After the measurement, if result λ λ was obtained, the quantum state is postulated to collapse to λ λ → → , in the non-degenerate case, or to P λ λ ψ ψ / ⟨ ⟨ ψ ψ , P λ λ ψ ψ ⟩ ⟩ , in the general case. The probabilistic nature of quantum mechanics thus stems from the act of measurement. This is one of the most difficult aspects of quantum systems to understand. It was the central topic in the famous Bohr–Einstein debates, in which the two scientists attempted to clarify these fundamental principles by way of thought experiments. In the decades after the formulation of quantum mechanics, the question of what constitutes a "measurement" has been extensively studied. Newer interpretations of quantum mechanics have been formulated that do away with the concept of " wave function collapse " (see, for example, the many-worlds interpretation). The basic idea is that when a quantum system interacts with a measuring apparatus, their respective wave functions become entangled so that the original quantum system ceases to exist as an independent entity (see measurement in quantum mechanics).