The Grandfather Paradox is one of the most famous paradoxes of theoretical physics, challenging the logical consistency of closed timelike curves (time travel to the past). If a traveler visits the past and prevents their biological grandfather from having children, how could the time traveler ever be born to travel back in the first place?
In the mid-1980s, Russian astrophysicist Igor Novikov proposed the Novikov Self-Consistency Principle, asserting that the probability of any event causing a paradox is strictly zero. Under this view, only self-consistent histories are physically possible.
Alternatively, the Many-Worlds interpretation of quantum mechanics suggests that traveling back in time branches into an alternate parallel quantum branch, leaving the original timeline intact. Such paradoxes continue to inspire physicists in unraveling quantum thermodynamics and the true nature of time.