The paper discusses a Gedanken experiment to measure the area of the apparent horizon of a black hole in the context of quantum gravity. The author, Michele Maggiore, explores a generalized uncertainty principle that emerges from model-independent considerations, similar to results obtained in string theory. This principle indicates that a minimum length, on the order of the Planck length, naturally arises in any quantum theory of gravity. The concept of a black hole is not operationally defined for masses smaller than the Planck mass. The study uses Hawking radiation as a key physical ingredient and examines the precision limits of measuring the radius of the apparent horizon. The results suggest that the concept of a horizon is uncertain and not defined at scales smaller than the Planck length, implying that black holes with masses below the Planck mass do not exist operationally. The generalized uncertainty principle is consistent with string theory predictions and implies a minimal observable length. The paper also introduces a function \( R_*(M) \) that interpolates between the Compton radius for small masses and the Schwarzschild radius for large masses, further supporting the idea that the concept of a horizon is not well-defined below the Planck scale.The paper discusses a Gedanken experiment to measure the area of the apparent horizon of a black hole in the context of quantum gravity. The author, Michele Maggiore, explores a generalized uncertainty principle that emerges from model-independent considerations, similar to results obtained in string theory. This principle indicates that a minimum length, on the order of the Planck length, naturally arises in any quantum theory of gravity. The concept of a black hole is not operationally defined for masses smaller than the Planck mass. The study uses Hawking radiation as a key physical ingredient and examines the precision limits of measuring the radius of the apparent horizon. The results suggest that the concept of a horizon is uncertain and not defined at scales smaller than the Planck length, implying that black holes with masses below the Planck mass do not exist operationally. The generalized uncertainty principle is consistent with string theory predictions and implies a minimal observable length. The paper also introduces a function \( R_*(M) \) that interpolates between the Compton radius for small masses and the Schwarzschild radius for large masses, further supporting the idea that the concept of a horizon is not well-defined below the Planck scale.