Two blind spots torture physicists: the birth of the universe and the center of a black hole. The former may feel like a moment in time and the latter a point in space, but in both cases the normally interwoven threads of space and time seem to stop short. These mysterious points are known as singularities.
Singularities are predictions of Albert Einstein’s general theory of relativity. According to this theory, clumps of matter or energy curve the space-time fabric toward themselves, and this curvature induces the force of gravity. Pack enough stuff into a small enough spot, and Einstein’s equations seem to predict that space-time will curve infinitely steeply there, such that gravity grows infinitely strong.
Most physicists don’t believe, however, that Einstein’s theory says much about what really happens at these points. Rather, singularities are widely seen as “mathematical artifacts,” as Hong Liu (opens a new tab), a physicist at the Massachusetts Institute of Technology, put it, not objects that “occur in any physical universe.” They are where general relativity malfunctions. The singularities are expected to vanish in a more fundamental theory of gravity that Einstein’s space-time picture merely approximates — a theory of quantum gravity.
But as physicists take steps toward that truer and more complete theory by merging general relativity and quantum physics, singularities are proving hard to erase. The British mathematical physicist Roger Penrose won the Nobel Prize in Physics for proving (opens a new tab) in the 1960s that singularities would inevitably occur in an empty universe made up entirely of space-time. More recent research has extended this insight into more realistic circumstances. One paper established (opens a new tab) that a universe with quantum particles would also feature singularities, although it only considered the case where the particles don’t bend the space-time fabric at all. Then, earlier this year, a physicist proved (opens a new tab) that these blemishes exist even in theoretical universes where quantum particles do slightly nudge space-time itself — that is, universes quite a bit like our own.
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