In quantum mechanics, the concept of an observer refers to a system that interacts with another system, thereby causing a collapse of its wave function and determining the outcome of measurements. However, when it comes to black holes, the situation becomes more complex.
The event horizon of a black hole is the boundary beyond which nothing, including light, can escape its gravitational pull. Within the framework of general relativity, once an object crosses the event horizon, it is effectively cut off from our observable universe.
In the context of black holes and quantum mechanics, the concept of the event horizon acting as an observer is often associated with the idea of black hole complementarity, proposed by theoretical physicist Leonard Susskind.
Black hole complementarity suggests that from the perspective of an observer falling into a black hole, the event horizon does not appear as a special place. Instead, the observer experiences a smooth transition through the event horizon without noticing any significant physical effects. This is known as the "frozen star" picture.
On the other hand, an observer situated outside the black hole would perceive the event horizon as a boundary beyond which information cannot be retrieved. According to this perspective, the event horizon plays the role of an observer in the sense that it effectively determines what information can or cannot be accessed by external observers.
It's important to note that the exact nature of how black holes and quantum mechanics reconcile is still an active area of research and subject to ongoing theoretical investigations. The concept of the event horizon acting as an observer is one proposed framework, but there are alternative interpretations and approaches as well, such as the firewall paradox and the holographic principle.