Gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe, such as the collision of two black holes or shockwaves from the cores of supernova explosions. Daniel Kennefick has written a remarkable history of an abstruse intellectual puzzle, the search for those waves.
For decades, physicists argued about whether such waves could exist at all. As soon as a settled theory of the waves emerged, about 1960, the search for them commenced. Almost half a century later, the quest continues, supported by huge detectors. The number of professionals who believe that gravitational waves will be detected within ten years is climbing steadily. Such detection would be timely: the centenary of the publication of Einstein's first paper on gravitational waves will be in 2016.
Einstein used analogy as a powerful tool in his development of general relativity. Gravitation and electromagnetism both follow the inverse square law, and both are field theories. James Clerk Maxwell's unification of electricity and magnetism had predicted the electromagnetic waves that were first detected by Heinrich Hertz. It was natural, therefore, that when Einstein developed a new theory of gravity, he quickly turned his attention to the phenomenon of gravitational radiation. He recognised that just as radio waves are generated when electrical charges accelerate, so waves of gravitational radiation should be produced by mass in motion. However, Einstein doubted that such waves would ever be observed, even from the most violent cosmic cataclysms such as supernova explosions, because the signal strength was predicted to be extremely weak. That is why experimentalists ignored gravitational waves. Why bother chasing an effect too small to be observed? In fact, a lively debate on the reality of the waves soon ensued, with Einstein himself expressing doubts on two occasions. Sir Arthur Eddington, the greatest relativist after Einstein, was the first sceptic: he quipped that "gravitational waves propagate at the speed of thought". Kennefick uses this phrase for his title to contrast the individual brilliance of physicists working on gravitational waves with the frustratingly slow progress of the field as a whole.
Two pivotal moments in gravitational research occurred in 1960s with the discoveries first of quasars and then of pulsars. Both possess very strong gravitational fields, and it was soon realised that they might provide the testing ground for gravitational theory. Quasars led to the development of the idea of the black hole, as well as the suggestion that gravitational waves might be a power source. Rather suddenly, general relativity was seen to be important in the new discipline of high-energy astrophysics.
In 1975, Joseph Taylor and his graduate student Robert Hulse announced the discovery of a binary pulsar system, consisting of two neutron stars in a mutual orbit not much bigger than the radius of the Sun. In 1993, they shared the Nobel Prize for Physics for this breakthrough.
It was soon noticed that the orbital period of the binary is declining at the rate of 75 milliseconds per year: the two collapsed stars are spiralling into an ever tighter orbit. This agreed almost exactly with the prediction from general relativity that the binary system would emit energy in the form of gravitational waves. The close agreement of theory and observation is an indirect proof of their existence.
Without doubt, this book is a masterpiece in the history of physics and Einstein studies. Kennefick covers a huge amount of ground in summarising the efforts of three generations of physicists on a quest that is yet to yield a positive detection. His new sources include two dozen oral history interviews he conducted in the mid-1990s. I particularly admire the skill with which the author has brought together the protagonists and sceptics in order to create a compelling account of the intellectual struggles.
The narrative is excellent in showing how theoretical physics works, being particularly strong in explaining the importance of collaborative work. Social aspects of research - who worked with whom, in what institutions, which scientific conferences did they attend, and so on - are covered in great depth.
Simon Mitton is a fellow of St Edmund's College, Cambridge.
Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves
Author - Daniel Kennefick
Publisher - Princeton University Press
Pages - 344
Price - £18.95
ISBN - 97806911170