Two mathematicians have proved that a straightforward question—how hard is it to untie a knot?—has a complicated answer.

Speed worked out with Houston Rockets star Kevin Durant in a video he posted Wednesday. He did a series of combine drills to compare himself to prospects coming into the league. The 20-year-old ran ...

If it were to become a reality, hypersonic flight, long the realm of science fiction, could revolutionize global travel, ...