Electromagnetic drive was originally proposed by Roger Sawyer already in 1999. It works by feeding light at microwave lenghts into a cavity where it bounces around and creates thrust. Some scientists have claimed they have succesfully created observable thrust with this method. Unfortunately, since there has not been any detected exhaustion this seems to break Newton's third law: everything has an equal and opposite reaction. It has naturally been deemed more probable that the observed thrust has been due to interference in the measuring equipment than a positive result that breaks a fundamental law of physics.
Now, a paper has been published
that shows how the drive could actually work without breaking this law
. The paper suggests that the exhaust is photons coming out in pairs that are out of phase. This means that these pairs cancel each other's electromagnetic signature out and they are not detected with the commonly used equipment. An interferometer could be used to detect these photons and if they are successfully detected it will result in people starting to take the engine seriously and probably put serious resources in developing it further.
This is one of those ideas that could revolutionize space exploration if
anything usable comes out of it. An EM drive doesn't need the huge and heavy fuel supply that the current drives use and is therefore much faster. For example, according to Sawyer's calculations the trip to Mars could take only 70 days.