The deepest problem: some perspectives on quantum gravity

Understand

Quantum gravity is likely the deepest problem facing current physics.

• While traditionally associated with short distance nonrenormalizability, it is evident that the long distance problem of unitarity, arising at high energies with black hole formation, is more profound.
• This reveals a conflict between foundational principles of quantum field theory: those of quantum mechanics, relativity, and locality.
• Difficulties modifying quantum mechanics suggest a "quantum-first" approach, with other principles as mathematical properties of a quantum space of states.