NASA EmDrive Paper Peer Reviewed

'EmDrive works just like any other engine,' Dr Arto Annila, physics professor at the University of Helsinki and lead author of the paper, told MailOnline.

'Its fuel is the input photons at microwave lengths.'

The researchers suggest the photons coming out of the machine interfere with each other, so that the overall effect seems as if nothing is there. 

'In the cavity the input photons will bounce back and forth, and invariably some of them will interfere completely destructively. Then the two photons will be exactly 180 degrees out phase. At the complete interference electromagnetic fields for the two photons will cancel exactly, but the photons themselves continue to propagate.'

The idea is the same as water waves travelling together, at the exact right time so that the crest of one wave is exactly at the trough of the other and cancelling each other out. 

The water does not go away, it's still there. In the same way the pairs of photons are still there and carrying momentum even though they cannot be seen as light. The paired photons without net electromagnetic field will escape from the cavity,' Dr Annila said. 'This efflux of paired photons is the exhaust of EmDrive.'

'When the cavity is asymmetric, like the tapered cone, the efflux of paired photons is also asymmetric. Therefore the loss of momentum carried by the paired photons is uneven. In other words, thrust is non-zero.'

Dr Annila came up with the idea along with Dr Erkki Kolehmainen, an organic chemistry professor at the University of Jyväskylä and Patrick Grahn, a multiphysicist at engineering software firm Comsol. 

'Thrust without exhaust is of course impossible,' the authors wrote in their. 'Yet, certain resonant cavities, when fueled with microwaves, deliver thrust without apparent exhaust.' 

Their theory suggest the exhaust produced by the EmDrive is there, but just cannot be seen. 

Dr Annila said the photons could theoretically be detected by an interferometer, the same instrument used to detect gravitational waves.

'My gut feeling is that it will be very difficult to detect such a small excess in energy density, especially when operating EmDrive steadily,' he said.

'Namely changes are more amenable to detection in any case. But still our idea about the exhaust can be useful to design the cavity for pairing photons better for an exit in a preferred direction, and hence to generate more thrust.'