The Grandfather Paradox
If you traveled back in time and killed your grandfather before your parent was born, you would never exist and therefore never travel back to kill him. So did he die or not?
This paradox, popularized in the early 20th century, is the central challenge for any theory of time travel that allows changes to the past. Every proposed resolution reveals something deep about the nature of time, causality, and whether the future is fixed.
Why it matters
The grandfather paradox isn't really about grandfathers. It's about causal loops and whether the past can be changed. Any version of time travel that allows retroactive causation produces some form of this paradox.
The problem has three escape routes, and each commits you to a different picture of reality.
Resolution 1: time travel is simply impossible
The paradox shows that backward time travel is logically incoherent. If time travel implies contradictions, it can't exist. Our physics seems to support this: closed timelike curves (the physicist's version of time travel) are technically allowed by general relativity but appear to require conditions that can't be created in practice.
Resolution 2: the past is immutable (consistent histories)
You can travel to the past, but you can never change it, because it already happened. If you go back and try to kill your grandfather, something will always prevent you. You'll trip, misfire, have a change of heart. The past you arrive in is already a past that included your visit without killing him.
Physicist Igor Novikov formalized this as the "self-consistency principle." Time travel is allowed, but only self-consistent, paradox-free histories can exist.
Resolution 3: parallel timelines
When you travel back and kill your grandfather, you split into a parallel timeline. In that branch, you were never born. That's fine, because you arrived from a different branch where you were. There's no paradox, only branching.
This is the many-worlds approach to time travel. It's internally consistent, though it multiplies timelines.
If you could travel back in time but couldn't change anything, if you'd be a ghost watching events unfold, would you still want to? And if you could change things, what would you trust yourself not to ruin?