The grandfather paradox
Travel back in time and stop your grandparents meeting. Do you still exist?
Science fiction from the 1920s onwards; studied by physicists since the 1980s
Kabir's time machine
In 2061, Kabir builds a working time machine in his garage in Lucknow. He decides to test it with the boldest experiment he can think of. He will travel back to 1985, to the wedding where his grandparents first met, and spill tea on his grandfather's shirt so that he leaves before being introduced.
If it works, his grandparents never meet. His parent is never born, and so Kabir is never born. But then nobody builds the time machine, nobody travels back, and nobody spills the tea.
So his grandparents do meet. So Kabir is born, builds the machine, travels back and spills the tea. So they don't meet…
Kabir sits in the time machine, finger over the button, wondering which version of history he is about to land in.
The puzzle
If you change the past in a way that prevents your own trip, the trip both happens and doesn't happen.
See it for yourself
What's really going on
Physicists take this seriously because Einstein's general relativity seems to allow, in principle, paths through spacetime that loop back to the past (called closed timelike curves). So what would stop the contradiction?
One idea, the Novikov self-consistency principle from the 1980s, says the universe only allows consistent histories. Kabir can travel back, but something always goes wrong: he slips, the tea cup is empty, the wedding is moved. Another idea says time travel creates a new branch of history: Kabir changes a different timeline, and his own past is untouched. A third possibility is that nature forbids time machines altogether. Stephen Hawking called this the chronology protection conjecture.
Unsolved. Nobody has built a time machine, so nobody knows which, if any, of these answers is correct.
Think about it
If history can't be changed, would a time traveller still have free will?