
suffered severely from diseases introduced from other
regions of the planet.
Such diseases are spread via the
global travels of humans and our cargo and also through
certain other disease vectors. Introduced diseases have been
extremely potent because the population receiving the
disease has no prior exposure to it and thus no build-up of
Indeed, disease introductions are blamed for loss
of human life so widespread as to have altered the broadest
contours of human history .
could introduce disease to humanity, then the impacts could
be - but wouldn't necessarily be - devastating. The disease
could quite easily be significantly different from anything
our immune systems have ever encountered before.
could also be entirely unfamiliar to our medical knowledge,
and it could potentially be highly contagious and highly
This combination of contagiousness (i.e. high R0
) and lethality (i.e. high mortality rate) is
unlikely in existing pathogens because such pathogens would
quickly kill their host population and then die out
Furthermore, if we had already encountered such
a disease on Earth, then we likely wouldn't be here anymore.
However, a disease from ETI would be new to us.
presumably would not be highly contagious and lethal to the ETI themselves or to the other organisms in their biosphere,
but it could be devastating to humans and the Earth system.
Then again, ETI biology may be so vastly different from
Earth biology that no significant interactions between
organisms occur. ETI may have their own contagious diseases
that are unable to infect humans or Earth-life because we
are not useful hosts for ETI pathogens.
After all, the ETI
diseases would have evolved separately from Earth biota and
thus be incompatible. So while there are reasons to believe
that an ETI disease which affected humanity would be
devastating, there are also reasons to believe that an ETI
disease would not affect humanity.
It is worth
noting that a disease brought by an ETI could harm us
without infecting us. This would occur if the disease
infects other organisms of interest to us.
For example, ETI
could infect organisms important to our food supply, such as
crop plants or livestock animals.
infection would be less likely to destroy humanity and more
likely to only harm us by wiping out some potentially
significant portion of our food supply. In a more extreme
case, ETI disease could cause widespread extinction of
multiple species on Earth, even if humans remain uninfected.
It may be
possible to protect humanity from diseases brought by ETI.
The most straightforward option is simply to prevent contact
between the ETI biosphere and Earth's biosphere. Several
calls for such prevention have already been advanced, often
under the rubric of planetary protection .
If we never
come into contact with an ETI biosphere, then we probably
cannot become infected by its diseases.
This fact has
implications both for how humanity handles communications
with ETI - for example, whether our communications encourage
contact - and for human space exploration policy - for
example, whether we send probes in search of ETI life, and
whether we send these probes back to Earth if life is found.
prevention fails and ETI disease is contacted, then
treatment may be aided by information about the biology of
ETI and other organisms in their biosphere.
information could be used to develop vaccines or other
countermeasures. However, our experience with novel diseases
on Earth, such as novel influenza strains, suggests that it
takes much less time for a disease to spread than for us to
find a cure.
The spread of ETI diseases may be even more
rapid and the cure even more difficult to develop.
Therefore, any head start we can get for our cure
development could be highly valuable. This in turn makes
remotely received information about ET biology (i.e. biology
of the ETI and others in their biosphere) valuable.
can receive information about ET biology before we make
physical contact - for example, if we can receive it via
electromagnetic transmission - then perhaps we can develop
adequate countermeasures to ET diseases before we encounter
The possibility that physical contact with ETI may
infect humanity with a deadly disease also suggests that we
may want to refrain from broadcasting any specifics of our
Malicious ETI that learn about our biology will
know how to best exploit our immune systems and may even
design a human-tailored biological weapon before coming to
destroy us. Thus, one possible METI strategy may be to
actively seek information about ET biology while carefully
guarding the details of human and Earth biology.
are not the only physical hazard we may unintentionally face
from ETI. A similar biological hazard is the invasive
Whereas a disease infects and harms an organism by
overwhelming its immune system, an invasive species affects
and harms an ecosystem by overwhelming its ecological
functions. The distinction between diseases and invasive
species is at most a blurry one.
A disease can at least
sometimes be classified as an invasive species. Some
diseases, such as viral diseases, are not well-classified as
species, while some diseases are not invasive because they
have a permanent and entrenched status within their host
Likewise, some invasive species are not diseases
per se but instead are harmful in other ways. For example,
an introduced predator is a disease only in a metaphorical
context of an encounter with ETI, the dynamics of invasive
species are similar to the dynamics of introduced diseases.
In both cases, humanity is particularly vulnerable due to
the extreme novelty of the introduced agent, because our
natural defenses and our skilled response efforts are
unaccustomed to the agent.
Also, in both cases, humanity
could benefit from preventing contact with the ET biosphere
and from remotely received information about the ET biology.
Although an invasive extraterrestrial species seems like it
should displace at least some portion of Earth's ecosystem,
it is also possible that such invasive species occupy a
completely different ecological niche than any extant life
on Earth. Thus, we may find that an extraterrestrial
invasive species takes up residence on our planet without
causing any destruction at all (analogous to a shadow
biosphere - see [33,64]).
non-biological physical hazard that we could face from
direct contact with ETI is unintentional mechanical harm.
For example, ETI might accidentally crush us while
attempting an unrelated maneuver. This scenario parallels
instances on Earth in which humans inadvertently destroy the
ecosystems of species that then go extinct.
All else equal,
humanity would generally prefer not causing the extinction
of species, but we often prioritize other matters. Indeed,
in many cases we may not have even realized that an
endangered species was present until after extinction has
Perhaps ETI could inadvertently destroy humanity
under analogous circumstances.
similar class of scenarios, ETI could inadvertently unleash
some harmful force into the galaxy through some act of
incompetence, quite possibly harming itself in the process.
For example, an otherwise benevolent extraterrestrial
civilization could accidentally unleash the extraterrestrial
equivalent of an "unFriendly Artificial Intelligence" (uFAI
This ET uFAI would be out of the control of its
(benevolent) makers and would likely destroy humanity as it
attempted to fulfill whatever objective function it happened
to have. The odds that this objective function will happen
to benefit humans seems extremely small. Indeed, it may be
difficult for humans to create such an objective function
even with considerable dedicated effort .
example, ETI that explore the galaxy using automated
self-replicating probes (also known as von Neumann probes)
may inadvertently unleash a catastrophic colonization wave
that rapidly spreads throughout the galaxy and destroys
other civilizations [10,26].
Such a scenario may arise
either from faulty design of automated probes or from the
malicious intent of artificially intelligent probes.
 suggests that such undesirable outcomes could be the
result of evolutionary dynamics in which the undesirables
are the strong which survive evolutionary pressures.
Finally, it is possible that ETI could render some portion
of the galaxy uninhabitable via an accident in a physics
experiment, just as there are concerns that certain human
physics experiments with particle accelerators could be
accidentally destructive .
Any of these scenarios would
involve the ETI accidentally harming humanity and probably
did not come into direct physical contact with ETI, it could
still be possible for ETI to unintentionally harm humanity.
This could occur if ETI send harmful information to humanity
via electromagnetic transmission.
A malicious ETI
broadcaster could, for example, send a message containing
harmful information that either damages human technology,
analogous to a computer virus, or coerces humans into a
seemingly benign but ultimately destructive course of
action, such as the construction of a dangerous device,
example, ETI might send information about its biology,
perhaps hoping that humanity could use this information to
protect itself against ET diseases or invasive species.
However, perhaps such an effort would backfire on humanity
if we use the information to create a disease, invasive
species, or other hazard.
The hazard would be created by
humans from the information received, and the creation could
be intentional or unintentional. But if the creation was
intentional, then it would be human intent, not ETI intent.
The possibility of an intentional or unintentional
informational hazard suggests that at least some care should
be taken in efforts to detect and analyze electromagnetic
signals sent from ETI.
one final information hazard scenario to consider. In this
scenario, contact with ETI serves as a demoralizing force to
humanity, with strong negative consequences.
history, contact between modern society and stone age
culture usually leads to the demise of the more primitive
Likewise, in the event of contact with ETI,
humanity may be driven toward global cultural collapse when
confronted with ETI technology, beliefs, and lifestyle .
Even if the ETI are friendly toward us and give us the
choice to accept or reject their knowledge, the vast
differences between our respective societies may force the
more primitive one (ours) into a demoralizing state of
For this reason, if ETI do already know
of our presence and if they wish to preserve the integrity
of our civilization, then they may choose to reveal
themselves to us slowly and gradually in order to avoid a
calamitous response .
7 - Conclusion
The outcome of contact
between humanity and ETI depends on many factors that cannot be
fully known at this time.
The scenario analysis presented in this paper therefore serves as a
means of training our minds to recognize patterns and analyze
outcomes before contact with ETI ever occurs.
Actual contact may not precisely follow the scenarios considered
here, but any amount of analysis to prepare ourselves for contact
will increase the likelihood of a positive outcome.
analysis presented here serves as a step toward developing a
comprehensive strategy for responding to contact with ETI.
Based on the
infeasibility of sustained exponential expansion through space, it
seems less likely that ETI will destroy us because of their lack of
resources. Nevertheless, ETI could still decide to harm us
intentionally because of their own ethical considerations, or they
may cause us unintentional harm through invasive species or cultural
It is also entirely possible that contact with ETI will
have little impact on Earth or humanity, especially if the form of
ETI life is vastly different from life on Earth.
SETI often assumes
that any two intelligent civilizations in the universe could
communicate, but we cannot neglect the possibility that the human
species will be completely unable to comprehend the language or
communication efforts of ETI.
The possibility of a neutral ETI
encounter, then, is just as worthy of consideration as a scenario
with friendly or hostile ETI.
suggests some immediate practical recommendations for humanity. One
recommendation is that messages to extraterrestrials should be
For example, prior messages have included
details of human biology, such as the numbers one through ten (our
base ten system is likely derived from the number of fingers on our
hands) and the form and structure of the DNA molecule. However,
details about our biology, though seemingly harmless, may actually
help certain ETI to cause us harm.
A malicious ETI listener may use
a message about human biology to design a potent biological weapon
for use against Earth. Since these messages will ultimately be sent
toward unknown ETI, we cannot know whether or not they might be
received by such a malicious ETI.
Therefore, caution is warranted.
For example, initial communication with ETI may be best limited to
simple mathematical discourse for security purposes until we have a
better idea of the type of ETI we are dealing with.
In our view,
decision making regarding messaging should factor in the
probabilities and magnitudes of possible message scenarios through a
formal risk analysis that could draw on the scenario analysis
recommendation is that humanity should avoid giving off the
appearance of being a rapidly expansive civilization. If an ETI
perceives humanity as such, then it may be inclined to attempt a
preemptive strike against us so as to prevent us from growing into a
threat to the ETI or others in the galaxy.
ecosystem-valuing universalist ETI may observe humanity's ecological
destructive tendencies and wipe humanity out in order to preserve
the Earth system as a whole.
These scenarios give us reason to limit
our growth and reduce our impact on global ecosystems. It would be
particularly important for us to limit our emissions of greenhouse
gases, since atmospheric composition can be observed from other
We acknowledge that the pursuit of emissions reductions and
other ecological projects may have much stronger justifications than
those that derive from ETI encounter, but that does not render ETI
encounter scenarios insignificant or irrelevant.
recommendation is that preparations for ETI encounter, whether
through METI, SETI, human explorations of space, or any other form,
should consider the full breadth of possible encounter
Indeed, perhaps the central conclusion of the analysis
presented here is that ETI contact could proceed in a wide range of
ways. It is inappropriate and inadequate to blindly assume that any
one specific scenario would result from contact. Until such contact
occurs, we simply do not know what would happen.
uncertainty, the broad scenario analysis presented here is an
important step towards helping us think through and prepare for
Despite its merits,
our scenario analysis remains fundamentally limited in several
important ways. As is common with scenario analysis in general, we
offer no quantification scheme for the probabilities of specific
scenarios. We also do not quantify the magnitude of the impacts
(benefit or harm) of specific scenarios.
The result of this is that
we are unable to produce a cumulative analysis of the risks and
rewards of contact with ETI or attempting to do so with METI. Such a
quantitative risk analysis would be of tremendous value for decision
Indeed, the need has been acknowledged for such
analysis in order to inform decisions about METI and other SETI
activities . However, the effort required for such an analysis
is far beyond the scope of what we can accomplish in a single paper
and thus must be left for future work.
The scenario analysis
presented here is an important step towards a quantitative risk
analysis, but it is not a complete risk analysis on its own.
caveat to our scenario analysis derives from the limits of our
knowledge about contact with ETI. Because we have no empirical data
about ETI, we must extrapolate from the information that we do have
available, including knowledge about the observable universe and
knowledge about ourselves.
We must bear in mind that our
observations are inevitably confined to human experience, and so our
extrapolations, no matter how generalized, may still contain
implicit anthropocentric biases.
It is entirely possible that ETI
will resemble nothing we have previously experienced or imagined, in
which case the contact may not resemble any scenario we could
This possibility does not mean that we should completely
dismiss any analysis of extraterrestrials, since there is also a
strong possibility that the contact would have some resemblance to
our scenarios. Nevertheless, the possibility that our experience and
imagination could come up severely short reminds us to use caution
in interpreting our analysis.
Until we actually detect ETI, we will
remain highly uncertain as to their nature and to the outcomes that
would follow from our contact with them.
One area for future
work concerns impacts (benefits and harms) to nonhumans. This paper
has focused on the impacts of contact to humanity. We have thus
neglected impacts to the ETI, to the rest of Earth, to the rest of
the galaxy, and possibly even to other entities as well.
on humanity to maintain a reasonably narrow scope for the paper, not
because we believe that impacts to nonhumans are unimportant.
Indeed, we feel strongly that consideration of impacts to nonhumans
represents an important area for future work.
An additional area
for future work concerns quantitative risk assessment. A
quantitative assessment of the scenarios presented in this paper
would be of tremendous use in developing strategies for responding
to contact with ETI.
However, because we have no observations of ETI, any attempt at quantitative analysis will struggle to assign
numerical probabilities to the qualities of an unknown ETI
civilization. Certain aspects of this problem, such as rates of
expansion and exploration, can be constrained with known physical
models, though, so at least some degree of quantification is
Additionally, continued exploration of our galaxy and
universe will reveal information that will further constrains some
of these scenarios such as,
the distribution of terrestrial planets
the prevalence of Earth-like atmospheric bio-signatures
existence of artificial radio signals
A complete quantitative
assessment of risk from an encounter with extraterrestrials may be
difficult to complete in the near future, but even incremental
progress will help us choose an optimal strategy if and when we make
actual contact with ETI.
Even if contact
with extraterrestrials never occurs, our scenario analysis still
acts as a set of future trajectories for human civilization.
thinking about the nature of extraterrestrials and intelligent life
in general is really an exercise in imagining the ways that future
humans could exist under different circumstances or in different
This scenario analysis therefore helps to illuminate
the consequences of particular decisions, such as the mode of
expansion or the ethical framework of an intelligent civilization,
and may help us distinguish between desirable and undesirable
trajectories for humanity.
As we continue the search for
extraterrestrials into the future, perhaps our thinking about the
different modes of contact will help human civilization to avoid
collapse and achieve long-term survival.
Jason Matheny and
Ilana Brito provided helpful assistance on disease spread dynamics.
Two anonymous reviewers provided helpful feedback on an earlier
version of this article.
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