The grandfather paradox is just one of the thorny logical problems that arise with the concept of time travel. But one physicist says he has resolved them.

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 ...

Vanderbilt University physicist Lorenzo Gavassino isn't so sure this is as big an obstacle as time-traveler pessimists have ...