If you don’t like limitations, physics is the worst. Relativity tells us that we can’t travel back in time, while quantum mechanics tells us that we can’t know position and momentum at the same time. But if you can’t beat them, why not just pit them against each other?
A team of Chinese physicists believe they have found a workaround to quantum mechanics' most famous principle, the uncertainty principle, by using relativity’s quirkiest solution: wormholes. The scientists think they have found a way to calculate the position and the momentum of a quantum state simultaneously, by grabbing a copy of the quantum state directly from the past.
The study, published by Nature Quantum Information, relies on the concept of closed-timelike-curves (CTCs). Although controversial, CTCs are a valid solution of Einstein’s theory of general relativity and allow time-travel through a wormhole connecting two different points in spacetime.
They are problematic because they defy classical logic, and the grandfather paradox is one example of the peculiar problems generated by a CTC. If you were on a CTC, you could go back in time and kill your grandfather before your father was born, violating the law of cause and effect.
While CTCs are not possible at our scale, they are perfectly workable at quantum mechanics level. Physicist David Deutsch, who first proposed them, has come up with a set of CTCs that has self-consistent solutions even though they defy causality. There are also curves that don’t defy causality, called open-timelike-curves (OTCs).
Closed-timelike-curves (top) and open-timelike-curves (bottom) are two mathematical solutions that allow time travel. Xiao Yuan et al./ Nature Quantum Information
Now, the researchers used both CTCs and OTCs to go beyond the limits of quantum mechanics. They were able to construct a perfect cloning machine, so the states of each particle can be copied perfectly, thus allowing the researchers to measure all the properties of the quantum state.
While this is a purely theoretical approach, other scientists are trying to test CTCs. A paper in Nature Communication has simulated experimentally the violation of causality in CTCs, by modeling the consequences of sending a photon back in time and making it interact with itself.
According to this research, the probabilistic nature of quantum mechanics allows for interactions between the present and past version of the same particles, defying classical logic but confirming what theoreticians were expecting. A quantum you traveling back to kill your quantum grandfather has a chance in doing so.
The results from both studies have a large impact both in practical and theoretical applications. For example, by using CTCs quantum computers could be made even more powerful than we believe they will be.
The application of CTCs in quantum mechanics also indicates that general relativity and quantum mechanics can sometimes work together, which might be the way to work out a complete theory that includes both.
No CTC or OTC has yet been found, so our time travel dreams are still beyond us. But maybe one day soon, wormholes will be common in computers just like transistors are today.