It's a form of redshift effect in curved spacetime. Any massive object stretches the spacetime in a way that the spectrum of the light emitted by that object sh

It's a form of redshift effect in curved spacetime. Any massive object stretches the spacetime in a way that the spectrum of the light emitted by that object shifts down from the point of view of a remote observer (remote - this is important). The larger the mass, the stronger the effect. A black hole would create an infinite redshift (due to singularity inside of it) -- but only if infinitesimal EM frequencies were possible, which in turn would imply continuous spectrum. The energy of photons is quantized, therefore no truly infinite redshift is possible. The same applies to other particles. Basically it's a raster effect that becomes apparent when you zoom up on a magazine photograph, or equivalently -- when you stretch a photograph printed on a rubber sheet, the only difference is that you stretch spacetime, not just space.