The Unchangeable Past
One proposed resolution to the grandfather paradox is that the past is fixed and unchangeable. This idea is formally known as the Novikov self-consistency principle, developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s. The principle applies to spacetimes that contain "closed timelike curves" (CTCs)—hypothetical paths through spacetime that loop back on themselves, which are permitted by Einstein's theory of general relativity.
According to Novikov's principle, any event that could cause a paradox has a probability of zero. If you traveled back in time to kill your grandfather, the laws of physics would conspire to make you fail. Your gun might jam, you could slip and fall, or the person you attack might turn out not to be your grandfather after all. The universe has a single, self-consistent history. You can participate in the past, but you cannot change it because your actions were always part of that history to begin with. This deterministic view ensures that no matter what a time traveler does, their actions will only fulfill the past as it already happened, not alter it.
The Branching Timeline
A second solution comes from the world of quantum mechanics, specifically the "many-worlds interpretation" (MWI). First proposed by physicist Hugh Everett in 1957, MWI suggests that all possible outcomes of a quantum event physically occur in separate, non-communicating parallel universes.
Applied to time travel, this theory suggests that your arrival in the past causes the timeline to branch into a new, separate universe. In this new timeline, you could successfully kill the man who is your grandfather's double. That universe would proceed along a new history where your parent, and consequently you, are never born. But this has no effect on your original timeline. You would continue to exist, but you would be stranded in a parallel reality that you created. Physicist David Deutsch applied MWI to time travel in 1991, proposing that a time traveler would simply end up on a different branch of history, neatly sidestepping any paradox.
The Protected Timeline
A third school of thought argues that time travel to the past is simply forbidden by the laws of physics. In 1992, physicist Stephen Hawking proposed the "chronology protection conjecture," which posits that the universe fundamentally prevents the formation of closed timelike curves accessible to macroscopic objects.
Hawking suggested that the very attempt to create or use a time machine would generate a feedback loop of vacuum fluctuations, a type of quantum radiation. The feedback would create an energy density so immense that it would destroy the wormhole or any other mechanism that would allow for backward time travel before it could be used. In a 1992 paper, Hawking humorously wrote that "it seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians." The conjecture states that any attempt to violate causality will fail because physics itself will intervene, not by making your gun jam, but by making the journey impossible in the first place.