March/April 1999 Table of Contents
the absence of knowledge of physical and cultural clues, communication
between two species can be almost impossible.
Historical Overview of Interplanetary Messages
showing pictures for interplanetary communication
earliest speculations about communication with extraterrestrial
intelligence (CETI) involved contact with the inhabitants of other
bodies of our Solar System, either our Moon or other planets. Astronomical
theories in the nineteenth century made plausible a belief in the
prevalence of life on non-terrestrial worlds circling our Sun. When
the focus turned to the possibility of communicating with the potential
denizens of these worlds, their relative closeness to the Earth
made it conceivable that signals could be detected through optical
that other intelligence could visually observe Earth, and given
that nineteenth century astronomers relied on optical telescopes
to survey the heavens, it is understandable that early proposals
for messages emphasized visible signaling. One popular type of proposal
involved displaying meaningful figures on a side of the Earth facing
the target moon or planet. In these plans, huge diagrams would be
etched on large expanses of land. For example, a visual representation
of a right triangle could be shown, with a square attached to each
side of the triangle to illustrate the Pythagorean theorem diagrammatically
1. In an early proposal to communicate with inhabitants of
the Moon, geometrical concepts would be shown directly. For example,
the Pythagorean theorem could be illustrated visually during the
daytime by clearing vast expanses of forest in Siberia to show
the areas surrounding a right triangle (right side of figure).
During the night, canals dug into the Sahara desert in the shape
of a circle could be filled with kerosene (left side of figure).
When lit, the flames would provide a pictorial signal of our existence.
Illustrations courtesy of author.
clearing gargantuan stretches of forest in Siberia, such geometrical
concepts could be illustrated for viewers watching the lighted side
of the Earth and, by creating large canals in the Sahara Desert
filled with lighted kerosene, a similar signal could be sent from
the dark side of the Earth (Figure 1). Among the early proponents
of directly displaying pictures to communicate with extraterrestrials
was the illustrious mathematician Karl Friedrich Gauss, who in 1826
was attributed with suggesting such an approach for communicating
with potential selenites - inhabitants of the Moon. As interplanetary
spacecraft became conceivable in the early twentieth century, the
same notion of directly showing pictures was proposed for vessels
bridging the space between worlds. A century after Gauss's proposal,
rocket pioneer Robert Goddard suggested that interplanetary craft
might bear metal plates inscribed with geometrical shapes and astronomical
objects to initiate CETI.
pictures for interplanetary communication
addition to proposals for sending pictures directly, others advocated
sending messages that would not be comprehensible without a reconstruction
of the message format. For example, in 1869 the polymath Charles
Cros suggested presenting pictorial information as sequences of
numbers (Figure 2). In this plan, several series of numbers would
be sent. As a clue to reconstructing the picture from these series
of numbers, the sum of the numbers in each series would be the same.
Recipients successful in decoding the message would first convert
these series into strings composed of beads (or their equivalents)
of two different colors, with the number of beads of a certain color
equal to the transmitted number for that part of the series. Finally,
if these strings of beads were aligned one under the other in the
order they were received, the reconstruction of the picture would
similar scheme was advanced in 1920 by H. W. Nieman and C. Wells
Nieman. Rather than transmitting series of numbers, however, the
Niemans suggested signaling with a series of "dots and dashes" sent
either by "wireless or light" (Figure 2). Each dot or dash could
be represented by a bead, with dots and dashes represented by different
color beads. Since each series would have the same number of combined
dots and dashes, the recipients would have a clue that they could
be aligned one under the other, as in Cros's proposal.
2. A pictogram representing a square can be described in several
ways. Cros suggested transmitting several series of numbers, with
the sum of each series being equal to the same number, as an aid
to decoding. In this example, the total of the numbers in each
series equals 11. When the series are "stacked," a two-dimensional
representation is created. Nieman and Nieman proposed a similar
approach, using signals of two different durations ("dots and
dashes"), which are represented as beads in black and white in
this reconstruction. Illustration courtesy of author.
slightly different approach was offered by Francis Galton in 1896.
Instead of encoding pictures as sequences of discrete units ordered
in a two-dimensional array, Galton favored starting with an introduction
to mathematics, and only later building up to pictorial representations
of objects by defining their outlines. Galton compared his "picture-writing"
to embroidery in which each "stitch" composing the outline of an
object would be defined in terms of its length and direction.
Gauss's plan, the proposals of Cros, the Niemans, and Galton add
the requirement of having the recipients reconstruct the format
of the message. As can be seen from the few proposals mentioned
so far, even when there is a goal of portraying information in a
two-dimensional array, there are additional questions about exactly
how the information should be encoded, and thus also how it would
for Interstellar Communication
showing pictures for interstellar communication
similar to those proposed in the nineteenth and early twentieth
centuries were independently espoused by scientists and engineers
working in the 1960s on what had become proposals for interstellar
(as opposed to interplanetary) communication. With diminished estimates
of the likelihood of intelligent civilizations elsewhere in our
Solar System, the search for extraterrestrial intelligence (SETI)
moved to other stars. (While the acronym "CETI" refers to either
sending messages to extraterrestrials or receiving signals from
them, the more recent term "SETI" typically refers only to the more
passive listening strategy.) As with messages proposed for communication
within our Solar System, we see two options for communicating with
pictures at interstellar distances:
without encoding them; or
using messages that require reconstruction, but that would be
designed to facilitate decoding.
first option-showing pictures directly-is exemplified by the messages
borne on the Pioneer and Voyager spacecraft. Although these craft
were not intended primarily for CETI, each carries a message to
any extraterrestrials who might intercept them in their travels
beyond the Solar System during the coming millennia. Each Pioneer
spacecraft carries a plaque that is inscribed with several diagrams
(Figure 3), including one of the spacecraft as it makes its way
through the Solar System (with planetary diameters indicated as
multiples of hydrogen wavelengths in binary numbers), another showing
the same spacecraft in greater detail, and an outline drawing of
a human female and male.
3. On the Pioneer plaque, an outline of the Pioneer
spacecraft is seen behind the figures of two humans. At the bottom
of the plaque, the same spacecraft is shown in a smaller scale
as it passes through the Solar System on its journey from Earth
(the third planet from the Sun). The figure with two connected
circles in the upper left-hand corner represents a hydrogen atom's
spin transition. The left central figure of fifteen converging
lines shows the Earth's location in time and space in relation
to prominent pulsars. Illustration courtesy of author.
the Voyager spacecraft each bear messages, some engraved on the
metal coverings of the recordings and others encoded on the record
disks themselves. Some of these engravings are the same as those
on the Pioneers, such as the diagram showing the location of Earth
and the time of launch by reference to several pulsars. Others are
unique to the Voyager record cover, for example, the outline drawing
illustrating how the stylus and recording are to be used together
(Figure 4). For the external diagrams on the Voyager, as well as
the pictures on the Pioneer plaques, the format need not be reconstructed.
As with Gauss's plan of portraying complete diagrams on the face
of the Earth, both the Pioneer and Voyager spacecraft carry direct
4. The cover over the Voyager recording bears two figures
(upper left) that provide instructions on how to place the stylus
on the recording. Diagrams in the upper right-hand corner illustrate
the signals produced by playing the record. As on the Pioneer
plaque, representations of the Earth's position and of a hydrogen
atom are also included. Illustration courtesy of author.
pictures for interstellar communication
Voyager spacecraft also bears a recording encoded in a format that
must be understood by the recipients before the recorded message
will be intelligible. The protective covering over each recording
is inscribed with pictorial instructions about how to place the
stylus on the record disk, and then how to turn the recording, resulting
in "playing" the pulses encoded on the disk (Figure 4). In addition,
an etching indicates how to align a series of pulses next to one
another, in a manner reminiscent of that suggested by Cros and the
Niemans. As a confirmation that the correct format is used, the
first image to be reconstructed from the recording-a circle-is also
shown on the protective cover. The first part of the recorded message,
which consists of pictorial information, quickly moves from a description
of a numbering system to diagrams of atoms and molecules, and then
on to a wide variety of pictures of the world as we know it.
what if we do not have physical contact with a spacecraft that bears
messages? What if we must overcome the distance between stars with
electromagnetic radiation, for example at radio or optical frequencies?
The vast interstellar distances separating transmitter and receiver
make proposals like Gauss's impossible. No longer is it possible
to show directly pictures of objects or concepts, at least not using
5. A hypothetical stocky biped is shown at the bottom center
of this pictogram created by Frank Drake in 1962. The fictional
being's solar system is also depicted, with the system's star
in the upper left-hand corner and its nine planets along the left
side. An oxygen atom is shown in the upper right-hand corner,
with its central nucleus surrounded by eight electrons. Similarly,
a carbon atom is represented at the top center with six electrons
orbiting its nucleus. Illustration courtesy of author. (cf.
with Figure 7)
standard response is to use a strategy similar to that proposed
by Cros, the Niemans, and Galton. Such was the approach taken by
astronomer Frank Drake in 1962 when he constructed a two-dimensional
"pictogram" similar to one we might some day receive from extraterrestrials
(Figure 5). As a clue to decoding the message, it consists of 551
bits of information. The only factors of 551 are the prime numbers
19 and 29, which are the lengths of the sides of the message. When
properly formatted, the message shows-among other things-a picture
of the hypothetical species sending the message, a diagram of its
solar system, and pictorial representations of carbon and oxygen
atoms to indicate elements important for this extraterrestrial biochemistry.
Electrical engineer Bernard M. Oliver, who constructed a similar
message with this format, justified the use of chemistry for inter-species
communication because, "The structure of atoms does not depend on
who studies them."
Drake actually transmitted a message from the Arecibo radio telescope
in 1974, he constructed a pictogram similar to his earlier creation,
but with more emphasis on terrestrial biochemistry (Figure 6). Similarly,
in my own work in the 1970s, I proposed sending sequences of pictograms
containing both pictures and words. When properly reconstructed,
such series of interrelated pictograms could convey more than isolated
6. The top three rows of the message sent from the Arecibo
telescope in 1974 list the numbers from one to ten in binary notation.
This forms the basis for numerically describing the chemical structure
of DNA, which is also illustrated iconically by the helical form
at the center of the message. Immediately below the helix is a
figure of a human being, and below that, a representation of our
Solar System, with the third planet (Earth) displaced toward the
human figure. Illustration courtesy of author.
it may be true, as Oliver noted, that the structure of atoms does
not depend on who studies them, scientific models of atoms may be
very much influenced by the characteristics of the scientists who
construct these models. And when two scientists differ in biology,
culture, and history as much as humans and extraterrestrials would
differ, these models of reality may vary considerably. This view
that extraterrestrials and humans may have such divergent ways of
conceptualizing the world that there can be no mutual understanding
is referred to as the Incommensurability Problem.
the core of the Incommensurability Problem is the view that no intelligent
species can understand reality without making certain methodological
choices, and that these choices may vary from civilization to civilization.
As philosopher of science Nicholas Rescher explains, extraterrestrials
could well have very different ways of doing science than humans.
If extraterrestrials have markedly different biologies and live
in considerably different environments than humans, they may well
have different goals for their science. In addition, they could
have radically different criteria for evaluating the success of
their science. Rescher notes, "Their explanatory mechanisms, their
predictive concerns, and their modes of control over nature might
all be very different." Finally, their means of formulating models
of reality might differ drastically from ours. For example, their
analog of arithmetic might not even be quantitative, but comparative.
Similarly, their conventions for picturing things could be very
different from those used by humans.
of Pictorial Representation
because extraterrestrials will have sight, having
this sensory modality may not in itself be sufficient to ensure
they will use pictograms. On the contrary, some would argue that
our emphasis on pictorial images for CETI is not so much a reflection
of the primacy of vision in humans, but rather a reflection of philosophical
assumptions about the proper means of gaining knowledge.
and historian of science Michel Foucault contends that our reliance
on science is based on studying the visible characteristics of objects
and was by no means a necessary development. Rather it reflects
a belief that originated in the seventeenth century that true knowledge
must be acquired from sight. This emphasis on vision led to eliminating
other senses as potentially valuable sources of scientific information.
If Foucault is correct, our reliance on visual information for scientific
reasoning might have taken a different turn, with instead our other
senses providing important data for science. Thus even without raising
the question of whether extraterrestrials will be able to see, we
may be wise not to overestimate the importance of pictorial representations
is all of this merely armchair philosophy that has no bearing on
real intelligent beings? To be more concrete, let's take a closer
look at contact between alien cultures here on Earth: interactions
between humans from different cultures. According to Jamake Highwater,
"We do not all see the same things. Though the dominant societies
usually presume that their vision represents the sole truth about
the world, each society (and often individuals within the same society)
sees reality uniquely."
To illustrate this point, Highwater tells of an
encounter between a Swiss artist and a Native American in the mid-nineteenth
century. As the European proceeded to sketch a picture of a man
on horseback, the Sioux proposed a more accurate way of portraying
the same subject. Rather than showing a side view of the man with
only one leg visible, as did the Swiss, the Sioux drew him with
both legs clearly present, though still in profile. Although the
European insisted that a side view demanded that only one leg be
shown because the other leg was hidden behind the horse, the Native
American calmly explained, "but, you see, a man has two legs." Reportedly,
the two never came to an agreement on the proper way to represent
a human being.
we consider the pictorial representations in pictograms for CETI,
how certain can we be that our particular conventions will be understood
by extraterrestrials? To exemplify this problem, artificial intelligence
expert Michael A. Arbib showed how Drake's message from 1962 might
be misinterpreted if it were read upside down by an extraterrestrial
very different in form from humans. In this orientation, the pictures
of carbon and oxygen might be mistaken for a creature with six legs,
and the picture of the biped might be seen as a communication satellite
(Figure 7). As Arbib summarizes his point: "While there may be some
chance of Drake's message being deciphered by an intelligence that
expects any biped-like shape to be an intelligent being, it is very
unlikely indeed to succeed with six-legged but large-brained creatures
7. Frank Drake's message from 1962 might well be misinterpreted
by a race of large-brained hexapods. This message is an upside-down
copy of the pictogram shown in Figure 5.
Illustration courtesy of author.
we cannot be certain of the nature of any recipients of our messages
a priori, it may prove difficult to construct pictures that will
be unambiguous. No matter how careful we are, to some extent extraterrestrial
viewers of our pictograms may project characteristics from their
own species-specific experiences onto our messages.
about more "objective" means of pictorial representation, such as
photographs? Is it not true, as Stanley Cavell maintains, that "Photography
overcame subjectivity in a way undreamed of by painting - by removing
the human agent from the task of reproduction"?
Snyder and Neil Walsh Allen claim that this is not the case. Instead,
they contend that photography does not succeed in providing a fundamentally
more objective image than that obtained in other visual arts such
as drawing and painting: "An image is simply not a property which
things naturally possess in addition to possessing size and weight.
The image is a crafted, not a natural, thing. It is created out
of natural material (light), and it is crafted in accordance with,
or at least not in contravention of, natural laws. This is not surprising.
Nor is it surprising that something in the camera's field will be
represented in the image; but how it will be represented is neither
natural nor necessary."
support this view, Snyder and Allen include (among other examples)
a discussion of photographing an object in motion. If we wish to
photograph a person running, for instance, our options as photographers
include the following ways of depicting motion:
blurred body against a stationary background
distinct body against a blurred background (by "panning")
body and background "frozen" by the camera, with motion implied
by the relative position of parts of the body
people accustomed to these conventions for depicting motion could
easily interpret each possibility, an extraterrestrial with different
conventions might have considerably more difficulty.
A General Theory of Signs
we think of interstellar messages in terms of classical information
theory, there is no innate relationship between the form of the
message and the content borne by the message. Instead, once the
information of the message is decided upon, an efficient means of
encoding it is sought. In this approach, there is a purely arbitrary
connection between content and form of the message. If instead we
consider messages from a semiotic perspective, we have a wider range
of possibilities for relating form and content. Semiotics is the
general study of signs, where a sign is something that represents
something else, the signified. For example, the words "the chair"
might represent the object you are currently sitting on, if indeed
you are now sitting. In this case, the words "the chair" are a sign
standing for the signified, in this case, a material object. One
of the tasks of semioticians is to categorize signs according to
the ways that the sign and the signified are related to one another.
In the case of the association between the sign "the chair" and
its signified object, this relationship is purely arbitrary. The
sign for this object could equally well be "the glumich." Thus,
in this example there is a purely conventional association between
the sign and the signified. In semiotic terms, when the association
between sign and signified is completely arbitrary, the sign is
referred to as a symbol. With symbols, there is no intrinsic connection
between the form of expression (the sign) and the content that is
expressed (the signified).
are, however, alternatives to the purely arbitrary connection between
sign and signified that is seen in symbols. One of these alternatives
is the icon, which is a sign that bears a physical resemblance to
the signified. For example, the profile of the man on a modern American
quarter is an icon for a specific man who was the first President
of the United States. We can also represent the same man with the
symbol "George Washington" (Figure 8). In this case, the image of
Washington is an icon because it physically resembles the signified
(a particular man with certain facial characteristics). With icons,
the form of the message reflects its content.
8. With a symbol, there is no physical resemblance between
the sign (in this case, the words "George Washington") and its
referent (a particular man with this name). By contrast, an icon
is physically similar to its referent. For example, the icon of
George Washington on the quarter resembles the appearance of a
specific man who was President of the United Sates. Illustration
courtesy of author.
can also be used when the signified is less concrete. For example,
the semiotician Ferdinand de Saussure noted that the concept of
justice is sometimes portrayed by the scales of justice. In this
case, the scales are an icon, because there is similarity between
the sign (the scales that balance two weights) and the signified
(the concept of justice, which involves a balance between transgression
Representations of Chemical Concepts
we return to the pictorial messages previously suggested for CETI,
we can see that they relied heavily on icons. For example, in his
message from 1962, Drake pictorially represented the Bohr model
of the atom, in which electrons are visualized as bodies moving
around a more massive nucleus. In addition to detailing the chemical
composition of deoxyribonucleic acid in his 1974 pictogram, Drake
also included an iconic representation of the double helix, thus
illustrating the structure of this macromolecule. Similarly, the
etched messages aboard the Pioneer and Voyager spacecraft convey
notions of time and distance in terms of characteristics of the
hydrogen atom, which is portrayed by an iconic line drawing.
we have seen, pictorial representations may not be the best way
to overcome the Incommensurability Problem. Yet there is something
very promising about using icons, signs that bear a similarity to
what they represent. To help understand what it would be like to
include nonpictorial icons for CETI, it is helpful first to realize
that icons are not specific to the visual sensory modality. Rather,
it is possible to have a sign that physically resembles the signified
in a nonvisual way. For example, the fly Spilomyia hamifera
beats its wings at a frequency very close to the wing-beat frequency
of the much more dangerous wasp Dolichovespula arenaria.
As a result, when one of these flies is in the vicinity of a group
of these wasps, the fly gains some immunity from attack by insect-eating
birds. The fly's mimicry of the wasps occurs within the auditory
modality; it is not attacked by would-be predators because it sounds
like the wasps. In short, the fly's defense strategy is based on
producing an auditory icon, in which the fly's wing-beating (the
sign) physically resembles the wing-beat of the wasps (the signified).
the same way, icons could function in any sensory modality. Given
that we are not sure which sensory modality will be primary for
extraterrestrials, a sign for interstellar communication that is
not reliant on any particular sensory modality would be preferable.
The critical difference between the iconic approach to communicating
chemical concepts that I discuss next and the standard iconic approaches
to CETI is the electromagnetic signal itself that acts as the sign,
in this case for an object that it resembles. Thus electromagnetic
radiation is used as an iconic representation, allowing a direct
communication of chemical concepts without encoding the message
into a format specific to a particular sensory modality. In short,
we are able to fulfill Gauss's goal of directly communicating messages,
but now over interstellar distances. Thus, the content of a transmitted
interstellar message can be shown directly through its form.
understand how we might use icons to communicate chemical concepts,
it is important to recall that each chemical element can be characterized
by a particular pattern of frequencies of radiation that it gives
off. This characteristic emission spectrum results from the transition
of electrons between orbitals. Specifically, as an electron moves
from a higher to a lower orbital, the atom releases radiation at
a set frequency. For any given transition, the frequency of radiation
is always the same because energy is released only in certain specifiable
quanta, or discrete units.
iconic approach to directly conveying chemical concepts could involve
transmitting signals at multiple frequencies, either simultaneously
or sequentially (see "Traversing the Galactic Darkness," p.14).
The frequencies of these transmissions would correspond to a few
of the emission lines that together best characterize each element.
For example, to transmit an icon representing hydrogen using infrared
wavelengths, we could sequentially transmit narrowband signals at
the following wavelengths: 1875.1 nanometers (nm), 1281.8 nm, 1093.8
nm, 1004.9 nm, and 954.5 nm. These are the wavelengths of the photons
that are emitted as electrons move from one orbital to a lower level
(Figure 9). Specifically, photons having these wavelengths are emitted
when electrons end up at the orbital level with quantum number n
= 3, after having moved down from five higher orbital levels. These
signals would be of very long duration and would be repeated many
times to increase the likelihood of their being detected. This would
be particularly important because of the wide range of frequencies
these signals would cover.
9. Each chemical element shows a characteristic pattern of
frequencies that results from its emission of photons. When an
electron moves from a higher to a lower orbital, a photon of a
specific wavelength is released. For example, when a hydrogen
atom's electron moves from the fourth orbital to the third, radiation
is emitted with a wavelength of 1875.1 nanometers. Hydrogen can
be represented iconically by transmitting a series of signals
at the frequencies corresponding to these orbital transitions.
This figure illustrates some of the wavelengths associated with
the Paschen series, in which electrons start or end at the third
orbital. Illustration courtesy of author.
this approach, rather than sending a signal with a purely conventional
structure, the structure of the signal would physically resemble
the concept being communicated. A range of concepts could be shown
with this method, from notions of energy levels and orbital transitions
to more complex concepts like chemical reactions. And although I
have emphasized phenomena associated with electronic transitions,
the same approach could be used for other transitions, for example,
between vibrational and rotational energy levels of molecules. Relying
more on iconicity that does not involve additional coding of the
message may help address the Incommensurability Problem. The message's
recipients are pointed directly toward the phenomena of interest,
and not toward our models of these phenomena. Rather than providing
a pictorial representation of, for example, the Bohr model of the
atom, we de-emphasize particular models of atoms and attempt to
show atomic phenomena directly.
Partial Conventionality of Icons
far we have spoken of a sign as if it is in a simple dyadic relationship
with its signified. By this analysis, icons seem superior to symbols
for establishing communication with extraterrestrials about whom
we know very little because there is a natural connection between
the sign and that which is signified. This contrasts with the conventional
relationship between sign and signified in symbols.
if we examine the problem from a more complete perspective, things
are not so simple. In reality, the sign and the signified are in
a triadic relationship between the sign, the signified, and the
interpreter of the relationship between the sign and the signified.
Thus, the similarity that exists between an icon and its referent
does not exist independently of the intelligence perceiving this
similarity. Although in iconicity there is a natural connection
between the sign and the signified, this connection cannot exist
without intelligence to observe the connection.
the problem of iconicity is that similarity is in the eye of the
beholder. And because we do not know what extraterrestrials will
be like, we cannot be sure that what to us seems an obvious similarity
will be seen as such by intelligence with a different biology, culture,
and history. Thus, judgment of similarity is not purely objective,
but is influenced by a variety of factors that impact conventions
have attempted a partial solution by de-emphasizing models of atomic
structure and focusing more on the phenomena by which we have constructed
our models. But we cannot be absolutely certain that even if extraterrestrials
conceive of the material basis of reality in terms of atomic and
molecular structure, they will necessarily attend to the phenomenon
of spectral emissions to construct their models. As much as we try
to present the phenomena in themselves, it is difficult to bracket
- or even identify - the presuppositions that we make when identifying
the phenomena we assume to be of universal significance. But with
each attempt that we make, we will increase our chances of constructing
messages that will be understood by extraterrestrials.
This article originally appeared in SETIQuest, Volume 4, Nos. 1
and 2. (c) 1998 by Helmers Publishing Co., Inc. All rights reserved.
Reprinted with permission of copyright holder. SETIQuest is no longer
published. On the Web, visit www.setiquest.com
to order individual issues or the entire set of 16 SETIQuest issues
published from 1994-8. A different version of this article appeared
in SPIE Proceedings, 1996, Volume 2704, pp. 140-9.
The title of this article was inspired by Harlow Shapley's book,
The View from a Distant Star: Man's Future in the Universe,
Basic Books, New York, 1963.
is a social scientist at the SETI Institute in Mountain View, California,
where he conducts research on the cultural aspects of SETI and composes
interstellar messages. He can be reached via email at firstname.lastname@example.org.