Encounter With Tiber Read online

  Almost every schoolchild has used the simple code in which A = 1, B = 2, and so forth, and most people know that newspaper and computer pictures are transmitted in pixels: two numbers to specify a position on the screen or in the picture, and another number to specify brightness (or for a color picture, three numbers to specify the brightness of the red, blue, and green components that mix to produce color). Indeed, in the 1930s Kurt Gödel and Alan Turing demonstrated that any information we can understand can be sent as a string of numbers. Thus, a message from another civilization—if that was what this was—ought to arrive in a string of numbers. But since the base of a number system is arbitrary (there’s no particular reason why it should be ten rather than twelve or five), it would be extremely unlikely that such a message would arrive in base ten numbers.

  Apparently what was coming in was groups of three high and low tones, or, as the Earth-based scientists were shortly to call them, beeps and honks. A group of three beep-or-honk choices could have eight different arrangements:

  beep beep beep

  beep beep honk

  beep honk beep

  beep honk honk

  honk beep beep

  honk beep honk

  honk honk beep

  honk honk honk

  And on some system or other, those probably represented the digits 0–7, which were the digits for a base eight system. The strings of digits would represent pictures or text.

  “Base eight,” Chris said. “So maybe they’ve only got a thumb and three fingers?”

  “Or eight tentacles,” François said. “Or one thumb and one finger on each of their four arms, or they’re all Buddhists and it’s terribly important to them to express things in terms of the Eightfold Way. Or they have three heads and those are all the combinations of nodding and shaking that are possible. It seems to me that for a while here we will be theorizing in advance of data.”

  “Just because it doesn’t have much foundation in fact doesn’t mean we can’t enjoy it,” Chris said. “I wonder if the USRA has anything they can tell me. I can get them by voice relay through the Internet.”

  But he wasn’t able to look into that possibility for three full days. Time in space is terribly precious, cannot be wasted, and if lost, must be replaced. Thus time is planned to the utmost and only the bare minimum needed for rest is left to individual initiative. Normally, because the hardware is expensive, and the equipment needed for making decisions is bulky, heavy, and better located on the ground, people on space missions don’t so much make decisions as execute them. Even (or especially) in emergencies, what ought to happen next is worked out on the ground, right down to how many times to turn a bolt, and the crew in space then carry out the directions with constant monitoring by radio. That doesn’t mean that no one ever does anything on their own initiative, but it’s rare, it’s not supposed to happen, and most of the time when it does happen it’s something brief.

  One reason why this is so is that time in orbit is unspeakably expensive. Just to get an astronaut, cosmonaut, or astro-F into orbit is expensive enough, but after that everything needed to keep him or her alive, for as long as the mission lasts, has to be gotten up to orbit as well. This is true whether the crew is working on an assigned project, resting, eating, or playing cards. The maximum possible work has to be squeezed out of a space crew, and it’s more likely that careful planning from the ground will result in more efficiency than the astronaut’s mere following of whim.

  But this was different; the mystery signal from Alpha Centauri had been a crisis, because no one knew what it was or how long it might last at the start. Now that the FSRT was running automatically, however, no matter how interesting the crew of the ISS might find the signal to be, they had to make up for dozens of lost person-hours on dozens of other important experiments, and everyone had to pitch in and get that work done before there would be any leisure to look at the serendipitous discovery.

  Lori had volunteered to stay an extra week to take up the slack, and even then they did not really have enough hands. Still, NASA understands that people must have rest, and after three days Chris found himself with a blessed four hours, not allocated to sleep, all his own.

  He promptly used it to do more work. “Jiro,” he asked, “I don’t suppose that there’s any way I can access the recorded data from the Alpha Centauri transmission?”

  “Of course there is,” Jiro said, grinning at him. “Join the club. We all spend our recreation time analyzing it. What’s your project?”

  “Well,” Chris said, “I have this idea that the source probably isn’t on either of the stars. That would be insanely unlikely. So I think it’s in orbit around them; and since I have several days’ worth of data now, I bet if I analyze it for Doppler shift, I can at least figure out which of the two stars it’s orbiting, and maybe even plot a rough orbit for it.”

  The Doppler shift is the change in frequency that occurs for waves originating from a moving object; the familiar example is the way that the sound of a car’s engine, as the car passes you, sounds at first like it’s rising in pitch, then like it’s falling. It happens because when the waves are coming from an object approaching you, each successive wave starts out a little closer to you, and thus has a shorter distance to travel and gets there sooner. And as the waves seem to “pile up” on top of each other, more of them arrive in a given time—and the number of waves arriving in a given time is the frequency.

  Similarly, if the object emitting the waves is moving away, each wave starts further away than its predecessor and takes longer to reach you; thus fewer and fewer of them arrive per second, and the frequency seems to decrease. High frequency sound is high pitched; low frequency sound is low pitched; and thus as the car passes you the sound seems first to rise and then to fall in pitch.

  The same phenomenon also happens with light, radio, X-rays, or any other form of electromagnetic radiation; high frequency light is violet, low frequency light is red. Thus by measuring changes in the apparent frequency of radio or light emitted by an object, you can measure whether an object is moving toward you or away from you, and how fast.

  For an orbiting body, this is enough, usually, to figure out a great deal about its orbit. The frequency will rise while the object is on the side of its orbit that has it approaching you; it will fall on the other side. If you know its speed at any point in the orbit, or better still over some part of the orbit, then since the speed with which a body moves in an orbit depends only on the shape of the orbit and the distance from the body it orbits around, the exact pattern of variation of frequency will give you an idea of the orbit’s shape, and from that you have a good guess at its distance. (Because Alpha Centauri is a double star, furthermore, Chris was able to know which of the two stars the body must be orbiting; the orbit of the two stars around each other revealed their masses, and the relation between orbital speed, shape, and distance depends on the mass of the star; the calculated orbit could only be consistent with one of the two stars.)

  The problem was complex and tricky in a way Chris liked, because before he could study Doppler effects in the frequency of the 0.96 mm radio signals from Alpha Centauri, he first had to factor out several known sources of Doppler effects: the motion of the ISS around the Earth (which created a 90-minute cycle), the motion of the Earth around the Sun, and the motion of Alpha Centauri A and Alpha Centauri B with respect to the Sun. His four hours of recreation were almost up before he finally began to see satisfactory results.

  Meanwhile he caught up on his reading about the mysterious signal. The message was repeating, starting over again about every eleven hours and twenty minutes. Each group of 16,769,021 base-eight numbers was taking about two and a half seconds to come in, so that there were 16,384 such groups in all. That was as much as anyone was saying in public.

  It was bizarre in many ways. For decades, dedicated amateur radio astronomers, and interested professionals who could squeeze in the time on the big dishes, had been trying to find so
mething, anything, that might look like a signal from another civilization. The prevalent idea had been that most likely, if we found it, it would not be a direct attempt to communicate with us, but a general announcement to the universe, perhaps as a beacon aimed at likely stars. Since they wouldn’t be trying to reach us in particular, all the astronomers who had thought about the problem had thought that we would need extremely sensitive detectors and big antennas to hear an alien civilization at all.

  Now, years after many of the SETI (Search for Extraterrestrial Intelligence) projects had been shut down for lack of funds, and the rest had struggled to get equipment sensitive enough to pick up faint signals from planets that might be hundreds of light-years away, suddenly here was a loud, clear signal coming in from the nearest star, something that almost anyone could get with even the crudest backyard receivers—and yet transmitted on the least convenient, most noise-prone wavelength imaginable.

  At last the analysis program passed its checkouts, and Chris told it to try the data. The program opened the file that contained the signal strengths and antenna positions, loaded the data into the appropriate internal matrix, and began processing. Various short messages indicating that the program had completed another step popped up on the screen; the program was executing properly.

  It finished, and displayed a set of equations plus the message “UNIQUE SOLUTION, 95% CERTAIN”—indicating that there was only one mathematically possible result to explain the data, and that the odds were nineteen to one or better that the solution was right.

  “Now, that’s weird,” François said, peering over Chris’s shoulder. “That looks like—er—”

  “It looks like it can’t possibly be an orbit around either star, or around their combined center of mass, or around any of their Lagrange points,” Chris finished for him. “Yes, I know.”

  “But the curve is beautifully smooth,” François pointed out. “It doesn’t look like garbage or error, it looks like—”

  Jiro too came over to kibbitz. “Hmm. Suppose you’re not dealing with one mass but two,” he said. “Suppose it’s a small body orbiting a larger body orbiting one of the stars. If you figure that—”

  “You’re right,” Chris said, and rapidly keyed in a test sequence. The program ran for several long minutes as it tried and discarded dozens of alternatives. Then quite suddenly, an animated display popped up on the screen, showing dots labeled A and B circling each other like wary boxers—and a strange lozenge-shaped spiral circling B.

  “Yep,” Chris said. “That’s a body in a highly elliptical orbit around another much larger body, which is orbiting B. Seems like a strange place for life, but that’s too good a picture not to be right.” He tapped the keys a few times, and said, “Well, if you figure that the planet is a gas giant, like Jupiter or Saturn, then an Earthlike planet in that kind of orbit around it would be marginally habitable—usually. It would get mighty cold on the long swings outward, and pretty hot whenever it swung in toward Alpha Centauri B. The real problem is, just guessing at it, I don’t think a moon around a gas giant could be stable in that position over a period of billions of years.”

  Jiro’s hands went to the keys. “Want to see if we can modify it to estimate a position and mass for the gas giant, and then check long-term stability on that elliptical orbit?”

  “Absolutely,” Chris said. “Funny thing, but that’s gravity for you—we can figure out exactly the mass of the planet, but we have no way of knowing whether what’s orbiting it is the size of a Volkswagen or the size of the Earth. The mass of the orbiting body doesn’t affect the period of the orbit, only the mass of the central body.”

  Jiro shrugged. “Just the way it works out. Well, our results do seem to be consistent. That central body seems to be about one hundred forty Earth masses—or one and a half times as big as Saturn, or less than half as big as Jupiter. Call it either a super-Saturn or a mini-Jupiter, eh?”

  Mission Control, seeing the importance of the results, authorized Jiro, François, and Chris to spend another two hours on the problem of the stability of the orbiting body, from which the broadcast was coming.

  At one time it was thought that all the planets in the solar system must be moving in exactly the same fashion every year, because it was assumed that even a slight annual change would accumulate until the planet was in a drastically different orbit, and sooner or later would accumulate so far that the planet would fall into the Sun, or swing far enough out to be lost forever, or eventually drift into an orbit that did repeat over time.

  But in the late 1980s, chaos theory—the mathematics that governs situations where a small difference at the beginning could have large and unpredictable effects at the end—brought astronomers to the realization that although each planet’s orbit was generally similar to the one from the year before, some of the planets and moons—especially in the more eccentric orbits, or in orbits far from the Sun (like Pluto’s)—could have chaotic orbits that never replicated perfectly but nevertheless kept the body in orbit for hundreds of millions of years. An orbit was “stable,” therefore, if the equations that described it would regularly return to the same sequence of values, so that the motion of the body would repeat, over and over. It was “unstable” if that never happened, and in that latter case, the more unstable (that is, the farther away from ever returning to the same place at the same speed moving in the same direction), the less likely it was to have lasted in its orbit for very long.

  The answer checked out but was highly unsatisfactory. “Well,” Chris said, “I guess what we’ve done is solved half the mystery. It couldn’t have been there much longer than forty million years. Therefore life didn’t evolve there; whoever and whatever they are, they aren’t from that moon. That’s almost reassuring, since a body with that orbit would be a hell of a place for habitability on a long-term basis. We must be getting transmissions from a beacon or something. Maybe it’s a distress call from an alien ship that was forced to land there, or a scientific probe reporting back what it’s found, or—”

  François looked up from the neighboring screen and said, “Not likely.”

  Chris stopped and said, “Why not?”

  “Because if you remember, one assumption we had to start with was that the signal was of constant strength; so I was rechecking that assumption against the computed orbit. And in one sense it is of constant strength—it’s all coming at the same power. But there’s been a very slow secular increase in signal strength going on, which is perfectly explained if you assume that we’re moving into the center of a beam. Which makes sense—even with a very large aperture, and a highly collimated beam, at that wavelength beam spread should work out to six or seven astronomical units either way from the Sun. You couldn’t point it directly at the Earth, perhaps, or there was no point in it—but if you pointed it at the Sun, the Earth would never be outside the beam.”

  Jiro and Chris gaped at him, and then Chris said what was obvious. “So they aren’t broadcasting to the whole universe. Whoever or whatever they are, they aimed that message right at us.”

  6

  AT THE END OF the next work period, when a group meal was scheduled, Tatiana Haldin said, “Well, congratulations; thanks to publication on the Internet, the three of you are now famous, and the rest of us will be wanting your autographs. You might be interested to know that the International Astronomical Union has already scheduled a meeting to name the gas giant that Dr. Terence has identified orbiting Alpha Centauri B.”

  “We don’t even know that it’s a gas giant,” Chris said, “and you can call me Chris. All we know is that it masses about a hundred and forty times what the Earth does. That’s not a lot to go on.”

  Denisov snorted. “And what else is a planet that big going to be made of? It’s big enough to have retained almost all its primordial hydrogen and helium, and since they make up the majority of matter in the universe, what else do you think it might be made of? Chocolate ice cream? And you should tell him, Tatiana, about the
campaign back in his country.”

  “I didn’t wish to embarrass him,” Haldin said mildly.

  “Well, I’ve always enjoyed embarrassing him,” Lori said, grinning. “Chris, there’s a bunch of nuts back home writing to Congress who want to name that gas giant after you. They’re calling themselves the Beta Centauri Terence Society.”

  Chris groaned. “How many different ways can the same people be dumb? In the first place, all of us had a hand in the discovery. And secondly, the star is Alpha Centauri B—the second largest star in the double star Alpha Centauri. Beta Centauri is a completely different star, in a different part of the constellation Centaurus, a lot farther away, just in sort of the same direction. And anyway, naming that planet, if that’s what it is, is the job of the International Astronomical Union. God, if you leave that kind of thing to the vote of the people, someday we’ll have planets named after Elvis and Cher.”

  Everyone laughed; Tatiana said, “I’m sure the new planet won’t have a name for a long time. The IAU is at least as political as any other scientific body; they’ll have to argue about whether to continue naming planets after gods, and if so, whose gods, and if whose, which of their gods, and so forth indefinitely.”

  François nodded. “If anyone asks me, I shall suggest ‘Marianne.’”