Physicists at University of Queensland, Australia have simulated time travel using particles of light. The researchers achieved this by simulating the behavior of a single piece of light–a particle of energy–traveling on a closed timelike curve (CTC)–a closed path in space-time. The work may help to understand the longstanding problem of how time-travel could be possible in the quantum world and how the theory of quantum mechanics might change in the presence of closed timelike curves.
The work also shows how many effects, forbidden in standard quantum mechanics, may be possible inside a CTC and how light would behave differently depending on how it was created.
In the study, the research team simulated the behavior of a single photon that travels through a wormhole and interacts with its older self. This was achieved, PhD student Martin Ringbauer told The Speaker, by making use of a mathematical equivalence between two cases. In the first case, photon 1 “travels trough a wormhole into the past, then interacts with its older version.” In the second case, photon 2 “travels through normal space-time, but interacts with another photon that is trapped inside a CTC forever” (as shown in the illustration at top of the article). “Using the (fictitious second case) and simulating the behavior of photon 2, we were able to study the more relevant case 1,” said Ringbauer.
“We used single photons to do this,” said UQ Physics Professor Tim Ralph, “but the time-travel was simulated by using a second photon to play the part of the past incarnation of the time travelling photon.”
The paper, “Experimental Simulation of Closed Timelike Curves,” was completed by University of Queensland’s Dr Matthew Broome, Dr Casey Myers, Professor Andrew White, in addition to Professor Ralph and Martin Ringbauer, supported by the Australian Research Council Centre of Excellence for Engineered Quantum Systems and Centre of Excellence for Quantum Computation and Communication Technology, and was published in Nature Communications.
In the team’s press briefing, Ringbauer commented on the relationship between the theory of general relativity and another important–but conflicting–theory, quantum mechanics. Time travel is thought to potentially help understanding the gap between the two schools of thought.
“The question of time travel features at the interface between two of our most successful yet incompatible physical theories – Einstein’s general relativity and quantum mechanics,” said Ringbauer.
Time travel in the quantum world may avoid general relativity paradoxes such as the grandparents paradox–a timetraveller preventing his grandparents from meeting and so preventing his own time travel.
The authors of the study believe that such paradoxes can be resolved in a quantum regime, because a quantum model of closed timelike curves–such as traversable wormholes–can be formulated consistently with relativity”
Ringbauer explained the concept to The Speaker this way: “General relativity predicts the existence of closed timelike curves (e.g. by following a path through a wormhole that connects two different temporal locations in space-time). This would allow travel back in time. In the classical world this is unlikely to be possible, since it causes paradoxes, such as the grandfather paradox. In the quantum world, however, these paradoxes are resolved and time-travel can be formulated in a self-consistent way.”
Part of the reason time travel could be freed from such paradoxes in the quantum world is that the properties of quantum particles are “fuzzy” and “uncertain,” and therefore there is “wriggle room” to avoid inconsistencies in such situations, according to Professor Ralph.
Although Ralph said that there was no evidence that nature behaved differently than the laws of standard quantum mechanics, it had not been tested in vastly different environments, such as near black holes, where the extreme effects of general relativity play a role.
This is the value of the study, said Ralph. “Our study provides insights into where and how nature might behave differently from what our theories predict.”
“We see in our simulation (as was predicted in 1991),” Ringbauer stated, “how many effects become possible, which are forbidden in standard quantum mechanics. For example it is possible to perfectly distinguish different states of a quantum system, which are usually only partially distinguishable. This makes quantum cryptography breakable and violates Heisenberg’s uncertainty principle. We also show that photons behave differently, depending on how they were created in the first place.”
By Justin Munce