Signatures of the extra dimensions required by string theory could be seen by future gravitational-wave detectors. That is the conclusion of David Andriot and Gustavo Lucena Gómez at the Max Planck Institute for Gravitational Physics in Germany, who have identified two ways in which gravitational waves could be affected by the consequences of string theory – a theoretical framework that invokes speculative concepts such as extra dimensions to try to fill important gaps in our understanding of physics, including the nature of quantum gravity.

Gravitational waves are ripples in space–time that are created when massive objects are accelerated under certain conditions. Andriot and Lucena Gómez calculate that adding N extra dimensions to 4D space–time results in a “breathing-mode” oscillation that would be present in a gravitational wave. The second distinct feature of extra dimensions, say the researchers, is a discrete set of higher-frequency signals accompanying a gravitational wave.

Third detector

The first gravitational-wave detection was made in 2015, when the LIGO observatory spotted a signal from a coalescing binary black hole. Andriot and Lucena Gómez say that LIGO’s current configuration of two detectors will not be able to detect the breathing mode. However, it is possible that a breathing mode could be detected once a third detector in Italy (called Virgo) reaches its full sensitivity in 2018.

As for the higher-frequency signals, Andriot and Lucena Gómez point out in the Journal of Cosmology and Astroparticle Physics that the trend in future detectors is towards lower frequencies, and therefore a special observatory would be needed to see that effect.