If you toss a rock upward, it will curve back down, pulled back by the earth’s gravity. But if you throw a rock fast enough, it won’t curve back down to earth; it will zoom out of the earth’s atmosphere and escape the earth’s gravitational pull. This is roughly what NASA does when it sends a spacecraft to Mars. The minimum speed you need to throw the rock to enable it to escape is called, naturally enough, the escape velocity. Black holes are so dense that if you get too close—past the so-called event horizon—the escape velocity is faster than the speed of light. Past the event horizon the
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