Hi, thank you for your interest in black holes!

I should preface by saying that I will use quotation marks around some concepts. Relativity makes a lot of things behave strangely, and some concepts we may take for granted in many parts of physics suddenly are no longer as well-defined. However, they can still be useful to talk about to help with intuition.

The short answer is that ‘surface gravity’ will actually decrease as the black hole mass increases! We can do some quick math to show this. A black hole’s Schwarzschild radius, which is a length that represents its ‘size,’ is related to its mass, so we say R_S~M. Surface gravity is related to an object’s mass, but inversely proportional to the square of the radius, so we say g~M/R_S². Combining these together, we have g~1/M, so the surface gravity is inversely related to mass.

Incidentally, this also means that the tidal force is also weaker close to the event horizon, a structure that is similar to the black hole’s ‘surface.’ As a result, the perils often associated with black holes like spaghettification are less severe for more massive black holes. In principle, for a massive enough black hole, you could survive falling in and passing through its event horizon. (Spoiler alert!!) If you’re a fan of the movie, it may interest you to know that this is the rationale used in Interstellar to explain why Cooper is able to survive his fall into Gargantua. They briefly reference it in the dialogue!

If it interests you, when we talk about black holes, especially behavior close to their event horizons, gravity is better described through the framework of general relativity, in which it’s described as a bending of spacetime. Like a marble rolling in a funnel, the gravitational ‘force’ that you feel is a consequence of the curvature of space and time taking you along a certain path as you are trying to travel in a straight line. As such, surface gravity, which is used to describe the force of gravity, is not a concept that we would actually use to describe the motion of objects close to a black hole. However, the case I had made above for surface gravity can also be made for the curvature at a black hole’s event horizon. As the black hole becomes more massive, the curvature at its surface will become weaker. Analogies like this are common in relativity, so there are often cases where intuition from non-relativistic physics holds up!

-Nick