Like stretch marks left on skin that expanded too quickly or cracks embedded in freezing ice, cosmic strings are artifacts of what the universe looked like in the moments before it rapidly changed ...

From H. G. Wells's The Time Machine to Christopher Nolan's Interstellar, the possibility of travelling through time has fascinated people for centuries. But, although it sounds like pure science ...

The curious minds at What If examine overwriting Einstein’s theory of relativity with time travel, imagining paradoxes, causality, and cosmic consequences.

Contents -- Acknowledgments -- Preface -- Dreaming of time travel -- Time travel to the future -- Time travel to the past -- Time travel and the beginning of the Universe -- Report from the future -- ...