Is emission-less propulsion possible? I believe it is...

[Bernardo de La Paz]

54. It's an EmDrive. Better tests eliminate it.

{emphasis added}

The EmDrive is a concept for a thruster for spacecraft, first written about in 2001.[2][3][4][5] It is purported to generate thrust by reflecting microwaves inside the device, in a way that would violate the law of conservation of momentum and other laws of physics.[6][7][8][9][10] The concept has at times been referred to as a resonant cavity thruster.[11][12]

There is no official design for this device. Neither person who claims to have invented it has committed to an explanation for how it could operate as a thruster or what elements define it, making it hard to say definitively whether a given object is an example of an EmDrive. However, over the years, prototypes based on its public descriptions have been constructed and tested.

In 2016, Harold White's group at NASA observed a small apparent thrust from one such test,[13] however subsequent studies suggested this was a measurement error caused by thermal gradients.[14][15] In 2021, Martin Tajmar's group at the Dresden University of Technology replicated White's test, observing apparent thrusts similar to those measured by the NASA team, and then made them disappear again when measured using point suspension.[1]

No other published experiment has measured apparent thrust greater than the experiment's margin of error.[16] Tajmar's group published three papers in 2021 claiming that all published results showing thrust had been false positives, explaining each by outside forces. They concluded, "Our measurements refute all EmDrive claims by at least 3 orders of magnitude."[1]