Back from the Horizon – No Black Holes?

When Stephen Hawking isn’t flying in zero gravity or throwing parties for time travelers, the Cambridge physicist is hard at work theorizing about the extremities of our universe. Much of Hawking’s scientific adoration comes from his groundbreaking work on black holes, and the book that established his popular reputation, “A Brief History of Time,” is largely concerned with the implications of these astronomical oddities. The buzz around Hawking’s most recent paper, then, is understandable, considering that it claims black holes do not exist.

Artist’s rendition of the Cygnus X-1 black hole sucking matter from its companion star. Courtesy of Stardate.

The traditional view of black holes can be traced back to 1676, when the Danish astronomer Olaf Roemer first proposed that light had a finite speed. After Isaac Newton outlined his theory of gravity, other scientists began to consider how this speed might be affected by the gravitational pull of large objects. If light was made of particles that had mass (which we now know to be untrue), as hypothesized by the English physicist John Michell, it might be possible for a body to be so massive that its gravitational pull would slow light down or even prevent it from moving altogether. In 1783, Michell published a paper suggesting that stars hundreds of times the size of the Sun could trap their own light, rendering them invisible to Earthbound observers.

It wasn’t until the 20th century that Michell’s ideas were revisited, this time in the context of Albert Einstein’s general theory of relativity. Einstein visualized gravity as a “warping” of space, where an object with mass deformed the space around it like a bowling ball depresses a taut rubber sheet. Under this framework, German astronomer Karl Schwarzchild proposed that a sufficiently massive object, compressed by gravity into a single point, would warp the space around it so that, beyond a certain point (called the Schwarzchild radius or the event horizon), light could not escape. Black holes could only be discovered through their effects on other astronomical entities, and the first well-established black hole was not discovered until 1971.

Hawking doesn’t dispute the existence of extremely dense, massive objects that are very difficult to detect from Earth, but he does challenge the idea that black holes are actually, well, black. His essential objection comes from the all-or-nothing nature of the event horizon. Under the traditional view of black holes, any information about the matter that passes the horizon is lost forever, as no light carrying the information could possibly escape. However, recent work in quantum mechanics suggests that some information can travel faster than light and would not be destroyed by the hole.

To solve this contradiction, Hawking has proposed that the event horizon should be replaced by what he calls the “apparent horizon.” With this hypothesis, light and mass are only trapped temporarily in the black hole before quantum effects spit them back out in a much more chaotic state. Mathematically proving the hypothesis, however, requires a theory of gravity that behaves accurately on the quantum mechanical level, and this, says Hawking, “remains a mystery.”

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