Sunday, May 11, 2008

Apocalypse now?

Back in 2000, when the Brookhaven Relativistic Heavy Ion Collider came on line, there was speculation that the high-energy subsatomic experiments might result in the catastropic destruction of the Earth. Implausibly, a subatomic-sized black hole or gravitational singularity, created in the accelerator, would accrete ordinary matter, eventually (in a flash) gobbling up the Earth. Or, alternately, a stable "strangelet" might accrete ordinary matter and convert it into strange matter. Poof!

The upshot of either scenario would be instantaneous destruction of the planet. Nothing happened.

Now, the even more powerful Large Hadron Collider at CERN near Geneva will soon come on line, and scaremongers raise the same concerns. It's rather like the anxieties of physicists before the first nuclear explosion in 1945: Would the blast at Alamogordo ignite the atmosphere by fusion and destroy the Earth? When the bomb exploded, the confidence of at least one physicist was briefly tested. Emilio Segre, an eyewitness and nuclear scientist, wrote: "We saw the whole sky flash with unbelievable brightness in spite of the very dark glasses we wore...I believe that for a moment I thought the explosion might set fire to the atmosphere and thus finish the earth, even though I knew that this was not possible." Why impossible? Physicists had done the calculations and decided the chances of catastrophe were infinitesimal.

The same is true for the Large Hadron Collider. We are assured that the risk is negligible. But not to worry. If the physicists suck the world into oblivion, it will happen so fast that we'll won't have time to wring our hands and rue.

Meanwhile, the world goes on, the planets whirl in their ancient tracks, the galaxies spin. Do scientists have their own theories about how it's all going to end? You bet. But don't bunker down yet. You have a few hundred billion years to get ready.

The universe began about 14 billion years ago in an expanding fireball of radiant energy. Space and time unfurled from a tiny seed of infinite energy, like a balloon inflating from nothing, cooling as it swelled. Energy became matter; matter became stars and galaxies, racing outward. Today, the galaxies continue to fly apart, impelled by their initial impetus, bearing clusters of galaxies to ever greater separations.

How will it end? There are two possibilities: Either the universe will expand forever, carrying the galaxies ever further apart, into cold and darkness, infinitely dispersed; or it will cease expanding and begin to contract, the galaxies drawing closer and closer, ending as it began in a blaze of radiant energy.

A whimper or a bang? A long glide into dark oblivion, or annihilation in a flash of blinding light? Three things are important to know: What is the rate at which the galaxies are flying apart? How much matter is acting to slow them down? Is there something called dark energy driving them apart?

During recent years, astronomers have made tantalizing progress toward finding answers. Several groups of researchers have been studying supernovas in distant galaxies. These extremely bright exploding stars can be observed billions of light-years away, and are used as indicators of the universe's expansion rate. The data indicate that the universe is not slowing down enough to make the galaxies fall back upon themselves. Perhaps it's even accelerating.

Other groups of astronomers have been comparing the actual distribution of galaxies to computer models for how the universe should evolve with different densities of matter. The best current fit between observation and calculation assumes that there is not enough matter to stop the expansion.

So, here's the astronomer's doomsday scenario:

Five billion years from now the sun will begin to swell into a red giant. Its surface will balloon outward toward the Earth, cooling and reddening (even as the core collapses and heats up). Mercury and Venus will be consumed, and a bloated red star will fill Earth's sky. Of course, all life on the Earth's surface will have been extinguished; as soon as the sun starts to swell, the atmosphere and oceans will be boiled away. After some hundreds of millions of years as a red giant, the sun will collapse to a glowing ember -- a white dwarf -- that will slowly fade from sight.

Will the doomed planet Earth have thrown off spores into interstellar space? Will our descendants have discovered ways to travel among the stars? Will Earthlings have been incorporated into a greater and longer-lasting galactic civilization? Your guess is as good as any, but the story is not over yet.

A hundred billion years will pass and the universe will be stretched exceedingly thin by continued expansion. Local clusters of galaxies will amalgamate into supergalaxies, and supergalaxies will drift far apart. Within the supergalaxies, the last dregs of energy will be squeezed out of star-birthing nebulas. No new stars will be born. The sky will grow increasingly dark.

All ordinary matter will be compacted in dead stars -- the cold cinders of white dwarfs, and those denser stellar remnants called neutron stars and black holes -- or cold lumps of rock (perhaps even ghostly, lifeless space ships) adrift in the darkness. Life, which requires the extraction of energy from its environment to survive, will be increasingly hard pressed for resources. Planet by planet, the last flickers of animation and spirit will be snuffed out.

Somewhere, in a last pool of cosmic warmth, perhaps in a faraway galaxy, a final organism will expire. Life, which for billions of years had burned among the stars like a cool blue flame, will flicker out.

Further Reading

Here is a neat animation that will give you a sense of scale for the LHC.

Discuss this essay and more over on the Science Musings Blog.