Final answer:
When a particle-antiparticle pair forms at the event horizon of a black hole, one particle may fall in while the other escapes, leading to a decrease in the black hole's mass, a concept known as Hawking radiation.
Step-by-step explanation:
The phenomenon you're asking about involves quantum mechanics and gravity, and takes place near the event horizon of a black hole. When a particle-antiparticle pair is formed at the event horizon due to the black hole's gravitational field, one scenario is that one member of the pair falls into the black hole while the other escapes.
This results in a net loss of energy for the black hole, causing it to shrink over time. This process is part of what's called Hawking radiation, theorized by Stephen Hawking, and plays a significant role in the potential evaporation of black holes over extremely long timescales.