You already know Einstein’s special relativity lets you zip forward in time. Just move fast enough, and the clock ticks slower for you than for everyone standing still. But the general theory? That’s where the math gets weird. It opens doors to the past.
Kurt Gödel, a logician and Einstein’s peer, found one such door in 1949. He made it a birthday present for his friend. The model was strange. A universe filled with dust-like particles moving like a thick liquid. Negative cosmological constant. It allowed for “timelike curves.”
Here is the kicker: You could travel along one of those curves. Start in one spot. Loop backward. End up exactly where you began.
Physicists initially dismissed these curves as mathematical junk. Unphysical artifacts of bad math. They assumed real universes wouldn’t support them.
Then Kip Thorne showed they were wrong.
In 1988, the future Nobel laureate found a new solution. Traversable wormholes. And they didn’t require a bizarre, dust-filled cosmos. They could exist in a realistic universe. This changed everything. Suddenly, time travel wasn’t just a logic puzzle. It was a physics problem with potential solutions.
Can you actually kill your grandfather?
The grandfather paradox is the elephant in the room. A writer named René Barjavel posed it in 1944. Go back. Kill your grandfather. Your father is never born. You are never born. How did you go back to kill him?
Russian physicist Igor Novikov tried to tame this beast. His self-conformity principle says you can go back, but you can’t change the story. You are part of the script. If you try to shoot your grandfather, the gun jams. You trip. You miss. The future remains exactly what it was.
You’ve seen this trope everywhere. Doctor Who. 12 Monkeys. The German series Dark. Characters try to fix a broken past. Their attempts create the break. It’s a closed loop.
Thorne and his team put this to the test. Not with people. With a ball.
Imagine a ball rolling into a wormhole. It comes out in the past. There, it collides with its own younger self, knocking the younger version away from the entrance. If the younger self never enters, the older self never arrives. Paradox.
The question for Thorne’s group in 1991 was simple but terrifying. Is there any starting position and speed for the ball where this loop cannot resolve?
They did the calculations. The answer was no.
For every initial condition, there was a self-consistent outcome. The ball always hits its past self in a way that ensures the loop holds. Thorne’s team couldn’t find a setup where the ball definitively prevents itself from entering. They couldn’t prove it mathematically for all cases. But their models strongly suggested self-consistent time travel is always possible.
There’s a complication, though. In some scenarios, infinite solutions existed. Which one happens? Nature doesn’t care. So Thorne brought in quantum mechanics. The ball exists in a superposition of all possible paths. It exits at every valid speed simultaneously. Even with the quantum fog, the wormhole physics held up.
Hawking’s empty party
Stephen Hawking wasn’t convinced.
Theoretical solutions were fine for math papers. They didn’t mean the universe would allow it. In 1992, Hawking proposed the “chronology protection conjecture.” Roughly translated: The laws of physics have a bouncer. And it won’t let you in.
Some fundamental theory, yet to be discovered, would prevent macroscopic time travel. Closed timelike curves would be forbidden. The universe keeps its history safe.
Hawking even made a joke out of it. He threw a party for time travelers. June 28, 2010. Champagne. Canapés. Balloons.
But he didn’t send the invitations until after the party was over.
The guest list was empty. No one showed up.
To Hawking, that was proof. If time travel were possible, someone would have seen the invite and come back. They didn’t. Therefore, it’s impossible.
Is he right? The math says no. The paradoxes suggest no. But the wormholes? They whisper that maybe, just maybe, the bouncer is asleep. Or maybe the bouncer doesn’t exist at all. We’re still waiting for the rest of the guest list.
