When the first pulsar signal was detected in the 1960s, for a short time it was referred to as "little green man 1", because the regular pulsing appeared to be a message from aliens. This regularity of pulsar signals means that these rapidly rotating neutron stars can be thought of as the most reliable clocks in the heavens. In this short film, we meet astronomers at the Jodrell Bank Observatory near Manchester in the UK to learn about an exciting application for these cosmic clocks – studying pulsar signals in the hunt for gravitational waves.

Gravitational waves are often referred to as ripples in space–time. "It's something that's most easily detected when you have massive objects, for example orbiting each other," explains Ben Stappers of Manchester University. "That generates a so-called ripple in space–time; a bit like if I threw a stone into a pond, you see the water ripples." Stappers is involved in a new project called the European Pulsar Timing Array (EPTA) that is looking for subtle variations in the arrival times of pulsar signals that could be the result of gravitational waves between the pulsar source and the receiver on Earth. The international project combines pulsar signals collected at radio-telescope facilities in the UK, the Netherlands, Germany, France and Italy.

The EPTA is not alone in looking for gravitational waves, as others are pursuing the same goal. One such experimental collaboration, who use the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope, claimed in March this year that it had found evidence for primordial gravitational waves because it had seen the polarization signal within the cosmic microwave background (CMB). Stappers comments on these results, saying that it encourages radio-astronomy community to speed things up and make a detection for itself. However, since the making of our film, which was recorded in April, significant doubt has been cast on the BICEP2 results because of inconsistencies with results collected by the Planck satellite, a space telescope that measures the CMB in detail.

If nothing else comes from the BICEP2 commotion, it has at least raised the public profile of the hunt for gravitational waves. You can find out more about how pulsars can help in this pursuit by reading the October issue of Physics World. Details of how to access the issue are here.