3. Extrasolar tests of gravity

By studying objects outside our Solar System, we can observe star systems with far greater gravitational fields. ‘Extrasolar tests of gravity’ considers stars of different sizes that have undergone gravitational collapse, including white dwarfs, neutron stars, and black holes. A black hole consists of a region of space-time enclosed by a surface called an event horizon. The gravitational field of a black hole is so strong that anything that finds its way inside the event horizon can never escape. Other star systems considered are binary pulsars and triple star systems. With the invention of even more powerful telescopes, there will be more tantalizing possibilities for testing gravity in the future.

Epilogue

Our understanding of gravity has developed rapidly over the past century. This began with Einstein’s revolutionary new theory and continued with sustained developments in both our understanding of the mathematics and observations that can be used to probe it. This VSI outlines how new gravitational effects have been predicted and observed in the Solar System; in exotic astrophysical systems; and in the Universe as a whole. The ‘Epilogue’ explains that although this VSI attempts to give some idea of the elegance of the concepts involved, and the details of the wondrous physics that results, it is inevitably left incomplete. The final words on gravity have yet to be written.

5. Cosmology

Cosmology began as a scientific discipline at the beginning of the 20th century, with the work of Albert Einstein and Edwin Hubble. Gravitational interaction is fundamental to cosmology, as gravity dominates over all other forces on large-scale distances. ‘Cosmology’ outlines the modern history of cosmology, discussing how studies have provided knowledge on the early Universe and its expansion. The Concordance Model proposes that only c.5 per cent of the energy in the Universe is in the form of normal matter; c.25 per cent is in the form of the gravitationally attractive dark matter; and the remaining c.70 per cent is in the form of the gravitationally repulsive dark energy. But there is still much to learn.

1. From Newton to Einstein

Gravity is the weakest of nature’s four fundamental forces, yet over large distances it dominates. This is because gravity, unlike the other forces in nature, is only ever attractive. The gravitational force between objects always increases as they become larger and have more mass. Despite the efforts of Isaac Newton and Albert Einstein, gravity remains an enigmatic puzzle. ‘From Newton to Einstein’ considers the pre-history of gravity including the ideas of Aristotle and Galileo. It describes Newton’s theory of gravity, first published in 1687. It finally explains Einstein’s theory of gravity, which supplanted Newton’s theory, and explains that is the curvature of space-time that is responsible for it all.

6. Frontiers of gravitational physics

‘Frontiers of gravitational physics’ considers some of the issues involved in the theoretical description of gravity. Since 1915, it has been Einstein’s theory that has shaped our understanding of the gravitational interaction, but a lot has happened in the world of theoretical physics since then. Quantum mechanics, devised by Bohr, Heisenberg, and Schrödinger, is incompatible with gravity and so has raised numerous questions. Several theories, including String Theory and Loop Quantum Gravity, have been proposed with some success. The concepts of cosmic inflation, the cosmological constant, and the possibility of multiple universes are also discussed. It concludes that with further astrophysical studies a new fundamental theory may explain it all.

4. Gravitational waves

As stars collapse they eject huge amounts of mass and energy; their gravitational field changes rapidly and, therefore, so does the curvature of the space-time around them. If the curvature of space-time is pushed out of equilibrium, by the motion of mass or energy, this disturbance travels outwards as waves. ‘Gravitational waves’ explains the effect of a gravitational wave: in a binary pulsar, the waves carry energy away from the system so that the two neutron stars slowly circle in towards each other. Gravitational waves were first detected in 2015 by the Laser Interferometer Gravitational-Wave Observatory in America. There are also plans to set up a detector in space.

2. Gravity in the Solar System

By observing the motion of planets and other objects in the Solar System (e.g. comets, asteroids, moons, and man-made spacecraft), we can learn a great deal about the behaviour of gravity. ‘Gravity in the Solar System’ reviews the experiments that have been undertaken to probe the foundational assumptions of gravity theories, including Newton’s law and Einstein’s theory. These foundational assumptions include the fact that the rest mass of an object should be independent of its position, and independent of its motion with respect to other bodies; the idea that the speed of light should be the same in every direction; and that all objects should fall at the same rate.