Friday, August 04, 2017

Seth Shostak and the Odds of SETI Success

"If this project is going to work at all it's going to work before you all become middle-aged." (Seth Shostak)

Seth Shostak thinks that, assuming SETI has the basically right idea, we will likely detect a signal from an alien civilization by 2035. It's important to keep in mind that this probability implies another: it is likely that a signal from an alien civilization will reach us during the next two decades. Indeed, I would argue that Shostak must believe that it is likely that the signal is already hitting us and we just haven't detected it yet. I think this assumption underpins all work in the SETI area: at any given moment it is very likely that a signal, though it may be difficult to detect, is striking the surface of the Earth.

According to the standard model, we're searching about 200 billion stars for somewhere between 10,000 and 1,000,000 "advanced technical civilizations", which Carl Sagan "operationally" defined as "societies capable of radio astronomy". Depending on how many civilizations there in fact are, Shostak argues, and given Moore's law of technological development, we should find the first of them by 2035 at the latest. That is, he is not thinking about the probability that a signal is hitting us; he is thinking about the probability of detecting one of the signals that, he assumes, is hitting us. If there are many, it won't take long. If there are few, it will take longer. But at some point we will have searched the entire probability space of the "cosmic haystack".

But consider our own detectability. Our ambient signal "leakage" from TV and radio (which would be very hard to distinguish from noise in any case) only reaches about 80 light years into space. There are only about 500 sun-like stars in that space and many more stars beyond it. The galaxy is 100,000 light years across. Our signal occupies hardly any of it. It gets even worse when we consider the few attempts at an intentional signal we've sent. In 1974 we sent the "Arecibo Message" for about three minutes. About forty light years away from Earth now, it occupies a tiny sliver of space only three-light minutes long. If it ever does hit a civilized planet, they have to be listening at the exact time that it does. If they blink for three minutes, they'll miss it.

So let's think about the space between us and any one of those "advanced technical civilizations". It is between 4 and 87,000 light years long, with more and more of the stars we're looking at being further and further away. Only .3% of the stars in the galaxy are within 5000 light years of us. And the closer the civilization is to us, the more likely it is that the signal hit us at the time of Socrates and is now a thousand light years away from us and receding! Moreover, we don't know when they might start signalling—a million years ago? A million years from now?

In other words, a million advanced technical civilizations cannot mean a million signals in our sky just waiting to be discovered like any other astronomical object. We can detect the light of stars and galaxies because it has been shining on us for billions of years and will continue to shine for billions more. But any imaginable signal from a technical civilization will have a finite duration. It is, after all, an artifact, a product of the history of the alien civilization. In my view, SETI forgets that alien intelligence, like ours, will be historically situated. When we have not found anything before Shostak's audience reaches middle-age it will prove, as he says, that it's not going to work at all. But we don't have to wait that long to realize that the project doesn't make any sense even on paper.

Wednesday, August 02, 2017

The SETI Coat of Arms

Somewhere along the line of my career I started to think, "Okay, maybe I won't find the signal (I can do the best job I can [but] I really can't control that). But if I can leave this field financially stronger than when we started—if we can find a way, by the time that I'm finished cheer leading, to have a stable financial funding for this kind of exploration—which may, indeed, be multigenerational—then I will have done something pretty damned good. And I can feel very good about that." (Jill Tarter)

This remark during last month's SETI Talk about Sarah Scoles' biography of Jill Tarter reminded me of a short story by Franz Kafka called "The City Coat of Arms". It is, of course, well worth reading in its own right and it is only about 500 words long so you might want to go ahead and find it and read it before reading the rest of this post. I don't want to spoil your enjoyment.

The story is about the building of the Tower of Babel. In fact, it's not so much a story as a page from a fictional history book. After noting that the building arrangements were "perhaps too perfect", our historian establishes the central premise:

The essential thing in the whole business is the idea of building a tower that will reach to heaven. In comparison with that idea everything else is secondary. The idea, once seized in its magnitude, can never vanish again; so long as there are men on the earth there will be also the irresistible desire to complete the building.

Anyone familiar with the rhetoric of SETI will see the connection. As Carl Sagan put it in his famous essay, "The Quest for Extraterrestrial Intelligence" (which, incidentally, uses a Kafka quote about the silence of the Sirens as its epigraph),

The search for extraterrestrial intelligence is the search for a generally acceptable cosmic context for the human species. In the deepest sense, the search for extraterrestrial intelligence is a search for ourselves. [...] It is difficult to think of another enterprise ... that holds as much promise for the future of humanity.

But Kafka understood, as Tarter also has come to understand, that once the, let's say, infinite promise of the project has taken hold of the imagination it does not need to succeed in any foreseeable future in order to retain its relevance, or, as Tarter rightly notes, its funding. Indeed, its significance for humanity will come from the silence, not the song, of sirens. Like Tarter, Kafka also seizes on the multi-generational aspect:

[O]ne need have no anxiety about the future; on the contrary, human knowledge is increasing, the art of building has made progress and will make further progress, a piece of work which takes us a year may perhaps be done in half the time in another hundred years, and better done, too, more enduringly. So why exert oneself to the extreme limit of one's present powers? There would be some sense in doing that only if it were likely that the tower could be completed in one generation. But that is beyond all hope.

Here, some SETI enthusiasts will say that the analogy breaks down. It is not they will insist "beyond all hope" that SETI will succeed in a generation. Yuri Milner has committed $100 million dollars to a ten-year push to find whatever there is to find—the Breakthrough Listen project. But I would encourage them to read a recent paper by Claudio Grimaldi. He shows quite convincingly that the probability of detecting a signal, even if the galaxy is teeming with intelligent life (Sagan estimated a million civilizations capable of radio astronomy) is exceedingly small. This is because the signals themselves, by definition transmitted on a "historical" timeframe, disappear in the astronomical volume of the galaxy. They simply can't fill in any reasonable volume of galactic space to make it very likely that the earth's orbit will pass through the decidedly finite volume occupied by the signal.

It is only if the alien civilization had in fact undertaken to build a lighthouse to reach the heavens, to signal continuously in all directions for millions and millions of years, that their signal would have any chance of reaching us at our particular moment in history, i.e., the ten-year funding framework of the Breakthrough Listen project. So Tarter is right, something more permanent needs to be established. But here, too, Kafka must be heard:

It is far more likely that the next generation with their perfected knowledge will find the work of their predecessors bad, and tear down what has been built so as to begin anew.

Again, there are echoes of the SETI rhetoric. We need to "speed up the search," as Shostak puts it. We need more powerful computers "on the back end" to analyze the data. We might not even know, yet, what to look for. The aliens might be messaging to us using, not radio waves, but laser beams or neutrino streams. We just don't know! On the one hand, this cornucopia of possibilities gives us hope. On the other hand, the hope is distributed across a probability space that requires millions of years (of listening and of sending) to achieve reasonable "coverage" (to use Grimaldi's term). "Such thoughts paralyzed people's powers," Kafka says of the builders of Babel,

and so they troubled less about the tower than the construction of a city for the workmen. Every nationality wanted the finest quarters for itself, and this gave rise to disputes, which developed into bloody conflicts. These conflicts never came to an end; to the leaders they were a new proof that, in the absence of the necessary unity, the building of the tower must be done very slowly, or indeed preferably postponed until universal peace was declared.

This is where it gets real for me. It is important to remember that SETI is not just a scientific field but a political situation. It must lobby for resources, as Tarter rightly points out, and it must manage its own internal conflicts and intrigues. Sarah Scoles, it should be remembered, has written not just about Jill Tarter but also about Geoff Marcy. She is not just interested in the former's heroic quest but that latter's infamous transgressions. She has, as it were, one auspicious and one drooping eye on astronomy. She is interested in the struggle for both truth and justice. She covers both the scientific discoveries and the political conflict.

"The time," says Kafka,

was spent not only in conflict; the town was embellished in the intervals, and this unfortunately enough evoked fresh envy and fresh conflict. In this fashion the age of the first generation went past, but none of the succeeding ones showed any difference; except that technical skill increased and with it occasion for conflict.

But, throughout it all, surely there is Sagan's "enterprise"—that great hope for humanity—animating these scientific and political projects? Well, one sometimes wonders. As Kafka puts it:

[T]he second or third generation had already recognized the senselessness of building a heaven-reaching tower; but by that time everybody was too deeply involved to leave the city.

It's this deep involvement that I discern in Tarter's remark, which she specifically frames in terms of her "legacy". It is no longer important to discover a signal. Indeed, Grimaldi may be right and the entire enterprise may be senseless. (I have my own back of the envelope calculation that suggests something similar.) What is important is to embellish "the city", to leave the field "financially stronger", as Tarter puts it, than we found it.

Remember that it may take generations. Tarter is still part of the first generation—those who came up with the idea of finding an intelligent signal and might yet die trying. There will now be a second and third generation, whose efforts may also fail, but will involve a great deal effort to justify increasing investments in time and technology. The odds for them will not improve markedly. In order to beat Grimaldi's house you need to listen for millions of years for signal that has been transmitted for just as long. Kafka anticipates the cultural impact of despair on such an astronomical scale:

All the legends and songs that came to birth in that city are filled with longing for a prophesied day when the city would be destroyed by five successive blows from a gigantic fist. It is for that reason too that the city has a closed fist on its coat of arms.

Or, in our case, all the movies are filled with gigantic asteroids that threaten humanity with extinction. Less dramatically, the SETI Institute's logo depicts, not so much a signal, as a question. Perhaps it should consider a fist with the tail of a comet?