Is time travel is real or rumour?

i personally think tat time travel jus a joke.. one famous astro physics expert from nasa told that time travel is impossible...by a simple theory....here it goes.. suppose let x n y be father n son.... y uses time machine n goes back to the time when his father y was a child....then he kills "y"..his father... then his father wud die in child stage itself n there is no possibility tat X may be born...then how cud he have travelled bac time..jus think... can you understand this...