Top Questions That Keep Physicists Awake at Night

We all have things that keep us up at night, as we try to solve the problems in our lives. But just think of the poor physicists: They are trying to solve the problems of the Universe! At a recent physics conference at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, a panel of scientists were asked what questions in physics kept them awake at night. Here are their answers:
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