The Search for Extraterrestrial Intelligence in the 21st Century

Neil Zimmerman January 22, 2010

A public lecture at the Department of Astronomy of Columbia University, 2010 Jan 22.

1 Abstract

Fifty years ago, scientist first began surveying the sky for interstellar messages encoded in radio waves. While the discovery of an alien civilization has eluded all efforts so far, there are reasons to remain optimistic about the prospect of detecting a signal of sentient origin this century. After reviewing the history of this field, I will explain the tools and strategies of new SETI programs.

2 Disclaimer

I’ll start by saying what this talk is not about. I won’t be talking about UFOs or abductions or other claims that Earth has been visited by aliens. Most astronomers remain sceptical of those conclusions. If you do believe we’ve had extraterrestrial visitors, that’s fine—I won’t try to change your mind. Instead, I’m interested in purely astronomical proof that advanced forms of life thrive far beyond our planet. That is, searching for evidence of alien civilizations using telescopes, the same tools that have worked so well unraveling other puzzles of the universe.

3 Light and Communication

Almost everything we know about outer space is based on collecting and in- terpreting light—light waves traveling patiently for centuries, millenia, or even billions of years until they are focused by our telescopes. The natural pro- cesses that generate light—electrons hopping between atomic energy states or spiraling around magnetic fields, nuclei vibrating and colliding—occur unbiq- uitously throughout the universe—in , around stars, and clouds of matter

1 in between—without the intervention of any organism. It is an elaborate sym- phony, encompassing a vast range of color and texture, but a lifeless one in the biological sense. How does the picture change when a species of life evolves to the point of being able to build and control its own light sources? Even going back to prehistoric times, humans have manipulated light to transmit information over long distances. Just last century, our lanterns reached the strength necessary to begin competing with the insentient ensemble of light, at least in our cosmic backyard. Earth’s broadcasts—though unintentionally—have begun inevitably spreading out into space like ripples on the surface of a pond, faintly bathing our neighboring stars and the unknown worlds they host. Gazing into the sky, we can turn the tables and ask—who is out there, making waves like us? What alien lantern flashes could be mixed in with the starlight, waiting to be uncovered and decoded? These are the questions that led to the beginning of the search for extraterrestrial intelligence (SETI) fifty years ago. There are a few vocabulary words in the language of physics worth going over, to lay the foundation of this topic. We classify light waves by how quickly their electric and magnetic force fields cycle. The full range—from the longest cycles to the shortest cycles—is called the electromagnetic spectrum. We use a unit called Hertz (Hz) to refer to the frequency, how many times per second the force field cycles. In the visible section, our eye manifests these differences as color. Violet light cycles faster than red light on the opposite side of the rainbow. But the spectrum of light extends far outside those familiar colors. For example, light waves that cycle at a frequency of millions or billions of times a second—in the shorthand of physics, Megahertz and Gigahertz—belong to the radio part of the spectrum. We sometimes use these numbers in everyday life without realizing what they mean. For example, everyone from New York knows that if you want to hear “blazing hip hop and R&B”, you set your FM dial to 97.1 MHz. By doing that, you tune in to a carrier signal that is cycling at 97,100,000 times a second. When you turn on a microwave oven, you’re actually cooking your food with light waves in the microwave part of the spectrum—a piece of electromagnetic real estate that happens to be crucial for SETI. An important fact to keep in mind is that radio waves are just a form of light. They are not sound waves, which are a completely different thing. What makes it confusing is that radio waves can be manipulated, modulated to represent sound and communicate it over long distances, but in a strict sense we can never “hear” radio waves. It is up to the receiving device to interpret the modulations, the wobbles in an FM broadcast carrier to convert it into music. If we cannot see or hear radio waves, how do we visualize them? One way is to plot how much of this invisible energy is passing through space over a specific range of frequencies. If we measured the energy of radio waves at this location in the range from 88 MHz to 108 MHz, the FM band, we’d see a series of spikes, one for each radio station at its home frequency. In astronomy we deal with enormous distances. Rather than measuring them in miles or kilometers, it is more convenient to talk about distances to stars in

2 terms of light years. By definition, a light year is the distance that light travels through empty space in one year, which amounts to about six trillion miles. Most of the stars in our home galaxy, the Milky Way, make up a spiral disk with a diameter of about 100,000 light years. Most of the stars you see in the night sky are a few dozen to a few hundred light years away.

4 Organizing Our Ignorace

Starting in the 1930s, astronomers had a major new tool at their disposal: radio telescopes. Using the same flavor of light that proved useful for communication on Earth, they opened up our eyes to phenomena in our universe that would be otherwise invisible. It didn’t take long for scientists to think of another way they could be used. In an issue of the science journal Nature from 1959, directly following a snoozer about beekeeping, you can find the radical paper by Giuseppe Cocconi and Philip Morrison that first proposed using radio telescopes to survey stars for transmissions from alien civilizations. With straightforward calculations, they demonstrate: if a civilization with our level of technology lived around a nearby , and they devoted a reasonable effort to directing a radio signal in our direction, and our best telescope was pointed at that star, then we could receive their message. In the most modest possible wording, they point out that evidence of intelligent life beyond our planet would be one of the most important discoveries in all of human history. They ended their article with wise words that continue to resonate with the philosophy of all efforts in this field: “The probability of success is difficult to estimate; but if we never search, the chance of success is zero.”[2] Before this paper came out, another scientist named had in- dependently thought of the same idea. In fact, he had quietly begun his own investigation at a radio observatory in West Viginia. He was afraid of gener- ating too much attention with such a contoversial idea, so he chose to do this very quietly on a shoestring budget. After Cocconi and Morrison’s article was published, the emboldened Frank Drake came out of the closet and announced what he was working on. It was an optimistic, can-do era, and most of them said, go for it! In fact some people helped him put his customized receiving gear together. With his setup, which he named Project Ozma, he observed two stars and analyzed their light at a frequency around 1.4 GHz. He checked for a narrow spike in frequency, as one of our own broadcasts might appear if it were highly amplified. Even though this first stab didn’t succeed in finding a signal, he inspired many other scientists to join his effort[3, p 19]. The following year, a group of astronomers interested in the ideas of Drake, Cocconi, and Morrison gathered to have a meeting at the in West Virginia. It was here that Frank Drake first presented his famous equation sizing up the factors that determine how much company we have:

N = R⋆ × fp × nh × fl × fi × fc × L

3 On the left hand side, N stands for how many civilizations there are in the Milky Way that are broadcasting their presence. This number depends on a chain of variables all multiplied together: R⋆, the rate at which new stars form; fp, the fraction of those new stars that bear planets; nh, the number of worlds in each that are habitable, or suitable for life; fl, the fraction of habitable planets on which life forms; fi, the fraction of life-bearing planets where intelligent life develops; fc, the fraction of intelligent species that develop the ability to communicate across space; lastly L, the average length of time such a civilization broadcasts into surrounding space[3, p xxxv]. What makes the fun but also frustrating is that we don’t know most of these variables, and valid guesses for these inputs vary by factors of a hundred or more. But it neatly organizes our ignorance. As we move to the right, the numbers that we need to plug in get more and more speculative. The first variable, R⋆, is the only one we know reasonably well: about ten new stars form in our galaxy each year. The next two terms—the fraction of stars with planets and the number of planets in each system that are habitable—are tantalizingly close to the reach of today’s astronomers. We already know that at least 40% of sun-like stars host their own planets. So within a few years, we can expect a decent estimate of fp. We may also have a strong estimate for the frequency of habitable planets in the next few years, thanks to the Kepler space mission. However, figuring out how often life forms on habitable worlds will take big steps in both telescope technology, biology, and planetary science. This is the exciting new intersection of disciplines we call . There’s a fair chance that fl be determined later in this century, after we have observatories in space dedicated to examining Earth-like planets orbiting other stars. We have reason to suspect that reasoning, thinking animlas develop more rarely than cruder forms of life. In Earth’s history, creatures with nervous systems didn’t appear until the 600 Million years ago, only 15% of the way back in geological time. Today we’re still outnumbered by organisms without brains, both by weight and number. And let me tell you, we need them, they don’t need us. Anything lurking in potentially habitable corners of the Solar System, like underground on Mars, or on or , is in all likelihood dumb. So fi may be a small number, but remains very uncertain, along with fc. The last variable, L, the lifetime of a broadcasting civilization, is another weak link—one that is fraught with concerns of our own future. The uncertainty of those last four variables especially allows a huge range of plausible estimates for the amount of communicating civilizations. As a result, one can respectably defend values of N smaller than one or as large as 100 million communicating civilizations. 100 million may sound like an outrageous number, but remember there are a lot of stars in the Milky Way, upwards of about 200 billion [8]. To put this in perspective, that optimistic guess for N implies that merely 0.05% of the stars are occupied with potential communicators. The reason it is worth speculating on these numbers is that the true value of N will decide how long the search will take to succeed, if we’re not alone. If species with advanced tenchonology tend to be self-destructive and survive for only short periods of time, or if complex animal life like us rarely develops,

4 then the distance to our nearest neighbors would be daunting even for our best technology. In that case, we might even have to look to other galaxies for company. In order for our neighborhood of the Milky Way—say, the region within 1,000 light years of the sun—to contain other communictating societies, their average survival time needs to be many times longer than the mere 70 year period that we’ve had radio transmitters and big telescopes and computer chips. There’s an important corollary to this: if we do find a pen pal in our vicinity, chances are that their civilization has been around much longer than ours. It would be very unlikely to make contact with a civilization of the same maturity as us. In other words, as leading SETI scientist Jill Tarter said, “we’re the youngest that can play this game”[7].

5 Beginning the Search

So how do we get into the game? Since the first work by Cocconi, Morrison, and Drake fifty years ago, other astronomers have joined in serious efforts to consider strategies to search for interstellar messages. One thing most contributors agree on is that light is the best medium to search with. As far as we know, physically traveling between stars with probes or spaceships requires an immense cost in energy and time. It would be conceited to assume that our galactic neighbors would go through such trouble to drop by our solar system anytime soon, out in the suburbs of the Milky Way. Light, on the other hand, has the advantage of allowing two parties to open a communication channel with by far the least effort from both sides. If an advanced civilization wanted to get our attention, it wouldn’t be diffi- cult for them to assemble a beacon that emits light in our direction. A dedicated hailing signal would be much easier for us to pick up than whatever transmis- sions happen to accidentally leak away from their home, since the energy would be directed towards us. There are two ways to design a beacon that stands out from the background of natural light. One is for the signal to be concentrated in a very narrow band of frequencies. Think of how the FM radio stations looked like spikes when their energy was plotted against frequency—that’s one sort of transmission that might make sense. Another way is for the signal to have fine structure in time, like a series of pulses. It is rare for natural light sources to behave like that, and if we picked up such a transmission we could examine it closely for patterns that really give away the fact that it is the product of an intelligent life form. There are a few reasons why Frank Drake carried out the first seach for radio waves at a frequency of 1.4 GHz. One is that the sky is really dark in the part of the spectrum between 1 GHz and 10 GHz. There is the least amount of background light from natural sources for a signal to compete with. Stars are dim in radio waves. At lower frequencies, there is contamination from radio waves emitted by free electrons. At higher frequencies, the molecules in our own atmosphere emit light that is difficult to see past. So we have a kind of natural “window” here. One problem was, 1 GHz to 10 GHz is actually enormous range

5 in frequency for 1960s equipment to deal with. It wasn’t feasible for Frank Drake to search all of the possible channels inside this window. So he only covered a very small fraction of the spectrum. But the number 1.4 GHz isn’t just a random guess. Cocconi and Morrison suggested this because it’s the very same frequency that clouds of neutral hydrogen gas throughout our galaxy glow in. They reasoned that this would be a sort of common ground for all technological civilizations that know a thing or two about radio astronomy. Over the years, dozens of surveys with radio telescopes used a similar ap- proach to Project Ozma, with some support from NASA. Over time, receiver technology improved, so astronomers could search more channels in much less than time than before, and with greater sensitivity. In the 1980s the SETI In- stitute, a non-profit organization, was founded to act as a home base for the efforts of a handful of scientists. Unfortunately, in 1993, NASA’s SETI efforts attracted criticism from Congress, and from then on was cut off from govern- ment funding. After this, smaller scaled-down SETI projects took place relying on private donations. There is one major hindrance of all the major SETI efforts in the last century. They all relied on using observatories that were built to share between many observing programs. This means that only a small fraction of time on these telescopes was devoted to SETI, and the surveys never reached their potential. To see what I mean, look at the numbers. To examine all of the targets within a 1,000 light year radius, you’d need to observe millions of stars. However, the most comprehensive radio survey to date, Project Phoenix, observed only 800 stars [4]. But not only is it question of how many stars you look at, but how deeply. Project Phoenix would not have been able to detect radio emissions leaking from a twin Earth ten light years away. The system was just not sensitive enough. So in this sense we’ve barely scratched the surface. We want to look at more stars with bigger eyes.

6 A New Generation of Tools

For two years starting in 1997, a small group of scientists and engineers, the SETI Science and Technology Working Group met and discussed the direction these efforts should move in. The main outcome was a recommendation to build a full-time SETI observatory using high-tech but low-cost, off-the-shelf parts. To maximize the senstivity of the search, they wanted as large an antenna collecting area as possible. But instead of one monster dish like the ones used in Green Bank or Arecibo, they realized it’d be more efficient to build an array of many satellite TV dishes converted into radio telescopes, and process their signals with standard computer hardware to effectively form one large antenna. The concept of combining antenna signals is not a new idea, but this is by far the highest performance per cost ratio that it has been done at before. Rather than using custom signal processing circuits, this design takes advantage of the exponential growth in consumer computer performance that has happened in the last few decades.

6 Construction began in 2004 after the project received a $13 million donation from Paul Allen, the co-founder of Microsoft. Hence its name, the Allen Tele- scope Array (ATA). Right now there are 42 antennas in operation, with plans to ultimately include 350. The expandability is another advantage of the array design. The ATA is currently surveying the part of the sky most densely filled with stars, towards the center of the Milky Way. With each pointing, it takes the biggest gulp out of the spectrum of any SETI project so far, prepared for signals all the way from 500 MHz to 11 GHz. In a way, the ATA is just a prototype for something bigger on the way in fifteen years. The Square Kilometer Array is the most ambitious radio telescope ever planned. It shares the same design strategy as the ATA, combining many medium-sized antennas, but with a total collecting area equal to one square kilometer, so at least a hundred times as powerful as the ATA. The SKA will be a wide-purpose astronomy facility supported by several countries around the world, not specifically made for SETI. However, due to the multi-tasking na- ture of the design, several observers will actually be able to conduct different programs simultaneously, as long as they’re looking in the same general direc- tion. So SETI will be able to piggyback without getting in the way of other astronomers. Ideally, we’d like to be able to check the entire the sky every moment, non- stop. If the occupancy fraction of stars is more like one in a million stars than the optimistic one in a thousand, then pencil-beam observatories like the ATA will take decades to succeed. But what if there are brilliant signals intermittently flashing at us from parts of the sky that are not being checked? One can imagine a situation where an advanced civilization is swinging a cosmic beacon to millions of stars, giving each one its turn. We’d have to be very lucky to be pointed in the right place at the right time. The reason that we aren’t doing this now is that it’s very technically difficult to monitor the whole sky. Dish-shaped antennas—like the one you’ve seen in all of the pictures so far—only pick up radio waves from one direction. The bigger they are, the smaller a piece of the sky they see. However, with differently shaped, little antennas, you can sense most of the sky equally well, without moving any parts. The tradeoff is that small antennas also have smaller col- lecting area. To some extent, this can be made up for by combining many little antennas into an array. But the other problem is, synthesizing a radio picture of the entire sky, every instant, over frequencies spanning the entire microwave window is an expensive computing problem [5]. In fact, too expensive to be pursued with the limited level of funds SETI survives on now. As the perfor- mance per cost of computers continues to double about every 18 months, there is no doubt this will eventually become affordable, but for another two decades the idea of an omnidirectional SETI observatory will likely remain on the shelf. We can’t be sure that radio waves will be the preferred flavor of light to communicate in our galaxy. In fact, some scientists have strongly argued that we’re more likely to succeed by searching for a beacon in the optical and infrared part of the spectrum. In the last twenty years, new kinds of lasers were invented that can give off extremely bright, but short bursts of light. Attaching a device

7 like that to a large telescope—using it backwards—our own laser pulse could easily outshine the Sun from the perspective of the recipient, for just a nanon- second. So if we can temporarily outshine the Sun, imagine what smarter aliens can do. It’s potentially an excellent way to communicate between stars. You need special equipment to search for these, because the cameras normally used in astronomy are too slow to pick up nanosecond bursts of light. Some SETI scientists, including founding father Frank Drake, have worked on developing high speed pulse detectors at the focus of telescopes, and surveying nearby stars for pulses of light. These optical sensors are working and ready to go, they just need more funding to be put into action.

7 Conclusions

There are more ideas than this, but it would take too long to go over all of them. I do want to emphasize that it makes sense to support several approaches happening at the same time. Diverse strategies compliment each other, since they are each matched to a different, and as fas as we know, equally defensible possibilities. But it’s not a wild goose chase. SETI steadily pursues a well- defined question: is the story of life’s evolution on Earth a freak event, or is it common throughout the galaxy? It is a challenging question for some people to consider, since it requires us to accept that we’re not necessarily privileged beings in the universe, that other animals far away could reach our level of knowledge, and surpass it. But it also uplifts us in a way. Success has the potential to unite humanity with a broader perspective of our place in the universe. One thing I am convinced of us is that people will not stop looking. The question is too old, and too compelling, for us to give up, now that we have finally have the tools to answer it. c Neil Zimmerman

References

[1] Bennett, Jeffrey; Shostak, Seth. Life in the Universe. 2nd ed. San Francisco, CA: Addison-Wesley, 2007. Print.

[2] Cocconi, Giussepe, and Philip Morrison. “Searching for Interstellar Commu- nications.” Nature 184 (1959): 844-846. [3] Ekers, Ronald D., et al., eds. SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence. Mountain View, CA: SETI Press, 2002. Print. [4] “Project Phoenix General Overview.” 2008. SETI Institute. 19 Jan 2010 http://www.seti.org/Page.aspx?pid=583

[5] Shostak, Seth. “The Future of SETI.” Sky & Telescope Apr 2001. http: //www.skyandtelescope.com/resources/seti/3304566.html.

8 [6] Soter, Steven. “SETI and the Cosmic Quarantine Hypothesis.”Astrobiology Magazine 17 Oct 2005. http://www.astrobio.net/index.php?option= com retrospection&task=detail&id=1745. [7] Tarter, Jill. “SETI Turns 50 - Five Decades of Progress in the Search for Ex- traterrestrial Intelligence.” Symposium: The Search for Life in the Universe. Space Telescope Science Institute. Baltimore, MD, 7 May 2009. [8] http://www.berkeley.edu/news/media/releases/2006/01/09 warp. shtml

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