What we learn in this chapter

What we learn in this chapter

From the author to the reader

Why this book is about gravity

During the 35 years that I have done research in gravitation, I have watched with amazement and delight as my colleagues in astronomy have, step-by-step, opened up almost the entire Universe to our view. And what a view! There are punctures in space called black holes that capture gas and stars with a relentless and unbreakable grip; there are 10km balls called neutron stars that are immense overgrown atomic nuclei with more mass than our Sun, that spin about their axes hundreds of times per second while emitting intense beams of radiation; there are bursts of gamma-rays from the most remote regions of the Universe that are so intense that they outshine the rest of the Universe for a short time; and most strikingly of all there was the beginning of time itself in an explosion of pure energy, driven by a force we do not understand, in which matter as we know it did not exist, in which even the laws of Nature themselves were mutable.

Some of the most exciting moments in the exploration of space in the last thirty years have been provided by a succession of unmanned spacecraft that have explored more and more remote reaches of the Solar System. The early Moon-orbiters, scouts for later Moon landers, were succeeded by spacecraft that visited Mercury Venus, Mars, Jupiter, Saturn, Uranus, Neptune, various comets, and the Sun itself.

But to explore the Solar System in this way requires stronger and stronger rockets, much stronger than are required simply to get a spacecraft away from the Earth's gravitational pull. In order to do the most with the rockets available to them, planetary scientists have used a remarkable trick, called the gravitational slingshot: they have used the gravitational pull of another planet, such as Jupiter, to give their spacecraft an extra kick in the direction they want it to go. In this chapter we will try to understand how this works, not only for getting spacecraft into the outer parts of the Solar System, but also for getting them very close to the Sun.

In this chapter we open the door to our own history. Surely one of the most satisfying discoveries of modern astronomy is how the natural processes of the Universe led to the conditions in which a small planet could condense around an obscure star in an ordinary looking galaxy, and life could evolve on that planet.

The evolution of life seems to have required many keys, but one of them is that the basic building blocks had to be there: carbon, oxygen, calcium, nitrogen, and all the other elements of living matter. The Universe did not start out with these elements. The Big Bang, which we shall learn more about in Chapter 24 to Chapter 27, gave us only hydrogen and helium, the two lightest elements. All the rest were made by the stars. Every atom of oxygen in our bodies was made in a star.

We are now ready to make another step outwards in our exploration of the Universe: we change from looking at stars to looking at galaxies. As we saw in Chapter 9, galaxies are vast collections of stars. Our own Galaxy, the familiar Milky Way, contains about 1011 stars. Figure 14.1 on the following page shows photographs of two typical galaxies. Galaxies are held together by the mutual gravitational attraction of all the stars. It is remarkable indeed that Newton's force of gravity, which he devised in order to explain what held the planets in their orbits, turns out to explain just as well what holds the whole Milky Way together.

Galaxies are more than just collections of stars. The collective gravity of all the stars makes the centers of galaxies very unusual places. Stars and gas crowd together so densely that in some cases they can form immense black holes, with masses of millions or even billions of stars.

We have now pushed our model of the history of our Universe back just about as far as we could hope to go: the Universe had a beginning, and that beginning was the source of all that happened afterwards – all matter, all stars, all galaxies, even life itself. Big Bang cosmology has placed modern physics in the remarkable position of being able in principle to trace back to the beginning every aspect of the world we live in, to say “This is where X came from, and this is how Y started”.

1. ▷ The background image on this page is a plot of the spatial distribution of galaxies in a thin wedge of space centered on our position, from the CfA Redshift Survey. Measured in 1985, this distribution gave astronomers their first indication that galaxies were grouped into chains as well as clumps. The human-like pattern (horizontal in this view, with the legs to the left) became a celebrity in its own right. Data from M Geller and J Huchra, image copyright South African Observatory (sao).

The tides wash the margins of all the great oceans, regulate the lives of sea urchins and fishermen, power the great bore waves on rivers like the St. John, the Amazon, and the Severn. For most of us the tides are romantic, primeval, poetic. Standing on an ocean beach, we might be impressed by this tangible manifestation of the gravity of the distant rock we call the Moon, but few of us would be led to reflect on how fundamental the tides are to an understanding of gravity itself. But fundamental is the right word. In the modern view, the real signature of gravity, the part of gravity that can't be removed by going into free fall, is the tidal force, whose most spectacular effect on Earth is to raise the ocean tides. In this chapter we will examine this aspect of gravity, starting with the simplest effects first and working our way up to ocean tides and then to tides elsewhere in the Solar System and beyond. We will return again and again in later chapters to the fundamental role of tides. Indeed, many astronomical systems transmit tidal forces as signals right across the Universe, signals that we call gravitational waves.

When Einstein began to develop his theory of gravity, he knew he had to build on special relativity, but he felt strongly that he also had to preserve Galileo's other great contribution to physics, the principle of equivalence (Chapter 1). As with special relativity, Einstein worked by blending the old and the new in equal proportions: special relativity combined the old principle of relativity with the new principle of the universality of the speed of light; in his new theory of gravity Einstein combined the old principle of equivalence with his new theory of special relativity.

1. ▷ Underneath the text on this page is the familiar Mercator projection map of the entire Earth. This map illustrates strikingly the fact that the surface of the Earth cannot be represented faithfully on flat paper.

Observational astronomy is based on the detection of radiation emitted by astrophysical objects. All morphological and physical information about astrophysical sources is derived from observation and analysis of the emitted radiation. Continuum radiation is a natural consequence of the principle that accelerating charges radiate. In this part of the book I will review and apply principles of electromagnetism to further our understanding of important astrophysical phenomena, demonstrating in the process that important radiation processes can be derived from the basic principle that accelerating charges radiate. To that end, I will develop the theory that describes bremsstrahlung and synchrotron radiation. A theoretical understanding of these two radiation mechanisms allows us to interpret the emission of a wide range of objects, ranging from distant radio galaxies to nearby HII regions. The specific astrophysical topics covered in this part of the book include: the radiative properties of pulsars, dispersion and Faraday rotation of electromagnetic radiation, active binary star systems, accretion disks, supernova remnants, particle acceleration, cosmic rays and active galaxies.

Many people assume that satellites orbit the Earth far above its surface, but the numbers tell a different story. Most satellites orbit at less than 300km above the ground. Compared with the radius of the Earth, 6400km, this is very small. Their orbits just skim the top of the atmosphere. We can expect, therefore, that the acceleration of gravity on such a satellite will not be very different from what it is near the ground. How then can it happen that the satellite doesn't fall to the ground like our cannonballs in the first chapter?

1. ▷ Communications and many weather satellites, which must be in “geostationary” orbits, are an important exception, being in distant orbits. We will return to these orbits in Chapter 4.

The answer is that it tries to, but the ground falls away as well. Imagine firing a cannon over a cliff. Eventually the ball will fall back to the height from which it was fired, but the ground is no longer there. The Earth has been cut away at the cliff, so the ball must fall further in order to reach the ground.

1. ▷ The picture behind the words on this page is the Hubble Space Telescope (HST), a satellite launched by the National Aeronautics and Space Administration (NASA), with participation from the European Space Agency (ESA) as well. […]