Michele Maggiore discusses a Gedanken experiment to measure the area of a black hole's apparent horizon in quantum gravity. Using general and model-independent considerations, he derives a generalized uncertainty principle (GUP) that aligns with results from string theory. The GUP suggests a minimum length scale of the order of the Planck length, implying that the concept of a black hole is not operationally defined if its mass is smaller than the Planck mass. In classical general relativity, the apparent horizon cannot be directly measured, but in a quantum theory of gravity, Hawking radiation allows an observer to receive signals from the horizon, enabling a direct measurement. The radius of the horizon and the black hole's mass and charge become experimentally determinable quantities. However, there is an intrinsic limitation to the precision of measuring the horizon's radius. The GUP is derived by considering the uncertainty in measuring the horizon's radius due to two sources: the resolving power of the microscope and the discontinuous change in the horizon's radius during measurement. These uncertainties lead to a lower bound on the measurement error, which is proportional to the Planck length squared divided by the wavelength of the emitted photon. The GUP is interpreted as a more general principle governing all measurement processes in quantum gravity. It implies a minimal observable length and agrees with results from string theory. The GUP also suggests that the concept of a horizon is not defined at scales smaller than the Planck length, and that black holes with masses smaller than the Planck mass are not operationally defined. The GUP is consistent with the idea that the Compton radius and the Schwarzschild radius are mutually exclusive attributes of an object, applicable in different mass regimes. The generalized uncertainty principle is expressed as a lower bound on the measurement error, with the minimal observable length emerging naturally from a quantum theory of gravity.Michele Maggiore discusses a Gedanken experiment to measure the area of a black hole's apparent horizon in quantum gravity. Using general and model-independent considerations, he derives a generalized uncertainty principle (GUP) that aligns with results from string theory. The GUP suggests a minimum length scale of the order of the Planck length, implying that the concept of a black hole is not operationally defined if its mass is smaller than the Planck mass. In classical general relativity, the apparent horizon cannot be directly measured, but in a quantum theory of gravity, Hawking radiation allows an observer to receive signals from the horizon, enabling a direct measurement. The radius of the horizon and the black hole's mass and charge become experimentally determinable quantities. However, there is an intrinsic limitation to the precision of measuring the horizon's radius. The GUP is derived by considering the uncertainty in measuring the horizon's radius due to two sources: the resolving power of the microscope and the discontinuous change in the horizon's radius during measurement. These uncertainties lead to a lower bound on the measurement error, which is proportional to the Planck length squared divided by the wavelength of the emitted photon. The GUP is interpreted as a more general principle governing all measurement processes in quantum gravity. It implies a minimal observable length and agrees with results from string theory. The GUP also suggests that the concept of a horizon is not defined at scales smaller than the Planck length, and that black holes with masses smaller than the Planck mass are not operationally defined. The GUP is consistent with the idea that the Compton radius and the Schwarzschild radius are mutually exclusive attributes of an object, applicable in different mass regimes. The generalized uncertainty principle is expressed as a lower bound on the measurement error, with the minimal observable length emerging naturally from a quantum theory of gravity.