A key prediction of Einstein's general theory of relativity - the 'Shapiro delay' - has been observed following highly accurate radio studies of a nearby pulsar. The theory forecasts that pulses of radiation streaming across the Universe should be impeded by relativistic distortions of space. Willem van Straten of Swinburne University of Technology in Australia and colleagues detected this effect while mapping the motion of the pulsar at the Parkes Observatory (W van Straten et al 2001 Nature 412 158).
Pulsars are rapidly spinning neutron stars that earn their name from the beams of radiation they emit, which appear as pulses to a stationary observer. Binary pulsars – systems in which a pulsar orbits another object – emit very regular pulses of radiation because their orbital and rotation periods are extremely regular. These properties make them excellent tools for probing the effects of general relativity.
At 450 light years from Earth, PSR J0437-4715 is the closest known binary pulsar, in which a pulsar orbits a white dwarf. Its proximity allows astronomers to measure its radio output from different angles, due to its motion relative to the Earth. Using this geometrical method, van Straten and colleagues calculated the orbit of the pulsar in three-dimensions. This also revealed the centre-of-mass of the system, from which the team calculated the masses of the pulsar and the white dwarf.
Next, van Straten and colleagues studied variations in the pattern of the radio pulses arriving on Earth. If the pulses emitted throughout the orbital period of the pulsar travelled through equivalent regions of space on their journey to Earth, they should all arrive at equal intervals.
But after eliminating geometrical effects, van Straten’s team found that the pulses took longer to arrive when the plane of the binary system was in their line of sight. Conversely, when viewing the plane ‘face on’, the pulses were not delayed. This phenomenon arises because when the plane is ‘edge on’, the signal from the pulsar passes through a region of space that is distorted by the gravity of the white dwarf. This distortion means that the signal takes a longer route to Earth, and therefore arrives later. This is the Shapiro delay.
General relativity also states that binary systems should gradually slow down, and emit the excess rotational energy as gravitational waves. This predicted increase in orbital period has been observed in previous experiments, but attempts to detect gravitational waves have so far failed. The length of the observed Shapiro delay was consistent with the amount of energy the binary system should be losing through gravitational waves.
“To our knowledge, this verification of the predicted space-time distortion is the first confirmation – outside the solar system – in which the orbital inclination was determined independently of general relativity”, say the authors.
The study also means that pulsar PSR J0437-4715 has the most accurately known location of any astronomical object. Now this system is known to exhibit the Shapiro delay, it is likely to be the subject of many more cosmological studies.
