# Can tides be raised on a black hole?

A tongue-in-cheek answer on Worldbuilding got me wondering: what happens to the shape of a black hole when other large masses happen near? The idea of distorting it is not completely out of the question since a rotating black hole will become oblate and develop an ergosphere.

A later, more serious question concerns the shape of the horizon with two black holes near each other.

If you take a simple model of a potential field, the isosurface of a given escape velocity will show tidal bulges. But with the connection between black hole surface area and entropy, I wonder how it can be changed without affecting some innate property of the BH, which an external object is not.

• Firstly, there is a singularity in the black hole. That is not a shape. The event horizon (which is just outside the black hole, or is it considered a part of it?), however, gets appropriately modified depending on the masses nearby. But that modification to the shape is almost negligible, unless there is another black hole nearby, in which case the black holes will eventually merge. – Kalpak Gupta Mar 15 '17 at 9:07
• @KalpakGupta that should be posted as an answer. – Asher Mar 15 '17 at 17:33
• It could be expanded into an answer. Since he’s saying yes, how does that work with the increased area being a measure of entropy? – JDługosz Mar 15 '17 at 20:02
• It does get distorted, negligible or not it changes something in the shape of the horizon. For an incoming particle of some mass it can cause a small sort of bubble to form, and as the particle leaves or gets absorbed it'll then straighten out. If it is a Kerr BH the particle could add or even take away (through the Penrose process) angular momentum and mass (there is a way to extract energy from a BH). And yes the horizon area has to remain the same or grow as it is proportional to the entropy. I am not sure what happens with a an N body system that is semi stationary, GR is hard for N bodies – Bob Bee Mar 16 '17 at 2:31
• Most calculable even if numeric work in this area is black hole perturbation theory. – Bob Bee Mar 16 '17 at 2:33