Tests of Einstein’s general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton’s theory1,2,3,4,5,6. Binary pulsars4 provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes7,8. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval9,10. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity11. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system12. In the absence of gravitational wave emission the outburst would have happened 20 days later13.
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This work is supported by the European community (project ENIGMA), the German Science Foundation, the Japanese Ministry of Education, Culture, Sports, Science and Technology, the Chinese Academy of Sciences, the Chinese National Natural Science Foundation, the Finnish Society of Sciences and Letters, the Finnish Academy of Science and Letters, the Jenny and Antti Wihuri Foundation, the Development Association Mansikka ry, and the municipality of Varkaus.
Author Contributions M.J.V. and H.J.L. were responsible for the interpretation of the data, K.N. and J.H. organized the observational fieldwork, and the other authors contributed data points.
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Valtonen, M., Lehto, H., Nilsson, K. et al. A massive binary black-hole system in OJ 287 and a test of general relativity. Nature 452, 851–853 (2008). https://doi.org/10.1038/nature06896
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