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Did You Know?
In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.

"Gamma-ray bursts continue to confound astrophysicists nearly a quarter century after their discovery. Despite intense study by observers and theorists alike, no one knows for sure what they are, where they come from, or even whether or not they are a single phenomenon."

So wrote University of Chicago astrophysicist Donald Q. Lamb in a December 1995 paper. It sums up what was known at the time about these mysterious bursts of high-energy radiation, first noticed in the late 1960s during monitoring for nuclear blasts. In his paper, Lamb argued that gamma-ray bursts originated nearby, cosmically speaking, in a diffuse halo of material surrounding our Milky Way galaxy. Other researchers argued for a "cosmological" source, far from our galaxy, in the distant reaches of the universe. But data to answer the question were scant. Nor did anyone know what extreme conditions could be causing the blasts. Theories abounded: Some 140 ideas were floated, and interest in finding an answer mounted.

To seek insight into the mystery, a Great Debate was staged in April 1995 in Washington, D.C. The date and the venue—Baird Auditorium in the Smithsonian Institution's Museum of Natural History—were significant. In the same auditorium 75 years earlier, prominent astronomers Harlow Shapley and Heber Curtis debated the burning issue of that day: the size of the universe and humanity's place in it. In 1920, many thought our solar system was at the center of the Milky Way galaxy and the universe was confined to that finite group of stars. It wasn't until years later that Edwin Hubble proved that a vast universe existed outside our galaxy. Now the debate was over where in that seemingly infinite expanse gamma-ray bursts originated.

There was a festive, anticipatory air as 350 astronomers, students, journalists, and the public filed into the auditorium on a Saturday afternoon. Bright buttons positing the source of the bursts were passed out, and those with an opinion wore their answer: galactic, cosmological, or a noncommittal "other." Lamb and Bohdan Paczyński, a professor of astrophysical sciences at Princeton University, squared off, with Paczyński presenting the cosmological view. Like the first debate in 1920, the second Great Debate was a courteous presentation of opposing views, discussion, and—in honor of the integrity of scientific process—a wrap-up with no vote one way or the other. An answer would come only with more data.

By now scientific investigation has supplied some of the answers. Paczyński was right: gamma-ray bursts originate in the far reaches of the universe, a result of unimaginable releases of energy during stellar explosions or collisions. But many questions remain. Perhaps some of them will fuel a third Great Debate, and people with gather at the Smithsonian in 2070 to tackle another cosmological quandary.

—Barbara L. Wyckoff