Gerard 't Hooft - Negative energies in quantum gravity

QGRAV2021 International Workshop on Quantum Gravity, Higher Derivatives & Nonlocality 8th to 12th March 2021 https://www.qgrav2021.com/​ https://www.qgrav2021.inpcs.net/ Abstract: Several approaches have been considered towards a coherent theory that attempts to reconcile gravitation with quantum mechanics. One often encounters the need to allow for negative energy states. Alternatively, one may introduce not only a lower bound for energy, providing us with a flat background metric being the vacuum state, but also an upper bound, giving us an `antivacuum' state. Vacuum and anti-vacuum are dual to one another, as we see in the Penrose diagram for eternal (or long-living) black holes. Such a model for the quantum black hole would solve the information problem and produce a unitary evolution law. Negative energy states also emerge in an attempt to employ local conformal symmetry to turn gravity into a renormalizable quantum field theory. An attractive feature of such a theory is that it might render all physical interaction constants to become determined and computable following renormalisation group equations. This would include all mass parameters and the cosmological constant. Yet negative energies generate new problems.