Black holes get an identity check
Scientists now say there's overwhelming evidence that they do exist
Astronomers are scientists of faith. How else could they believe in such an incredible thing as a black hole? It's an object that has collapsed to such density nothing escapes its strong gravity. Astronomers can't see it. They have no direct evidence that it exists. Yet their faith now is so strong it's reshaping their views of how galaxies form, how energy flows through the cosmos, perhaps even how our universe was born.
Einstein hated the concept, even though his theory of general relativity predicts it. Kip Thorne loves it. The California Institute of Technology astrophysicist says "we have close to a 100 percent case that [collapsing objects] leave black holes."
Cornell University theorist Eanna Flanagan nudges even closer to certainty, saying "I'm 100 percent convinced personally." They expressed their confidence during a presentation organized by Cornell's Astronomy Department in Ithaca, N.Y., last month.
Until a few years ago, even such true believers wouldn't have been that confident. New circumstantial evidence encourages many astronomers to consider the reality of black holes beyond reasonable doubt.
Some of that evidence is illustrated here. A reputed black hole at the heart of a nearby galaxy shoots a high-speed jet thousands of light years across space. A black hole feeding hungrily in another galaxy burps out a bubble of hot gas that is too much to swallow. Studies of how galaxies and black holes age suggest an intimate relationship.
To understand why such things excite astronomers requires a nodding acquaintance with some of the mind-bending notions the theory of space-bending black holes entails. Black holes can be as small as several solar masses or as gigantic as the billion-solar-mass black holes at the core of many galaxies. Massive or not, there's no matter in the structure of a black hole. As Dr. Thorne explains, "A black hole is made wholly and entirely solely by the warpage of space and time."
There is no gravitational force in general relativity. The gravity of a mass manifests itself in this warpage. Nothing escapes a black hole because, at its perimeter, space is so strongly curved that all paths lead into the hole. Thorne prefers to think in terms of time. He explains that time flows inward at the edge of a black hole. Nothing can escape because the future of everything on the perimeter lies inside the hole.
If that sounds weird, consider what happens just outside the perimeter. Matter under a black hole's influence orbits just as planets orbit a star. That matter can also spiral down and be swallowed up. But near the hole itself, it's not just matter swirling around. A rotating black hole drags space itself around with it, carrying along any matter in that space. Thorne describes this as "a motion of space around a black hole in a way that's similar to the winds around a tornado." He adds that space around a black hole not only spins around, it slides into the black hole. Some space slides inward at light speed as seen from the outside.
There's enormous energy involved. Thorne says the space vortex energy represents 29 percent of the black hole's mass. He notes that a black hole is many times more efficient at turning mass into energy than are nuclear processes.
Not everything caught in a black-hole dance is sucked inside. The system has to get rid of excess matter and rotational momentum. Magnetic forces in the swirling dust and gas circling the hole escape in powerful jets and outflowing bubbles. The interaction of such material with itself and with interstellar or intergalactic gas generates X-rays, gamma rays, radio waves, and sometimes spectacular light shows.
The Hubble Space Telescope and other orbiting observatories give astronomers a front-row seat. So, too, do arrays of radio telescopes, whose continent-spanning ranges gives them a detail-resolving ability equivalent to that of a single gigantic telescope the size of Earth. These new detailed views strengthen astronomers' faith in black holes.
Consider the false color image of the jet shooting from the heart of galaxy M87, some 50 million light-years away (photo above). It's a visual depiction of what arrays of radio telescopes in Europe and the United States have detected. Hubble Telescope images confirm the view. When the image was published a year ago, research team member Bill Junor at the University of New Mexico in Albuquerque explained that the group traced the jet to within a few hundredths of a light-year - an astronomical hair's breadth - of the galaxy's core where a 3-billion-solar-mass black hole is thought to lie. What he calls this "unprecedented level of detail" is consistent with black-hole theory.
The Hubble Telescope image of a gas bubble erupting from galaxy NCG 4438 is also 50 million light-years distant. That, too, is what astronomers expect from the presumed black hole at the galaxy center. Releasing the image last June, NASA explained that the bubble rises from the white region beneath it. This is a so- called accretion disk, where material is accumulating around the black hole and from which some material is ejected.
One of the questions about galaxies that harbor black holes is whether the holes came first and galaxies formed around them or whether holes and galaxies grew together. Astronomers locate the presumed black holes and judge their size by the way stars and other matter swirl around the galaxy centers. Recent analyses of comparable galaxies of different ages and sizes are illustrated in the chart shown on page 17. In publishing it last June, NASA noted that the tight correlation between the size and age of the galaxies' central bulges and of the presumed black holes strongly implies that holes and galaxies grow and age together.
Commenting on this, John Kormendy of the University of Texas at Austin said this correlation implies that the final mass of a galactic black hole is not set at its birth but is determined by the galaxy formation process. Although that process is not understood, what happens to galaxies and to their black holes seems to be one and the same thing.
Encouraged to believe black holes exist and are important, astronomers now want direct evidence to test the theory. Thorne notes that gravity waves - waves of distortion in space-time - generated by black holes will give such direct insight. It could happen in this decade.
Modern gravity-wave observatories are much more sensitive than tone-deaf predecessors, which failed to find a signal. One such observatory began operation in Japan last year. Several more are under construction in Germany, Italy, and the US.
Direct black-hole insight will raise a fundamental scientific challenge. Physics, as now known, doesn't work well inside these incredible objects. The great laws of conservation of energy and momentum are inoperative. Relativity theory predicts what's called a singularity - a point-like place of great mass - where all theory breaks down at a black-hole center. This is the part Einstein disliked.
Thorne says it's essential to understand what happens there or, if singularity is a false concept, to learn what should replace it.
To know that, he adds, is to know the universe's birth. The big bang was the greatest singularity of all.
(c) Copyright 2000. The Christian Science Publishing Society