Borderlands of Science
Page 25
7.8
7,000
9.00
Manganese steel
7.8
16,000
21.00
Drawn steel wire
7.8
42,000
54.00
Kevlar
1.4
28,000
200.00
Iron whisker
7.8
126,000
161.00
Silicon whisker
3.2
210,000
660.00
Graphite whisker
2.0
210,000
1,050.00
TABLE 8.2
Beanstalks around the solar system.
Body
Radius of stationary
Taper factor
satellite orbit (kms)
Mercury
239,731
1.09
Venus
1,540,746
1.72
Earth
42,145
1.64
Luna
88,412
1.03
Mars
20,435
1.10
Jupiter
159,058
842.00
Callisto
63,679
1.02
Saturn
109,166
5.11
Titan
72,540
1.03
Uranus
60,415
2.90
Neptune
2,222
6.24
Pluto*
20,024
1.01
* Since Pluto's satellite, Charon, seems to be in synchronous orbit,
a beanstalk directly connecting the two bodies is feasible.
CHAPTER 9
Far-Out Alternatives
9.1 Problems of interstellar travel. One of the strongest of today's limitations on science fiction writers is the pesky constancy of the speed of light. If you can't go faster than that, light-speed limitation is—to put it mildly—an inconvenience for travel to even the nearest stars.
To many people, travel to the stars may not seem so difficult. After all (the logic goes) a dozen humans have already been to the Moon and back. We have sent landers to Mars, and we plan to do so again. Our unmanned probes have allowed us to take a close look at every planet of the Solar System except Pluto.
After interplanetary travel surely comes interstellar travel. If we have been able to do so much in the forty years since the world's space programs began, shouldn't an interstellar mission be possible in a reasonable time . . . say, thirty or forty years from now?
In a word, no.
For travel on Earth, different transportation systems can be nicely marked by factors of ten. Up to two miles, most of us are (or should be) willing to walk. For two to twenty miles, a bicycle is convenient and reasonable. A car is fine from twenty to two hundred, and above that most of us would rather fly or take a train.
Away from Earth, the factor of ten is no longer convenient. Our closest neighbor in space, the Moon, is about 240,000 miles away, or 400,000 kilometers. A factor of ten does not take us anywhere interesting. Nor does a factor of a hundred. We have to use a factor of 1,000 to take us as far as the Asteroid Belt.
Ten thousand times the distance to the Moon takes us four billion kilometers from Earth, to the outer planets of the Solar System. We are still a long way from the stars. For that we need another factor of 10,000. Forty trillion kilometers is about 4.2 light-years, and that is close to the distance of Alpha Centauri. Thus, the nearest star is about 100,000,000 times as far away as the Moon.
Want to visit the center of our galaxy, a common drop-in point for science fiction travelers? That is almost 10,000 times as far away as Alpha Centauri, a trillion times as far away as the Moon. Getting to the Moon, you may recall, was considered a big deal.
Numbers often have little direct meaning. Perhaps a more significant way of thinking of the distance to the stars is to imagine that we have a super-transportation system, one that can carry a spacecraft and its crew to the Moon in one minute. Anyone interested in solar system development would drool at the very thought of such a device. Yet we will take 190 years to reach Alpha Centauri, and most of the stars that we think of as "famous" are much farther away: 1,300 years trip time to Vega, over 20,000 years to Betelgeuse. Galactic center? Sorry, that's going to take a couple of million years.
We need a faster-than-light drive. But before we consider exotic alternatives, let's take one more look at what we might do within the confines of the laws of physics as they are known today. If nothing can travel faster than light, can we use light itself as our tool?
We can, provided we are willing to send and receive signals, rather than material objects. That is what SETI, the Search for Extraterrestrial Intelligence, is all about.
9.2 The Search for Extraterrestrial Intelligence. ET made it look easy. You collect a few bits and pieces of electronics, join them together in some mysterious way, and lo and behold, you have a transmitter that will send a signal to the stars. You switch on, and wait for your friends to show up.
ET did not ask the assistance of Earth scientists in sending his message; but suppose that he had. Suppose that we were asked to send a message to the stars, one that could be received and interpreted many light-years away. What techniques would we choose, and how would we go about it?
The idea of sending messages to beings on other worlds is an old one. In the 1820s, the mathematician Carl Friedrich Gauss proposed to lay out huge geometrical figures on the surface of Earth. He argued that these, seen through telescopes by the inhabitants of other planets, would give proof that Earth harbored intelligent life. The principal pattern, created by the layout of large fields containing crops of different colors, would show a right-angled triangle with each side bordered by squares. We would provide graphic evidence that Earthlings (though not, apparently, many of America's high school students) are familiar with the theorem of Pythagoras.
Gauss had in mind the nearer planets of the solar system, since even with big telescopes the biggest fields on Earth could not be seen from farther away than Mars or Venus. Nonetheless, given its limitations, Gauss's idea is not impossible. It represents wonderfully advanced thinking for its time.
Similar suggestions involving the lighting of great fires in the Sahara Desert were made later in the nineteenth century. By 1900, extraterrestrial communication had become a popular subject. In that year the French Academy of Sciences offered a prize of 100,000 francs to the first person making contact with another world. The planet Mars was specifically excluded, since that was considered too easy.
These early proposals for extraterrestrial communications all had one thing in common: they assumed that visible light would be the best way to communicate over great distances. At first that seems a fair assumption, even when we extend our goal from interplanetary to interstellar space talk. We live on a planet orbiting a fairly typical star. Our eyes have evolved to be sensitive to the light of that star, as modified by passage through the Earth's atmosphere. Other beings, born on planets that circle other stars, are likely to have developed organs of sight. It would be most efficient for them to have developed maximum sensitivity in roughly the same wavelength region as us. Therefore we should be able to communicate by optical techniques, using the part of the electromagnetic spectrum visible to humans.
This sounds reasonable, but it misses a key point. Visible wavelengths are not the best ones for interstellar communication, precisely because visible light is so abundant throughout the universe. We can certainly send a signal, but another being will have trouble distinguishing it from natural signals that every planet, star, and galaxy emits or reflects at the same wavelengths.
Detection would be a formidable task. There is just too much clutter in the spectral window between 0.40 and 0.70 micrometers, where we ourselves see. Our message will be lost
in the background noise that Nature is generating all around us.
What we need is a signal that will not be confused by emissions from stars, planets, interstellar dust clouds, galaxies, or any other natural source in the universe. We must find a "quiet" part of the spectrum, in which Nature does not make strong signals of her own; and we need a region where other beings would find logical reasons to send and look for signals.
This sounds like a difficult proposition, but fortunately such a region does exist.
9.3 The choice of signal carrier. If we sit down to make a list of the properties that any signalling system should have for communication over interstellar distances, we find that our signal must satisfy these requirements:
1) It should possess characteristics that allow it to be readily distinguished from naturally generated emissions;
2) It should not be easily absorbed by interstellar dust and gas;
3) It should be easy to detect;
4) It should be easy to generate with modest amounts of power;
5) It should travel at high speed.
We assume that no signal can travel faster than the speed of light, so anything traveling at light-speed will be our first preference.
That at once rules out certain signaling methods. For example, the Pioneer 10 and 11 and the Voyager 1 and 2 spacecraft are on trajectories carrying them out of the solar system. They are on their way to the stars, and they even contain messages intended for other beings. However, they travel horribly slowly. It will be hundreds of thousands of years before they reach the nearest stars. Thus they, and any other spacecraft described in Chapter 8, are too slow for interstellar messages.
The speed requirement does nothing to limit our choice within the electromagnetic spectrum. Everything from X-rays and gamma rays to visible light and long-wavelength radio waves travel in vacuum at the same speed; our other four criteria must be employed to select a preferred wavelength.
The first systematic examination of the whole spectrum, to see what is best for interstellar communication, was done by Philip Morrison and Giuseppe Cocconi (Morrison and Cocconi, 1959). As Morrison has remarked, they started out thinking that gamma rays would be the best choice, and only later broadened their viewpoint to include the whole electromagnetic spectrum.
After making their study, they concluded that there are indeed preferred wavelengths for interstellar communication, wavelengths that in fact satisfy all five of the criteria listed above. Morrison and Cocconi also addressed the question of how the signals might be generated and received.
Left out of consideration—deliberately—was the question of who might be sending signals to us. As Morrison put it, informally, "See, you were thinking that in order to call somebody up, you have to have somebody to call. I'm saying that before you call, you have to have a telephone system. We got our initial idea from the telephone system, not from thinking that anyone is there. We don't know how to estimate the probability of extraterrestrial intelligence . . . but if we never try, we'll never find it."
The inability to estimate that probability has not stopped people from trying. Suppose we write an equation giving the number of technologically advanced civilizations sending out messages in our galaxy as a product of seven independent factors: 1) the number of stars in our galaxy; 2) the fraction of such stars with planets; 3) the average number of planets orbiting any star that are suitable for the development of life; 4) the fraction of planets where life actually develops; 5) the fraction of life-bearing planets that develop intelligent life; 6) the fraction of intelligent life forms who actually seek to communicate with other forms; and 7) the fraction of the planet's lifetime occupied in the communicating phase.
This is known as the Drake Equation. It was proposed by Frank Drake, often considered the father of SETI. In 1960, using an 85-foot radio telescope in Green Bank, West Virginia, he was the first person to seek radio signals from extraterrestrial intelligences.
There are a few things to note about this equation. First, it is not a physical law, but merely an enumeration of factors. Second, if any factor is zero, the left hand side and hence the number of signals is zero. Third, only the first factor, the number of stars in our galaxy, is known to even one significant figure. The rest are little more than blind guesses.
Although thousands of pages have been written about the Drake Equation and its factors, I don't think it tells us much. The right attitude was expressed by Freeman Dyson, in his book Disturbing the Universe (Dyson, 1979): "I reject as worthless all attempts to calculate from theoretical principles the frequency of occurrence of intelligent life forms in the universe . . . Nevertheless, there are good scientific reasons to pursue the search for evidence of intelligence. . . ."
Morrison and Cocconi examined the whole electromagnetic spectrum. They reported their results in Nature magazine, and asserted that the microwave region, the one that we use for terrestrial radio and radar, is the best place to put your signal. This wavelength regime is markedly quieter (less cluttered by natural signals) than the gamma ray, X-ray, ultraviolet, visible, or infrared ranges. Nature seems to have overlooked this region for stars and planets, to the point where Earth, with its copious emissions of man-made radar, radio, and television signals, is by far the most powerful source in the solar system. At microwave wavelengths Earth is brighter than Jupiter or even the sun, although the latter is a beacon millions of times brighter at visible wavelengths.
Further, even within the microwave region, there is a definite preferred window, a "quiet spot" between 30 centimeter wavelength (1 gigaHertz frequency) and 0.3 centimeter wavelength (100 gigaHertz frequency). Wavelength and frequency are inversely related, since frequency times wavelength=the speed of light. Thus either wavelength or frequency can be used equally well to define a range of the spectrum. When we speak of radio or radar we usually work in terms of frequencies; for visible or infrared light, we generally use wavelengths.
If we want to send or receive signals from the surface of the Earth, rather than out in space, then the absorption properties of our atmosphere must be taken into account. We also have to note that man-made signals from radio and television and radar form a possible source of noise for external signals. This finally reduces the quietest region to a "terrestrial microwave window" from 1 to 10 gigaHertz (30 to 3 centimeters).
Below 1 gigaHertz, the natural synchrotron radiation of the galaxy provides unwanted noise. Above 20 gigaHertz, the quantum noise of spontaneous emission dominates; but between 1 and 10 gigaHertz the only significant noise is the cosmic background radiation, peaking at a temperature of 2.7 Kelvin and an associated frequency of 25 gigaHertz, but still appreciable between 1 and 10 gigaHertz.
By fortunate coincidence, conveniently within this valley of quiet lie two significant spectral lines: at 1.420 gigaHertz (21 centimeters) we find the radiation emission of neutral hydrogen, and at 1.662 gigaHertz (18 centimeters) the emission of the hydroxyl radical. Together, hydrogen and the hydroxyl radical combine to form water, the basis for all life as we know it. As Project Cyclops, an early study of search methods for extraterrestrial intelligence, stated with memorable imagery:
"Nature has provided us with a rather narrow band in this best part of the spectrum that seems specially marked for interstellar contact. It lies between the spectral lines of hydrogen (1420 megaHertz) and the hydroxyl radical (1662 megaHertz). Standing like the Om and the Um on either side of a gate, these two emissions of the disassociation products of water beckon all water-based life to search for its kind at the age-old meeting place of all species: the water hole" (1972; cited in NASA SP-419, 1977, edited by Philip Morrison).
If we are going to use radiation to send our interstellar signal then this place, the "water hole," provides the best set of frequencies. Moreover, signals in this region can be generated easily, with standard radio equipment; they can be beamed in any direction that we choose; and they will be detectable over stellar distances with the transmission power available to us today.
There is still a problem: deciphering a possible message.
A signal is not acceptable as artificial (remember the pulsars) until it is decoded. Of course, a "message" in the usual sense is not needed; it would be quite sufficient if the pulses that we receive were, say, the prime numbers, or numbers followed by their squares.
In the early days of SETI, Frank Drake devised a short message containing some basic information about us. He sent it to a number of his colleagues, telling them that it was a message and inviting them to decipher it. Not one of them succeeded. Can you? Drake's "message" is given in TABLE 9.1 (p. 246).
The messages sent out on the Voyager spacecraft had the same problem. They included music, the sound of rain and cars, and a statement from President Jimmy Carter; the sign of intelligence, perhaps, but one difficult to interpret, even for its senders.
Note the difference between detecting extraterrestrial signals, and sending signals for others to receive. These two different problems are often confused, but SETI is the search for extraterrestrial intelligence (we sit and listen, but we don't send any signals ourselves), and CETI is communication with extraterrestrial intelligence (we also send our own messages).
The same instruments may be used either to send or to receive signals. A radio telescope can listen, by placing detection equipment at its focal point; or it can send, by placing a transmitter at the same focal point. The signal can be sent to any preferred direction in space.
The 1,000-foot radio telescope at Arecibo in Puerto Rico has been used in both modes; to listen for signals from many places, and to beam a coded signal to the Hercules globular star cluster, M13, 25,000 light-years from Earth. A radio telescope a little bigger than the one at Arecibo Observatory would be able to detect that same signal when it reaches M13, 25,000 years from now.