Laser Interferometer Gravitational-Wave Observatory (LIGO) Co-Founder Rainer Weiss uses a visual aid as he speaks about the discovery of gravitational waves during a news conference at the National Press Club in Washington, 11th February 2016. ©Andrew Harnik/AP/Press Association Images 150 years since James Clerk Maxwell showed that electric and magnetic fields create waves of light, and 100 years after Einstein created his theory of gravity (general relativity) gravitational waves have been discovered. The announcement came at the American Association for the Advancement of Science, in Washington DC on Thursday 11th February. This is the climax of an experiment that began in 2002, which was then rebuilt to a new sensitivity in September last year. Almost immediately last autumn, rumours began that the detector was responding to gravitational waves. Now, at last, we have the confirmation of this major missing piece of Einstein’s general relativity theory. For almost a century, physicists have debated whether the gravitational field creates waves analogous to what happens for electromagnetic fields? If so, what are they like; do they travel at the same speed as light or, as the sceptic Arthur Eddington famously opined: “at the speed of thought.” In the opinion of many scientists, the discovery of gravitational waves would be the final piece in the jigsaw of general relativity theory and a certainty for a Nobel Prize. In Einstein’s universe, space and time are intimately entwined into an elastic medium—“space-time”—According to general relativity, space-time becomes warped when massive bodies, such as planets and stars, are present. If the mass that is the source of a gravitational field suddenly shifts, as in a supernova explosion, or when two black holes swirl and merge, gravitational waves should spread throughout the medium. This sort of behaviour is familiar in electromagnetism, thanks to Maxwell. An oscillating charge in a radio antenna, for example, emits waves in the form of electromagnetic “dipole” radiation. At first sight, gravitational waves and electromagnetic waves appear to be analogous. However, Einstein himself was cautious. He recognised that there is a crucial difference between gravity and electromagnetic forces: electrical charges and magnets can attract or repel, depending on the relative polarity of the two charges, whereas gravity appears to be universally attractive: there is no “antigravity.” It is the duality inherent in electromagnetism that leads to dipole radiation; there is no such analogue for gravity. Einstein duly recognised that gravitational waves are not analogous to electromagnetic waves, and even doubted whether they exist at all. Solving the equations of general relativity and correctly interpreting the results have been among the most difficult tasks in the physical sciences. Some theorists have argued that solutions, which at first sight appear to imply the existence of gravitational waves, are mere artefacts of the mathematics and not indications of reality. So the search for gravitational waves is more than merely an exercise in ticking the boxes. If, as now seems likely, gravitational waves do exist, there is the question of what speed they travel. Ask most scientists and they will respond: “at the speed of light.” True, a fundamental plank of special relativity theory is that light travels at a universal speed in a vacuum; but that theory excludes gravity. While it is often presented as self-evident, at least in popular texts, that gravitational waves both exist and travel at light-speed, not everyone has been convinced. Eddington insisted that there is no reason why there should be only one universal speed in nature, and set out to find the answer by solving the equations. He showed that Einstein’s equations predict the existence of gravitational waves that travel at the same speed as light, but with a caveat: only if the waves are low intensity. Waves of light do not interact with themselves during their transit through a vacuum; gravitational waves, by contrast, can interrupt their own flight. If gravitational waves are very strong, they can change the fabric of space-time so much that the waves become scattered. Some parts of the wave may backtrack and arrive later than the main wave. The modern belief that gravitational waves exist came after the discovery of the first binary pulsar in 1974. Its orbital period was measured over several years, and found to be slowing down. This turned out to agree with what Einstein’s theory predicted if the pulsar was radiating gravitational energy. Although this was not a direct observation of gravitational waves, it led to the received wisdom that these phantoms indeed exist and reinvigorated attempts to detect them directly. A problem has been that even the most powerful waves at their sources in deep space are expected to be exceedingly tenuous by the time we receive them. Any disturbance in space-time would alter the fabric of space-time by a distance smaller than the nucleus of an atom. The ability to detect such a signal, and also to be able to distinguish it from noise such as earth tremors, has pushed technology for decades. Previous claims have foundered. Enter LIGO: Laser Interferometer Gravitational Observatory. Viewed from above, LIGO appears as two tubes, which form a cross, each arm being some 4 kilometres long. Laser beams are split in two so as to travel along each of the arms, where they are reflected back and forth by mirrors. The reflected beams meet at the point where the two arms cross. The two waves mutually destruct at this point, which is dark. If a gravitational wave passes through the apparatus, the two arms will be disturbed slightly and the laser beams will have travelled different distances. Their waves will no longer match and a light detector will record the fact. This is how the delicacy of interference between light beams can reveal subtle changes in distance on the scale of a billionth of the diameter of an atom. An added beauty of LIGO is that it consists of twins. One 4km cross is in Hanford, Washington State, its twin is 3000 km away, in Louisiana. Signals in both detectors, within about one-hundredth of a second of one another, would reduce the problem of local noise and also suggest the passage of a gravitational wave travelling at (about) light speed. Triangulation also enables the direction or source of the wave to be determined. Having done that, “conventional” astronomers can peruse the heavens to see what electromagnetic signals are associated with the gravity-quake. A new era in astronomy could be at hand.