Throughout most of this book we have been considering the cutting edge of experimental gravitational physics research, ranging from tests on the scale of millimetres to those on the size of the entire observable Universe. In this chapter we will consider 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. This theory treats space and time as a single object, and lets the properties of that object be determined by the matter that exists within it. I hope that the reader has, by now, been convinced of the phenomenal success of Einstein’s theory. It is truly extraordinary that a single theory should be able to explain such a wide array of physical efects. Einstein’s theory, however, is unlikely to be the final word in our understanding of gravity. A lot has happened in the world of theoretical physics since 1915, and much of it suggests that we should expect there to be an even more fundamental theory.
Quantum mechanics and gravity
Not long after Einstein published his theory of gravity, at the beginning of the 20th century, the world of theoretical physics was forever changed (once again) by scientists such as Bohr,Heisenberg, Schr’dinger, and others. Since Newton, and up until this point, physics had been thought to be deterministic. That is, if you know enough information about the position and motions of all objects in the Universe, then you should be able to predict the future with arbitrarily high precision. Physical theories that work in this way are now referred to as classical theories. Einstein’s relativity theory is an example of a classical theory. The revolution led by Bohr, Heisenberg, and Schr’dinger created another type of theory, quantum mechanics. The new quantum theory was based on probability, and resulted in a description of nature in which it was only ever possible to calculate the odds that certain events will happen in the future, and within which it is never certain what the future holds.