# Why can't supermassive black holes merge? (or can they?)

Supermassive black holes are usually found at the center of galaxies, including our own, and during a galaxy merger they end up beginning a dance of death, spinning around each other in a near-endless waltz, until finally merging. However, researchers are currently unclear as to the time it takes for black holes to merge -- or indeed, if they merge at all.

"It's a major embarrassment for astronomy that we don't know if supermassive black holes merge," said Jenny Greene, a professor of astrophysical sciences at Princeton and co-author of the study. "For everyone in black hole physics, observationally this is a long-standing puzzle that we need to solve."

This puzzle is dubbed the "final-parsec problem." Some astronomers believes that once two supermassive black holes get close enough together, reducing their distance to 1 parsec (3.2 light years), they may dance for an eternity.

Question: If it turns out that supermassive black holes can't merge, or have a tough time doing so, what might the reasons be?

• It's safe to use Newtonian approximations when the separation between the BHs is large. To get a rough idea of where relativistic effects become too large to ignore, use the Schwarzschild time dilation ratio $\sqrt{1-r_s/r}$, where $r_s$ is the Schwarzschild radius, and $r$ is the distance. – PM 2Ring Jul 12 '19 at 13:39