Theories of Gravity
For almost a century Einstein's general relativity (GR) has been the last word on gravity. However, we know it cannot be the ultimate theory of gravity, since it fails at the centres of black holes, where it predicts infinite densities and fields. Furthermore it appears to be incompatible with quantum mechanics. It is not clear at what energy scale GR ceases to be a good description of gravity. However, there are many alternative theories of gravity that predict deviations from GR at the sort of energy scales that prevail in the current Universe (i.e., much below the Planck scale). These theories tend to predict several subtle effects on the orbits of binary pulsars, particularly pulsars in asymmetric binaries (e.g., pulsars with white dwarf companions):
- Emission of dipolar gravitational waves. This would be detectable as an orbital decay larger than that expected from the emission of quadrupolar gravitational waves predicted by GR. So far, the experiments most sensitive to this phenomenon have not detected it.
- Violation of the Strong Equivalence Principle. In binary star systems consisting of a pulsar and a white dwarf and moving in the gravitational field of the Milky Way, this would be visible as a small temporal change in the orbital eccentricity. Certain aspects of the strong equivalence principle can be tested especially well with the help of pulsar PSR J0337+1715, which is part of a triple system together with two white dwarfs.
- Violation of Local Lorentz Invariance. This would be detectable as a small change in the orbital eccentricity of a binary pulsar with a tight orbit, and/or as free precession in isolated pulsars. The best limits are consistent with GR.
The investigation of these effects requires a much higher sensitivity than previous experiments. To achieve this, we need better accuracy in measuring the arrival times of the pulsar signals at the radio telescope ("pulsar timing"). A future verification of one of these effects would disprove GR and extend the physics beyond our previous understanding of the universe. On the other hand, the more accurately GR is confirmed, the more one can restrict or exclude many competing theories. For several of these alternative gravitational theories, pulsars provide the best tests, orders of magnitude better than other experiments, e.g. in the Solar System or with gravitational-wave detectors. This includes theories that have been developed as alternatives to dark matter and / or dark energy. Consequently, accurate testing of GR is important not only for our understanding of gravity and the laws of physics, but also for our understanding of the nature of our universe.
What are we doing about improving timing precision?
The precision of pulsar timing is presently limited by two factors:
- Statistics: pulsars are exceedingly faint radio sources, requiring the use of the world's largest radio telescopes.
- Systematics: as the Earth and the pulsar move, their line of sight is always sampling different regions of the interstellar medium which always have slightly differing electron densities.These unpredictable variations introduce an extra source of uncertainty in the measurement of the times of arrival of the radio pulses.