Portrait of a Black Hole
By adapting a global network of telescopes, astronomers will soon get their first look ever at the dark silhouette of a black hole.
You have probably seen the TV commercial in which a cell phone technician travels to remote places and askes on his phone, "Can you hear me now?" Imagine this technician traveling to the center of our Milky Way galaxy, whererin lurks a massive black hole, Sagittarius A (Sgr A*), weighing as much as 4.5 million suns. As the technician approached within 10 million kilometers of the black hole, we would hear his cadence slow down and his voice deepen and fade, eventually turning to a monotone whisper with diminishing reception. If we were to look, we would see his image turn increasingly red and dim as he became frozen in time near the black hole's boundary, known as the ever horizon.
The technician himself, however, would experience no slowing of time and would see nothing strange at the location of the event horizon. He would know he had crossed the horizon only when he heard us say, "No, we cannot hear you very well!" He would have no way of sharing his last impressions with us -- nothing, not even light, can escape from gravity's extreme pull inside the event horizon. A minute after he crossed the horizon, the gravitational forces deep inside the hole would tear him apart.
In real life we cannot send a technician on such a journey. But astronomers have developed techniques that will soon allow them, for the first time, to produce images of a black hole's dark silhouette against a backdrop of hot glowing gas.
Wait, you say. Haven't astronomers reported lots of observations of black holes, including all sorts of pictures? That is true, but the pictures have been of gas or other material near a black hole, with the hole itself an invisible speck, or of huge outpourings of energy presumed to come from a black hole. In fact, we do not even know for sure whether black holes really exist. [see "Black Stars, Not Holes," by Carlos Barcelo, Stefano Liberati, Sebastiano Sonego and Matt Visser; SCIENTIFIC AMERICAN, October].
Astronomers have detected objects in the sky that are sufficiently massive and compact that, if Einstein's general theory of relativity is correct, they must be black holes, and it is customary to talk of them as if they were (as we do in this article). But until now we could not tell if these objects had the defining characteristic of a black hole -- a horizon through which material can flow only one way. This question is not merely a matter of esoteric curiosity, because such horizons are at the heart of one of the deepest puzzles in theoretical physics. And images showing the dark silhouettes of black holes' event horizons would help us understand the extraordinary astrophysical processes taking place in their neighborhood.
Driving Questions
Event horizons are a source of fascination because they represent a fundamental inconsistency between two great triumphs of 20th-century physics: quantum mechanics and relativity. Time reversibility is an essential feature of the quantum-mechanical description of physical systems; every quantum process has an inverse process, which may be used, in principle to recover any information that the original process may have scrambled. In contrast, general relativity -- which explains gravity as arising from teh curvature of space and predicts the existence of black holes -- admits no inverse process to bring back something that has fallen into a black hole. The need to resolve this inconsistency between quantum mechanics and gravitation has been one of string theorists' primary motivations in their quest for a quantum theory of gravity -- a theory that would predict the properties of gravitation as arising from interactions following the laws of quantum mechanics.
On a more basic level, physicists would like to know if Einstein's general relativity really is the theory of gravity, even where it predicts shocking deviations from classical, Newtonian theory -- such as the existence of event horizons. Black holes have the twin virtues of corresponding to extraordinary simple solutions to Einstein's equations of gravity (a black hole is completely characterized by just three numbers -- its mass, charge and spin), as well as being places where gravity differs the most from Newtonian theory. Thus, black holes are prime locations for seeking evidence of deviations from Einstein's equations under extreme conditions, which could provide clues toward a quantum theory of gravity. Conversely, the equations' success near black holes will dramatically extend the regime in which we know general relativity works.
Pressing astrophysical questions about what happens in the vicinity of black holes also demand answers. Black holes are fed by infalling material such as gas and dust. The matter gains vast quantities of energy as it falls closer to the hole's horizon, producing heat 20 times more efficiently than nuclear fusion, the next most potent energy generator known. Radiation from the hot, spiraling gas makes the environment near black holes the brightest objects in the universe.
Astrophysicists can model the accreting material to some extent, but it is unclear how gas in the accretion flow migrates from an orbit at a large radius to one near the horizon and how, precisely, it finally falls into the black hole. Magnetic fields, created by charged particles moving in the accretion flow, must play a very important role in how the flow behaves. Yet we know little about how these fields are structured and how that structure affects black hole's observed properties. Although computer simulations of the entire accreting region are becoming feasible, we theorists remain decades away from true ab initio calculations. Input from observations will be vital for inspring new ideas and deciding among competing models.
More embarrassing to astrophysicists is our lack of understanding of black hole jets -- phenomena in which the forces near a supermassive black hole somehow conspire to spew out material a ultrarelativistic speeds (up to 99.98 percent of light speed). These amazing outflows traverse distances larger than galaxies, yet they originate near the black hole as intense beams collimated tightly enough that they could thread the solar system -- the eye of a galactic needle. We do not know what accelerates these jets to such high speeds or even what the jets are made of -- are they electrons and protons or electrons and positrons, or are they primarily electromagnetic fields? To answer these and other questions, astromomers desperately need direct observations of the gas in a black hole's vicinity.
Stalking the Behemoth from Afar
Unfortunately, such observations are difficult for several reasons. First, black holes are extremely small by any astronomical measure. They appear to come in two main varieties: stelar-mass black holes, the remnants of dead massive stars, with typical masses of five to 15 suns, and supermassive black holes, located at the centers of galaxies and weighing millions to 10 billion suns. A 15-solar-mass black hole's event horizon would be a mere 90 kilometers in diameter -- far too tiny to be resolved at interstellar distances. Even a one-billion-sun monster would fit comfortably inside Neptune's orbit.
Second, a black hole's small size and intense gravity make for extremely fast motion -- matter very near a stellar-mass black hole can compete an orbit in less than a microsecond. It takes highly sensitive instruments to observe such rapid phenomena. Finally, only the small subset of black holes that have large reservoirs of nearby gas to accrete are visible at all; the vast majority of black holes in the Milky Way are, as yet, undiscovered.
Rising to these challenges, astronomers have developed a variety of techniques that, short of providing direct images, have provided information about the properties and behavior of matter orbiting close to suspected black holes. For instance, astronomers can weigh a supermassive black hole by observing stars near it, much like using planets' orbits to weigh the sun. In distant galaxies, individual stars near a supermassive hole cannot be resolved, but the spectrum of their light indicates their distribution of velocicities, which yields a mass for the hole. The supermassive black hole Sgr A* at the center of the Milky Way is close enough for telescopes to resolve individual stars near it, producing the best mass estimate of any black hole to date [see box on page 47]. Unfortunately, these stars are far outside the region that interests us most, where general relativistic effects become significant.
Information about how much spin stellarmass black holes have has come from binary systems in which a black hole and an ordinary star orbit each other close enough for the hole to slowly feed on its companion. Analysis of the x-ray spectra and orbital parameters for a handful of such systems indicates that the holes have 65 to 100 percent of the maximum spin permitted by general relativity for a hole of a given mass; very high spin seems to be the norm.
Light (ranging from radio waves to x-rays) and energetic jets are not the only things emitted by black holes. When two black holes collide, they shake the fabric of spacetime around them, producing gravitational waves that propagate out like ripples on a pond. These ripples of spacetime should be detectable at vast distances, albeit requiring incredibly sensitive instruments. Although observatories already operating have yet to detect any gravitational waves, the method offers a revolutionary new way to study black holes. [see "Ripples in Spacetime," by W. Wayt Gibbs; SCIENTIFIC AMERICAN, April 2002].
A Window with a View
Despite providing a wealth of information, none of the techniques we have described thus far offer an image of a black hole's event horizon. Now, however, thanks to very recent advances in technology, direct imaging of a black hole's horizon is imminent. The black hole to be imaged is the behemoth in our backyard, Sgr A*. At a distance of only 24,000 light-years, Sgr A* occupies the largest disk on the sky of any known black hole. A 10-solar-mass black hole would have to be 1/100th as far away as the nearest star to appear as big. And although supermassive black holes much larger than Sgr A* exist, they are millions of light-years away.
The dark silhouette of a distant black hole is more than doubled in apparent size thanks to the bending of light rays by the hole's gravity. Even so, Sgr A*'s horizon will appear to span a mere 55 microarcseconds -- as small as a poppy seed in Los Angeles viewed from New York City.
The resolution of all modern telescopes, as impressive as they are, is fundamentally limited by diffraction, a wave-optics effect that occurs as light passes through the finite aperture presented by the size of the telescope. Generally, the smallest angular scale resolvable by a telescope can be decreased by making the telescope larger or by capturing shorter wavelength light. At infrared wavelengths (which, conveniently, pass through dust clouds that hide Sgr A* at visible wavelengths), an angular scale of 55 microarcseconds would require a telescope seven kilometers across. The shorter wavelengths of visible or ultraviolet light would somewhat reduce this gargantuan requirement but not by enough to be any less ridiculous. Considering longer wavelengths might seem pointless -- millimenter radio waves, for instance, would require a telescope 5,000 kilometers across. But it just so happens that Earthsize radio telescopes are already operating.
A technique called very long baseline interferometry (VLBI) combines the signal detected by an array of radio telescopes sprinkled around the globe to achieve the angular resolutions that an Earth-size radio dish would achieve. Two such arrays of telescopes have been operating for more than a decade -- the Very Long Baseline Array (VLBA), with dishes in the U.S. as far afield as Hawaii and New Hampshire, and the European VIBI Network (EVN), with dishes in China, South Africa and Puerto Rico, as well as Europe. You may remeber seeing a much smaller system, the Very Large Array in New Mexico, in movies such as Contact and 2010.