When the aliens of Independence Day started their rampage, they had a clear goal—to exploit the Earth’s resources, before discarding it like an old apple core. Compare this with one of my favorite doomsday devices (along with Stanley Kubrick’s whimsical creation in Dr. Strangelove)—the neutronium planet killer in the classic Star Trek episode “The Doomsday Machine.” This machine destroyed the civilization that created it, and once there was nothing at home left to destroy, it wandered through the galaxy, offing whatever other planets it found. I am particularly taken with this idea because of the complete purposelessness of the destruction; I fully expect that this will be the nature of our own planet’s end.
Here’s a tricky question. If I were to turn off the nuclear reactions that power the Sun (as, for example, was done to an unfortunate star in Star Trek VII: Generations), how long would it take for the Sun to stop shining? The right answer is surprising. In the nineteenth century, two giants of theoretical physics, Lord Kelvin in England and Heinrich Helmholtz in Germany, each tried to determine the answer. Their question was equivalent but slightly different, because no one at the time knew anything about the nuclear reactions that power the Sun. Kelvin and Helmholtz both assumed that the source of the Sun’s heat was its own ongoing gravitational collapse, and that it was gradually shrinking and cooling as it radiated heat. If the Sun’s own mass was its power source, they wanted to know how long the Sun would burn after it was first formed. The answer they derived was between 30 million and 100 million years.
This was a truly amazing result, I think. It implies that if I were to turn off the Sun today, it would continue to burn for at least 30 million years, powered by gravitational collapse alone, before dying out like an ember! (The sudden shutdown in Generations would never have happened, but the reality would have been far too slow to interest even an audience of dedicated Trekkers.) Thirty million years or so may seem like an awfully long time, but in fact it got Kelvin and Helmholtz into hot water (forgive the pun). Since they didn’t know of any internal power source, they reasoned that because the Sun was still shining, it definitely had to be less than 100 million years old. The problem with this was that even at the time Kelvin and Helmholtz did their calculations it was known from fossil evidence that the Earth was much older than 100 million years.
Creationists love it when sound scientific reasoning produces a cosmic paradox. But what such paradoxes really provide is an opportunity for discovery. The fact that the Sun had to be at least as old as the Earth suggested that there was an internal power mechanism that kept it shining. Fewer than 50 years after the Kelvin-Helmholtz estimate, natural radioactivity was discovered in the laboratory, and fewer than 50 years after that, nuclear power was harnessed. In 1938, the great theoretical physicist Hans Bethe, who is still alive and calculating today, finally showed that nuclear reactions could power the Sun, a theoretical discovery for which he later received the Nobel Prize.
Incidentally, for those of you who like to debate with the creationists who believe that the solar system is only between 5,000 and 7,000 years old, here is some useful ammunition. Guess how long it takes radiation emitted deep inside the solar core to make its way to the solar surface? Again, the answer is surprising: it takes almost 10,000 years! The reason is simple. The Sun is very big: its radius measures some 432,000 miles, and a quantum of radiation emitted in its interior travels on average about 1 centimeter before it hits something and is scattered in another direction. The random walk that ensues takes about 10,000 years (again, on average) to progress to the surface. This means that if the Sun were only 5,000 years old, it would not yet be shining—at least not with anything like its present consistent brightness!
It is these two factors—the random walk and the competition between gravitational contraction and nuclear burning—that determine how fast the Sun will burn its nuclear fuel. And it is this rate of nuclear burning that sets a limit on the term of life on Earth.
Since the origin of the solar system some 4.5 billion years ago, the Earth has been a slave to the Sun. Every process, every major event in our terrestrial history, has been dependent on our closest cosmic stellar companion. The average energy received on Earth every day from the Sun is tremendous—about 1,350 watts per square meter. Day in, day out, for 4.5 billion years the Sun has been bathing the Earth with almost 100 million billion watts of radiation. This solar radiation makes life possible on Earth, but it takes its toll on the Sun.
The 100 million billion billion total watts of power the Sun has been pumping out since its formation is, as we have seen, not directly due to the energy released by collapsing dust and gas. Instead, relentlessly (and, when viewed on an atom-by-atom basis, slowly), more than 1038 hydrogen nuclei in the solar core are converted each second into nuclei of the next lightest element, helium—enough nuclear reactions to power almost a million 10-megaton hydrogen bombs per second. The incredible pressure generated by these reactions is enough to balance the gravitational attraction that would otherwise cause the Sun to collapse inward.
As a result of this nuclear burning, the solar core is inexorably converting from mostly hydrogen to mostly helium. As the relative abundances change, the whole structure of the Sun changes in response. Over the course of a human lifetime, this change is not noticeable (although there are some changes that are, like the sunspot cycle, whose 13-year periodicity is still not understood). However, over cosmic time the Sun’s structure has changed considerably. Since life arose on Earth, the luminosity of the Sun has increased by almost 25 percent, for example. And so too, eventually, the Sun will run out of its hydrogen fuel. In spite of the complexity of the reactions taking place in the solar interior, it is a relatively straightforward matter to determine when the hydrogen fuel will run out: simply divide the total energy the Sun produces per second by the energy produced each time 4 hydrogen nuclei fuse to form a nucleus of helium, in order to get the number of hydrogen atoms being burned per second, and then divide that by the amount of hydrogen left in the Sun’s core. The answer is about 5 billion years.
Unlike many larger stars, however, the Sun will not end its life with a bang but with a whimper. The exhaustion of its hydrogen fuel does not leave the Sun with nothing to burn. Helium is itself susceptible to nuclear burning, at a much higher temperature, to form yet heavier elements, such as boron, carbon, oxygen, and nitrogen. How does the core of the Sun—the region in which nuclear burning takes place—reach the higher temperatures at which the nuclear burning of helium can take place? Simple. The core contracts because of the Sun’s own gravity, and the pressure and temperature of the gas inside increases in response until the temperature for helium burning is reached.
Now, the rest of the Sun does not stand idly by while all this excitement occurs in its core. During the final stages of hydrogen burning, as the core begins to contract, the Sun’s outer layers puff up, due to the extra release of heat from the core. The size of the Sun will increase many times, turning it into what is called a Red Giant. While this is merely one of a series of metamorphoses the Sun will experience during its lifetime, it is a particularly important one for Earth, since in the puffing-up process the solar surface will increase enough to envelop the Earth’s orbit. From then on, our tiny speck in the solar system will be no more.
It may sound fantastic to think of the Sun puffing up by such a great factor, so let me tell you something even more fantastic. The largest known star is Mu Cephei, which has a radius of 11 astronomical units. An astronomical unit is the distance from the Earth to the Sun—93 million miles—so this star would encompass our solar system out to Saturn. I find that remarkable to contemplate.
The Sun will continue to evolve after it gobbles the Earth; eventually, helium will burn to form carbon, carbon will burn to form oxygen, and so on, until the nuclear fusion process reaches iron. At this point, nuclear burning stops, since iron, the most tightly bound nucleus in existence, cannot release energy by fusing to form a heavier nucleus. Thus the nuclear fight ag
ainst the force of gravity will be lost, and the Sun will collapse inward to form a dark star known as a white dwarf, slowly radiating away its stored energy. Eventually it will die out like an ember and join the blackness of space around it. What remains of the Earth will have merged with its stellar host; like a member of the Borg collective, Earth will have utterly lost its identity.
This ultimate calamity is so far removed that it’s pretty well irrelevant to us, our children, our children’s children, and their children… and on down the line. But even if we are lucky enough to survive all the other challenges to our continued existence, our species’ days are numbered.
That is, of course, if we remain on Earth—a Big IF. I imagine that we will have chosen to leave well before the Sun blows up, if our species persists long enough to develop the means to leave—another Big IF. Since the burgeoning speciation at the dawn of the Cambrian, some 540 million years ago, there have been 5 mass extinctions, during which a significant fraction of species alive at the time disappeared. The largest occurred at the end of the Permian, around 250 million years ago, when up to 96 percent of all species on the planet became extinct. The most famous extinction is surely the one in which the dinosaurs perished, 65 million years ago, at the boundary between the Cretaceous and Tertiary periods. There is good evidence that this extinction followed a collision between the Earth and an extraterrestrial object, probably an asteroid or a comet.
In order to explain these extinctions, biologists, geologists, and physicists have been examining all possible causes, and more candidates are being discovered all the time. The list of plausible potential threats to life on Earth is getting long enough so that one wonders how we have managed to survive thus far. How might we go? Let me count the ways:
1. Human Folly: This is the most immediate threat, although it may not be a global one. By this I mean that even in the event of a global thermonuclear exchange, some humans (and many other species of life) may survive. The conditions under which the unfortunate survivors will eke out their existence will be ugly, but such is life. A more deadly threat, I believe, is posed not by global war but by global complacence. We are currently polluting our water, filling our atmosphere with greenhouse gases, reproducing our numbers without regard to Earth’s resources, and so on. The changes we are making act slowly on the scale of a human generation, but when you add it all up, we are in the middle of the biggest mass extinction in the Earth’s history; close to 30,000 species a year are becoming extinct. We appear to be doing a better job of this than any of the natural disasters that have occurred since the Cambrian. We are unlikely to entirely wipe out our own species by this global complacence, but we may make life on Earth so unpleasant that it is preferable to leave.
2. Extraterrestrial Impact: As noted earlier, the collision of a large asteroid or comet with the Earth is the current best candidate for the Cretaceous-Tertiary mass extinction. While comparable impacts are rare, with a frequency of perhaps one every 100 million years, they are also inevitable. The advance notice we might receive of the approach of such an object would probably be months or years; we might by then possess the technology to destroy the intruder before the collision. If we don’t, the Earth could become virtually uninhabitable for humans.
3. Supernovae: When a star 10 times as massive as our Sun reaches the final stage of nuclear burning to form an iron core, the gravitational pressure becomes so great near the center that the interior of the star collapses in mere seconds to form an incredibly dense object known as a neutron star. In the process, the outer shell of the star is blown off in one of the most spectacular fireworks demonstrations in the universe. The brightness of a supernova can exceed, for a period of days, the brightness of an entire galaxy. There are thought to be two or three such explosions in the Milky Way galaxy per century. The reason we don’t usually see them is that (surprisingly) in spite of their intrinsic brightness, the dust in our galaxy obscures the visual signal. The last supernova to be recorded in our own galaxy was observed in 1604 by the great Johannes Kepler. Now, our Sun travels around the outskirts of our galaxy at about 200 kilometers per second—fast enough to perform a full revolution every 200 million years or so. During this time, our stellar neighbors change. If, in the course of our trip around the center of the galaxy, we were to pass within even a few tens of light-years of an exploding star, the results could be traumatic, to say the least: Earth might be knocked out of its orbit—or vaporized. Advance warning of an impending nearby supernova might be possible, depending upon the observing technology of our civilization at the time—although it is difficult to imagine what might be done to protect us from the consequences, if we are around to experience them.
4. Neutron Star Collision: Neutron stars formed in supernovae are sometimes found orbiting in binary systems, either with another neutron star or a star that is still burning its nuclear fuel. Sometimes—perhaps once every 100,000 to 1,000,000 years in a given galaxy—these two partners, losing energy and spiraling inward, will collide in a massive fireball. This may sound so infrequent as not to matter. However, over the past couple of decades, satellites originally designed for the Cold War monitoring of possible nuclear weapons tests have scanned the skies for X rays and gamma rays (which are more energetic than X rays). The results have been surprising. Short gamma-ray bursts of very high energy, lasting from seconds to days, have been observed all over the sky. Because of their uniform distribution, astronomers speculated that they were at cosmological distances—that is, not confined to our own galaxy. In this case, the energy they release would be tremendous. The hypothesis that they are cosmological was confirmed in 1997, when Caltech astronomers observed a visual counterpart to a gamma-ray burst during the burst phase and determined that this object was some 2 billion light-years from us. The best current explanation for the phenomenon involves collapsing neutron-star binary systems. It seems lately that each time a new class of energetic astrophysical object is discovered, someone speculates on a possible link to mass extinctions on Earth. One such group has calculated that if a neutron-star binary system collapsed in our region of the galaxy (perhaps once every few 100 million years), the high-energy cosmic rays released by the event would provide a lethal radiation dose to most of humanity.
5. Old Age: Finally, barring any extreme catastrophes of the type mentioned above, the Earth may become inhospitable to life simply by evolving in its own quiet way. For example, its molten iron core is thought to be responsible for the magnetic field that surrounds the planet. This magnetic field deflects most potentially harmful cosmic rays. As the Earth cools, its core will cool with it. Once the core solidifies, the charged currents that now flow to create the magnetic field will disappear. Whether this will take longer than the 5 billion years we have left on Earth is not yet clear.
And let’s not forget our friend the Moon. As I noted in chapter 1, its tidal forces are ever so gradually slowing the Earth’s period of rotation. Over the course of billions of years, the length of Earth’s day will increase until it coincides with the Moon’s orbital period. Earth’s climate is bound to become unlivable long before this synchrony is reached.
Well, if you gotta go, you gotta go. If humanity is to survive these disasters, we will have to embark on a cosmic voyage to another world, or build our own and travel around in it. As I hope I’ve made clear, almost all the barriers to interstellar travel discussed earlier were based on round-trip travel, on the timescale of a few generations. Once we decide to leave Earth forever, the requirements change considerably. Speed is not an issue, for example. If we are heading nowhere in particular, then it doesn’t matter how fast we get there. What we will require is a self-sustaining environment large enough to generate artificial gravity by rotation and to shield us from harmful cosmic rays (or powerful enough to generate large magnetic fields to deflect them). These are no small requirements, but I like to think that with several million years to get ready, even creatures with as notoriously little foresight as human beings might be
up to the task.
Which brings me back to Independence Day once again. Perhaps 15-mile-wide ships are impractical to zip around the atmosphere in. But just as planet Earth is a self-sustaining spacecraft as it travels around the Sun, which in turn travels around our own galaxy every 200 million years, the man-made spaceships of our future, in which we will venture beyond our solar system, may also be mammoth systems, designed to house not one generation but thousands, and designed not for combat but for survival. I trust that if our spaceships make it to a safe harbor across far reaches of the cosmic ocean, we will present ourselves more generously than the visitors in Independence Day.
However, it is equally likely—or perhaps more likely—that the resources and the organizational and logistic skills necessary for us to leave our world in one piece will remain beyond our grasp. Will all remnants of our existence then perish with us, and with our Sun? Not necessarily. A large comet, or astrophysical shock wave, striking the Earth would not only do incalculable damage but would also eject a great deal of matter into space. Among this stuff would undoubtedly be the organic materials that provide the blueprint of our existence. Just as the organic basis of our DNA may derive from interstellar pollution, perhaps one day we will bequeath our own organic material to the universe.
Beyond Star Trek Page 8