General relativity (GR) forms the backbone of big bang cosmology and provides a remarkably accurate description of the dynamics of the universe. Yet scientists expect future measurements to reveal discrepancies at some point because of GR’s incompatibility with quantum mechanics. A recently-discovered triple star system containing a binary white dwarf/pulsar orbited by another white dwarf (all confined to a region smaller than Earth’s orbit around the Sun) will allow astronomers to administer tests more sensitive than previously possible.
“Can you hear me now?” Now associated with a popular cellular provider commercial, this query represents the efforts of that company to test and improve their coverage. While holding a cell phone, the test man actively seeks out remote and challenging locations to demonstrate the superior reception compared to competitors. Scientists apply a similar method to test models—even highly successful ones like general relativity. A newly discovered triple star system will provide one of the most challenging and difficult tests yet.
When describing the central features of the theory of general relativity, scientists will often say that the laws of gravitation are independent of the motion or location of the experimental setup. Known as the equivalence principle (specifically the strong equivalence principle), this statement also implies that the gravitational effects on a body don’t depend on the nature or internal structure of that object. For example, a white dwarf typically contains a mass similar to the Sun inside a volume similar to Earth’s. In contrast, a neutron star also contains a mass similar to the Sun but with a diameter of about 12 miles! To appreciate the difference in these objects’ internal structure, consider their gravitational binding energy (the energy required to pull all the mass apart, out to an infinite distance): 0.1 for the neutron star and 10-6 for the white dwarf.
The triple star in question, J0337+1715, houses a white dwarf and neutron star in close orbit with a period of 1.6 days. A second white dwarf orbits these two objects with a period of 327 days. Thus, the outer white dwarf’s strong gravitational field provides a natural laboratory that allows astronomers to see if the inner white dwarf and neutron star behave as predicted by the equivalence principle.1
In theory, any three-body system can test the equivalence principle. Previously, scientists have used solar system bodies. However, the gravitational binding energy of the planets and moons ranges from 10-9 down to 10-11. The much larger difference in binding energies for J0337+1715 (five orders of magnitude compared to two for solar system bodies) and the larger absolute value for the neutron star (0.1 compared to 10-9) give significantly greater sensitivity to violations of the equivalence principle than any other tests done to date.
The neutron star in J0337+1715 is also a millisecond pulsar that emits strong, periodic radio pulses more than 300 times a second. Violations of the equivalence principle will result in specific signatures in the timing of these pulses. Astronomers have not acquired enough data yet to search for these signatures, but they continue making observations to remedy that deficit.
GR provides the basis for big bang cosmology, which also corresponds well to the Bible’s description of the universe. Yet scientists know that a more complete theory must exist because of GR’s incompatibility with quantum mechanics. Hopefully, these future tests will point in the direction of the more-complete theory and ultimately provide a better understanding of whether our universe arose by itself or if it owes its existence to a Divine Creator.
- S. M. Ransom et al., “A Millisecond Pulsar in a Stellar Triple System,” Nature, published online January 5, 2014, https://www.nature.com/nature/journal/vaop/ncurrent/full/nature12917.html.