Time Travel as a Means of Hacking Computers

In physics, time travel is technically allowed by the theory of relativity. The results that demonstrate time travel are called closed timelike curves (or CTC), which are paths that begin and end at the same coordinates in spacetime.

Though technically allowed, the closed timelike curves in relativity do create all sorts of problems for physics, because the initial conditions are no longer static. All predictivity seems to be lost. David Deutsch was able to describe a way to avoid this in 1991, by explaining how the particles begin traveling in a loop that causes them to interact in the initial conditions in exactly the same way they "originally" did, creating sort of a "Groundhog Day" effect.

According to a new paper in Physics Review Letters, it looks like there's a new addition to the problems caused by closed timelike curves: they can be used to break potential quantum encryption techniques. These systems involve using a quantum-mechanical system to encrypt information in a computer system. Here's how the Physics Review Focus newsletter describes the new findings:

Todd Brun of the University of Southern California in Los Angeles and his colleagues have now found a way to use states defined by the Deutsch formulation to decode quantum-encrypted messages. Such a message could be sent as a series of particles, each in quantum state "zero," quantum state "one," or a combination state called a superposition. The intended recipient measures each particle but needs additional information after-the-fact from the sender to distinguish the superpositions from the non-superpositions. But a spy who could distinguish "on the fly" between, say, a zero and a superposition state could intercept the message and also send particles to the recipient that mimic the originals, thereby avoiding detection.

For the spy to accomplish this, the researchers imagine a particle entering a CTC so that it travels around and back in time, allowing it to interact with its future self, so to speak, before going on its way again. They describe an interaction that, in the simplest example, leaves a particle in the zero state unchanged but transforms a superposition of zero and one into a pure one state. A standard measurement by the spy that distinguishes one from zero can then reveal with complete certainty whether the initial state was zero or a superposition.

Ordinarily such a transformation wouldn't be possible without advanced knowledge of the incoming state. The trick, Brun explains, is that the particle interacts with the transformed version of itself that comes back from the future. Brun says the scheme doesn’t violate any laws of physics, but he admits that the logic is hard to grasp. Compared with regular chronological reasoning, he says, "it's definitely cheating."

Related Articles:

• Physical Review Focus - Time Travel Beats Quantum Mechanics
• Physics Review Letters - Localized Closed Timelike Curves Can Perfectly Distinguish Quantum States by Todd A. Brun, Jim Harrington, and Mark M. Wilde
• Physics Review D - Quantum mechanics near closed timelike lines by David Deutsch
• What are Quantum Computers?