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Cultural historian Jacques Barzun once described science as "the glorious entertainment." There could be no better example than black holes.

In 1916, Albert Einstein published his theory that gravity is the bending of space caused by the presence of massive objects. Others soon suggested that the gravitational pull of a sufficiently massive ("heavy") but compact object might warp space so radically that even light would be unable to escape. For decades, scientists debated whether the theory actually predicted such "black holes," although Einstein's opinion was that they didn't exist. Einstein's theory was murky concerning happenings inside such an object's "event horizon," the boundary of the spatial region from which nothing could escape. The theory seemed to predict the massive object could quickly collapse into a single geometric point. Einstein's theory went haywire at such a "singularity," equivalent to trying to divide a number by zero (a mathematically prohibited procedure).

Physicists debated this question for decades. It wasn't until 1964 that British physicist Roger Penrose invented new mathematical formulations of the theory that made logical sense of black hole singularities. According to my Department of Physics colleague Daniel Kennefick, quoted in Scientific American, "It turned out ... that they didn't really understand the structure of infinity, and Penrose solved that problem." Penrose's work made black holes acceptable, eventually landing him half of this year's physics Nobel Prize.

But such approval could not have come without substantial evidence. Science operates on the basis of theory (creative logical reasoning) and evidence. Without evidence, good ideas are mere "hypotheses" -- educated guesses.

Hard evidence for black holes began accumulating in 1964, when Cygnus X-1 was discovered. Cygnus X-1 is a double star (two star-like objects rotating around each other) 6,000 light-years from Earth (the distance light travels in 6,000 years). One object is "compact" -- much smaller than an ordinary star -- and known to be 15 times heavier than our sun. It's thought to be too small and massive to be anything other than a black hole. Its companion is a giant star that orbits the compact object. Cygnus X-1 sends out X-rays that are presumably emitted by gases that the black hole sucks from the companion star. Currently, 11 other black hole candidates, similar to Cygnus X-1, have been discovered in our Milky Way galaxy.

Far beyond the Milky Way, giant energy emissions in other galaxies, emissions known as "quasars," appear to originate in a type of black hole that is far heavier than these 12. It's now thought that nearly all galaxies harbor such "galactic black holes" at their center. As described in these pages on April 30, 2019, a science team recently used a globe-spanning array of microwave receivers known as the "event horizon telescope" to capture the first and only optical image of a giant black hole in the center of a distant galaxy.

As you might have guessed by now, one of these monsters resides at the center of our own galaxy, a mere 25,000 light-years away. In 2012, astrophysicists observed our galactic black hole ripping apart an entire star as the star approached the black hole's event horizon. Such ripping is caused by gravitational "tidal forces" that pull more strongly on the star's near side than on the star's far side, much as Earth's moon pulls more strongly on Earth's near side and thus stretches Earth, making it slightly egg-shaped and raising tides on both the near and far sides.

This is where the other half of the Nobel Prize comes in. During the 1990s, astrophysicists Reinhard Genzel of Germany and Andrea Ghez of the United States found direct evidence of a galactic black hole with an event horizon 15 million miles in diameter (the size of the planet Mercury's orbit around the sun) and a mass of four million suns. This black hole has sucked in some four million star-like objects during our galaxy's 13-billion-year history! This work provided our best evidence supporting Penrose's theoretical conclusions.

It wasn't easy. Ghez (the fourth woman to win a physics Nobel Prize) and Genzel each independently led teams that painstakingly followed the path of a particular star for 15 years as it orbited once around the Milky Way's center. This complete orbit around a smallish dark patch of apparently empty space showed unmistakably that the star was moving so fast, and in such a tight orbit, that it could only be moving under the inward pull of a galactic black hole, and it won a Nobel Prize for Ghez and Genzel.

Art Hobson is a professor emeritus of physics at the University of Arkansas. Email him at [email protected]

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