"What happens when cause eliminates its own effect?"
The Scenario: You build a time machine and travel back to 1950, before your parents were born. There, you encounter your grandfather as a young man.
The Paradox: If you prevent your grandfather from meeting your grandmother, your parent would never be born—and neither would you. But if you were never born, you couldn't travel back in time to prevent anything. So did you prevent it or didn't you?
The Grandfather Paradox creates an impossible causal chain:
There's no stable state. Every outcome undermines itself!
Physics itself prevents paradoxes. No matter how hard you try, something will stop you—the gun jams, you slip, you can't find them. The universe enforces consistency.
When you travel back, you create a new timeline (World B). You can change that world, but your original timeline (World A) remains unchanged. You just can't return to it.
Whatever you do in the past was always part of history. Maybe your intervention is what caused your grandparents to meet! Your actions are predestined, not paradoxical.
You don't travel in your timeline—you travel to a parallel universe that happens to be 75 years behind. Nothing you do affects your original reality.
Physicist David Deutsch proposed that quantum mechanics resolves the paradox:
You leave Timeline A at the moment of departure. Your arrival in the past creates Timeline B. In Timeline A, you simply vanished in 2025. In Timeline B, a stranger appeared in 1950. Neither contradicts the other.
The grandfather paradox isn't just philosophy—it touches real physics:
The consensus: if time travel is possible, paradoxes are somehow prevented—either by physics, by branching timelines, or by constraints on what time travelers can do.
The Grandfather Paradox appears throughout science fiction, though often with creative solutions: