Miscellaneous articles

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 regular internet rumour is that radio astronomers searching for signals from extraterrestrial civilisations have detected an alien broadcast. The rumour has apparently always been 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, we'd understand from the mere detection of such a message 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 also being the answer to the other.

However, the advantage of attempting to answer the latter question is that we've examples of messages that have 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 solar system. Mission completed, they then continued under their own momenta on a trajectory out of the solar system, towards the stars.

A painting of one of the Pioneer probes.

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

Attached 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'.

A photograph of the pioneer plaque attached to Pioneer 10.

The plaque attached to Pioneer 10 (Credit: NASA)

A close-up photograph of the plaque.

A photograph of the plaque that was 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. And 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 one of the designers of the plaque, the scientist and writer Carl Sagan, the purpose of the plaque is to tell any aliens who find it ‘a little bit of where we are, when we are, and who we are’.

A photograph of Carl Sagan holding the plaque.

Carl Sagan holding the plaque (Credit: NASA)

However, the Cambridge University philosophy 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 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 coded 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 consider 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, grammar and, in the case of writing, punctuation marks 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 that isn’t true. 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 the whole probe would be completely destroyed upon hitting a planet’s atmosphere at a tremendous speed.

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, 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 is able to send, and the relatively limited capacity of the probe’s power source, which will very likely be fully drained.

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', it's obviously 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 attached to.

Also, there's no plausible reason for us to fix to the probe 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's also no plausible cause of 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.

This positioning of the plaque also rules-out the ‘mere decoration’ interpretation. Plus, it'll surely seem implausible that we'd want to use this extraordinary moment of first contact to merely present some of our art and not communicate any message at all, especially given that such art obviously could easily only be aesthetically pleasing from our point of view.

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

In short, the aliens will know that if the plaque carries a coded message, then that 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 the academic paper 'A Message from Earth' their thinking behind each part of the message. Understanding some parts requires knowledge of atomic physics and astrophysics, and so I'll focus on the two parts which don’t require such uncommon knowledge to understand.

Who we are

The markings on the plaque that will be the easiest for the aliens to interpret will be the outline of the probe the plaque is attached to. Superimposed onto that drawing are the drawings of a man and a woman.

A reproduction of the line drawings of a nude man and woman.

The part of the plaque showing our appearance

And Crane asks how the aliens will be able to work-out that these markings represent us and not something else. However, it’ll actually be quite easy for the aliens to work out that these markings depict us – as Crane himself unintentionally helps to demonstrate by providing an unconvincing example of what 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 won't 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.

Consider again the plaque as a whole:

A photograph of the plaque.

The drawing of us clearly has two distinct attributes: it’s composed of irregular lines, in stark contrast to the geometrical lines and curves which make-up all of the other parts of the message, and it has a greater level of detail than the other parts.

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

All of this will together make it obvious to the aliens that these markings are a naturalistic drawing of us, and that the outline of the probe is being used to indicate our size, given that its presence doesn’t 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 of good will, or even as a gesture of anything, then they equally have no reason to interpret it as a gesture of any of the above negative attitudes.

Also, regarding point 1 above, given that the aliens will know that the message is addressed to any civilisation that happens to find the probe, they’d have no reason to think that we’d want 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.

Regarding the possibility of the aliens wrongly interpreting the raised hand as the normal position of this part of the body, we continually move our bodies into different positions, and so although we don’t normally have one of our hands raised, neither do we normally adopt any other bodily position.

Therefore Crane should be equally concerned about the aliens wrongly concluding that we’re normally standing, and with both hands down by our sides, as the woman 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.

And even if they do wrongly conclude that the raised hand is the normal position of this part of the body, that wouldn't actually matter greatly.

Regarding '... they would have to recognise that the raised hand was a sign of peaceful greeting rather than ... the normal position of this part of the body' – my emphasis – the intended interpretation of the raised hand actually isn't critical to this introductory message. Indeed, as Sagan and co explained in their paper:

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 parody version of the plaque, with the man instead lying horizontally while levitating.

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)

Where we are

The aliens will also undoubtedly correctly interpret the markings which make-up the diagram of our solar system.

A reproduction of the diagram of our solar system.

The part of the plaque showing our solar system

Crane argues that these markings 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 above – and that we’ll know that they’ll know that they live in, or at least originate from, a star system – see point 5 above. Therefore it’ll be obvious to the aliens that the large circle followed by a line of smaller circles of varying size is a diagrammatic cross-section of our home star system – see points 2 and 4 above.

Also, because they’ll know we’ll have designed the message in a way that minimises ambiguity – point 3 above – they’ll know that these markings won’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 small circle from the large circle, and with the line joining the two obviously representing the probe’s flight path.

Also, the aliens' conclusion that the markings which make-up the drawing of the man and woman depict us – for the reasons I gave above – is another reason why they'll not misinterpret the diagram of our solar system as Crane suggests.

And yet another reason why the possibility of such a misinterpretation can be ruled-out is that, as I explained, science and technology – which the Pioneer probes are a product of – is dependent on an ability to probe and manipulate the world, which a species of limbless disc- or ball-shaped creatures wouldn't be able to do.

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 the Sun – is 𝗜𝗜, and alongside Earth – third from the Sun – is 𝗜𝗜𝗜, and alongside Saturn – sixth from the Sun – is 𝗜𝗜𝗜𝗜𝗜𝗜𝗜.

Given that these markings will, to the aliens, not unambiguously resemble anything recognisable that could be physically located next to each planet – see points 3 and 4 above – 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 numerals representing physical quantities.

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

Of course, these numerals are contrary to the principle, in point 2 above, that the aliens mustn’t require 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 mean.

They’ll need to work-out both what the quantities are and what they refer to. The latter will be obvious. Knowing the different distances of each planet from the star they orbit will greatly help to identify the star system – see points 1 and 5 above – and the absence of these markings next to that star will confirm that the quantities are indeed such distances.

Regarding what the quantities are, they'll need to work-out 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 I, two by II, three by III, and so on. However, this system is obviously only practical for representing small numbers.

An alternative system 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 optimal system is one that uses different symbols for different numbers only up to a certain number, and then uses different combinations of these symbols for larger 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, and 9. When we reach ten, we return to the start of this sequence, but also put the symbol 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, which is 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, and so change the symbol there 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, 103 … . And so on.

Therefore, although the decimal system does use a different symbol for each number, the symbols for numbers above nine are made-up of different combinations of the symbols for zero to nine.

The decimal system is the base 10 numeral system, given that each column resets upon reaching ten. 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 used in most countries 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 numerals using the binary system – see point 3 above.

The aliens 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.

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 the decimal representation of twelve, the 0 in 012 is unnecessary, so the leftmost 0 in 01100 is unnecessary in the binary representation of twelve.

And by analysing the numerals alongside each planet they'll find that the leftmost column is always 𝗜, whereas the rightmost column can be either or 𝗜. 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 𝗜𝗜next to the first planet out from the Sun represent ten, and the markings 𝗜--𝗜𝗜 next to the second planet out represent nineteen, and the markings 𝗜𝗜𝗜next to the third planet out represent twenty-six, and so on.

And, given that there are no suitable units of distance available anywhere on the plaque, the aliens will know that these quantities represent relative, rather than actual, distances. Also, 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 sum, contrary to Crane's amused rejection of the idea that the Pioneer plaque will be decipherable by aliens, they'll actually be able to understand the coded message on the plaque with relative ease, because the code was designed to be easily cracked. And the ease with which a message of introduction will be read in the markings will itself 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'd be able to understand an initial communication from aliens – 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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I found the quote at the beginning of this article in the book Cosmos, by Carl Sagan.


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.