On January 24, the journal Nature published an article entitled "There are no black holes." In a brief article published on arXiv, a scientific preprint server, Stephen Hawking, currently Director of Research at the Centre for Theoretical Cosmology at the University of Cambridge, proposed a theory of black holes that could reconcile the principles of general relativity and quantum physics.
"According to Einstein's theory of general relativity, a black hole is kind of cosmic central vacuum cleaner that swallows everything in its reach and lets nothing escape. It emits no radiation," says Robert Lamontagne, an astrophysicist at the Department of Physics, Université de Montréal, and Executive Director of the Observatoire du Mont-Mégantic.Since it is not visible and has no boundaries as such, a black hole is classically defined by an area of space called the "event horizon," where nothing can escape. "Beyond this horizon, matter and light flow freely, but as soon as the horizon's intangible boundary is crossed, matter and light become trapped," he says.
However, if we use quantum mechanics to describe a black hole, the laws of thermodynamics must apply. In this description, a black hole emits particles in the form of radiation and, ultimately, evaporates. Hawking himself predicted this in the 1970s.
"Following through with Hawking's argument, we conclude that if there is evaporation there must be a boundary to the event horizon, a place of transition between the inside and outside of the black hole," says Lamontagne. "A high energy envelope, a firewall, which burns up matter, is proposed."
However, this scenario poses a problem: if the firewall exists, we should be able to see it. Furthermore, the existence of a firewall around a black hole is inconsistent with the theory of general relativity.
While the two major theories, that of general relativity (a theory of gravity) and quantum mechanics (a description of the microscopic world), work well in their respective fields, they are not universal: neither can explain alone how black holes work.
"The Holy Grail would be to find THE theory that would unify the other two. And Stephen Hawking has come back with a new proposal," says Lamontagne. Roughly, Hawking suggests that if the firewall is not visible, it is because its position fluctuates constantly and rapidly. "Hawking says, and this is purely hypothetical, that the fabric of space and time is in turmoil and we cannot define its whereabouts."
In short, since we cannot change the principles of either quantum mechanics or general relativity, Hawking proposes to slightly modify the description of black holes. Hence his remark that black holes do not exist the way we thought they did, as we thought we knew them.
In our galaxy, black holes are less numerous than suggested by sci-fi movies. The largest black hole near us is at the center of our galaxy - the Milky Way. It is 30,000 light-years from Earth. Its mass is about one million times that of the Sun, and it occupies a space equivalent to our solar system.
"We cannot see it directly but we have located it because of effects we can observe using various technological methods: it constantly deviates the trajectories of stars in its vicinity," says Lamontagne. Moreover, in 2014, a huge cloud of gas will fall toward this "nearby" black hole. "This is exciting from an astronomical point of view because we will be able to examine the phenomenon for 10 to 20 years to come."
The image at the top of the page shows a rapid X-ray flare that was observed from the direction of the supermassive black hole that resides at the center of our galaxy. This violent flare captured by NASA's Chandra X-ray Observatory has given astronomers an unprecedented view of the energetic processes surrounding this supermassive black hole.
A team of scientists led by Frederick K. Baganoff of MIT detected a sudden X-ray flare while observing Sagiattarius A*, a source of radio emission believed to be associated with the black hole at the center of
"This is extremely exciting because it's the first time we have seen our own neighborhood supermassive black hole devour a chunk of material," said Baganoff. "This signal comes from closer to the event horizon of our Galaxy's supermassive black hole than any that we have ever received before. It's as if the material there sent us a postcard before it fell in."
In a just few minutes, Sagittarius A** became 45 times brighter in X-rays, before declining to pre-flare levels a few hours later. At the peak of the flare, the X-ray intensity dramatically dropped by a factor of five within just a 10-minute interval. This constrains the size of the emitting region to be no larger than about 20 times the size of the "event horizon" (the one-way membrane around a black hole) as predicted by Einstein's theory of relativity.
The rapid rise and fall seen by Chandra are also compelling evidence that the X-ray emission is coming from matter falling into a supermassive black hole. This would confirm the Milky Way's supermassive black hole is powered by the same accretion process as quasars and other active galactic nuclei.
Dynamical studies of the central region of our Milky Way Galaxy in infrared and radio wavelengths indicate the presence of a large, dark object, presumably a supermassive black hole having the mass of about 3 million suns. Sagittarius A* is coincident with the location of this object, and is thought to be powered by the infall of matter into the black hole. However, the faintness of Sagittarius A* at all wavelengths, especially in X-rays, has cast some doubt on this model.
The latest precise Chandra observations of the crowded galactic center region have dispelled that doubt, confirming the results of the dynamical studies. Given the extremely accurate position, it is highly unlikely that the flare is due to an unrelated contaminating source such as an X-ray binary system.
"The rapid variations in X-ray intensity indicate that we are observing material that is as close to the black hole as the Earth is to the Sun," said Gordon Garmire of Penn State University, principal investigator of Advanced CCD Imaging Spectrometer (ACIS), which was used in these observations. "It makes Sagittarius A* a uniquely valuable source for studying conditions very near a supermassive black hole."