8. Gravitational waves

In 1916, Einstein published his theory of general relativity, which incorporated fundamentally new ideas about the nature of gravity, including that gravitational effects take time to travel. He also showed that under some circumstances, objects lose energy by emitting ‘ripples’ in time and space: gravitational waves. ‘Gravitational waves’ explains how these waves are very weak and only the most powerful events in the Universe generate strong enough versions to be detected. Gravitational waves differ from other kinds of waves as their only effect is to cause objects to move together and then apart again. They provide a unique new window on the Universe, allowing us to look deeper and further than ever before.

6. Electromagnetic waves

‘Electromagnetic waves’ considers the history of the scientific investigation into the electromagnetic spectrum, including Einstein’s insight into the quantized nature of electromagnetic radiation. It explains that the only difference between light, radio waves, and all the other forms of electromagnetic radiation is the length of the fictitious-but-convenient waves or, equivalently, the energy of the photons involved. These different energies lead to different mechanisms for the formation and absorption of the different kinds of radiation, and it is this which gives rise to their different behaviours. Radio waves, microwaves, infrared radiation, light, ultraviolet light, X-rays, and gamma rays are all discussed.

4. Seismic waves

Sound waves travel very easily underground, often for many thousands of kilometres. These are usually referred to as a kind of seismic wave and are most often triggered by earthquakes, which result from a sudden slip of tectonic plates, down to about 700 kilometres below the Earth’s surface. ‘Seismic waves’ describes the four types of seismic wave generated by earthquakes: P-waves (primary waves), S-waves (shear waves), Love waves (usually the most powerful and destructive of seismic waves), and Rayleigh waves, which are created when P and S waves reach the Earth’s surface together, combining to form undulating ground rolls. Free vibrations and star waves are also described.

7. Quantum waves

At the beginning of the 20th century, the wave theory of light was facing insuperable problems. The ultimate death blow was Einstein’s explanation of the photoelectric effect. ‘Quantum waves’ discusses the work of Louis de Broglie, Niels Bohr, and Einstein. It considers the uncertainty and indeterminacy of electron motion, wave function, quantum tunnelling, and the phenomenon of entanglement. Quantum theory shows that electromagnetic radiation and electrons are particles. If string theory is correct, it shows that the wave concept is much more than a powerful tool to model the way the world works. It means that, for all its staggering complexity, the Universe is nothing more than a vast network of interacting waves.

5. Biological waves

There are many instances when the application of simple wave theory can lead to powerful insights and testable predictions. ‘Biological waves’ considers some representative samples of different kinds of wave that are of biological significance. It discusses the different types of brain waves, first discovered in the 1920s by Hans Berger, including alpha, beta, delta, theta, and gamma waves; the rhythm of heart beats; the wave motion of peristalsis that travels all the way down the alimentary tract from throat to rectum; the different wave motion in the locomotion of animals such as worms, snakes, centipedes, and millipedes; and the patterns made by groups of animals such as starling flocks.

2. Water waves

Water waves may in fact be the most complex of all; out at sea, the water surface is a summation of an ever-changing mix of waves of many sizes, speeds, and directions, thanks largely to the fact that some waves can last for weeks, and in that time they can travel thousands of kilometres. Until recently, many wave parameters were hard to measure, but accurate measurements can now be made by a number of techniques including dual frequency radar altimetry. ‘Water waves’ describes many different waves and how they build, including capillary waves, breakers, edge waves, harbour resonance, seiches, tides, and tsunamis.

3. Sound waves

Sound is a small fraction of the pressure wave spectrum. In terms of wavelength, we can hear far more than we can see: while deep red light has waves about twice as long as those of deep violet, the lowest-pitched sound waves we can hear are about ten times longer than the highest. ‘Sound waves’ explains infrasound, the lower frequencies detectable by touch rather than hearing, and the audio range, describing the fundamental differences between sound power and loudness, and frequency and pitch. It also considers sound underwater. Sonar systems can be passive systems that simply detect sounds, while active sonar transmits sounds and detects those that are reflected.

1. Waves in essence

Most waves can be defined by just a few parameters: period, frequency, wavelength, amplitude, particle velocity, phase velocity, and group velocity. ‘Waves in essence’ explains these parameters in turn and then goes on to discuss the spreading and fading of waves and the complexities of waves that arise through their interactions with objects and other waves resulting in diffraction and interference. It also describes the difference between longitudinal and transverse waves and the important wave phenomena of refraction and reflection. It then outlines the fundamental difference between pressure waves like sound, ocean, and seismic waves, and electromagnetic waves, which include light and radio waves. All electromagnetic radiation is made of particles called photons.