Solution The Galaxy Is a Dangerous Place

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I am become death, the destroyer of worlds.

Bhagavadgita

A key realization of modern astronomy is that the Universe is a perilous place. We now know that violent phenomena are common and pose a variety of threats. A stray black hole wandering into a planetary system would devour the planets and any life they harbored. (We know black holes exist. some astronomers estimate that a million of them may be wandering through interstellar space. Could one of them be heading our way?) Neutron stars called magnetars would pose an interesting threat if they came too close. (on 27 August 1998, several orbiting detectors recorded radiation from the magnetar SGR1900+14. The radiation came within 30 miles of Earth's surface. Fortunately, our atmosphere shielded us, as it does from a variety of forms of cosmic radiation. SGR 1900+14 is 20,000 light years away, so would our atmosphere have saved us had the magnetar been closer?199) A galaxy might possess a violently active nucleus, which is quite deadly. (The central region of our own Galaxy, although not as active as objects like blazars, for example, is nevertheless inhospitable. Close to the center, stars are so crowded that the night sky would be bright enough to read by; closer still, and you meet the accretion disk of a million-solar-mass black hole. This is why the inner edge of the GHZ is defined by the point where the violent central regions are no longer a threat.)

Could this be the explanation of the Fermi paradox? Might the random violence of an uncaring Universe explain the silence? Are civilizations destroyed before they can reach us?

The three mechanisms mentioned above — stray black holes, magnetars, and active galactic nuclei — do not by themselves, or as a group, explain why our Galaxy is silent. Black holes and magnetars might pose a threat to individual stars or stellar groups over the course of the Galaxy's lifetime, but they cannot act as a Galaxy-wide sterilizing agent; and while the center of the Galaxy is probably a place to avoid, it seems to provide no threat to life way out here in the spiral arms, some 30,000 light years away

figure 50 Black holes may be lurking out there in interstellar space.

figure 51 A Hubble Space Telescope image of the center of the galaxy ngc253. The central region of this galaxy is violently energetic, and is not likely to be a hospitable place for life.

from the action. On the other hand, two other mechanisms — supernovae and gamma-ray bursters — might resolve the Fermi paradox.

Supernovae

A supernova is the cataclysmic explosion of an aging star. Such explosions are powerful and occur rather frequently on an astronomical timescale: the Galaxy on average is host to one or two supernovae per century.

There are two types of supernova. A Type Ia supernova results when a white dwarf in a binary system reaches a critical mass after sucking material from its companion. A violent thermonuclear explosion ignites and blows the star to bits. A Type II supernova occurs in the later stages of the life of massive stars. When the core of a massive star no longer produces enough energy to support itself against the relentless force of gravity, the star collapses under its own weight. The core forms a dense neutron star or even a black hole; the outer layers of the star rebound from the core at high velocity and head off into space, where they become part of the interstellar medium. (Life on Earth would not exist were it not for an ancient Type II supernova that seeded space with heavy elements cooked up in its core.) The details of the two types of explosion are different, but both types radiate large amounts of energy. Over the course of a few weeks, a supernova can release as much as 1044 J in a variety of forms.

figure 52 Ozone depletion over the South Pole in 2000. A nearby supernova could reduce ozone levels over the entire globe.

A nearby supernova might be disastrous for life on Earth. One estimate is that any supernova exploding anywhere within about 30 light years of Earth could destroy most surface life on our planet.

However, the mechanism of destruction is not obvious. For example, although a Type Ia supernova is intrinsically the brightest type of supernova, even at maximum brightness it would have to be no farther away than one light year to appear as bright as the Sun. On an astronomical scale this is extremely close, so we have nothing to fear from supernova optical photons. Type II supernovae emit vast numbers of neutrinos, and perhaps the large neutrino flux from a nearby supernova could have deleterious effects upon organisms. But it is difficult to believe that neutrino fluxes can lead to mass-extinction events. No, the real threat is the enormous amount of gamma-radiation that a nearby supernova would dump into Earth's atmosphere. Direct gamma-radiation from the explosion would probably not harm us, because the upper atmosphere provides an effective shield. However, the gamma-rays would cause atmospheric nitrogen to dissociate, the nitrogen would then react with oxygen to form nitric oxide, and the nitric oxide would react with ozone — thus rapidly depleting the ozone layer. Ozone levels could be reduced by as much as 95% for several years. With Earth's ozone layer down, surface life would have nothing to protect it from lethal UV rays from the Sun. The supernova, in other words, kills by a classic one-two punch: first the gamma-radiation lowers our defenses, then the solar UV radiation devastates multicellular life.

As we will discuss later, there have been several mass-extinction events since multicellular life took to the land. Can any of these be blamed upon the effects of a local supernova? It is difficult to say with certainty. As seems increasingly probable, the last mass extinction — the one in which dinosaurs perished — was due to the effects of a meteor impact. Perhaps the other great die-offs were caused by similar impacts; or perhaps they were due to climate change; or perhaps they were just chaotic events that can happen in complex systems. There is no evidence linking mass extinctions to the after-effects of supernovae. Even if supernovae can cause mass extinctions, it is not certain whether the extinctions pose a long-term threat to the emergence of intelligence. Perhaps, indeed, supernovae are necessary for intelligent life. Maybe, to use Cramer's phrase, they constitute another "pump of evolution." For the moment, though, let us assume that a nearby supernova can cause a mass-extinction event, and that such an event slows the development of intelligent life.

Since all stars, including the Sun, move through space, over the course of aeons random stellar motions will bring the Sun close to a supernova. Eventually, a supernova will explode close to Earth. (In case any readers are worried, no star presently within 60 light years of us will go supernova within the next few million years.) The critical question is: how often is a supernova event likely to occur close enough to Earth to cause a mass-extinction event? Typical estimates are that a supernova event will occur within 30 light years of Earth on average every couple of 100 million years. If that is true, we have another question to ask. Why are we here?

One answer to this question could be simply that the calculations of the frequency of supernovae are wrong; or (which is quite likely) perhaps we do not fully understand the effects of a nearby supernova. In this case, there is no implication for the Fermi paradox. But perhaps we are here because Earth has been extremely lucky; perhaps Earth has not seen a really close supernova since the emergence of life on land. If this is true, then we can resolve the Fermi paradox by saying that every other life-bearing planet has been less fortunate than Earth.

However, resorting to luck is a poor sort of explanation. And there is no astrophysical evidence to suppose Earth has been particularly fortunate with regard to supernovae. If we have been lucky, then there is no reason to suppose that, in the past, other regions of the Galaxy did not also have a run of good luck. Indeed, if we accept that intelligent life is common, then supernovae are just not effective enough to explain the Fermi paradox. Inevitably, by the blind workings of chance, some civilizations will never come close to a supernova and will thus have the time to develop space travel. And once they colonize other parts of the Galaxy, no supernova can stop them. (Hence, the threat from supernovae is another motivating factor for ETCs to engage in interstellar colonization! Once a civilization has colonized stars within a radius of about 30 light years of the home world, they will survive the effects of a local supernova.)

What we need if we want to explain the Fermi paradox is a mechanism that can affect life on every planet in the Galaxy, without exception. If there were some mechanism that generated a sufficiently powerful Galaxy-wide sterilizing event it could operate fairly infrequently (every few hundred million years, say) and remain an explanation for the Fermi paradox. Multicellular life would be eradicated before intelligence had the chance to arise; a civilization could never advance to the stage where it might develop effective countermeasures to the threat. Putative ETCs would not have had billions of years to colonize the Galaxy; instead, they would have the few hundred million years since the last sterilizing event. In essence, the "Universal Clock" would be reset every time a sterilizing event took place.

It seems unbelievable that any phenomenon could cause such widespread devastation. Unfortunately, astronomers now know of a potential Galaxy-wide sterilizing mechanism: the devastating power of a gamma-ray burster (grb).

Gamma-Ray Bursters

Gamma-ray bursters were discovered by accident more than 30 years ago, but until recently their origin was completely unknown.200 Even now, the precise physical origin of GRBs is a matter for intense debate among astronomers. Whatever the progenitor event may be, the important fact about a GRB is this: the GRB fireball is the most powerful phenomenon in the known Universe. A GRB pours out more energy in a few seconds than the Sun will generate in its entire lifetime. A GRB shines so brightly that our detectors can see them from halfway across the Universe. All the GRBs we have detected so far seem to have occurred in distant galaxies; if one occurred in our Galaxy, it would be bad news. We need to ask two questions. First, how frequently do GRBs occur in our Galaxy? Second, if our Galaxy hosted a GRB event, just how bad would things be?

Calculating the frequency of occurrence of Galactic GRBs is a typical Fermi problem! It happens that a galaxy hosts a GRB about once every 100 million years. Interestingly, this rough timescale is pretty much the timescale between mass-extinction events on Earth. People have suggested, therefore, that GRBs might be responsible for mass extinctions.

The Frequency of Gamma-Ray Bursters

A gamma-ray detector such as BATSE (Burst and Transient Source Experiment) on board NASA's orbiting Compton Gamma Ray Observatory detects on average one GRB per day. BATSE covers about one third of the sky, and therefore about three GRBs occur in the Universe each day — or about 1000 each year. As a rough estimate, we can suppose that there are 1011 galaxies in the Universe; so on average there are 10~8 GRBs per galaxy per year. In other words, to a first approximation with which Fermi would be happy, a typical galaxy will host a GRB about once every 100 million years. (This calculation assumes that GRBs emit their energy equally in all directions. If GRBs emit their energy in a beam, as some theories suggest, then the GRBs we detect would be those with beams that happen to be pointing toward us. The total GRB-event rate would therefore be much higher, to account for those GRBs with beams pointing in some other direction. For our purposes, though, we do not need to consider this point.)

The awesome power released by GRBs means that, even if one occurred at a large distance from Earth, our planet would still be bathed in radiation. Moreover, the same GRB could cause devastation throughout the Galaxy. The pessimists suggest that a GRB could sterilize the Galaxy.

Nevertheless, this suggestion is very much open to debate. Gamma-ray bursters are undeniably more powerful than supernovae, so they could be at much greater distances and still inflict the same sort of damage to the ozone layer, through the same processes. But there is a difference.

While a supernova event occurs over a rather long period of time, a GRB pumps out most of its energy in less than a minute. Therefore only half of a planet will be affected directly by a burster; the other half is safe from the blast, as it is shielded by the mass of the planet. Of course, the damage from the affected side of the planet might propagate and cause worldwide

figure 53 Did a gamma-ray burster kill the dinosaurs? In this artist's impression, T. Rex looks up at the brief flash of a burster. A much more probable scenario, however, is that a meteor impact caused the mass extinction at the end of the Cretaceous period. Whether gamma-ray bursters (or supernovae) caused earlier mass extinctions is not known.

figure 53 Did a gamma-ray burster kill the dinosaurs? In this artist's impression, T. Rex looks up at the brief flash of a burster. A much more probable scenario, however, is that a meteor impact caused the mass extinction at the end of the Cretaceous period. Whether gamma-ray bursters (or supernovae) caused earlier mass extinctions is not known.

destruction; and secondary effects might cause further problems. But with our present state of knowledge, it is just as easy to argue that a planet's ozone layer would protect surface life from the effects of a GRB — unless the GRB occurs too close, of course, in which case the planet is toast.

Suppose we accept that a GRB can destroy all higher life-forms throughout a galaxy. Combine this with the prediction from some theories of GRB formation that bursters were more frequent in the past, and you have the resolution of the Fermi paradox proposed by James Annis.201 The proposal is simple: In the past, GRBs effectively sterilized planets before any life-forms in a galaxy had the chance to develop intelligence. Only now that the event rate has decreased, and GRBs are less common, has there been time for technologically advanced civilizations to arise. With Annis' proposal, there is nothing necessarily special about Earth or humanity; there may be tens of thousands of ETCs in our Galaxy at or near the same stage of development. All of them will have had the same time as life on Earth to develop: the amount of time since the last GRB exploded in the Galaxy.

Personally, I think it unlikely that GRBs are capable of sterilizing whole galaxies, and thus I do not accept that GRBs by themselves resolve the Fermi paradox. It is undeniable, however, that GRBs occur and are astonishingly powerful; they would certainly sterilize any planet unlucky enough to be nearby. The SETI optimists — those who argue that intelligent, technologically advanced civilizations are common — thus have to face an unpalatable conclusion: over the course of a Universal Year, many of those civilizations must have been within distance of a GRB. Countless numbers of advanced civilizations must have been consumed by fire.202

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