This, from Wired on July 31, was typical: "Nasa validates 'impossible' space drive." Or this, from ExtremeTech on August 1, "NASA tests ‘impossible’ no-fuel quantum space engine – and it actually works."
The (supposed) space drive at issue bounces microwaves around a specially shaped chamber, and in the process is said (somehow) to produce a net thrust in one direction. The nature of the impossibility? That the drive -- if it works as advertised -- violates conservation of momentum.
Physics has gone a long time with every bit of evidence showing momentum is conserved. Always.
|Space drive: old school|
Newer, ion thrusters forgo chemistry but exploit the same action/reaction mechanism. That is, they use electromagnetic fields to propel Xenon ions. In both cases, matter ("reaction mass") is expelled from the spacecraft. Momentum is conserved.