The Breakthrough Starshot program has gotten a lot of press for planning to accelerate tiny (a few grams) probes to 10-20% C with powerful laser pulses, but a new idea proposes building a giant particle accelerator in space to push much larger probes (up to 1000kg) up to speed with a relativistic electron beam. Such a setup might get a large-ish probe to Alpha Centauri within 40 years.