In a 1969 paper, Roger Penrose wrote, “I only wish to make a plea for black holes to be taken seriously and their consequences to be explored in full detail. For who is to say, without careful study, that they cannot play some important part in the shaping of observed phenomena?”1 His success at getting researchers to consider black holes in earnest was acknowledged with half of this year’s Nobel Prize in Physics.

With decades of evidence, it may be hard to imagine that physicists ever doubted their existence. That evidence includes work by Reinhard Genzel and Andrea Ghez, who received the other half of this year’s prize (see the story on page 17), and the Event Horizon Telescope image released last year (see the Quick Study by Dimitrios Psaltis and Feryal Özel, Physics Today, April 2018, page 70). General relativity, which admits the possibility of...

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