| Summary: | Why we need to quantise everything, including gravity Quantum gravity—the theory that will merge quantum theory and general relativity—is yet to be found. Some physicist even claim that it is not needed after all: quantum theory and general relativity can coexist in their respective domains. In contrast, a powerful line of argument in favour of quantum gravity states that a gravitational classical system coupled to a quantum system must itself be quantum. These arguments have a long history, starting in the early days of quantum theory. Yet, they have limited applicability because they rely on restrictive assumptions. This paper proposes a radically different approach. The key feature is an information-theoretic characterisation of both the quantum and the classical systems, which makes the argument in the most general form ever proposed. Any system displaying quantum effects is capable of transmitting its quantum properties to the classical systems interacting with it via copy-like operations.
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