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On blind dates, we search for others that resemble us, at least at some level. This is true in our personal life but even more so on the galactic dating scene, where we have been seeking a companion civilization for a while without success. While developing our own radio and laser communication over the past seven decades, the Search for Extraterrestrial Intelligence (SETI) focused on radio or laser signals from outer space—two kinds of electromagnetic “messenger” that astronomers use to study the cosmos.

Over the same period, we have been also launching probes, like Voyager 1 and 2, Pioneer 10 and 11 and the New Horizons spacecraft, towards interstellar space. These could eventually reach alien civilizations, passively announcing our existence. But in 1960, at the dawn of the space age, Ronald Bracewell noted in a Nature paper that a physical space probe could also search for technological civilizations across interstellar distances. SETI should therefore explore this technique as well—a timely notion in the era of multimessenger astronomy, ushered most recently by the detection of gravitational waves.

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