three-body calculations suggest how black holes get close enough to merge
A statistical solution to the infamous three-body problem of classical physics could explain why the LIGO-Virgo gravitational-wave detectors have observed numerous black-hole mergers.
The three-body problem involves three classical objects (such as stars, planets or even black holes) orbiting and interacting with one another. In principle, the behaviour of a three-body system at a future time in its evolution is uniquely determined by the initial conditions of the system. However, infinitesimal changes in these initial conditions can accumulate over time to become huge differences in outcomes. As it is never possible to measure initial conditions with infinite precision, it is therefore never possible to use them to predict long-term outcomes – a signature of deterministic chaos.
A general closed-form solution of the three-body problem does not exist, but if the objects are very different in mass, the system can be approximated by two-body problems with small perturbations from the third object. Things become more daunting, however, when the three masses are similar. Now, astrophysicists in the US have built computer models of such “non-hierarchical” triple systems. Created by Nicholas Stone of the Hebrew University of Jerusalem (who did the work while at Columbia University in New York) and Nathan Leigh of University of Concepción in Chile, the simulations could help to explain why LIGO-Virgo have seen an abundance of gravitational-wave signals from merging black holes.
