Stephen Hawking Makes Pioneering Discoveries in Gravitational Field Theory

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Stephen Hawking Makes Pioneering Discoveries in Gravitational Field Theory

Overview

Black holes, formed by stars that have collapsed under their own weight, are perhaps the strangest physical objects in the universe. Their gravitational fields are so strong that neither light nor matter can escape. At their center are thought to be singularities, where the laws of physics cease to function normally. Within singularities, scientists believe they can find the keys to the universe's birth and death.

Background

The concept of black holes was first described in 1783 by the Reverend John Mitchell, a British amateur astronomer. Using the simple concept of escape velocity, the speed at which an object must move to escape a planet or star's gravitational field, he theorized that light might also be held to these rules.

Albert Einstein's (1879-1955) theory of general relativity laid the keystone for the discovery of black holes. His system for describing gravity was quickly explored by many other scientists. One of these was Karl Schwarzschild (1873-1916). While serving the German army in the trenches of the Russian front during World War I, Schwarzschild theorized that a star could collapse under its own weight, crushing itself down until its gravitational field became so strong that neither light nor matter could escape. Physicist John Wheeler named these objects "black holes" in 1969. The point to which an object must be crushed to hold back light is now called the "Schwarzschild radius." (The Earth, for instance, would have to be crushed to a diameter of about 2 centimeters to hold back light.)

Stars normally burn hydrogen and helium, the two lightest elements, producing energy in a process called nuclear fusion. As the energy they emit radiates outwards, it prevents the star from collapsing under its own gravity. After a star has exhausted its hydrogen and helium, it begins to burn heavier elements such as carbon and oxygen. Soon this fuel is exhausted as well. Then, when the radiated energy can no longer hold up against gravity, the star quickly collapses inward.

Black holes form at this point—when a massive star reaches the end of its life. Two theories have evolved to explain their formation. In the first, a large star (more than 25 times the mass of the Sun) collapses under its own weight and crushes itself down to a black hole. In the second, a star explodes violently in a type II supernova. The star, having exhausted its nuclear fuel, then quickly collapses. The energy from this compression triggers one last massive burst of fusion reactions as the star crushes itself. The resulting energy blows apart the star, letting off almost as much energy as it radiated in its entire lifetime in just a few seconds. What is left at the center collapses inward to form a neutron star. As nearby matter falls back on to the neutron star and increases its mass, the star will form a black hole if it becomes heavy enough to collapse further.

Black holes may also have been formed shortly after the Big Bang. These can range from microscopic to massive sizes. Astrophysicists now believe that the centers of many galaxies contain black holes with masses equivalent to that of millions of suns. They are fed continually by rich fields of gas and dust as well as entire stars that fall into the center.

Since black holes cannot be detected by any telescope or sensor, they can only be observed indirectly. Accordingly, astronomers look for the effects black holes have on surrounding gas, dust, and stars. A black hole's tremendous gravity can pull in nearby gas and dust. Dust gives off increasing amounts of energy as it falls toward the hole, energy that can be detected as x rays. The first such black holes were found in the 1990s. Since then, many more have been detected. Black holes at the center of galaxies are identified by observing rapidly swirling stars near the galactic center as well as bipolar jets of x rays moving away from the plane of the galaxy.

Impact

Until the 1960s black holes remained interesting quirks of physics. Only after Stephen Hawking discovered that black holes radiate heat did their importance in the evolution of the universe come to be appreciated. Until then it seemed as if they were eventually going to sop up all the matter in the universe, leaving nothing but black holes.

Physicists then faced a paradox: how could black holes seemingly violate the second law of thermodynamics (all objects lose energy as heat)? Black holes grew larger and larger whether or not they were absorbing matter, without losing energy. A series of discoveries by Hawking and others showed that this assumption was false.

In 1970 Jakob Bekenstein, a graduate student of John Wheeler, was the first to suggest that black holes had a temperature. This meant that they must also emit particles, something most physicists believed a black hole was unable to do. Bekenstein's ideas were roundly dismissed by the physics community, especially Hawking. Shortly thereafter, Russian physicist Yakov Zeldovich published a similar paper showing that a rotating sphere should emit particles, at least until it stopped rotating.

In 1972 Hawking, with Brandon Carter and Jim Bardeen, wrote a contradictory paper explaining the mechanics of black holes. They were trying to show how a black hole's surface area could increase without violating certain physical laws. They realized that their equations matched perfectly with the nineteenth-century equations governing thermodynamics, but at the time they noted that the similarities were only superficial, not identical. Later they would realize their mistake.

The turning point came in 1973 when Hawking decided to examine Zeldovich's equations on his own. To his "surprise and annoyance" he found that black holes do emit particles, and would do so even after they stopped rotating. Apparently, Bekenstein had been right—black holes do indeed have a temperature.

Hawking's discovery showed that black holes are, in fact, dynamic objects. Instead of acting like mindless vacuums sucking up matter, black holes actually have a temperature, given by this tiny amount of radiation. The temperature is minute, only a few millionths of a degree above absolute zero. But it is enough—this small amount of radiation means that black holes will slowly but surely lose mass until they evaporate into a puff of x and (maybe) gamma rays.

These emitted particles are created by a property of quantum mechanics in which subatomic particles flash in and out of existence in fractions of a second. Normally they disappear just as quickly as they are formed. But Hawking found that if these particles form just inside the event horizon of a black hole, they can tunnel across the event horizon and travel away from the black hole. The energy of these particles gives the black hole its temperature. In his honor, this radiation is now called "Hawking radiation." (Tunneling is another quantum property by which particles can cross barriers that they normally do not have enough energy for.)

This discovery was one of the most important physical theories of the century. Not only was it a brilliant solution to a difficult problem, it was the first physical theory to merge thermodynamics, quantum mechanics, and general relativity into a single system. It was the first step towards a "theory of everything" that would simultaneously describe the microscopic world of atoms and the macroscopic world of planets, galaxies, and the universe. Since then, many physicists have taken on the challenge of developing theories that describe both of these worlds. The biggest problem they face is how to reconcile "smoothness" and "roughness." Einstein's general relativity theory treats the four dimensions of space and time and their curvature by gravity as a simple, smooth force. Quantum mechanics, in contrast, sees the world in discrete, bumpy jumps called quanta.

In addition to searching for a theory of everything, Hawking and others also tackled the "information puzzle" hypothesis, first brought up by Hawking in 1976. The problem involves what happens to the information given into a black hole. All particles that fall into a black hole have "information" as defined by certain quantum mechanical properties. Physicists are still debating and theorizing about what happens to this information: does a black hole completely absorb it and keep it hidden from the universe forever, or is this information somehow reexpressed by the Hawking radiation the black hole emits?

The solution to this problem may rely upon the fate of the black hole. At the center of the black hole resides a singularity, where the curvature of space and time become infinite and Einstein's general relativity is of no use in describing space and time. The properties of this singularity, which are being studied by Hawking and others, may resolve the information puzzle-the singularity may hold all the information inside itself. Or, conversely, the information may be gone.

Hawking had a long-standing bet regarding this singularity with physicist Kip S. Thorne and John Preskill of the California Institute of Technology. Hawking bet that we will never be able to observe the singularity, that it will vanish with the rest of the black hole as it evaporates. This view is the "cosmic censorship" hypothesis, first stated by mathematician Roger Penrose (1931- ). Thorne took the other side, believing that we will be able to observe the naked singularity. The bet was partially settled in 1997 when Matthew Choptuik showed with supercomputer simulations that under certain circumstances the singularity could be revealed.

Studying singularities is important because the universe may have begun in a similar state. The Big Bang theory states that the universe began as a tightly compressed object that exploded outward to create everything we can see today. The properties of this initial ball of matter may have been just like those of a singularity. Understanding the properties of singularities better will help us define the original state of the universe.

PHILIP DOWNEY