Drew Brisbin, an astronomer working in a desert in Chile has this thought experiment for us.
Can we tap ionized particles in the interstellar medium as a way of exchanging momentum for propulsion?
His answer was shared on Centauri Dreams in a post titled, SWIMMERs: A Thought Experiment on the Potential and Limitations of Propellantless Interstellar Travel.
Parallel Spirals 