A painstaking 21-year study of a pulsar nearly 4000 light-years away has reinforced confidence in the universality of gravity. The observations, made by astronomers using the National Science Foundation’s Green Bank Telescope in West Virginia and the Arecibo Observatory in Puerto Rico, have provided the best constraints ever made outside of the Solar System on the value of G, the gravitational constant.
Pulsars, which have large masses and are yet only 20-25 kilometres in diameter, are the dense core remnants of stars that have undergone the supernova phase. As pulsars rotate, their magnetic poles sweep out across space and emit radio waves at an incredibly consistent rate, making them exceptional cosmic timepieces and perfect for studies of this kind.
The pulsar used in this experiment, creatively named PSR J1713+0747, is particularly bright and stable and follows a 68-day orbit around its white dwarf star companion. This unusually wide orbit minimises the effect that gravitational radiation – the conversion of orbital velocity to gravitational waves – can have on data, meaning that the timing of the radio wave emissions can be measured much more accurately than in cases where the orbital change is more significant.
Despite the mounting evidence, the universality of the gravitational constant cannot be taken for granted. According to study co-author Ingrid Stairs of the University of British Columbia in Canada, “theories of gravity that are different from general relativity often make such predictions [that there could be ‘special’ times or locations with different gravitational behaviour]”.
However, the results of this experiment support the idea that the gravitational constant is exactly that – constant. Scientists will continue to probe our understanding of the fundamental forces, but so far everything points to an unrelenting consistency of gravity across the Universe.