system
Given the following post, respond as if you're commenting on Reddit:
human
Could an object orbit within the event horizon of a supermassive black hole?

Since we theorise that tidal forces exerted by gravity just inside the event horizon of a supermassive black hole would have a negligible effect on matter, could a space station orbit for 500 years with people living their everyday lives without noticing the effects of the black hole. If such an orbit were possible, would it last indefinitely?
gpt
No, it's not possible.  Inside the event horizon of a black hole all particles are forced to move inward radially in an analogous way to how all particles must move forward in time outside the event horizon.  So no orbits are possible.  There are actually well-defined limits to where orbits are possible or not.  The most famous of these is called the innermost stable circular orbit (ISCO).  For a non-spinning black hole, the ISCO is 3 times larger than the Schwarzschild radius (the distance.  So it's actually not possible to orbit the black hole in some places outside the event horizon!  For a spinning black hole, the ISCO actually matches the event horizon at the equatorial plane if you're orbiting in the same direction that the black hole is spinning.

There are a few other important distances, one example of which is the photon sphere, where photons can travel in a ~~stable~~ circular orbit.  That turns out to be 1.5 times the Schwarzschild radius for a non-spinning black hole.

EDIT: As sto-ifics42 pointed out, orbits at the photon sphere are not stable.  The photon sphere is the point closest to the black hole for which a circular orbit is possible.  Another interesting distance is the so-called marginally bound orbit at twice the Schwarzschild radius.  Circular orbits here are unstable because they are below the ISCO.  This means that any small perturbation gets amplified exponentially.  For particles at or below the marginally bound orbit, they have enough energy for these perturbations to cause them to escape to infinity!