Gravitational Waves and Perspectives for Quantum Gravity

Understand

Understanding the role of higher derivatives is probably one of the most relevant questions in quantum gravity theory.

• Already at the semiclassical level, when gravity is a classical background for quantum matter fields, the action of gravity should include fourth derivative terms to provide renormalizability in the vacuum sector.
• The same situation holds in the quantum theory of metric.
• At the same time, including the fourth derivative terms means the presence of massive ghosts, which are gauge-independent massive states with negative kinetic energy.