Here's the story – our protagonist rewinds history, locates baby Hitler, and averts global war by putting him on a path to peace … but, oh noes! This sets off a domino chain of events that stops our hero from being born, or worse, kicks off the apocalypse.
Unintended 'butterfly effect'-style consequences of time travel might be a juicy problem in science fiction, but physicists now have reason to believe in a quantum landscape, tweaking history in this way shouldn't be a major problem.
Since going back to a previous moment in time is still in the 'too hard' basket, a pair of physicists from the Los Alamos National Laboratory in the US went with the next best thing and created a simulation using an IBM-Q quantum computer.
"On a quantum computer, there is no problem simulating opposite-in-time evolution, or simulating running a process backwards into the past," says theoretical physicist Nikolai Sinitsyn.
It's hardly as complex as a Universe of human actors and historical events, but a small stage made up of correlated quantum states is plenty for researchers to replay events with tiny changes to see how things play out.
"So we can actually see what happens with a complex quantum world if we travel back in time, add small damage, and return. We found that our world survives, which means there's no butterfly effect in quantum mechanics," says Sinitsyn.
Even if you've never heard of the butterfly effect, it's a common trope in time travel fiction you no doubt will have stumbled across. Ray Bradbury referred to it in his 1952 short story, A Sound of Thunder, by having a character change the future by simply stepping on a butterfly.
Outside of fiction, the physics of chaos theory contains its own reference to the fragile butterfly's powerful effect on history, famously framed in the title of a 1972 talk by MIT meteorologist Edward Lorenz, Does the flap of a butterfly's wings in Brazil set off a tornado in Texas?
Similar to Bradbury's doomed butterfly, chaos theory's version has far-reaching consequences that we'd have no hope of ever predicting thanks to the sheer complexity of knock-on effects.
But in our classical physics world, we may think of a domino chain. Each domino is discreet, and its actions are predictable (even if the ultimate impact of the entire chain grows beyond the reach of maps, models, and algorithms).
Quantum states play by an entirely different set of rules, and up until now, no one was sure how the butterfly effect would play out in a quantum world.
In this case, Sinitsyn and his colleague, Bin Yan, wanted to know what would happen if they rewound the interactions that entangle quantum waves of possibility – units of 'superposition' we call qubits – and then introduced the quantum analogy of a butterfly stomp. Would the future remain intact?
For those interested in the technical details, a number of entangled qubits were run through a set of logic gates before being returned to their initial setup.
Back at their starting point, a measurement was made, effectively turning its beautiful wave of 'maybes' into solid 'actuality', stomping out its superposition. The whole setup was then allowed to run again.
"We found that even if an intruder performs state-damaging measurements on the strongly entangled state, we still can easily recover the useful information because this damage is not magnified by a decoding process," says Yan.
Turns out our twitching butterfly is meaningless in the quantum world.
The researchers speculate that the qubit's tangled history isn't a delicate bunch of variables prone to disruption, but is instead exactly what preserves its future. The more complex its journey backwards in time, the more determined our time-travelling qubit is to return to the present with its information intact.
Before you get too excited about the implications of this research, let's be clear. This isn't a flux capacitor in the making, sorry. But it just might have some interesting applications in future quantum systems, perhaps as a way to test if they're still playing by quantum rules.
"We found that the notion of chaos in classical physics and in quantum mechanics must be understood differently," says Sinitsyn.
This research was published in Physical Review Letters.