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\nBlack holes are among the most fascinating objects in the Universe. Enclosing huge amounts of matter in relatively small regions, these compact objects have enormous densities that give rise to some of the strongest gravitational fields in the cosmos, so strong that nothing can escape \u2013 not even light.\n<\/p>\n
\nThis artistic impression shows two black holes that are spiralling towards each other and will eventually coalesce. A black hole merger was first detected in 2015 by LIGO<\/a>, the Laser Interferometer Gravitational-Wave Observatory, which detected the gravitational waves \u2013 fluctuations in the fabric of spacetime \u2013 created by the giant collision.\n<\/p>\n \nBlack holes and gravitational waves are both predictions of Albert Einstein\u2019s general relativity<\/a>, which was presented in 1915 and remains to date the best theory to describe gravity across the Universe.\n<\/p>\n \nKarl Schwarzschild derived the equations for black holes in 1916, but they remained rather a theoretical curiosity for several decades, until X-ray observations performed with space telescopes could finally probe the highly energetic emission from matter in the vicinity of these extreme objects. The first ever image of a black hole\u2019s dark silhouette, cast against the light from matter in its immediate surrounding, was only captured recently by the Event Horizon Telescope<\/a> and published just last month.\n<\/p>\n \nAs for gravitational waves, it was Einstein himself who predicted their existence from his theory, also in 1916, but it would take another century to finally observe these fluctuations. Since 2015, the ground-based LIGO and Virgo observatories have assembled over a dozen detections, and gravitational-wave astronomy<\/a> is a burgeoning new field of research.\n<\/p>\n \nBut another of Einstein\u2019s predictions found observational proof much sooner: the gravitational bending of light, which was demonstrated only a few years after the theory had appeared, during a total eclipse of the Sun in 1919<\/a>.\n<\/p>\n \nIn the framework of general relativity, any object with mass bends the fabric of spacetime, deflecting the path of anything that passes nearby \u2013 including light. An artistic view of this distortion, also known as gravitational lensing, is depicted in this representation of two merging black holes.\n<\/p>\n \nOne hundred years ago, astronomers set out to test general relativity, observing whether and by how much the mass of the Sun deflects the light of distant stars. This experiment could only be performed by obscuring the Sun\u2019s light to reveal the stars around it, something that is possible during a total solar eclipse.\n<\/p>\n \nOn 29 May 1919, Sir Arthur Eddington observed the distant stars around the Sun during an eclipse from the island of Pr\u00edncipe<\/a>, in West Africa, while Andrew Crommelin performed similar observations in Sobral, in the north east of Brazil. Their results, presented six months later, indicated that stars observed near the solar disc during the eclipse were slightly displaced, with respect to their normal position in the sky, roughly by the amount predicted by Einstein\u2019s theory for the Sun\u2019s mass to have deflected them.\n<\/p>\n \n\u201cLights All Askew in the Heavens,\u201d headlined the New York Times in November 1919 to announce the triumph of Einstein\u2019s new theory. This inaugurated a century of exciting experiments investigating gravity<\/a> on Earth and in space and proving general relativity more and more precisely.\n<\/p>\n \nWe have made giant leaps over the past hundred years, but there is still much for us to discover. Athena<\/a>, ESA\u2019s future X-ray observatory, will investigate in unprecedented detail the supermassive black holes that sit at the centre of galaxies. LISA<\/a>, another future ESA mission, will detect gravitational waves from orbit, looking for the low-frequency fluctuations that are released when two supermassive black holes merge and can only be detected from space.\n<\/p>\n \nBoth missions are currently in the study phase, and are scheduled to launch in the early 2030s. If Athena and LISA could operate jointly for at least a few years, they could perform a unique experiment: observing the merger of supermassive black holes both in gravitational waves and X-rays, using an approach known as multi-messenger astronomy.\n<\/p>\n \nWe have never observed such a merger before: we need LISA to detect gravitational waves at the onset of the merger and tell us where to look in the sky, then we need Athena to observe it at high precision in X-rays to see how the mighty collision affects the gas surrounding the black holes. We don\u2019t know what happens during such a cosmic clash so this experiment, much like the eclipse of 1919 that first proved Einstein\u2019s theory, is set to shake our understanding of gravity and the Universe.\n<\/p>\n \nOn Earth, we deal with gravity every day. We feel it, we fight it, and we investigate it. Space agencies such as ESA routinely launch spacecraft against our planet\u2019s gravity, and sometimes these spacecraft borrow the gravity of planets to reach interesting places in the Solar System. We study the gravity field of Earth from orbit, and fly experiments on parabolic flights, sounding rockets and the International Space Station to examine a variety of systems under different gravity conditions. On the grandest scales, our space science missions explore how gravity affects planets, stars and galaxies across the cosmos and probe how matter behaves in the strong gravity field created by some of the Universe\u2019s most extreme objects like black holes. Join the conversation online this week following the hashtag #GravityRules<\/i>\n<\/p>\n