Communicating With Aliens

Is there a fundamental barrier to us understanding each other?

An earlier version of this article was published in the journal Think in 2007.

Any messages transmitted from outer space are the responsibility of the BBC and the Post Office. It is their responsibility to track down illegal broadcasts.

– UK Ministry of Defence, as reported in The Observer, 26 February 1978

A rumour recently spread around the internet that radio astronomers searching for signals from extraterrestrial civilisations had detected an alien broadcast. The rumour was false, but the possibility of such a message being detected in the future raises an interesting question: would we be able to understand it?

Of course, the mere detection of such a message would immediately communicate to us that we’re not alone in the universe. But would we be able to decipher its contents, or could there perhaps be some fundamental barrier to us understanding a message from an alien intelligence?

A message from Earth

Underlying this question is the more general question of whether any two alien species of at least our level of intelligence, and scientific and technological advancement, would be able to communicate on first contact. Therefore, the reverse of the above question – Would an alien species of at least our level of intelligence and advancement be able to understand a message from us? – is fundamentally the same question, with the answer to either question also being the answer to the other.

However, the advantage of attempting to answer the latter question is that we have examples of messages that have actually been sent. Therefore, to address the question in the introduction I’ll address its reverse.

In 1972 NASA launched the Pioneer 10 and 11 space probes, which explored the outer planets of our solar system. Mission completed, they then continued under their own momenta on a trajectory out of the solar system, towards the stars.

An artist’s impression of one of the Pioneer probes passing Jupiter (Credit: NASA)

Bolted to the side of each probe is a small engraved metal plaque introducing us to any intelligence that finds this celestial ‘message in a bottle’.

The plaque attached to Pioneer 10 (Credit: NASA)

A photograph of the plaque attached to Pioneer 10 (Credit: NASA)

On the right side of the plaque there’s a line drawing of a man and a woman standing in front of the probe. Along the bottom there’s a diagram of our solar system. Above that is a diagram which both locates the Sun in our galaxy and identifies the epoch in which we live. And above that is a diagram of a fundamental physical process in nature called the hyperfine transition of hydrogen. This diagram defines the units for certain values given elsewhere on the plaque.

In the words of the designers of the plaque, which included the late scientist and writer Carl Sagan, the purpose of the plaque is to tell any aliens who find it ‘where we are, when we are, and who we are’.

Carl Sagan (Credit: NASA)

However, the philosopher professor Tim Crane wrote that he finds the idea that the Pioneer plaque would be decipherable by aliens ‘very humorous’. His reasoning can be summarised as follows:

While we ourselves know that the markings on the plaque represent things, and what things they represent, and that a message is being conveyed by their representation, the aliens will obviously not have access to what was going through our heads when we created the plaque. Therefore, how will they be able to decipher this coded message if they don’t have access to the code, or even know there’s a message to be deciphered?

However, Sagan was gifted not just as a scientist but also as a communicator, winning the Pulitzer Prize for non-fiction. Also, the message he helped design is aimed at alien scientists, not alien philosophers. When we study the markings on the plaque as carefully as any alien scientists who find the plaque undoubtedly will, and with the scientific knowledge that they’ll undoubtedly have, it becomes clear that the design of the plaque is much more clever, and subtle, than Crane assumes.

What they’ll know about what we know

Communication using language obviously requires the receiver to have an understanding of the words, punctuation marks and grammar of the language being used. And perhaps Crane is so accustomed to using language to communicate that he’s assuming that all communication depends upon the use of conventions already understood by both parties. But this isn’t the case. Contrary to Crane’s above reasoning, the aliens will be able to work-out quite a lot about what will have been going through the heads of the creators of the Pioneer plaque.

The intended target of the message is an alien species of at least our level of intelligence and advancement. Therefore if the message is received by aliens of lower intelligence and advancement, who then fail to understand the message, that wouldn’t constitute a failure of Sagan and co to achieve their aim.

Also, there doesn’t anyway seem to be a way for the plaque to come into the possession of aliens who haven’t developed space travel, given that it would melt due to friction upon entering a planet’s atmosphere. In fact, any recipients of the plaque will almost certainly have developed interstellar space travel, given that the chances of the probe even entering a star system are negligible, due to the enormous distances between stars.

Upon capturing the probe, the alien scientists will, being scientists, undoubtedly carefully scrutinise it. And with their scientific knowledge they’ll easily be capable of determining how each part works and therefore what it was designed to do. They’ll also notice the relative weakness of the signal the radio transmitter would have been able to send, and the relatively limited capacity of the probe’s power source. From all of this they’ll easily infer that this machine was designed to merely explore the home star system of its creators, gathering data and radioing it back to the home planet, and that once the probe had completed its mission it would have travelled out of that star system under its own momentum.

All of this knowledge, combined with the realisation that the plaque is unrelated to the functioning of the probe, will undoubtedly lead the aliens to conclude that the obviously deliberate markings on the plaque are most likely an introductory message to any intelligence that happens to find the probe.

Crane argues that the aliens could just as easily conclude that the markings are ‘random scratches on the plate, or mere decoration’.

Regarding 'random scratches', this is an extremely unlikely interpretation given that it's implausible that something scratching the metal in a random way would create perfectly straight lines and a line of perfect circles, and even an outline of the probe that the plaque is bolted to. Also, there's no credible reason for the creators of the probe to fix to it a small metal plate that has random scratches on it, or that was originally blank but acquired such scratches on its journey. In the latter case, there also doesn’t seem to be any likely cause of such scratches in the vacuum of space. The only conceivable form of physical contact is from micrometeoroids, which would simply pit the metal. Also, the plaque was deliberately attached to the probe with its etched side facing into the probe, to protect the message from micrometeoroids.

Regarding ‘mere decoration’, there's also no credible reason why the creators of the probe would want to use this extraordinary moment of first contact to merely present some of their art and not communicate any message, especially given that such art could easily only be aesthetically pleasing from its creator's point of view.

At the very least, it will be seen as a reasonable possibility that the markings are an introductory message, and an awareness of any other possibilities will surely not deter the aliens from trying to decipher the markings in case there’s indeed a message. And, in doing so, they’ll have several pieces of knowledge to guide them:

    1. They’ll know what the contents of such a brief introductory message will most likely be, and not be. For example, while the creators of the probe may provide information about their location in the galaxy, or their appearance, they’re unlikely to provide, in such a brief introductory message, information about their diet, or about other organisms on their planet.

    2. They’ll know that we’ll know that understanding such a message mustn’t require a prior understanding of the meaning of arbitrary symbols – such as words – and only drawings and diagrams can therefore be used.

    3. They’ll know that we’ll know that such a message must be designed in a way that minimises ambiguity over its contents.

    4. They’ll know that we’ll know that understanding such a message mustn’t require any prior knowledge of unique aspects of our world.

    5. They’ll know that we’ll know the minimum level of scientific knowledge of a species that has reached a level of technology that has enabled them to retrieve the probe from interstellar space.

    6. They’ll know that we’ll know that, for the same reason, they must possess a certain minimum level of intelligence – these six points being examples of that intelligence.

In short, the aliens will know that if the plaque carries a coded message, then the message will have been designed to be as easily decipherable as possible by a species with their level of intelligence and knowledge.

Sagan and co explained in an academic article their thinking behind each part of the message. However, understanding some parts of the message requires knowledge of atomic physics and astrophysics. The rest of this article will focus on the two parts which don’t require such uncommon knowledge to understand, and which are also the two parts that Crane uses to illustrate his criticism.

Who we are

The set of markings on the plaque that will be the easiest for the aliens to interpret will be the outline of the probe the plaque is bolted to. Superimposed onto that drawing are the drawings of a man and a woman. And Crane asks how the aliens will be able to work-out that these markings represent us and not something else.

The part of the plaque showing our appearance

However, it’ll actually be quite easy for the aliens to work out that these markings depict the creatures that created the probe – as Crane himself indirectly helps to demonstrate, by providing an unconvincing example of what else they could confuse the markings for.

He claims there’s no reason why the aliens would conclude ‘that the drawing of the man and woman symbolise life-forms rather than chemical elements…’. However, first, the aliens will know that these markings are unlikely to be arbitrary symbols that represent chemical elements – see point 2 above. Second, the markings couldn’t be mistaken for drawings of chemical elements, because there’s no resemblance. Third, the markings also couldn’t be mistaken for diagrams of chemical elements, because even such a simplified representation would have at least some resemblance to what it represents.

Upon looking again at the plaque as a whole:

it’s clear that this part of the message has two distinct attributes: it’s composed of irregular lines, in stark contrast to the geometrical lines and curves of which all of the other parts of the message are composed, and has a greater level of detail than the other parts.

Also, it will seem likely to the aliens that we would try to give an impression of our appearance in such an introductory message – see point 1. And there’s nothing else that these markings could likely represent – see points 2 to 5 – nor anything else on the plaque that most likely represents a creature – see point 3, and the next section.

All of this will together make it obvious to the aliens that these markings are most likely a naturalistic drawing of the creatures that made the plaque, and that the outline of the probe is being used to indicate their size, given that its presence doesn’t seem to serve any other likely purpose.

It’s of course true that, as Crane points-out, the raised hand of the man is unlikely to be recognised as a sign of friendship. He writes:

And – perhaps most absurd of all – even if they did figure out what the drawings of the man and woman were, they would have to recognise that the raised hand was a sign of peaceful greeting rather than of aggression, impatience or contempt, or simply that it was the normal position of this part of the body.

However, if the aliens have no reason to interpret the raised hand as a gesture, and still less one of good will, then they’ll equally have no reason to conclude it’s a gesture of any of the above negative attitudes.

Also, given that the aliens will know that the message is addressed to any civilisation that happens to find the probe – see point 1 – it’s unlikely that they’d think that we’d be motivated to express aggression, contempt or – ‘perhaps most absurd of all’ – impatience towards a species we’ve never met, and whose existence and location we’ve no knowledge of.

Perhaps Crane also fears that the aliens will think that the man is gesturing all three attitudes, and that the inclusion in the message of details of where to find us will lead them to conclude that these angry aliens are so impatient for an interstellar dust-up that they’ve resorted to trying to randomly provoke their galactic neighbours.

Also, the interpretation of the raised hand as a gesture of friendship is clearly not critical, contrary to what Crane writes. Indeed, as Sagan and co explained in their article:

A raised outstretched right hand has been indicated as a ‘universal’ symbol of good will in many human writings; we doubt any literal universality, but included it for want of a better symbol. It has at least the advantage of displaying the opposable thumb.

Although, alternative designs have been suggested:

A proposed solution, by Edward Tufte, to how the Pioneer plaque ‘might have escaped from its conspicuously anthropocentric gestures by showing instead the universally familiar Amazing Levitation Trick’. Copyright Edward Tufte – reproduced by permission.

Similarly, regarding Crane’s concern that the aliens might wrongly conclude that the raised hand is ‘the normal position of this part of the body’, it wouldn’t actually matter greatly if they did.

Also, we continually move our bodies into countless different positions, and so although we don’t normally have one of our hands raised, neither do we normally adopt any other particular bodily position. Therefore, Crane should be equally concerned about the aliens wrongly concluding that we’re often standing, and with both hands down by our sides, as the women is.

But the aliens will themselves have movable limbs of some sort: the development of science and technology – including the building of spacecraft – is dependent on an ability to probe and manipulate the world. Therefore, they’ll know they shouldn’t make any assumptions about ‘normal’ bodily positions from the drawing.

Where we are

It also seems unlikely the aliens won’t be able to interpret the diagram of our solar system as such.

The part of the plaque showing our home star system

Crane argues it could just as easily be interpreted as depicting ‘the shape of the designers of the spacecraft’.

But the aliens will know that we may provide information about where we live in such an introductory message – see point 1 – and that we’ll know that they’ll know that they live in, or at least originate from, a star system – see point 5. Therefore, it’ll surely be obvious to the aliens that the large circle followed by a line of smaller circles will very likely be a diagrammatic cross-section of our home star system – see points 2 and 4.

Also, because they’ll know we’ll have designed the message in a way that minimises ambiguity – point 3 – they’ll be confident these markings don’t represent something else in a way that could be so easily confused for a star system.

And the correct interpretation of these markings will be further confirmed by the small outline of the probe, with its antenna pointing towards the third planet out from Sun, and with the line joining the two obviously representing the probe’s flight path.

Also, the much greater likelihood of the drawing of the man and woman representing the creators of the probe – for the reasons given above – will also reduce the chances of the aliens misinterpreting the diagram of our solar system in this way.

Moreover, this misinterpretation is unlikely simply because, as explained, science and technology, which the Pioneer probes are a product of, is dependent on an ability to probe and manipulate the world, which surely wouldn’t be possible for a species of limbless disc- or ball-shaped creatures.

However, there’s one aspect of this part of the message that’ll require more thought to decipher, although not much more.

Alongside each planet is a row of the same two markings – ‘I’ and ‘–’ – repeated in different combinations. For example, alongside Mercury – first from Sun – is ‘II–’, and alongside Earth – third from Sun – is ‘III–’, and alongside Saturn – sixth from Sun – is ‘IIIIIII’.

Given that these markings will, for the aliens, not unambiguously resemble anything recognisable that could be physically located next to each planet – see points 3 and 4 – and also that rows of different combinations of the same two markings are also alongside other parts of the message, it’ll be obvious to the aliens that these markings are very likely numerals representing physical quantities.

They’ll know that these markings are unlikely to be our names for these planets, given that what we happen to call them will obviously not be of much use to them, whereas certain physical quantities relating to the planets will help them identify our star system – see point 1.

Of course, these numerals are contrary to point 2, that understanding the message mustn’t require prior knowledge of the meaning of arbitrary symbols. However, unlike an engraving of ‘Greetings!’, the aliens will easily be able to work-out what these symbols represent.

They’ll need to work-out both what the quantities are and what they refer to.

The correct answer to the latter will be fairly obvious. Knowing the different distances of each planet from Sun will greatly help to identify our star system – see point 5 – and the absence of these markings next to Sun will confirm that the quantities are indeed such distances.

Regarding the question of what the quantities are, this in turn raises the question of what kind of numeral system is being used.

The simplest numeral system possible involves representing a number by repeating a particular symbol the relevant number of times. For example, the number one could be represented by ‘|’, two by ‘||’, three by ‘|||’, and so on. However, this system is obviously only practical for representing small numbers.

One alternative is to use a different symbol for each number. However, this system is also impractical, because we’d have to remember an impossibly large number of symbols, even for everyday use.

The answer is to use different symbols for different numbers only up to a certain number, and then use different combinations of these symbols for greater numbers.

For example, the decimal system, which we use in everyday life, uses different symbols for each number from zero to nine – which in most countries are, in increasing order, ‘0’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’, ‘7’, ‘8’, ‘9’. When we reach ten, we return to the start of this sequence, but also put a ‘1’ in the ‘tens column’, to the left of the ‘ones column’: ‘10’, ‘11’, ‘12’, ‘13’ … . So, for example, ‘13’ represents one ten plus three ones: thirteen. And when we reach twenty, we again restart the sequence in the ones column, but increase the number in the tens column by one, to ‘2’. And when re reach one hundred, we restart both of these columns, and also put a ‘1’ in the ‘hundreds column’, to the left of the other two: ‘100’, ‘101’, ‘102’ … . And so on. Therefore, although the decimal system does use a different symbol for each number, the symbols for numbers above nine consist of different combinations of the ten symbols for zero to nine.

The decimal system is the ‘base ten’ numeral system, and the base of a numeral system can be any number – you would simply require that number of different symbols. The ‘base 2’, or ‘binary’, system is the second-most simple numeral system, given that it only requires two symbols, to represent zero and one. Each column in the binary system resets upon reaching two, with the column to the immediate left increasing by one. Therefore, instead of having columns representing, from right to left, ones, tens, hundreds, thousands, and so on, the columns in binary represent ones, twos, fours, eights, sixteens, and so on. Zero to ten are therefore represented in the binary system, using the numerals we use for zero and one, as follows: ‘0’, ‘1’, ‘10’, ‘11’, ‘100’, ‘101’, ‘110’, ‘111’, ‘1000’, ‘1001’, ‘1010’ – pronounced, respectively, ‘zero’, ‘one’, ‘one zero’ (not ‘ten’), ‘one one’ (not ‘eleven’), ‘one zero zero’ (not ‘one hundred’), and so on.

Given that the binary system is the simplest practical numeral system, and that the markings that apparently represent quantities on the plaque consist of just two symbols, it’ll be obvious to the aliens that these markings are very likely numerals using the binary system – see point 3.

They also won’t have any difficulty working-out which symbol represents zero and which represents one, and nor that the columns increase in magnitude from right to left rather than vice versa. This is because they’ll know that the column of greatest magnitude will always contain one, and never zero, which would be redundant. That is, just as, in our decimal representation of twelve, the ‘0’ in ‘012’ would be unnecessary, so the leftmost 0 in 01100 isn’t necessary in our binary representation of twelve. And the aliens will, by analysing the numerals alongside each planet, notice that the leftmost column always has the same symbol – ‘I’ – whereas the rightmost column can have either symbol. Therefore, they’ll know both that the leftmost column is the column of greatest magnitude, and that ‘I’ represents one, while ‘–’ represents zero.

The aliens will then know that the markings next to the first planet out from Sun – ‘II–’ – represent ten, and those next to the second planet out – ‘I­­––II’ – represent nineteen, and those next to the third planet out – ‘III–’ – represent twenty-six, and so on. Also, there are no suitable units of distance available anywhere on the plaque, and so the aliens will know that these quantities represent relative, rather than actual, distances. And once they see that the nine quantities increase with each planet out from the home star, their confidence in their interpretation of these markings will be strengthened even further.

Understanding them

In this way, the aliens will very likely crack the coded message on the Pioneer plaques with ease, because the code was designed to be easily cracked. And the very ease with which a message of introduction will be read in the markings will increase even further their confidence in that interpretation of those markings. Therefore, Sagan and co were far from the naive optimists Crane believes them to have been.

So, the answer to the original question of whether we would, at least in theory, be able to understand an initial communication from an alien species – whether the drawings, diagrams, and basic symbols, that make-up the message are marked on a surface or transmitted via radio waves – is a qualified ‘yes’: as long as it’s our scientists, and not our philosophers, who are given the task of deciphering the message.

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The content of this article can change, and so referencing of it should state the date of reading. Also, you can save the current version in the Internet Archive, and then link to the archived copy.


I found the quote at the beginning of this article in the book Cosmos, by Carl Sagan.