Daily Archives: 17 July 2014

Time travel model tests quantum theory

Even though the laws of physics seem to permit time travel, many physicists and non-physicists still worry about the paradoxes that arise when you try to change the past (for an example see… well, pretty much any movie involving time travel). But quantum mechanics can give a way out of the mess, and an experiment performed at the University of Queensland has tested what happens when you try this.

Although not a dinkum time machine, the experiment simulated the effect of sending a photon (a particle of light) back in time by using two photons, one acting as the past version and one as the future version, and having them interact (Martin Ringbauer, Matthew A. Broome, Casey R. Myers, Andrew G. White & Timothy C. Ralph 2014, “Experimental simulation of closed timelike curves”, Nature Communications 5, Article number: 4145, doi:10.1038/ncomms5145).

Diagram of the time travel simulation experiment
A diagram of the experimental set-up, in which two single photons, generated in a nonlinear β-barium-borate crystal, are sent by optical fibres into the top and bottom paths. The bottom photon represents the time traveller, which is polarised according to a theoretical quantum consistency condition. The two photons interact in the middle in a polarising beam splitter and then are detected by photo-diodes at the outputs (Image Ringbauer et al., via Nature)

To make it act like a time machine, they imposed theoretical conditions established in 1991 by David Deutsch (Deutsch 1991, “Quantum mechanics near closed timelike lines”, Physical Review D 44, 3197, DOI:10.1103/PhysRevD.44.3197 [PDF 4.7 MB]).

In Deutsch’s theory, any attempt to meddle with the past gives mixed results, i.e. a quantum mixture of meddled and non-meddled. These separate possibilities exist simultaneously, and are often interpreted as alternative timelines created by the act of travelling back in time.

There are other theories of time travel,  such as those that forbid any changes to the past that haven’t already happened, but Deutsch’s model is a particularly popular one.

The study of even a simulated time machine gives clues to how our understanding of physics may have to change to accommodate such bizarre circumstances.

To find out more about it, we spoke to one of the experimenters, PhD student Martin Ringbauer. Continue reading Time travel model tests quantum theory