Death By Black Hole Part 10

Now those were some dumb aliens.

They must not have been looking at other planets en route to Earth because Jupiter, for example, contains over two hundred times the entire ma.s.s of Earth in pure hydrogen. And I guess n.o.body ever told them that over 90 percent of all atoms in the universe are hydrogen.

And how about all those aliens that manage to traverse thousands of light-years through interstellar s.p.a.ce, yet bungle their arrival by crash-landing on Earth?

Then there were the aliens in the 1977 film Close Encounters of the Third Kind Close Encounters of the Third Kind, who, in advance of their arrival, beamed to Earth a mysterious sequence of repeated digits that encryption experts eventually decoded to be the lat.i.tude and longitude of the aliens' upcoming landing site. But Earth longitude has a completely arbitrary starting point-the prime meridian-which pa.s.ses through Greenwich, England, by international agreement. And both longitude and lat.i.tude are measured in peculiar unnatural units we call degrees, 360 of which are in a circle. Armed with this much knowledge of human culture, it seems to me that the aliens could have just learned English and beamed the message, "We're going to land a little bit to the side of Devil's Tower National Monument in Wyoming. And since we're coming in a flying saucer we won't need the runway lights."

The award for dumbest creature of all time must go to the alien from the original 1979 film Star Trek, The Motion Picture Star Trek, The Motion Picture. V-ger, as it called itself (p.r.o.nounced vee-jer) was an ancient mechanical s.p.a.ce probe that was on a mission to explore and discover and report back its findings. The probe was "rescued" from the depths of s.p.a.ce by a civilization of mechanical aliens and reconfigured so that it could actually accomplish this mission for the entire universe. Eventually, the probe did acquire all knowledge and, in so doing, achieved consciousness. The Enterprise Enterprise stumbles upon this now-sprawling monstrous collection of cosmic information at a time when the alien was searching for its original creator and the meaning of life. The stenciled letters on the side of the original probe revealed the characters stumbles upon this now-sprawling monstrous collection of cosmic information at a time when the alien was searching for its original creator and the meaning of life. The stenciled letters on the side of the original probe revealed the characters V V and and ger ger. Shortly thereafter, Captain Kirk discovers that the probe was Voyager 6 Voyager 6, which had been launched by humans on Earth in the late twentieth century. Apparently, the oya oya that fits between the that fits between the V V and the and the ger ger had been badly tarnished and was unreadable. Okay. But I have always wondered how had been badly tarnished and was unreadable. Okay. But I have always wondered how V-ger V-ger could have acquired all knowledge of the universe could have acquired all knowledge of the universe and and achieved consciousness yet not have known that its real name was achieved consciousness yet not have known that its real name was Voyager Voyager.

And don't get me started on the 1996 summer blockbuster Independence Day Independence Day. I find nothing particularly offensive about evil aliens. There would be no science-fiction film industry without them. The aliens in Independence Day Independence Day were definitely evil. They looked like a genetic cross between a Portuguese Man of War jellyfish, a hammerhead shark, and a human being. While more creatively conceived than most Hollywood aliens, their flying saucers were equipped with upholstered high-back chairs and arm rests. were definitely evil. They looked like a genetic cross between a Portuguese Man of War jellyfish, a hammerhead shark, and a human being. While more creatively conceived than most Hollywood aliens, their flying saucers were equipped with upholstered high-back chairs and arm rests.

I'm glad that, in the end, the humans win. We conquer the Independence Day aliens by having a Macintosh laptop computer upload a software virus to the mothership (which happens to be one-fifth the ma.s.s of the Moon) to disarm its protective force field. I don't know about you, but I have trouble just uploading files to other computers within my own department, especially when the operating systems are different. There is only one solution. The entire defense system for the alien mothership must have been powered by the same release of Apple Computer's system software as the laptop computer that delivered the virus.

Thank you for indulging me. I had to get all that off my chest.

LET US a.s.sUME, for the sake of argument, that humans are the only species in the history of life on Earth to evolve high-level intelligence. (I mean no disrespect to other big-brained mammals. While most of them cannot do astrophysics, or write poetry, my conclusions are not substantially altered if you wish to include them.) If life on Earth offers any measure of life elsewhere in the universe, then intelligence must be rare. By some estimates, there have been more than 10 billion species in the history of life on Earth. It follows that among all extraterrestrial life-forms we might expect no better than about 1 in 10 billion to be as intelligent as we are, not to mention the odds against the intelligent life having an advanced technology and and a desire to communicate through the vast distances of interstellar s.p.a.ce. a desire to communicate through the vast distances of interstellar s.p.a.ce.

On the chance that such a civilization exists, radio waves would be the communication band of choice because of their ability to traverse the galaxy unimpeded by interstellar gas and dust clouds. But humans on Earth have only understood the electromagnetic spectrum for less than a century. More depressingly put, for most of human history, had aliens tried to send radio signals to Earthlings we would have been incapable of receiving them. For all we know, the aliens have already done this and unwittingly concluded that there was no intelligent life on Earth. They would now be looking elsewhere. A more humbling possibility would be if aliens had become aware of the technologically proficient species that now inhabits Earth, yet they had drawn the same conclusion.

Our life-on-Earth bias, intelligent or otherwise, requires us to hold the existence of liquid water as a prerequisite to life elsewhere. As already discussed, a planet's...o...b..t should not be too close to its host star, otherwise the temperature would be too high and the planet's water content would vaporize. The orbit should not be too far away either, or else the temperature would be too low and the planet's water content would freeze. In other words, conditions on the planet must allow the temperature to stay within the 180 degree (Fahrenheit) range of liquid water. As in the three-bowls-of-food scene in the fairy tale Goldilocks and the Three Bears, Goldilocks and the Three Bears, the temperature has to be just right. When I was interviewed about this subject recently on a syndicated radio talk show, the host commented, "Clearly, what you should be looking for is a planet made of porridge!" the temperature has to be just right. When I was interviewed about this subject recently on a syndicated radio talk show, the host commented, "Clearly, what you should be looking for is a planet made of porridge!"

While distance from the host star is an important factor for the existence of life as we know it, other factors matter too, such as a planet's ability to trap stellar radiation. Venus is a textbook example of this "greenhouse" phenomenon. Visible sunlight that manages to pa.s.s through its thick atmosphere of carbon dioxide gets absorbed by Venus's surface and then reradiated in the infrared part of the spectrum. The infrared, in turn, gets trapped by the atmosphere. The unpleasant consequence is an air temperature that hovers at about 900 degrees Fahrenheit, which is much hotter than we would expect knowing Venus's distance to the Sun. At this temperature, lead swiftly liquefies.

The discovery of simple, unintelligent life-forms elsewhere in the universe (or evidence that they once existed) would be far more likely and, for me, only slightly less exciting than the discovery of intelligent life. Two excellent nearby places to look are the dried riverbeds of Mars, were there may be fossil evidence of life from when waters once flowed, and the subsurface oceans that are theorized to exist under the frozen ice layers of Jupiter's moon Europa. Once again, the promise of liquid water defines our targets of search.

Other commonly invoked prerequisites for the evolution of life in the universe involve a planet in a stable, nearly circular orbit around a single star. With binary and multiple star systems, which comprise about half of all "stars" in the galaxy, planet orbits tend to be strongly elongated and chaotic, which induces extreme temperature swings that would undermine the evolution of stable life-forms. We also require that there be sufficient time for evolution to run its course. High-ma.s.s stars are so short-lived (a few million years) that life on an Earthlike planet in orbit around them would never have a chance to evolve.

As we have already seen, the set of conditions to support life as we know it is loosely quantified through what is known as the Drake equation, named for the American astronomer Frank Drake. The Drake equation is more accurately viewed as a fertile idea than as a rigorous statement of how the physical universe works. It separates the overall probability of finding life in the galaxy into a set of simpler probabilities that correspond to our preconceived notions of the cosmic conditions that are suitable for life. In the end, after you argue with your colleagues about the value of each probability term in the equation, you are left with an estimate for the total number of intelligent, technologically proficient civilizations in the galaxy. Depending on your bias level, and your knowledge of biology, chemistry, celestial mechanics, and astrophysics, you may use it to estimate from at least one (we humans) up to millions of civilizations in the Milky Way.

IF WE CONSIDER the possibility that we may rank as primitive among the universe's technologically competent life-forms-however rare they may be-then the best we can do is keep alert for signals sent by others because it is far more expensive to send than to receive them. Presumably, an advanced civilization would have easy access to an abundant source of energy such as its host star. These are the civilizations that would be more likely to send rather than to receive. The search for extraterrestrial intelligence (affectionately known by its acronym "SETI") has taken many forms. The most advanced efforts today use a cleverly designed electronic detector that monitors, in its latest version, billions of radio channels in search of a signal that might rise above the cosmic noise. the possibility that we may rank as primitive among the universe's technologically competent life-forms-however rare they may be-then the best we can do is keep alert for signals sent by others because it is far more expensive to send than to receive them. Presumably, an advanced civilization would have easy access to an abundant source of energy such as its host star. These are the civilizations that would be more likely to send rather than to receive. The search for extraterrestrial intelligence (affectionately known by its acronym "SETI") has taken many forms. The most advanced efforts today use a cleverly designed electronic detector that monitors, in its latest version, billions of radio channels in search of a signal that might rise above the cosmic noise.

The discovery of extraterrestrial intelligence, if and when it happens, will impart a change in human self-perception that may be impossible to antic.i.p.ate. My only hope is that every other civilization isn't doing exactly what we are doing because then everybody would be listening, n.o.body would be receiving, and we would collectively conclude that there is no other intelligent life in the universe.

TWENTY-SEVEN.

OUR RADIO BUBBLE.

For the opening scene to the 1997 film Contact Contact, a virtual camera executes a controlled, three-minute zoom from Earth to the outer reaches of the universe. For this journey, you happen to be equipped with receivers that enable you to decode Earth-based television and radio broadcasts that have escaped into s.p.a.ce. Initially, you hear a cacophonous mixture of loud rock music, news broadcasts, and noisy static as though you were listening to dozens of radio stations simultaneously. As the journey progresses out into s.p.a.ce, and as you overtake earlier broadcasts that have traveled farther, the signals become less cacophonous and distinctly older as they report historical events that span the broadcast era of modern civilization. Amid the noise, you hear sound bytes in reverse sequence that include: the Challenger Challenger shuttle disaster of January 1986; the Moon landing of July 20, 1969; Martin Luther King's famous "I Have a Dream" speech, delivered in August 28, 1963; President Kennedy's January 20, 1961, inaugural address; President Roosevelt's December 8, 1941, address to Congress, where he asked for a declaration of war; and a 1936 address by Adolf Hitler during his rise to power in n.a.z.i Germany. Eventually, the human contribution to the signal disappears entirely, leaving a din of radio noise emanating from the cosmos itself. shuttle disaster of January 1986; the Moon landing of July 20, 1969; Martin Luther King's famous "I Have a Dream" speech, delivered in August 28, 1963; President Kennedy's January 20, 1961, inaugural address; President Roosevelt's December 8, 1941, address to Congress, where he asked for a declaration of war; and a 1936 address by Adolf Hitler during his rise to power in n.a.z.i Germany. Eventually, the human contribution to the signal disappears entirely, leaving a din of radio noise emanating from the cosmos itself.

Poignant. But this scroll of acoustic landmarks would not unfurl exactly as shown. If you somehow managed to violate several laws of physics and travel fast enough to overtake a radio wave, then few words would be intelligible because you'd hear everything replayed backward. Furthermore, we hear King's famous speech as we pa.s.s the planet Jupiter, implying Jupiter is as far as the broadcast has traveled. In fact, King's speech pa.s.sed Jupiter 39 minutes after he delivered it.

Ignoring these facts that would render the zoom impossible, Contact Contact's opening scene was poetic and powerful, as it indelibly marked the extent to which we have presented our civilized selves to the rest of the Milky Way galaxy. This radio bubble, as it has come to be called, centers on Earth and continues to expand at the speed of light in every direction, while getting its center continuously refilled by modern broadcasts. Our bubble now extends nearly 100 light-years into s.p.a.ce, with a leading edge that corresponds to the first artificial radio signals ever generated by Earthlings. The bubble's volume now contains about a thousand stars, including Alpha Centauri (4.3 light-years away), the nearest star system to the Sun; Sirius (10 light-years away), the brightest star in the nighttime sky; and every star around which a planet has thus far been discovered.

NOT ALL RADIO signals escape our atmosphere. The plasma properties of the ionosphere, more than 50 miles up, enable it to reflect back to Earth all radio-wave frequencies less than 20 megahertz, allowing some forms of radio communication, such as the well-known "short wave" frequencies of HAM radio operators, to reach thousands of miles beyond your horizon. All the broadcast frequencies of AM radio are also reflected back to Earth, accounting for the extended range that these stations enjoy. signals escape our atmosphere. The plasma properties of the ionosphere, more than 50 miles up, enable it to reflect back to Earth all radio-wave frequencies less than 20 megahertz, allowing some forms of radio communication, such as the well-known "short wave" frequencies of HAM radio operators, to reach thousands of miles beyond your horizon. All the broadcast frequencies of AM radio are also reflected back to Earth, accounting for the extended range that these stations enjoy.

If you broadcast at a frequency that does not correspond to those reflected by Earth's ionosphere, or if Earth had no ionosphere, your radio signals would reach only those receivers that fell in its line of "sight." Tall buildings give significant advantage to radio transmitters mounted on their roofs. While the horizon for a 5'8" person is just 3 miles away, the horizon seen by King Kong, while climbing atop New York City's Empire State Building, is more than 50. After the filming of that 1933 cla.s.sic, a broadcast antenna was installed. An equally tall receiving antenna could, in principle, be located 50 miles farther still, enabling the signal to cross their mutual 50-mile horizon, thereby extending the signal's reach to 100 miles.

The ionosphere reflects neither FM radio nor broadcast television, itself a subset of the radio spectrum. As prescribed, they each travel no farther on Earth than the farthest receiver they can see, which allows cities that are relatively near each other to broadcast their own television programs. For this reason, television's local broadcasts and FM radio cannot possibly be as influential as AM radio, which may account for its preponderance of politically acerbic talk shows. But the real influence of FM and TV may not be terrestrial. While most of the signal's strength is purposefully broadcast horizontal to the ground, some of it leaks straight up, crossing the ionosphere and traveling through the depths of s.p.a.ce. For them, the sky is not the limit. And unlike some other bands in the electromagnetic spectrum, radio waves have excellent penetration through the gas and dust clouds of interstellar s.p.a.ce, so the stars are not the limit either.

If you add up all factors that contribute to the strength of Earth's radio signature, such as the total number of stations, the distribution of stations across Earth's surface, the energy output of each station, and the bandwidth over which the energy is broadcast, you find that television accounts for the largest sustained flux of radio signals detectable from Earth. The anatomy of a broadcast signal displays a skinny and a wide part. The skinny, narrow-band part is the video carrier signal, through which more than half the total energy is broadcast. At a mere .10 hertz wide in frequency, it establishes the station's location on the dial (the familiar channels 2 through 13) as well as the existence of the signal in the first place. A low-intensity, broadband signal, 5 million hertz wide, surrounds the carrier at higher and lower frequencies and is imbued with modulations that contain all the program information.

AS YOU MIGHT guess, the United States is the most significant contributor among all nations to Earth's global television profile. An eavesdropping alien civilization would first detect our strong carrier signals. If it continued to pay attention, it would notice periodic Doppler shifts in these signals (alternating from lower frequency to higher frequency) every 24 hours. It would then notice the signal get stronger and weaker over the same time interval. The aliens might first conclude that a mysterious, although naturally occurring, radio loud spot was rotating into and out of view. But if the aliens managed to decode the modulations within the surrounding broadband signal they would gain immediate access to elements of our culture. guess, the United States is the most significant contributor among all nations to Earth's global television profile. An eavesdropping alien civilization would first detect our strong carrier signals. If it continued to pay attention, it would notice periodic Doppler shifts in these signals (alternating from lower frequency to higher frequency) every 24 hours. It would then notice the signal get stronger and weaker over the same time interval. The aliens might first conclude that a mysterious, although naturally occurring, radio loud spot was rotating into and out of view. But if the aliens managed to decode the modulations within the surrounding broadband signal they would gain immediate access to elements of our culture.

Electromagnetic waves, including visible-light as well as radio waves, do not require a medium though which to travel. Indeed, they are happiest moving through the vacuum of s.p.a.ce. So the time-honored flashing red sign in broadcast studios that says "On the Air" could justifiably read "Through s.p.a.ce," a phrase that applies especially to the escaping TV and FM frequencies.

As the signals move out into s.p.a.ce they get weaker and weaker, becoming diluted by the growing volume of s.p.a.ce through which it travels. Eventually, the signals get hopelessly buried by the ambient radio noise of the universe, generated by radio-emitting galaxies, the microwave background, radio-rich regions of star formation in the Milky Way, and cosmic rays. These factors, above all, will limit the likelihood of a distant civilization decoding our way of life.

At current broadcast strengths from Earth, aliens 100 light-years away would require a radio receiver that was fifteen times the collecting area of the 300-meter Arecibo telescope (the world's largest) to detect a television station's carrier signal. If they want to decode our programming information and hence our culture, they will need to compensate for the Doppler shifts caused by Earth's rotation on its axis and by its revolution around the Sun (enabling them to lock onto a particular TV station) and they must increase their detection capacity by another factor of 10,000 above that which would detect the carrier signal. In radio telescope terms, this amounts to a dish about four hundred times Arecibo's diameter, or about 20 miles across.

If technologically proficient aliens are indeed intercepting our signals (with a suitably large and sensitive telescope), and if they are managing to decode the modulations, then the basics of our culture would surely befuddle alien anthropologists. As they watch us become a radio-transmitting planet, their attention might first be flagged by early episodes of the Howdy Doody Howdy Doody show. Once they knew to listen, they would then learn how typical human males and females interact with each other from episodes of Jackie Gleason's show. Once they knew to listen, they would then learn how typical human males and females interact with each other from episodes of Jackie Gleason's Honeymooners Honeymooners and from Lucy and Ricky in and from Lucy and Ricky in I Love Lucy I Love Lucy. They might then a.s.sess our intelligence from episodes of Gomer Pyle Gomer Pyle, The Beverly Hillbillies The Beverly Hillbillies, and then, perhaps, from Hee Haw Hee Haw. If the aliens didn't just give up at this point, and if they chose to wait a few more years, they would learn a little more about human interactions from Archie Bunker in All in the Family All in the Family, then from George Jefferson in The Jeffersons The Jeffersons. After a few more years of study, their knowledge would be further enriched from the odd characters in Seinfeld Seinfeld and, of course, the prime-time cartoon and, of course, the prime-time cartoon The Simpsons The Simpsons. (They would be spared the wisdom of the hit show Beavis and b.u.t.thead Beavis and b.u.t.thead because it existed only as a nonbroadcast cable program on MTV.) These were among the most popular shows of our times, each sustaining cross-generational exposure in the form of reruns. because it existed only as a nonbroadcast cable program on MTV.) These were among the most popular shows of our times, each sustaining cross-generational exposure in the form of reruns.

Mixed in among our cherished sitcoms is the extensive, decade-long news footage of bloodshed during the Vietnam war, the Gulf wars, and other military hot spots around the planet. After 50 years of television, there's no other conclusion the aliens could draw, but that most humans are neurotic, death-hungry, dysfunctional idiots.

IN THIS ERA of cable television, even broadcast signals that might have otherwise escaped the atmosphere are now delivered via wires directly to your home. There may come a time when television is no longer a broadcast medium, leaving our tube-watching aliens to wonder whether our species went extinct. of cable television, even broadcast signals that might have otherwise escaped the atmosphere are now delivered via wires directly to your home. There may come a time when television is no longer a broadcast medium, leaving our tube-watching aliens to wonder whether our species went extinct.

For better or for worse, television might not be the only signals from Earth decoded by aliens. Any time we communicate with our astronauts or our s.p.a.ce probes, all signals that do not intersect the craft's receiver are lost in s.p.a.ce forever. The efficiency of this communication is greatly improved by modern methods of signal compression. In the digital era, it's all about bytes per second. If you devised a clever algorithm that compressed your signal by a factor of 10, you could communicate ten times more efficiently, provided the person or machine on the other side of the signal knew how to undo your compressed signal. Modern examples of compression utilities include those that create MP acoustic recordings, JPEG images, and MPEG movies for your computer, enabling you to swiftly transfer files and to reduce the clutter on your hard drive.

The only radio signal that cannot be compressed is one that contains completely random information, leaving it indistinguishable from radio static. In a related fact, the more you compress a signal, the more random it looks to someone who intercepts it. A perfectly compressed signal will, in fact, be indistinguishable from static to everyone but the person who has the preordained knowledge and resources to decode it. What does it all mean? If a culture is sufficiently advanced and efficient, then their signals (even without the influence of cable transmissions) might just disappear completely from the cosmic highways of gossip.

Ever since the invention and widespread use of electric bulbs, human culture has also created a bubble in the form of visible light. This, our nighttime signature, has slowly changed from tungsten incandescence to neon from billboards and sodium from the now-widespread use of sodium vapor lamps for streetlights. But apart from the Morse code flashed by shuttered lamps from the decks of ships, we typically do not send visible light through the air to carry signals, so our visual bubble is not interesting. It's also hopelessly lost in the visible-light glare of our Sun.

RATHER THAN LET aliens listen to our embarra.s.sing TV shows, why not send them a signal of our own choosing, demonstrating how intelligent and peace loving we are? This was first done in the form of gold-etched plaques affixed to the sides of the four unmanned planetary probes aliens listen to our embarra.s.sing TV shows, why not send them a signal of our own choosing, demonstrating how intelligent and peace loving we are? This was first done in the form of gold-etched plaques affixed to the sides of the four unmanned planetary probes Pioneer 10 Pioneer 10 and and 11 11 and and Voyager 1 Voyager 1 and and 2 2. Each plaque contains pictograms conveying our base of scientific knowledge and our location in the Milky Way galaxy while the Voyager Voyager plaques also contain audio information about the kindness of our species. At 50,000 miles per hour-a speed in excess of the solar system's escape velocity-these s.p.a.cecraft are traveling through interplanetary s.p.a.ce at quite a clip. But they move ridiculously slow compared with the speed of light and won't get to the nearby stars for another 100,000 years. They represent our "s.p.a.cecraft" bubble. Don't wait up for them. plaques also contain audio information about the kindness of our species. At 50,000 miles per hour-a speed in excess of the solar system's escape velocity-these s.p.a.cecraft are traveling through interplanetary s.p.a.ce at quite a clip. But they move ridiculously slow compared with the speed of light and won't get to the nearby stars for another 100,000 years. They represent our "s.p.a.cecraft" bubble. Don't wait up for them.

A better way to communicate is to send a high-intensity radio signal to a busy place in the galaxy, like a star cl.u.s.ter. This was first done in 1976, when the Arecibo telescope was used in reverse, as a transmitter rather than a receiver, to send the first radio-wave signal of our own choosing out to s.p.a.ce. That message, at the time of this writing, is now 30 light-years from Earth, headed in the direction of the spectacular globular star cl.u.s.ter known as M13, in the constellation Hercules. The message contains in digital form some of what appeared on the Pioneer Pioneer and and Voyager Voyager s.p.a.cecraft. Two problems, however: The globular cl.u.s.ter is so chock full of stars (at least a half-million) and so tightly packed, that planetary orbits tend to be unstable as their gravitational allegiance to their host star is challenged for every pa.s.s through the cl.u.s.ter's center. Furthermore, the cl.u.s.ter has such a meager quant.i.ty of heavy elements (out of which planets are made) that planets are probably rare in the first place. These scientific points were not well known or understood at the time the signal was sent. s.p.a.cecraft. Two problems, however: The globular cl.u.s.ter is so chock full of stars (at least a half-million) and so tightly packed, that planetary orbits tend to be unstable as their gravitational allegiance to their host star is challenged for every pa.s.s through the cl.u.s.ter's center. Furthermore, the cl.u.s.ter has such a meager quant.i.ty of heavy elements (out of which planets are made) that planets are probably rare in the first place. These scientific points were not well known or understood at the time the signal was sent.

In any case, the leading edge of our "on-purpose" radio signals (forming a directed radio cone, instead of a bubble) is 30 light-years away and, if intercepted, may mend the aliens' image of us based on the radio bubble of our television shows. But this will happen only if the aliens can somehow determine which type of signal comes closer to the truth of who we are, and what our cosmic ident.i.ty deserves to be.

SECTION 5.

WHEN THE UNIVERSE TURNS BAD.

ALL THE WAYS THE COSMOS WANTS TO KILL US.

TWENTY-EIGHT.

CHAOS IN THE SOLAR SYSTEM.

Science distinguishes itself from almost all other human endeavors by its capacity to predict future events with precision. Daily newspapers often give you the dates for upcoming phases of the moon or the time of tomorrow's sunrise. But they do not tend to report "news items of the future" such as next Monday's closing prices on the New York Stock Exchange or next Tuesday's plane crash. The general public knows intuitively, if not explicitly, that science makes predictions, but it may surprise people to learn that science can also predict that something is unpredictable. Such is the basis of chaos. And such is the future evolution of the solar system.

A chaotic solar system would, no doubt, have upset the German astronomer Johannes Kepler, who is generally credited with the first predictive laws of physics, published in 1609 and 1619. Using a formula that he derived empirically from planetary positions on the sky, he could predict the average distance between any planet and the Sun by simply knowing the duration of the planet's year. In Isaac Newton's 1687 Principia Principia, his universal law of gravity allows you to mathematically derive all of Kepler's laws from scratch.

In spite of the immediate success of his new laws of gravity, Isaac Newton remained concerned that the solar system might one day fall into disarray. With characteristic prescience, Newton noted in Book III of his 1730 edition of Optiks Optiks: The Planets move one and the same way in Orbs concentric, some inconsiderable Irregularities excepted, which may have arisen from the mutual actions of...Planets upon one another, and which will be apt to increase, till the system wants a Reformation. (p. 402) (p. 402) As we will detail in Section 7, Newton implied that G.o.d might occasionally be needed to step in and fix things. The celebrated French mathematician and dynamicist Pierre-Simon Laplace had an opposite view of the world. In his 17991825 five-volume treatise Traite de mecanique celeste Traite de mecanique celeste, he was convinced that the universe was stable and fully predictable. Laplace later wrote in Philosophical Essays on Probability Philosophical Essays on Probability (1814): (1814): [With] all the forces by which nature is animated...nothing [is] uncertain, and the future as the past would be present to [one's] eyes. (1995, Chap. II, p. 3) (1995, Chap. II, p. 3) The solar system does, indeed, look stable if all you have at your disposal is a pencil and paper. But in the age of supercomputers, where billions of computations per second are routine, solar system models can be followed for hundreds of millions of years. What thanks do we get for our deep understanding of the universe?

Chaos.

Chaos reveals itself through the application of our well-tested physical laws in computer models of the solar system's future evolution. But it has also reared its head in other disciplines, such as meteorology and predator-prey ecology, and almost anyplace where you find complex interacting systems.

To understand chaos as it applies to the solar system, one must first recognize that the difference in location between two objects, commonly known as their distance, is just one of many differences that can be calculated. Two objects can also differ in energy, orbit size, orbit shape, and orbit inclination. One could therefore broaden the concept of distance to include the separation of objects in these other variables as well. For example, two objects that are (at the moment) near each other in s.p.a.ce may have very different orbit shapes. Our modified measure of "distance" would tell us that the two objects are widely separated.

A common test for chaos is to begin with two computer models that are identical in every way except for a small change somewhere. In one of two solar system models you might allow Earth to recoil slightly in its...o...b..t from being hit by a small meteor. We are now armed to ask a simple question: Over time, what happens to the "distance" between these two nearly identical models? The distance may remain stable, fluctuate, or even diverge. When two models diverge exponentially, they do so because the small differences between them magnify over time, badly confounding your ability to predict the future. In some cases, an object can be ejected from the solar system completely.

This is the hallmark of chaos.

For all practical purposes, in the presence of chaos, it is impossible impossible to reliably predict the distant future of the system's evolution. We owe much of our early understanding of the onset of chaos to Alexander Mikhailovich Lyapunov (18571918), who was a Russian mathematician and mechanical engineer. His 1892 PhD thesis "The General Problem of the Stability of Motion" remains a cla.s.sic to this day. (By the way, Lyapunov died a violent death in the chaos of political unrest that immediately followed the Russian Revolution.) to reliably predict the distant future of the system's evolution. We owe much of our early understanding of the onset of chaos to Alexander Mikhailovich Lyapunov (18571918), who was a Russian mathematician and mechanical engineer. His 1892 PhD thesis "The General Problem of the Stability of Motion" remains a cla.s.sic to this day. (By the way, Lyapunov died a violent death in the chaos of political unrest that immediately followed the Russian Revolution.) Since the time of Newton, people knew that you can calculate the exact paths of two isolated objects in mutual orbit, such as a binary star system, for all of time. No instabilities there. But as you add more objects to the dance card, orbits become more and more complex, and more and more sensitive to their initial conditions. In the solar system we have the Sun, its eight planets, their 70+ satellites, asteroids, and comets. This may sound complicated enough, but the story is not yet complete. Orbits in the solar system are further influenced by the Sun's loss of 4 million tons of matter every second from the thermonuclear fusion in its core. The matter converts to energy, which subsequently escapes as light from the Sun's surface. The Sun also loses ma.s.s from the continuously ejected stream of charged particles known as the solar wind. And the solar system is further subject to the perturbing gravity from stars that occasionally pa.s.s by in their normal orbit around the galactic center.

To appreciate the task of the solar system dynamicist, consider that the equations of motion allow you to calculate the net force of gravity on an object, at any given instant, from all other known objects in the solar system and beyond. Once you know the force on each object, you nudge them all (on the computer) in the direction they ought to go. But the force on each object in the solar system is now slightly different because everybody has moved. You must therefore recompute all forces and nudge them again. This continues for the duration of the simulation, which in some cases involves trillions of nudges. When you do these calculations, or ones similar to them, the solar system's behavior is chaotic. Over time intervals of about 5 million years for the inner terrestrial planets (Mercury, Venus, Earth, and Mars) and about 20 million years for the outer gas giants (Jupiter, Saturn, Ura.n.u.s, and Neptune), arbitrarily small "distances" between initial conditions noticeably diverge. By 100 to 200 million years into the model, we have lost all ability to predict planet trajectories.

Yes, this is bad. Consider the following example: The recoil of Earth from the launch of a single s.p.a.ce probe can influence our future in such a way that in about 200 million years, the position of Earth in its...o...b..t around the Sun will be shifted by nearly 60 degrees. For the distant future, surely it's just benign ignorance if we do not know where Earth will be in its...o...b..t. But tension rises when we realize that asteroids in one family of orbits can chaotically migrate to another family of orbits. If asteroids can migrate, and if Earth can be somewhere in its...o...b..t that we cannot predict, then there is a limit to how far in the future we can reliably calculate the risk of a major asteroid impact and the global extinction that might ensue.

Should the probes we launch be made of lighter materials? Should we abandon the s.p.a.ce program? Should we worry about solar ma.s.s loss? Should we be concerned about the thousand tons of meteor dust per day that Earth acc.u.mulates as it plows through the debris of interplanetary s.p.a.ce? Should we all gather on one side of Earth and leap into s.p.a.ce together? None of the above. The long-term effects of these small variations are lost in the chaos that unfolds. In a few cases, ignorance in the face of chaos can work to our advantage.

A skeptic might worry that the unpredictability of a complex, dynamic system over long time intervals is due to a computational round-off error, or some peculiar feature of the computer chip or computer program. But if this suspicion were true, then two-object systems might eventually show chaos in the computer models. But they don't. And if you pluck Ura.n.u.s from the solar system model and repeat the orbit calculations for the gas giant planets, then there is no chaos. Another test comes from computer simulations of Pluto, which has a high eccentricity and an embarra.s.sing tilt to its...o...b..t. Pluto actually exhibits well-behaved chaos, where small "distances" between initial conditions lead to an unpredictable yet limited set of trajectories. Most importantly, however, different investigators using different computers and different computational methods have derived similar time intervals for the onset of chaos in the long-term evolution of the solar system.

Apart from our selfish desire to avoid extinction, broader reasons exist for studying the long-term behavior of the solar system. With a full evolutionary model, dynamicists can go backward in time to probe the history of the solar system, when the planetary roll call may have been very different from today. For example, some planets that existed at the birth of the solar system (5 billion years ago) could have since been forcibly ejected. Indeed we may have begun with several dozen planets, instead of eight, having lost most of them jack-in-the-box style to interplanetary s.p.a.ce.

In the past four centuries, we have gone from not knowing the motions of the planets to knowing that we cannot know the evolution of the solar system into the unlimited future-a bittersweet victory in our unending quest to understand the universe.

TWENTY-NINE.

COMING ATTRACTIONS.

One needn't look far to find scary predictions of a global holocaust by killer asteroids. That's good, because most of what you might have seen, read, or heard is true.

The chances that your or my tombstone will read "killed by asteroid" are about the same for "killed in an airplane crash." About two dozen people have been killed by falling asteroids in the past 400 years, but thousands have died in crashes during the relatively brief history of pa.s.senger air travel. So how can this comparative statistic be true? Simple. The impact record shows that by the end of 10 million years, when the sum of all airplane crashes has killed a billion people (a.s.suming a death-by-airplane rate of 100 per year), an asteroid is likely to have hit Earth with enough energy to kill a billion people. What confuses the interpretation is that while airplanes kill people a few at a time, our asteroid might not kill anybody for millions of years. But when it hits, it will take out hundreds of millions of people instantaneously and many more hundreds of millions in the wake of global climatic upheaval.

The combined asteroid and comet impact rate in the early solar system was frighteningly high. Theories and models of planet formation show that chemically rich gas condenses to form molecules, then particles of dust, then rocks and ice. Thereafter, it's a shooting gallery. Collisions serve as a means for chemical and gravitational forces to bind smaller objects into larger ones. Those objects that, by chance, accreted slightly more ma.s.s than average will have slightly higher gravity and attract other objects even more. As accretion continues, gravity eventually shapes blobs into spheres and planets are born. The most ma.s.sive planets had sufficient gravity to retain gaseous envelopes. All planets continue to accrete for the rest of their days, although at a significantly lower rate than when formed.

Still, billions (likely trillions) of comets remain in the extreme outer solar system, up to a thousand times the size of Pluto's...o...b..t, that are susceptible to gravitational nudges from pa.s.sing stars and interstellar clouds that set them on their long journey inward toward the Sun. Solar system leftovers also include short-period comets, of which several dozen are known to cross Earth's...o...b..t, and thousands of asteroids that do the same.

The term "accretion" is duller than "species-killing, ecosystem-destroying impact." But from the point of view of solar system history, the terms are the same. We cannot simultaneously be happy we live on a planet; happy that our planet is chemically rich; and happy we are not dinosaurs; yet resent the risk of planetwide catastrophe. Some of the energy from asteroid collisions with Earth gets dumped into our atmosphere through friction and an airburst of shock waves. Sonic booms are shock waves too, but they are typically made by airplanes with speeds anywhere between one and three times the speed of sound. The worst damage they might do is jiggle the dishes in your cabinet. But with speeds upwards of 45,000 miles per hour-nearly seventy times the speed of sound-the shock waves from your average collision between an asteroid and Earth can be devastating.

If the asteroid or comet is large enough to survive its own shock waves, the rest of its energy gets deposited on Earth's surface in an explosive event that melts the ground and blows a crater that can measure twenty times the diameter of the original object. If many impactors were to strike with little time between each event, then Earth's surface would not have enough time to cool between impacts. We infer from the pristine cratering record on the surface of the Moon (our nearest neighbor in s.p.a.ce) that Earth experienced an era of heavy bombardment between 4.6 and 4 billion years ago. The oldest fossil evidence for life on Earth dates from about 3.8 billion years ago. Not much before that, Earth's surface was unrelentingly sterilized, and so the formation of complex molecules, and thus life, was inhibited. In spite of this bad news, all the basic ingredients were being delivered nonetheless.

How long did life take to emerge? An often-quoted figure is 800 million years (4.6 billion-3.8 billion = 800 million). But to be fair to organic chemistry, you must first subtract all the time Earth's surface was forbiddingly hot. That leaves a mere 200 million years for life to emerge from a rich chemical soup, which, as do all good soups, includes water.

Yes, the water you drink each day was delivered to Earth in part by comets more than 4 billion years ago. But not all s.p.a.ce debris is left over from the beginning of the solar system. Earth has been hit at least a dozen times by rocks ejected from Mars, and we've been hit countless more times by rocks ejected from the Moon. Ejection occurs when impactors carry so much energy that smaller rocks near the impact zone get thrust upward with sufficient speed to escape the gravitational grip of the planet. Afterward, the rocks mind their own ballistic business in orbit around the Sun until they slam into something else. The most famous of the Mars rocks is the first meteorite found near the Alan Hills section of Antarctica in 1984. Officially known by its coded, though sensible, abbreviation, ALH-84001, this meteorite contains tantalizing, though circ.u.mstantial, evidence that simple life on the Red Planet thrived a billion years ago. Mars bears boundless geological evidence for a history of running water that includes dried riverbeds, river deltas, and flood plains. And most recently the Martian rovers Spirit Spirit and and Opportunity Opportunity found rocks and minerals that could have formed only in the presence of standing water. found rocks and minerals that could have formed only in the presence of standing water.

Since liquid water is crucial to the survival of life as we know it, the possibility of life on Mars does not stretch scientific credulity. The fun part comes when you speculate whether life arose on Mars first, was blasted off its surface as the solar system's first bacterial astronauts, and then arrived to jump-start Earth's own evolution of life. There's even a word for the process: panspermia. Maybe we are all descendants of Martians.

Matter is far more likely to travel from Mars to Earth than vice versa. Escaping Earth's gravity requires over two-and-a-half times the energy than that required to leave Mars. Furthermore, Earth's atmosphere is about a hundred times denser. Air resistance on Earth (relative to Mars) is formidable. In any case, bacteria would have to be hardy indeed to survive the several million years of interplanetary wanderings before landing on Earth. Fortunately, there is no shortage of liquid water and rich chemistry on Earth, so we do not require theories of panspermia to explain the origin of life as we know it, even if we still cannot explain it.

Ironically, we can (and do) blame impacts for major episodes of extinction in the fossil record. But what are the current risks to life and society? Below is a table that relates average collision rates on Earth with the size of impactor and the equivalent energy in millions of tons of TNT. For reference, I include a column that compares the impact energy in units of the atomic bomb that the United States dropped on the city of Hiroshima in 1945. These data are adapted from a graph by NASA's David Morrison (1992).

Once per...

Asteroid Diameter (meters) (meters) Impact Energy (Megatons of TNT) (Megatons of TNT) Impact Energy (A-Bombs) (A-Bombs) Month