27 ALIEN PLAGUE A biological attack from space need not be deliberate. British astronomer Fred Hoyle, whose “steady state” cosmology was for years a serious challenger to the Big Bang theory, was convinced that comets are full of viruses that cause flu epidemics when they rain down on Earth. His ideas, once ridiculed, have been partially vindicated by the discovery that pieces of Mars have landed on Earth, and by studies showing that extremely hardy microbes could survive extended voyages through space. NASA takes the idea of alien pathogens seriously enough that it has an elaborate quarantine protocol for any space sample returned here. The possibility that our planet was seeded with microbial life from Mars (or even, per Hoyle, from a comet) means that we cannot be certain that space microbes would be incompatible with our biology. Odds: still low, given the sketchy evidence that space microbes play any role in terrestrial disease.
26 SUPERBOMB The atomic bomb is hardly the ultimate tool of destruction. Matter-antimatter reactions are a much more effective way to tap E = mc2. Splitting a uranium atom converts only about 0.1 percent of its mass into energy, but mixing matter and antimatter is 100 percent efficient. About 100 pounds of antimatter would exceed the yield of all the world’s atomic weapons. Fortunately, we don’t know how to store more than a few atoms’ worth of it at a time. Another possibility if you want to destroy the world: Use a giant particle accelerator to fabricate a mini black hole. If it had more mass than that of a large mountain, it would be stable and would immediately sink through the ground, consuming the planet from within until there was nothing left but an Earth-mass black hole, about one-third of an inch wide. Odds: high for a weapon more destructive than the H-bomb, very low for the black hole in particular.
25 WEATHER-CONTROL MISHAP Efforts to control the weather have a poor track record (China attempted it during the 2008 Olympics, but nobody can tell if the project worked). Still, some researchers are talking seriously about global climate modification as a way to counteract climate change. These “geoengineering” schemes include pumping light-reflecting sulfur compounds into the stratosphere and placing giant sunshades in space. What could go wrong seems more obvious than what could go right. This approach could also lead to another doomsday weapon: Shoot an enormous cloud of chaff into a stable spot between Earth and the sun and you could dim the whole world, possibly triggering an ice age. Odds: very low. Geoengineering schemes would be tested extensively first; a chaff bomb is a slow, cumbersome way to attack.
24 TIME TRAVEL This is one of the favorite tropes in science fiction: Hop into your time machine, visit the past, and you could wipe out the present—intentionally or not (watch out for that butterfly). The current thinking in physics is that time travel might actually be possible. Now the catch: The same theories say that a time machine could go back only to the moment when it was activated. Even if genuine travel to the past is possible, Seth Lloyd of MIT argues that quantum rules would prevent the traveler from doing anything that contradicts what he already knows about the present. Where is the fun in that? Odds: indeterminate, but logic dictates that destructive interference with the past is impossible.
23 STRANGE MATTER Ordinary atoms contain particles called quarks, which come in two varieties: up and down. There are other kinds of quarks that we don’t encounter because they normally form unstable particles. But in 1984 Ed Witten, the godfather of string theory, proposed that there might be a stable state of matter that consisted in part of a third type of quark, called a strange quark. This hypothetical type of matter, called a strangelet, could possibly be created naturally by an extremely energetic cosmic ray, or deliberately in a particle accelerator even more potent than the Large Hadron Collider. Once unleashed, a strangelet could act as a kind of subatomic infection, converting any ordinary matter into strange matter. Eventually it would consume the whole planet, leaving a lump of strangeness in its wake. Odds: very low. Cosmic rays have been striking Earth for billions of years with no such effect.
22 DARK MATTER CLUMP Dark matter is the heavy but invisible (and as yet undetected) stuff believed to hold galaxies together. If a clump passed near our sun, its pull could shake loose comets from the outer solar system, sending some of them crashing into Earth. Australia-based astronomer Kenji Bekki claims that one such passage happened millions of years ago, forming a ring of stars called Gould’s Belt. If a dark cloud headed right into (and through) Earth, that might have dire consequences too. In theory, dark matter particles could interact with each other, giving off gamma radiation. Afsar Abbas, a physicist in India, suggests that the radiation would not only cause a wave of mutations but also heat up Earth’s interior and trigger massive volcanism, leading to a double extinction. Odds: indeterminate. Dark matter is so elusive that an event could be starting right now and we might not know it.
21 SOLAR SHUTDOWN Back in the 1970s, when it seemed that the sun was not emitting the expected number of particles known as neutrinos, some solar physicists proposed that our star might go through million-year stretches of reduced activity, during which time its brightness could drop by perhaps 40 percent. Although the initial evidence for a solar shutdown evaporated, the mechanism remains possible. Such a dip would put our planet in a deep freeze, and in fact paleontologists now find evidence of one such episode of extreme cold (nicknamed “Snowball Earth”) about 650 million years ago. If the sun dimmed significantly today, the oceans would gradually freeze solid, and most multicellular life on Earth—humans included—would probably go extinct. Odds: low. There is no evidence that other sunlike stars experience such shutdowns, though there is not much proof that they don’t.
Next: Handicapping the Other 20 Doomsdays