The EM Drive

There has been a lot of hype lately concerning The EM Drive, a supposed microwave based thruster that needs no propellant and May "violate the laws of physics." Call me just a teeny bit skeptical:

NASA has tested an "impossible" electric space drive that uses no propellant – and found it works even when it is designed not to.

The system is designed to use microwave energy reflected along a specially designed chamber to produce thrust. The idea first appeared as the Emdrive by British inventor Roger Shawyer in 2001, who designed a motor that he showed could produce power in this way. But critics scoffed, saying it would violate the laws of momentum.

The EmDrive, we're told, generates thrust by using the properties of radiation pressure. An electromagnetic wave has a small amount of momentum which, when it hits a reflector, can translate that into thrust, Shawyer found, and this apparently can be used to power flight in the near-frictionless environment of space.

The idea languished, but a decade later the Chinese Academy of Sciences published a paper saying that it too had built an EmDrive-like which, when fed 2.5kW, generated 720mN of thrust – a tiny amount, admittedly.

But this got the attention of NASA boffins, who in 2013 commissioned a series of tests on the drive and got some surprising results.

In an eight-day trial held by US engineering firm Cannae, researchers found that by using a reflective chamber similar to that proposed by Shawyer, the team was able to use solely electrical input to generate 30 to 50 micro-Newtons of thrust. Again, incredibly tiny, enough to move a grain of sand, but apparently significant.

If it really works, maybe we could power it with cold fusion.

I guess I should mention that my skepticism is based on the notion that it violates conservation of momentum. My guess is that if it works, there is some momentum being transferred, probably to some nearby object, via quite conventional known physics.