Do black holes really exist?
22:16 18 June 2007
NewScientist.com news service
Stephen Battersby

Black holes might not exist – or at least not as scientists have imagined, cloaked by an impenetrable "event horizon". A controversial new calculation could abolish the horizon, and so solve a troubling paradox in physics.

The event horizon is supposed to mark a boundary beyond which nothing can escape a black hole's gravity. According to the general theory of relativity, even light is trapped inside the horizon, and no information about what fell into the hole can ever escape. Information seems to have fallen out of the universe.

That contradicts the equations of quantum mechanics, which always preserve information. How to resolve this conflict?

One possibility researchers have proposed in the past is that the information does leak back out again slowly. It may be encoded in a hypothetical flow of particles called Hawking radiation, which is thought to result from the black holes' event horizons messing with the quantum froth that is ever-present in space.

But other researchers argue the information may never have been cut off in the first place. Tanmay Vachaspati and his colleagues at Case Western Reserve University in Cleveland, Ohio, US, have tried to calculate what happens as a black hole is forming. Using an unusual mathematical approach called the functional Schrodinger equation, they follow a sphere of stuff as it collapses inwards, and predict what a distant observer would see.

They find that the gravity of the collapsing mass starts to disrupt the quantum vacuum, generating what they call "pre-Hawking" radiation. Losing that radiation reduces the total mass-energy of the object – so that it never gets dense enough to form an event horizon and a true black hole. "There are no such things", Vachaspati told New Scientist. "There are only stars going toward being a black hole but not getting there."

Dark and dense
These so-called "black stars" would look very much like black holes, says Vachaswati. From the point of view of a distant observer, gravity distorts the apparent flow of time so that matter falling inwards slows down. As it gets close to where the horizon would be, the matter fades, its light stretched to such long wavelengths by the dark object's gravity that it would be nearly impossible to detect.

But because the pre-Hawking radiation prevents the formation of a black hole with a true event horizon, the matter never quite fades entirely. As nothing is cut off from the rest of the universe, there is no information paradox.

The idea faces firm opposition from other theoretical physicists, however. "I strongly disagree," says Nobel laureate Gerard 't Hooft of Utrecht University in the Netherlands. "The process he describes can in no way produce enough radiation to make a black hole disappear as quickly as he is suggesting." The horizon forms long before the hole can evaporate, 't Hooft told New Scientist.

Lab test
Steve Giddings of the University of California in Santa Barbara, US, is also sceptical. "Well-understood findings apparently conflict with their picture," he told New Scientist. "To my knowledge, there hasn't been an attempt to understand how they are getting results that differ from these calculations, which would be an important step to understanding if this is a solid result."

There could be a way to test the new theory. The Large Hadron Collider being constructed at CERN in Geneva might just be capable of making microscopic black holes – or, if Vachaspati is right, black stars. Unlike the large, long-lived black holes in space, these microscopic objects would evaporate fast. The spread of energies in their radiation might reveal whether or not an event horizon forms.

Alternatively, colliding black stars in space might reveal themselves, as Vachaspati says they would churn out not only gravitational waves (like colliding black holes) but also gamma rays. He suggests that they could be responsible for some of the gamma-ray bursts seen by astronomers.

Journal reference: Physical Review D (In press)
http://space.newscientist.com/article/dn12089-do-black-holes-really-exist.html