Frontiers | Pages 200-201 | See Linked Version

How Protons Decay

Protons seem like bastions of stability in the nuclei of atoms. However, theories predict that all protons will disintegrate into tiny waste products far in the future, perhaps a trillion trillion trillion years from now. Physicists are watching for these ultrarare decays, but so far they have eluded detection.

According to one theory, a decaying proton unleashes a predictable blizzard of exotic and short-lived particles. The strong nuclear force that binds the three quarks composing a proton (1) breaks down, allowing two quarks (2) to spontaneously merge into a massive particle called a leptoquark (3). This unstable particle ejects two flecks of antimatter: a positron (4) and an antiquark (5). The antiquark merges with the remaining quark (6) from the former proton to create yet another transient particle, a pi zero (7). Finally, this particle disappears in a flash of gamma-ray photons (8). These high-energy rays of light could form pairs of electrons and positrons (9), which might destroy each other and release still more photons. These fleeting bits may be all that remains when solid matter as we know it vanishes from the cosmos.

leaves in the wake of a passing truck. Comets could bombard any planets orbiting the stars for millions of years. The gravitational mayhem could fling some planets from their solar systems entirely. Living in the Milky Way during that era will require adaptability and a ready fleet of interstellar ships, to be sure.

To gaze into an even bleaker future, we must stop being parochial and let our minds wander beyond the Milky Way. We also need to think in units of time that don't enter most dinner conversations. The smallest stars burn their nuclear fuel so slowly that they will survive for about 10 trillion years. By that time galaxies will start shutting down their star formation as supplies of hydrogen dwindle. Galaxies also will be extraordinarily far apart by then as the expansion of the universe accelerates. After 100 trillion years, no more stars will shine. Black holes, neutron stars, and white dwarfs, long since cooled to blackness, will drift within the dark remains of galaxies.

Occasional collisions will spark spectacular flashes of light. Then, after about a trillion trillion trillion years, matter itself will decay. Current theories predict that protons are unstable on these timescales. When they vanish into blips of gamma rays and tiny waste products of matter, nothing made of atoms will remain.

The sole denizens of this dark universe will be black holes. But they too disappear, in the process called Hawking radiation--a quantum-mechanical annihilation of particles that slowly evaporates a black hole and emits gamma rays. The largest black holes of all, those at the centers of galaxies today, will take a staggeringly long time to evaporate: a googol, or 1 followed by 100 zeros, years. When the last black hole shrinks and destroys itself in a bright burst of gamma rays, we may then declare that the universe has died--not with a whimper, but a bang.

These fantastic outcomes are all grounded in scientific knowledge that generations of astronomers and physicists have assembled with care. Yet when we look to the future or gaze deep into space, we still face profound limits to our knowledge. Those barriers are erected by our narrow abilities to imagine how the universe works. Two millennia ago astronomers believed that there were just 2,000 stars in all the heavens. They were all equally far away from Earth, it seemed, placed on a large spherical shell that marked the boundary of the cosmos. Could Ptolemy, laboring to understand the universe in the second century A.D., ever have imagined that there are 100 billion stars in our galaxy alone, all at different distances? Could he have grasped 100 billion other galaxies beyond the Milky Way?

Today, we labor to answer the great astronomical questions of our own era. Thousands of years from now astronomers may smile back at us as they learn about our futile efforts to fit the universe into a model barely resembling a larger reality. We hope we are not completely off track. But even if we are, we build the lower rungs of the ladder of astronomical knowledge for future generations--just as Ptolemy did for us 18 centuries ago. Ultimately, the greatest progress will come when we realize that the questions we must answer are the ones we have yet to ask.