Well, the 21st of May 2011 has come and gone and most of us are still here. According to Peter Segel of NPR’s Wait, Wait, Don’t Tell Me that must mean that either we didn’t make the cut, or else this is heaven. To judge from the world news, the latter possibility is certainly not the case. Those who believe the former, according to Harold Campings’ interpretation of Scripture, must now look forward to five months of torment until the world is destroyed by a great earthquake and everyone remaining is sent to hell. I’m not sure about the months of torment - although I suppose Congress will find a way - but as for the destruction of the world, it is not going to happen like that. Here is a physicist’s survey of how the world will end.

1. By fire. The Sun is a gravitationally-controlled nuclear fusion reactor that transmutes hydrogen into helium. When the hydrogen in the Sun’s core is depleted, pressure in the core drops and the core collapses under its weight, thereby heating surrounding shells of gas, which then expand. The Sun becomes a bloated “red giant” star, its outer surface passing beyond Earth to about the orbit of Mars. Although there is observational evidence from other stellar systems that a planet can be engulfed by its star and not be vaporized, the searing temperatures of the solar interior will evaporate the oceans, strip the Earth of its atmosphere, and destroy every living organism that inhabits the planet. Eventually (after a few million years) the Sun will shed its outer layers to form a “white-dwarf”, a small, intensely hot, bright stellar cinder about the size of the Earth, with a scorched, utterly barren Earth orbiting around it. This scenario is certain to happen if nothing else intervenes beforehand to destroy the world. Physicists predict this destruction by fire to take place in about 5,000 million years, give or take a few hundred million. That should leave enough time for Congress to work out the budget for 2012.

2. By ice. The gravitational attraction that keeps planets in elliptical orbits about the Sun also acts between planets and leads to perturbations in these orbits. Under appropriate circumstances, a very massive planet like Jupiter can eject a much smaller planet like Earth entirely from the solar system. Recent observations by means of gravitational lensing seem to indicate that there are billions of orphan planets wandering through the universe. So far, Jupiter has acted like a big brother, protecting the Earth by flinging away most of the large objects that otherwise would have pummeled and cratered us. However, if the ratio of the Earth’s period of revolution (1 year) to Jupiter’s period (about 11.859 earth years) should become sufficiently close to a ratio of whole numbers, then it may be the Earth that Jupiter flings out of the Solar System. In the cold depths of space, the Earth’s oceans and atmosphere will freeze and all life (apart, perhaps, from microbial spores) will end. This destruction by ice is not as certain as the destruction by fire, and, although it may occur sooner, it is difficult to predict when. Conceivably, if it does occur, humans might be able to forestall their demise for a short while by using the energy of nuclear fission, if only they could agree on where to put the spent fuel rods.

Far more probable, however, is the third scenario.

3. By Exhaustion of Resources. Long before either of the two preceding scenarios take place, the population of the Earth will rise to a level unsustainable by the rate at which the Sun provides energy available for food production. Earth receives energy from the Sun at the rate of about 1.4 kilowatts per square meter at the top of the atmosphere. A typical adult human requires energy at the rate of about 2000 Calories per day (0.09 kilowatts) for basic living needs. When account is taken of the loss of solar energy by reflection (Earth’s albedo), the fraction of the Earth’s surface that is land area, the fraction of land area that is arable, the efficiency of photosynthesis, the flow of food energy between trophic levels, and other modes of energy diffusion (such as leaving some energy for other creatures on the planet), one arrives at a maximum sustainable human population of a few tens of billions of people – and not necessarily at a decent quality of life, either. The present human population is approaching 7 billion and growing exponentially so that the difference between any two reasonable estimates of the quantity “few” would be reached in a small number of generations. No amount of human ingenuity can circumvent the limits to human population imposed by the physical laws governing solar irradiance and the mathematical consequences of continued exponential growth on a finite planet.

In well under a thousand years (physicists think in long terms) fossil fuels and fissionable nuclear fuels will have become commercially exhausted (no matter how much oil you think may lie under Alaska or the Gulf of Mexico), commercial fusion will probably remain a pipe dream, the air and oceans will be grossly polluted, and food and potable water will be scarce commodities whose acquisition will be determined largely by violence and war. By that time the Earth would indeed have become a hell and those selected for “rapture” will not be shaking hands with Jesus somewhere up in the sooty atmosphere, but, at best, will be on space ships to another planet.