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as a singularity. The black-hole solutions of general relativity, ignoring quantum-mechanical effects as described later, are completely stable. Once massive black holes form, they should theoretically remain forever, and subsequent processes (for example, the accumulation of matter) only increase their size. Two black holes are able to coalesce and form a single, larger black hole, as LIGO has detected, but a single black hole could not split up into two smaller ones. However, in 1974, English theoretical physicist and mathemati- cian Stephen Hawking showed that when quantum effects are properly taken into account, a black hole should seem to emit thermal radiation. The radiation would result from the quan- tum-mechanical prediction that pairs of particles spontaneous- ly form in the vacuum of space. These particles usually instantly annihilate each other, one being matter and the other being antimatter. Sometimes, though, a particle pair should form with one particle inside the black hole's event horizon and the other outside this boundary. The particle outside the event horizon would escape into space as so-called Hawking radiation, taking with it a tiny portion of the black hole's mass. Over time, Hawking's hypothesis suggests that black holes can shrink and eventually disappear—so long as they are emitting Hawking radiation faster than they are accumulating matter or energy. Yet wherever in space a black hole might be located, it would receive significant, ambient radiation from other stars and the big bang itself through the cosmic microwave back- ground. Because this ambient radiation is thought to be much larger than any emitted Hawking radiation, the black hole would not shrink. Even if the ambient radiation somehow did not reach the black hole, the time required for a typical black hole (formed from the collapse of a star) to completely decay is much longer than the age of the universe. The upshot: In the history of the universe, no stellar black holes have disappeared due to Hawking radiation. The possibility remains that there are less massive black holes than those formed in stellar collapses. Theoretically, for in- stance, black holes of any mass could have been created at the beginning of the universe in the big bang. For smaller-mass black holes, the Hawking radiation process would be quite im- portant. For example, a black hole created with mass of 1.72 × 1011 kg (3.8 × 1011 lb), about the mass of a large hill, would have radiated away all of its mass just recently (assuming that no mass had been accreted in the meantime). Black holes created with a mass smaller than this value would have disappeared earlier in the universe's history, and those with a larger mass would still exist. The final stage of a black hole's evaporation would be quite violent and would take place quickly. As a black hole radiates, it loses mass, and its temperature rises. But a higher temperature means that the black hole radiates and loses mass at a faster rate, raising its temperature even further. The final burst, as the black hole gives up the remainder of its mass, would be a spec- tacular event. The final emission would contain all radiation and particles that could exist, even those not generated by existing accelerators. At present, there is no evidence that points to the existence of black holes with small masses or having evaporated via a Hawk- ing radiation process. Some physicists have suggested that the Large Hadron Collider—the most powerful particle accelerator built to date and that has operated since 2010—might pro- duce miniature black holes that immediately evaporate due to Hawking radiation and thus be indirectly detectable. Black Hole (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this. + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this.