Astronomy and Space Science: Black Holes

Introduction

Although the ideas and arguments surrounding the ultimate fates of massive stars had been debated since the time of Isaac Newton (1643–1727), it wasn't until 1969 that American physicist John A. Wheeler (1911–) coined the term “black hole” to describe the remnants of such stars. The modern theoretical prediction of black holes came as a consequence of Albert Einstein's (1879–1955) general theory of relativity and his introduction of the concept of four-dimensional space-time, that is, the fusion of the three spatial directions of everyday experience with the dimension of time. A spectacular consequence of relativity theory was the prediction that the remnants of massive stars can ultimately collapse into black holes, objects with gravitational fields so intense that not even light can escape. During the latter half of the twentieth century, the understanding, description, and discovery of black holes became one of the preeminent quests of astronomers and cosmologists seeking to understand the structure of the cosmos. Although black holes must by definition be observed by their effects, not directly, there is strong evidence today for the existence of many such objects. Indeed, astronomers now state that a large black hole is probably located at the center of every large galaxy and that many smaller black holes are scattered throughout the stars.

Historical Background and Scientific Foundations

In 1963, New Zealand astrophysicist Roy Patrick Kerr (1934–) used Einstein's field equations to predict the existence of rotating black holes. In the late 1960s, Wheeler and German-born Canadian scientist Werner Israel (1931–) made independent calculations and predictions regarding the effects of shape and asymmetry on the formation and activity of black holes.

Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910–1995) was the first to describe how stars evolve into supernova, white dwarfs, neutron stars, or black holes. Chandrasekhar mathematically proved in the 1920s and 1930s that the remains of massive stars should produce what we now call black holes, although the full significance of his work was not appreciated until the 1950s. In 1983 he was awarded a Nobel Prize in Physics for his pioneering work on black holes.

Black Hole Formation

Throughout the life of a star, a tensional tug-of-war exists between the compressing force of the star's own gravity and the expanding pressures generated by nuclear reactions at its core. Typically, after burning first its hydrogen and then the helium, which is the ash or byproduct of its hydrogen-burning process, a star runs out of useable nuclear fuel. The spent star then contracts under the pull of it own gravity. Modern astrophysics predicts one of three fates for such a collapsing star—which one it undergoes depends on its mass.

A star less than 1.44 times the mass of the sun (a mass threshold termed the Chandrasekhar limit) collapses until the pressure in the increasingly compacted electron clouds at its core is high enough to balance the collapsing gravitational force. Such stars become white dwarfs, small dim stars contracted to a radius of only a few thousand kilometers—roughly the size of a planet. This is the fate of most stars in the visible universe. If the star retains between 1.43 and roughly three or four times the mass of the Sun, the pressure of its core electrons is insufficient to stop the gravitational collapse. In such stars, contraction continues to produce a neutron star only a few kilometers in radius. Within a neutron star, the nuclear forces and the repulsion of the compressed atomic nuclei balance the crushing force of gravity. With even more massive stars, however, there is no force that can withstand the gravitational collapse. Such stars continue their collapse to form a singularity—a point at which the equations of general relativity break down, predicting infinite density. The gravitational field of such an object warps space-time so intensely that not even light can escape. Such an object and its vicinity are termed a black hole.

In 1939, American physicist J. Robert Oppenheimer(1904–1967), who ultimately supervised the United States's project Trinity during World War II (which built the first atomic bombs), made detailed calculations reconciling Chandrasekhar's predictions with general relativity theory.

Singularities and Event Horizons

At the center of every black hole there lies one of the most intriguing objects in all physics—the singularity. In the late 1960s, English mathematician and physicist Stephen Hawking (1942–) along with English mathematician Roger Penrose (1931–) drew on both quantum and relativity theory to show that within a black hole there must exist a singularity (a geometric point without space) of infinite density. Penrose advanced a scientific and philosophic concept known as the Law of Cosmic Censorship, an assertion that regions around singularities (i.e., black holes) are regions of space cut off from direct human observation.

An event horizon is the boundary of a black hole, a spherical surface enclosing the singularity and a surrounding volume of space. The radius of the event horizon surrounding a given black hole is termed the Schwarzschild radius, named after the German astronomer Karl Schwarzschild (1873–1916), who studied the properties of geometric space around a singularity. Light is bent by any gravitational field; within a black hole, gravity is so strong that light is bent back upon itself. That is, inside the event horizon, the gravitational attraction of the singularity is so strong that the required escape velocity is greater than the speed of light. As a consequence, no object, not even light itself, can escape from the region of space inside the event horizon. The event horizon is also an observational boundary: No information generated within the black hole can escape to be observed by us.

Observing Black Holes

Because no information can pass through an event horizon, the existence of singularities cannot be proved by direct observation. Essentially, all we can know with any certainty regarding the processes of the singularity are the external gravitational effects it produces. Astronomers can only speculate as to whether there is some process or set of unknown physical laws whereby nature could avoid the formation of the seemingly counterintuitive concept of the singularity.

The collapse of a star into a black hole has two predicted effects on the light reaching us from such a star: the star grows fainter and redshifts (that is, begins to appear more red) because less light is able to escape and because the photons of light that do escape lose energy and thus are shifted toward the red end of the spectrum. Eventually the star will “wink out” when light can no longer escape—although accelerated matter in the vicinity of the black hole may continue to radiate, perhaps violently.

The existence of black holes is inferred by their effects on surrounding matter. In binary star systems in which one star is a black hole, for example, although the black hole cannot be directly observed, its gravitational strength can be detected by its effect on the rotation of its companion star. In addition, an accretion disk—flat ring of matter spiraling toward a central attracting body—may form as matter accelerates toward the event

horizon of the black hole. As the matter in the accretion disk spirals down toward the black hole, it is heated to high temperatures and emits high-energy electromagnetic radiation (e.g., X rays and gamma rays). Astronomers consider it likely that a black hole is present wherever a star orbits around an unseen companion and there is high-energy electromagnetic radiation from an unidentified source near the center of rotation.

Most astronomers today agree that such a mechanism, working on a galactic scale, accounts for the phenomena known as quasars (short for quasi-stellar radio sources). A massive black hole, if located near the center of a young galaxy, can consume much of the galaxy's plentiful supply of interstellar gas and stars, sucking it up by sheer gravitational force. Such a supermassive black hole, perhaps weighing tens of billions of times as much as our sun, accelerates so much matter so strongly that tremendous amounts of energy would be emitted. In some cases, more energy is radiated than by entire galaxies, all from an area roughly the size of our solar system. In some cases, a quasar radiates about 100 times as much energy as our entire Milky Way galaxy.

Starting with Dutch-born American astronomer Maarten Schmidt's (1929–) work, studies of Quasar 3C273 show that it blasts such tremendous amounts of energy in the form of visible and x-ray light, that the observational data can only be accounted for by the presence of a supermassive black hole with galactic matter orbiting it in an accretion disk. As electrons in the accretion disk are accelerated to speeds near the speed of light, they emit radio waves by the process termed synchrotron radiation. These are the characteristic strong radio emissions that originally drew quasars to the attention of astronomers.

Along with strong radio emissions, observations of NGC 4261 with the Hubble Space Telescope show enormous plumes shooting from its core—a phenomenon that could be associated with a truly black hole a billion times more massive than our sun. The ring seen in the Hubble images of NGC 4261 is thought to be the accretion disk surrounding the black hole. Cyg X-1 (so named because it was the first X-ray source discovered in the constellation of Cygnus) is also identified by most astronomers as containing a black hole.

In the last two decades of the twentieth century, astronomers discovered a handful of binary star systems (systems in which two stars orbit each other) in which one partner is a conventional visible star and the other is an object visible only at short wavelengths (e.g., X rays). In these systems, the motion of the visible star shows that its unseen companion has a mass considerably greater than 3 to 5 solar masses. In 1994, the Hubble Space Telescope provided virtually conclusive evidence for the existence of a supermassive black hole at the center of the M87 galaxy. Similar evidence indicates that a black hole lies at the center of our own Milky Way galaxy; current theory holds that every large galaxy has a black hole at its center.

Although there is no upper limit to the size of a black hole, quantum theory predicts that a black hole can be no smaller than about 1^– 33 cm in radius. Note that this contradicts the prediction from general relativity of zero radius and infinite density. As of the early 2000s, physicists were still striving to produce a physical theory that would reconcile the relativistic theory of gravitation to quantum mechanics: presently known physical law simply cannot describe what happens at the center of a black hole, though it has been successful at describing most of what goes on in the vicinity of a black hole.

Although matter cannot escape a black hole, the formation of virtual particles (particle-antiparticle pairs that exist so briefly that their masses can not be measured) may allow for an important phenomenon called Hawking radiation, named after Stephen Hawking, that would allow matter to leak from black holes. If a particle-antiparticle pair comes into being near (but outside) the event horizon of a black hole and one of the particles crosses the event horizon, the partner particle cannot be annihilated and thus becomes a real particle with mass and energy; since the total amount of mass and energy in the universe must remain constant over time, the particle falling into the black hole must have negative energy and mass. The black hole thus loses mass; to an observer, it appears that the black hole has emitted a particle and lost that much mass. If Hawking radiation exists—it had not, as of 2008, yet been observed—then black holes, despite all their galaxy-gobbling power, are mortal, and must eventually evaporate. However, the process is very slow: a black hole the mass of the sun would take 10⁶⁶ years to evaporate via Hawking radiation.

Some astronomers suggest that singularities (and the black holes that surround them) may not always have been exclusively created by the death of massive stars. In the early universe, black holes could have formed as a consequence of the collection and collapse of large volumes of interstellar gas that may now lie at the center of quasars and galaxies.

Modern Cultural Connections

Concepts stemming from the physics of black holes have impacted physics, cosmology, and philosophy.

Black holes almost certainly exist: they are predicted by theory and explain many observations for which no other explanation is known. Their impact on astronomy

has been great, suggesting new classes of observations and new explanations for extreme phenomena such as quasars. Based on observations made using the Hubble Space Telescope, astronomers now report that all or almost all galaxies have a supermassive black hole at their center, often containing billions of times as much mass as our sun. Furthermore, larger galaxies contain larger black holes. This might seem obvious—bigger galaxy, more massive black hole—but in fact, it is not easy to explain why larger galaxies should contain more massive black holes. Relative to the size of a galaxy, a black hole and its accretion zone are vanishingly small, so that most of the matter in any galaxy is beyond the gravitational reach of its central black hole; when, then, if most of the galaxy's mass is forever out of the hole's reach, does a larger galaxy have a larger black hole? Theorists seeking to understanding the evolution of galaxies have thus been presented both with new data and with new mysteries.

The existence of black holes opens the possibility of the existence of another bizarre phenomenon, the wormhole. A wormhole, if any exists, is an opening in space-time that connects two distance parts of the universe, traversing the intervening distance: It would be possible, in theory, for matter to pass from one end of the wormhole to the other faster than even light could travel between the two points in ordinary space. Wheeler, American physicist Kip S. Thorne, and others have studied the properties that would be

associated with these connections to the black holes (also termed Einstein-Rosen bridges). Wormholes, although predicted by several physical theories, have not been experimentally observed.

If they do exist, then wormholes may mean that time travel is not physically impossible (though it is likely to remain forever impractical, since gravitational stretching would destroy any object as large as a human being long before it reached a black hole's event horizon). If time travel can occur, even by subatomic particles alone, then it may be that time-travel paradoxes in which a cause-and-effect process interferes with itself cannot be absolutely ruled out. The theoretical possibility of such paradoxes is now incorporated into all serious attempts to understand the nature of time and causality (the nature of cause-and-effect), which are fundamental to all physics.

In 2007, physicists using the Pierre Auger Cosmic Ray Observatory in Argentina showed that supermassive black holes at the centers of nearby galaxies are the source of rare ultrahigh-energy particles from space that occasionally impact Earth, known as ultrahigh-energy cosmic rays. This class of superpowerful cosmic rays was first observed in the 1960s. A single such subatomic particle may carry as much energy as a fast-moving baseball.

In popular culture, black holes have been a source of much fascination but little insight. For many non-astronomers, the main effect of contemplating black holes has been reinforcement of the idea, which entered industrial culture with the bizarre claims of relativity and quantum mechanics in the early twentieth century, that the universe is a fundamentally weird or alien place. This notion is not, however, a claim made by physics itself but an emotional or cultural reaction to the claims made by physics. It would be equally valid to see the world of everyday life as being just as “real” as the strange phenomena of quantum mechanics and cosmic physics. A black hole is no more real, no more representative of what the universe is really like, than a donut hole.

In any case, popular fascination with the imagination-stretching facts of black-hole physics is widespread. For example, various distorted descriptions of black holes and worm holes as time-travel or space-travel machines are common in science fiction. The use of microscopic black holes as inexhaustible power sources has also been proposed by some writers. Such objects would, in concept, release energy as matter was dropped into them, much as radiation is released by matter as it spirals toward the event horizon of a stellar-size black hole. However, all practical applications of the properties of black holes remain purely speculative. The technological obstacles to any such application are literally astronomical (some time-travel concepts would require the use of more energy than exists in the whole universe); there is little likelihood that such applications will ever be realized. Also, because the existence of wormhole and time-travel effects have yet to be verified experimentally, future developments in physics may show that they are truly impossible, not just impractical.

See Also Astronomy and Space Science: Galactic Astronomy; Astronomy and Space Science: Pulsars, Quasars, and Distant Questions; Physics: Special and General Relativity.

bibliography

Books

Hawking, Stephen. A Brief History of Time. New York: Bantam Books, 1998.

Hoyle, Fred. Astronomy and Cosmology. San Francisco: W.H. Freeman, 1975.

Sagan, Carl. Cosmos. New York: Random House, 1980.

Thorne, Kip S. Black Holes and Time Warps: Einstein's Outrageous Legacy. New York: W.W. Norton, 1994.

Trefil, James S. Space, Time, Infinity: The Smithsonian Views the Universe. New York: Pantheon Books, 1985.

K. Lee Lerner