Why Quantum Gravity Can't Calculate Anything Useful

Physics is a practical science, used to calculate useful quantities from given inputs. In fundamental physics, as theories become more reductionist, the inputs to these calculations tend to become increasingly restrictive. For instance, while classical physics calculates how objects move, particle physics derives this from the interactions of individual particles within the object. Extrapolating from this, we might expect quantum gravity to calculate something even more abstract.

However, there is a roadblock with quantum gravity: the union between relativity and quantum physics means that we don't yet know what the input to quantum gravity even is. On the one hand, relativity requires all real physical inputs to be independent of perspective. On the other hand, the many possibilities in quantum physics do depend on observers. This conflict essentially eliminates all reasonable candidates for observables, which means that a theory of quantum gravity might not be able to compute anything specifically relevant to our world.

So, how do we understand this paradox, and what should quantum gravity compute? We'll explore this in this episode.