This new propulsion system could rewrite the rules of spaceflight—not to mention completely defy conventional physics.

After a widely reported set of recent studies all but killed the controversial EmDrive, the propulsion device’s inventor has fired back by pointing out what he calls critical errors in the research.

EmDrive is a new type of rocket engine that requires no propellant, a far-fetched dream to many people, but one that has been in the works since 1999 when it was first proposed by British scientist ...