This is an updated and edited version of a past post on my blog. The part about chirality has been added and the rest of the post has been edited.
What do aliens want?
The search for extraterrestrial intelligence (SETI) has traditionally hinged on detecting electromagnetic waves, most commonly radio waves but also infrared and x-ray radiation as well as optical pulses at specific frequencies. But in the absence of knowledge about the specific nature of extraterrestrial civilizations, we need to explore all sources of communication possible and not just ones based on electromagnetic waves. Thus the message we would send or receive could and should include everything from symbolic signals to actual physical samples of material signifying the presence of intelligent life. SETI is an endeavor fraught with such momentous potential significance that it would be foolish to hinge it on physics alone. We need to employ other sciences in its service.
For doing this it’s extremely valuable to turn the question around and ask what we would do if we were to announce our presence. What kind of messages would we send to a potential ET listener? This line of questioning is valuable but it always includes the significant pitfall of suffering from anthropocentrism. It’s all too easy to believe that ET thinks just the way we do. Nonetheless, thinking from a human perspective opens the way toward understanding various potential forms of communication. So with the caveat that we should not constrain ET to fit our shoes too well, it’s worth pursuing this line of thinking.
Assuming that the listening civilization is at least as advanced as ours and possibly more and assuming that they are actively listening and sending, there are a few critical requirements. One is that the message should be, to the best of our knowledge, as non-anthropomorphic and objective as possible. For instance mathematics is safely considered to be the universal language for describing nature and it is often assumed that ET would use some kind of mathematics to describe and interpret nature. Similarly whatever physical message we send should appeal to universal laws and constituents of matter that could be considered agnostic to human beliefs and definitions. Once this condition is satisfied, the second requirement of course is that the message should be unambiguously construed as ‘artificial’ and not of natural provenance. There should be clear evidence of deliberate ‘design’ in its construction.
This requirement is harder to satisfy than it seems. As we are well aware, there are numerous examples of natural entities which suffer from the ‘illusion of design’. Seashells, snowflakes, the myriad anatomical structures inside organisms and life itself all suffer from the illusion of design. No wonder that creationists and intelligent design proponents have seized on all of these and declared them to be the work of an intelligent designer. In fact if we didn’t know better about the process of evolution and natural selection that has fashioned these complex structures, we too would think of them as designed, and indeed we did until Darwin came along and produced his great piece of work (Richard Dawkins’s book “The Blind Watchmaker” has a great chapter on these illusions of design). So when selecting a message to transmit, we need to be careful that it can be clearly distinguished between one which is natural but creates an illusion of design and one which must be actually designed by intelligent beings like ourselves. This requirement for making sure that a message looks designed has led the radio astronomy camp to suggest sending out messages that communicate prime number sequences. If after waiting for some time, we receive a message containing the next prime number in the sequence, we could be almost certain that the message was sent by a civilization which had discovered mathematics and factorization and which therefore could be considered ‘intelligent’.
Based on this background, I asked myself the following question:
‘As a chemist and especially as an organic chemist, how would I transmit a molecular message to an alien civilization such that the message would almost certainly be construed as designed by an intelligent being?’
Here are two potential solutions to the problem. My focus is on the recognition of the messages as unambiguously artificial once they are found. The initial communication of these messages to their intended recipients is a very different kind of problem which I don’t address in this post.
Hope from an isotope
Organic chemists are well aware of differences between naturally occurring and artificially synthesized molecules. Chlorophyll, penicillin and quinine are examples of naturally occurring molecules while nylon, Viagra and LSD are unambiguously synthetic. Thus a chemist’s impulsive reaction might be to suggest sending samples of nylon or LSD out to potential ET listeners as decidedly ‘designed’ entities. But recall what we said about creating the illusion of design. Viagra may be man-made, but there’s really no reason why it cannot be made by nature in principle, even if it may be very unlikely in practice. Nature is wonderfully adept at producing an astonishing variety of molecular structures. For all we know, we might find Viagra someday as a vital communication molecule in some obscure marine sponge. To provide the strongest evidence of artificial design, we need to send a molecular message that is unlikely to be naturally designed not just in practice but also in principle.
That’s when it hit me that we could make a good case for an unambiguously designed message by transmitting molecules that have all or many of their hydrogen atoms replaced by deuterium. Recall that deuterium (D) and tritium (T) are the two isotopes of hydrogen, differing by the presence of one neutron. But they are spread out exceedingly thin among the major isotope of hydrogen (H) that we all know and love. Hydrogen is the most abundant element in the universe but deuterium comprises 1 atom in about 6000 of hydrogen and amounts to only 0.02%, while tritium is even scarcer. These isotopic abundances of D and T are constrained by the fundamental laws of physics governing nuclear stability and are extremely unlikely to be different under any circumstances anywhere in the universe. Given the universal low abundance of D, the probability of, say, a molecule of benzene containing only D being synthesized naturally in the universe under any conditions is vanishingly small. On the other hand, organic chemists can and do make molecules containing D using their bag of chemical tricks. The discovery of a deuterated molecule in outer space should thus point almost unambiguously to an artificial origin. The molecule need not even be fully deuterated since even partial deuterium enrichment is very unlikely to occur naturally. Full replacement by deuterium however would cinch the evidence.
Along with radio and infrared waves, we should thus also try to probe the presence of deuterated compounds in deep space. Fortunately we have several spectroscopic techniques to detect deuterium that include extremely sensitive mass spectrometry methods. The problem with deuterium is that it might be hard to detect against the abundant background of normal hydrogen. Tritium could have been possibly used to circumvent this problem since its radioactivity would make it stand out against the background. Unfortunately the half-life of tritium is only 12 years so it’s useless as an emissary of interstellar communication.
For any intelligent civilization, the advantages of sending out deuterated molecules would be many. For one thing, virtually any all-D molecules would do the trick. Simple deuterated molecules are as unlikely to have been naturally synthesized as complex deuterated molecules. As noted above, even all-D benzene could be a signature of intelligent life. So would all-D methane. Using simple heavy water (D2O) is another option. This wide berth in picking the exact molecular structures frees us up to focus on optimizing other important properties of the molecules like resistance to the rigors of outer space (extreme heat, cold, radiation etc.). Since even simple deuterated molecules would serve our purpose, chemists won’t have to go to great lengths in terms of the actual synthesis. Plus as mentioned above, even partial enrichment in D would work. Another idea comes from the comments section of my original posts where some readers recommended transmitting radio signals at the frequency of deuterium itself or deuterated water.
This strategy of transmitting isotopically enriched molecules could be extended to other elements. What about carbon, the element of life? The most abundant isotope of carbon is carbon-12 (12C). 13C makes up about 1% of the rest while radioactive 14C comprises as little as 0.0000000001%. Just like T, 14C is a potentially valuable but unfortunately useless isotope because of its half-life of 5700 years. That leaves 13C as the ideal candidate. Similar to D, the probability of a molecule naturally enriched in 13C is tiny, and therefore the discovery of a 13C enriched molecule would also strongly suggest an artificial origin. But 13C has other advantages. It is a magnetically active isotope of carbon which can be detected using Nuclear Magnetic Resonance (NMR) spectroscopy; organic chemists use it all the time in deducing the structures of complex molecules by enriching them in 13C. A molecule enriched in 13C would not only signify an artificial origin but it would also provide the added bonus of revealing its own identity through NMR spectroscopy. Just like a message containing prime numbers would reveal knowledge of mathematics, a message containing a swarm of 13C-enriched molecules would reveal knowledge of chemistry and NMR spectroscopy. A civilization that has discovered nuclear magnetic resonance could be considered reasonably intelligent.
I propose therefore that the search for extraterrestrial intelligence should include the search for molecules enriched in deuterium, carbon-13 and minor isotopes of other elements in addition to more traditional signals like electromagnetic radiation.
Looking-glass molecules to the rescue
In Alice in Wonderland, Alice steps through the looking glass and suspects that “Perhaps looking-glass isn’t good to drink”. In saying this Alice (and Lewis Carroll) were ahead of their times since modern chemistry confirms their suspicion; looking-glass milk would likely be made of right-handed amino acids and proteins which our bodies constructed from left-handed proteins would not be able to process.
The presence of biomolecules having a certain handedness or “chirality” is something that we take for granted and is a fundamental concept in chemistry. Chirality refers to two molecules which have otherwise identical properties - like your right and left hands - which are non-superimposable mirror images of each other. With very few exceptions, all amino acids that sustain life are left-handed (L) and all sugars are right-handed (D). Some appeals to parity arguments from theoretical physics have tried to account for this bias, but currently there seems to be no real reason why we could not imagine a looking- glass world made out of left-handed sugars and right-handed amino acids.
Could we exploit our knowledge of chirality to send an unambiguously artificial chemical message to ET? Isolated D and L amino acids have both been discovered in space and are thought to be produced by abiogenic processes. Some meteorites are thought to contain a slight excess of L amino acids, but the discovery of either form by itself won’t point to synthetic origins since both can be produced by cosmic processes with equal probability. Let’s start by considering a peptide as a string of Ds and Ls. We have already noted that a string composed entirely of Ds or Ls, for instance
may not be very helpful since D and L amino acids found in space could possibly form peptides by simple condensation reactions (although the discovery of a long peptide may itself be revealing)
How about a mixture of D and L amino acids in a short peptide? Mixtures might suggest deliberate tinkering, but even there the exact composition matters. Let’s look at a mix of Ds and Ls. Now things get more interesting since one can think of constructing different patterns. For instance consider the following strings:
In the first case D and L alternate. The second case presents two Ds alternating with one L. The third one has the patterns “2D-1L” and “2L-1D” alternating with each other. The point here is that one can make the exact pattern as complicated and repetitive as possible. The more repetitive and complicated it is, the smaller the probability that it could have arisen by chance. The trick is to find a pattern that’s as far from random as possible. Increasing the length of the peptide string creates more opportunities for complex patterns, although this could lead to deterioration in other qualities such as stability (greater risk of breakage) and volatility. Discovering a small peptide floating in space or on a meteorite with a rather creative repetitive pattern of L and D amino acids would make an abiogenic origin unlikely.
We could get more creative. In a rather obvious modification, D could stand for a 0 and L could stand for a 1 so that Ds and Ls correspond to binary code. The letters could also stand for the dots and dashes in Morse code although Morse is likely to be a unique human invention. You could have Ds and Ls that are prime numbers, Ds and Ls that are square roots of successive integers or Ds and Ls that represent the Pythagoras theorem for a unit rectangle. Once mathematics enters the picture, you could make your Ds and Ls do almost anything that would be rightly construed as intelligence. Another consideration would be the information content of the amino acid string. Computer scientists have come up with various metrics for quantifying information content and complexity; for instance, the information content of both a completely random string (“DDDD…”) as well as a string that can be represented by a single simple rule (“2Ds follow 1L”) is low. Maximum information content arises somewhere between complete randomness and complete order. In designing their amino acid strings, chemists could use these information theoretic principles to modulate the information content of the message.
At some point the construction of the most deliberately designed string of Ds and Ls would be limited not by specific patterns but by other considerations like physical properties, cost and transport constraints. The point I want to make is that because of its universal nature, chirality could be successfully employed to transmit an unambiguously synthetic message.
If we wished to communicate our existence and intelligence to other civilizations, we would not constrain ourselves to physics and astronomy but would also employ chemistry, biology and every other tool at our disposal. The discovery of intelligent life in the universe is too important to be left to the vagaries of a single or a few approaches. One of the signs of intelligence is the ability to make the most out of diversity. We can only expect other intelligent civilizations to behave accordingly.