What is the grandfather paradox of time travel?

The grandfather paradox is a puzzle about cause and effect in backward time travel. A traveler goes into the past and kills their own grandfather before the grandfather has fathered the traveler's parent. The traveler then could never have been born, so could never have made the trip, so the grandfather would not have been killed. The effect cancels its own cause, which is why the puzzle is described as a problem for causality, the idea that a cause must precede its effect. [1] [2] [3]

The Stanford Encyclopedia of Philosophy calls it the most important objection to the logical possibility of backward time travel, and says it has convinced many people that such travel is impossible. The argument runs that changing the past is impossible, yet if a traveler reached the point of facing the grandfather with a loaded weapon, nothing would remain to prevent the impossible event. So the line must be drawn earlier, and the trip itself must be impossible. Isaac Asimov cited such paradoxes as a sign that true time travel is impossible, and Stephen Hawking proposed a chronology protection conjecture, under which the laws of physics prevent closed timelike curves from appearing. [4]

One response is that the past cannot be changed, so the traveler simply fails. Carl Sagan described this idea as nature contriving to stop the act: the gun jams, or the grandfather bends down to tie a shoelace. The Novikov self-consistency principle, which appears in one physics model of closed timelike curves, forbids paradoxes from arising at all. The Stanford entry treats mysterious chaperones or time guardians as hard to take seriously, but notes a better version from the philosopher David Lewis: the traveler fails for some commonplace reason, such as a jammed gun, a distracting noise or a banana peel. [5] [1] [4]

A second problem remains: the traveler seems both able and unable to kill the grandfather. Lewis answered that what someone can do depends on which facts are held fixed. Relative to the traveler's aim, skill and weapon, they can; relative to the fact that the grandfather survived, they cannot. This is an equivocation rather than a contradiction, and the Stanford entry says Lewis's solution has been widely accepted. Kadri Vihvelin disagrees, arguing that the statement that the traveler can kill the grandfather is simply false, because in worlds like ours the traveler would not succeed had they tried. [4]

Other possibilities are that traveling into the past leads to another timeline, or that physics prevents backward time travel entirely. One explainer stresses that the paradox itself cannot establish which of these, if any, is correct. Sagan also raised the idea of a new causal scheme in which events leading to the traveler's adult life differ. [6] [5]

The paradox shows that backward time travel cannot simply let someone alter an established past in ways that contradict the history they came from. It does not by itself prove time travel impossible. Whether it is possible depends on which resolution, if any, is accepted, and the search results do not show a single settled answer. [6] [4]