Hawking and the Laws of Thermodynamics
Hawking was very interested in the discovery of quasars, and he was one of the scientists who believed that they must be powered by massive black holes in order for them to give out so much energy and to be visible at such a great distance. In the 19th century a branch of physics known as thermodynamics was developed to help improve the efficiency of the steam engine. Thermodynamics had fallen out of fashion by the time of Hawking but it still contained some interesting ideas, and one of them was the concept of entropy. It is difficult to give a precise definition for entropy, but the closest we can get is to call it the degree of disorder. Left to its own resources entropy always increases, and it requires the expenditure of energy to reduce the disorder. In the 19th century thermodynamics suggested that as the entropy increased the universe would slowly run down until it was the same temperature everywhere, then there would be no available energy left to reorganize it.
Through his studies Hawking knew that black holes could not emit any radiation and it followed that they could either remain the same size forever or grow larger by swallowing up extra matter—but a black hole could not shrink in size. The rule was almost the same as that of the second law of thermodynamics wherein the entropy of a closed system could only remain the same or become greater. Hawking then realized that the mathematics of the two was identical, and he came to the conclusion that the surface area of the black hole was a measure of its entropy. Hawking also knew that the temperature of a black hole was in inverse proportion to its mass. He realized that the laws of thermodynamics suggested exactly the same thing. The smaller a black hole the greater its temperature. When he came to consider very small black holes—smaller in size than the proton—he realized that they must be very hot indeed.
Hawking next turned his mind to singularity theory. A singularity is a point at which physical quantities such as density and temperature become infinite, and there had long been a theory that for these quantities to reach infinity was just not possible. The saying that “Nature abhors a singularity” was the opposite of the saying that “Nature abhors a vacuum,” but they were also parallel statements. Hawking worked closely with Roger Penrose (b. 1931) on the theory, and both men agreed that singularities must exist at the center of a black hole and that they also existed in the first instant of the Big Bang. One argument they put forward was that it was impossible to see beyond the horizon of a black hole and therefore the existence of a singularity inside it did not matter because there was no way it could affect the rest of the universe.
Black Holes Meet Quantum Mechanics
Then Hawking’s researches came up with an astonishing result. It had been known since the first theories of the black hole were expounded that nothing could possibly escape from inside the event horizon. Hawking, however, produced a theory to show that this was not always true; under certain conditions a black hole could in fact emit radiation. He arrived at this result after applying the laws of quantum mechanics to the problem. It was also known that under certain conditions particles could be generated from pure energy.
Astronomers have found evidence for what is called “virtual particle” production. Nature allows pairs of “virtual” particles to form spontaneously at any point in space. Each pair consists of a particle and its antiparticle, for example an electron and a positron, a proton and an antiproton, or a pair of photons (the photon is its own antiparticle). Normally, when the particles are formed, they annihilate each other after a time interval of about 10−21 seconds. If the particles appear near the event horizon of a black hole, however, then the gravitational field of the black hole changes the virtual particles into real particles. One of the particles is captured by the black hole and is never seen again. The other has sufficient energy to escape. Thus, seen from a distance, the black hole seems to emit a particle. It has also swallowed a particle so we might think that it has grown in size. In fact the reverse is true. The black hole has expended energy on the virtual particles and the net result is that it has lost energy (or mass if you prefer to think in terms of mass). Thus the spontaneous pair production process causes the black hole to slowly wither away.
Even stranger things could happen when black holes and quantum mechanics come together. Hawking tried to work out the properties of very small black holes. Readers will remember that a black hole with the same mass as the Earth would be about 1.7 centimeters (0.7 in) in diameter. In an earlier chapter we also saw how a very large black hole can have such a low density that the whole universe could in fact be a black hole. Now we must consider the extreme opposite: a black hole smaller than a proton. As the mass of a black hole is reduced so is the radius, but although the mass reduces the density increases as the radius decreases. Hawking calculated that a black hole weighing about a billion tonnes, the size of a mountain on Earth, would be contained inside a sphere smaller than a proton! The only conditions where the pressure was high enough to compress something the size of a mountain into a space the size of a proton was during the early phases of the Big Bang. He reasoned that there could be many black holes of this size left behind by the Big Bang when the universe was first created. These microscopically small black holes were amazing and mind-boggling objects. How could the mass of a mountain be compressed into a single proton? The black hole evaporates slowly but the nature of its final demise is in dispute. Every black hole has an infinite singularity at its center; we do not know if this singularity creates a large explosion or if the black hole disappears peacefully. Quantum theory predicts that the black holes were not always stable and that they could explode spontaneously with the release of an incredible amount of energy. Hawking thought they were nevertheless sufficiently stable to still be around billions of years after the Big Bang.
Cosmologists Take Center Stage
Interpreting the observations and images from space often requires the application of very exotic physics and esoteric mathematics. Hawking and Penrose wanted very much to find and observe an exploding black hole to prove their theories. They knew that by measuring the frequencies and amplitudes of the radiation and by a careful examination of the spectrum they would learn much about its history. They became convinced, much against current thinking at the time, that it was in fact possible to obtain information from a black hole about what had fallen into it. Thus in the late 20th century and into the third millennium the origins of the universe became the realm of the cosmologist more than any other kind of astronomer.
Hawking has published a great number of academic papers, but he has also written for the popular press. In 1979 he was the co-author with German physicist Werner Israel (b. 1931) of the book General Relativity, an Einstein Centenary Survey. Hawking’s best-known publication, however, is A Brief History of Time published in 1988 and which remained for several years in the list of bestsellers. It was this book that helped solved the financial problems associated with Hawking’s long-suffering family and the growing expense needed for his nursing and medical care. In 2001 his book The Universe in a Nutshell was published; it became another bestseller, providing glossy full-color pictures of the latest ideas in cosmology. Stephen Hawking is the only man in modern times to approach Albert Einstein as a scientific icon, and it may be a long time before we see his successor.
Hawking is the Lucasian Professor of Mathematics at the University of Cambridge—the chair occupied by Isaac Newton in the 17th century. His ambition is very similar to that of Albert Einstein: he seeks to find the holy grail of science, a theory to unify all the existing theories into something that describes the whole of the universe.
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ASTRONOMY IN THE SPACE AGE
By the late 1950s, a momentous new development in space exploration had been achieved: at that time it was not only possible to send spacecraft into orbit but also for humans to experience the great void of space at first hand. The space age had begun. It even became possible for people to set foot on another world. Soon scientists were looking beyond the M
oon and sending probes to the planets in the remote reaches of the solar system.
The second half of the 20th century saw the first attempts by engineers and scientists to explore the solar system with the use of spacecraft. In 1957 the world was astounded when the Russians announced that their first satellite, Sputnik 1, was in orbit around the Earth. The Russians soon followed this achievement by becoming the first nation to send a living animal, the dog Laika, into orbit. On April 12, 1961, the Russian spacecraft Vostok 1 orbited the Earth in a flight lasting 108 minutes with the cosmonaut Yuri Gagarin (1934–68) on board. Gagarin thus became the first human to enter space.
Landing on the Moon
As might be expected, the Americans were severely taken aback by these Russian space achievements, and in an attempt to claw back the initiative the US president John F. Kennedy (1917–63) announced a very ambitious project to put a man on the Moon by the end of the decade. There followed the so-called space race between America and Russia, with the early honors going to the Russians. In 1959 the Soviets achieved the first fly-past of the Moon, followed by the first hard landing on the Moon’s surface, and then the first orbit of the Moon. During the orbit, the probe took remarkable photographs of the Moon’s far side—it was the first time this view of the Moon had ever been seen because it always presents the same face to us as it orbits the Earth.
The financial cost of the space race was extremely high, and for this reason the Soviets could not hope to stay ahead of the Americans for very long. The Americans soon had satellites orbiting the Earth as well as piloted orbital flights. By the middle of the decade the American Apollo program was well under way. On July 20, 1969 Neil Armstrong became the first person to step on the surface of the Moon. The Apollo program achieved most of its aims, and out of the seven lunar missions (Apollo 11 to Apollo 17) only the ill-fated Apollo 13 did not reach the Moon. Apollo 13 remains as the greatest drama in the early history of space flight, after the dramatic return of the astronauts to Earth following a failure in the command module.
When samples of Moon rock were returned by the Apollo missions and the close-up views of the lunar surface were studied there was a great scientific return for the money spent on the space effort, but there was little of direct commercial value. After the Moon landing the next step in the exploration of space no longer involved piloted missions. Life support systems were very costly and heavy, and it was far more efficient and less dangerous to explore the solar system by means of robot spacecraft and to transmit the findings back to Earth.
Exploring Mars
Mars was the next target. In the 1960s several of the Mariner missions mapped practically the whole of the surface of Mars and produced strong evidence that the surface had supported sustained water flow at some time in the past. They were followed in the mid-1970s by the Viking landers which touched down on Mars to become the first craft to send back views from the planet’s surface. As early as 1877 the Italian astronomer Giovanni Schiaparelli (1845–1910) had studied Mars through his 20-centimeter (8 in) telescope and discovered what he thought were sets of lines crisscrossing the surface. The American astronomer Percival Lowell (1855–1916) examined this suggestion further, and by the end of the century he had produced an image of the red planet showing a network of canals, built perhaps to carry water from the poles to the Martian desert for irrigation. This imaginative interpretation of the geological features of Mars developed by Schiaparelli and Lowell was soon discredited, but when the Viking orbiter crafts produced the first detailed maps of the Martian surface they found valleys that could only have been created by running water at some time in the remote past. There were also plains and craters on the surface, as well as mountains and extinct volcanoes. One volcano, Olympus Mons, is far bigger at 15 miles (25 km) in height than any others in the solar system. Mars also has canyons even greater than the Grand Canyon in the USA.
Martian Adventures
At the end of the current decade we will know far more about Mars than we know at present. While Mars has been a primary target for planetary exploration, unfortunately many missions have proved unsuccessful, contact with the spacecraft being lost at launch, en route or crash-landing onto the surface.
However, orbiting crafts and landers are both providing more and more data to piece together the puzzle of Mars. The Reconnaissance orbiter was launched in August 2005 and is now providing more detailed mapping of the Martian surface from orbit.
The Opportunity and Spirit rovers have spent four years exploring Mars and their examination of surface rocks have provided the best evidence yet that Mars was once covered by oceans of liquid water. Each rover has traveled for several miles, and in 2006 Opportunity reached the edge of Victoria Crater, after spending many months exploring the smaller Endurance Crater. The rover had to shelter in a crevice while waiting for a large dust storm to clear. A safe path was found, and Opportunity entered into Victoria Crater. It is hoped that the crater will show evidence of how it was formed, possibly providing clues to the ancient surface history of Mars itself. The far rim of the crater, lying about 800 meters (2,625 ft) away and rising about 70 meters (230 ft) above the crater floor, can be seen in the distance. The alcove in front has been given the name Duck Bay.
The Phoenix lander is currently en route, and will search likely sites for signs of water. Two further Scout missions are planned to arrive in 2013 and 2018 and they will eventually provide much better exploration data. The geological survey of the whole of Mars will be complete within a few years, by which time the scientists will attempt much closer surveys of the more interesting areas. Geologists on Earth want a sample of Martian rock and the Mars Science Laboratory, to be launched in 2009, will help to provide one. It is hoped that a sample can be brought back to Earth by 2014.
Exploring Venus
Venus was the next planet to be explored by probes. The planet had traditionally been thought to be the one most similar to Earth, albeit with a slightly smaller mass and a much warmer climate. The Russian Venera probes reached the surface of Venus in the 1970s to early 1980s and were able to determine conditions underneath the thick layer of cloud that obscures the planet. The temperature was a searing 470 °C (880 °F), and the atmosphere was 96 percent carbon dioxide and about 4 percent hydrogen. It was also about 90 times denser than the Earth’s atmosphere. None of the Venera probes lasted longer than about two hours, succumbing quickly to the extreme heat and pressure at the surface. There had been much volcanic activity on Venus, producing large quantities of sulfur in the atmosphere. Indeed, the dense clouds that enveloped the planet were found to consist mostly of sulfuric acid. Radar surveys showed many gently rolling hills on the surface, many volcanoes and two “continents.” There were many craters including a huge impact crater on the surface, christened Klenova, which was 88 miles (142 km) across. It was a great disappointment to discover that the surface of the planet of love could hardly have been a more hostile environment.
Exploring Mercury
In 1974 the Mariner 10 probe reached Mercury and went into an orbit around the planet that enabled three close approaches to be made. The probe mapped a remarkable 45 percent of the surface during these orbits. The surface proved to be heavily cratered like the Moon. Unfortunately the planet was no more hospitable than Venus. The very thin atmosphere consisted of hydrogen, helium, sodium, potassium and a trace of oxygen. As a result of the thin atmosphere the daytime temperatures reached about 350 °C (662 °F), but at night they fell to about 170 below zero. Features examined included the Caloris Basin, about 808 miles (1300 km) in diameter and surrounded by a ring of mountains. It was also possible for the probe to measure the length of the day on Mercury. The year on Mercury has long been known to be the equivalent of 88 Earth days, but the solar day turned out to be 176 Earth days—two Mercurian years! The first fly-by of NASA’s Messenger mission, launched in 2004, was achieved in January 2008, with two more fly-bys to come before it enters Mercury’s orbit in 2011.
Exploring Jupi
ter
The development and launch of the very sophisticated Voyager probes in the 1970s meant that exploration of the outer planets of the solar system became possible. The two Voyager probes visited Jupiter in 1979 and Saturn in 1980 and 1981, with Voyager 2 traveling to study both Uranus and Neptune during the late 1980s. These missions were followed by the Galileo spacecraft’s visit to Jupiter, and Cassini’s sojourn at Saturn in later decades. To save fuel, these probes were designed to reach the outer reaches of the solar system by using the gravitational pull of the other planets to produce a sort of slingshot effect, helping to propel them on their journey. Very sharp and interesting images of the gas giants and their satellites (moons) were transmitted back to Earth. Clearly visible on the surface of Jupiter—the biggest planet in the solar system—is the Great Red Spot. This feature is so large that it was seen in the 17th century. It is about 15,500 miles (25,000 km) long by 7, 456 miles (12,000 km) wide—large enough to swallow two Earths. It takes about six days to make a rotation, and although it changes shape over a period of time it has been a permanent feature on Jupiter for over 300 years. It is best described as a colossal hurricane or typhoon. The energy needed to maintain it comes from inside the planet, and it is part of an ever-changing atmosphere around Jupiter.
The Story of Astronomy Page 21