Which of these bizarre phenomena, if any, can really exist in our universe? Black holes, down which anything can fall but from which nothing can return; wormholes, short spacewarps connecting regions of the cosmos; singularities, where space and time are so violently warped...
Ever since Einstein's general theory of relativity burst upon the world in 1915, some of the most brilliant minds of our century have sought to decipher its mysteries. Some of them--like black holes and time machines--are so unthinkable that Einstein himself rejected them.