Non-Linear Quantum Mechanics

We add non-linear and state-dependent terms to the Hamiltonian of quantum field theory. The resulting low-energy theory, non-linear quantum mechanics, is causal, preserves probability and permits a consistent description of the process of measurement. We explore the consequences of such terms and show that non-linear quantum effects can be observed in macroscopic systems even in the presence of de-coherence. We will describe recent experimental efforts to measure effects which had otherwise been weakly bounded. We also expose a fundamental vulnerability of any non-linear modification of quantum mechanics - these modifications are highly sensitive to cosmic history and their locally exploitable effects can dynamically disappear if the observed universe has a tiny overlap with the overall quantum state of the universe, as is predicted in conventional inflationary cosmology. We identify observables that persist in this case and discuss opportunities to detect them, for example in tests of strong field general relativity and current probes of the equation of state of the universe.