> Unless they believe the machine breaks some of the fundamental physics laws, the device must emit something.
I presume you're referring to the third law of motion, but if so, you've mischaracterized it. Nothing in the laws of motion state that engines must "emit" something. My car drives along quite efficiently without relying on any emission of mass. So does a sailboat.
Of course, these are still third-law compliant. My car exerts force against the earth (slowing it slightly), the sailboat takes energy from the wind.
To be third-law compliant, this alleged "reactionless" drive would need to apply force against something. But ejecting matter or energy to gain moment in the opposite direction is only one way to do that.
The biggest red flag about this is that there's no physical explanation. It's true that an explanation would imply some new physics. But it wouldn't be breaking the existing physical laws of motion, it would just be discovering some new thing to apply force against (virtual particles, or dark matter/energy, or whatever).
Actually, the conservation of momentum is local, so the car must emit some particles. The bottom of the tires emits (virtual) photons that are absorbed by the road. When you see it globally, the tires just push the road. (You can't go to the local mechanical shop and talk about virtual photons.)
In the middle of the space, where there is only vacuum around, you can't use the "virtual particle that get absorbed almost immediately" trick. So you need to emit some real particles (like a photon).
To explain this experiment, you can't push against the virtual particles, because they collapse and create some real particle (like a photon) to carry the momentum away and then you get the momentum/energy upper bound. (It's more complicated, particles appear and disappear, but the net effect is the same.)
Most of the "explanations" use virtual particles, but as an alternative explanation it's possible that this is pushing "dark matter". Perhaps this is a neutrino fan. (I'm not sure if neutrinos are technically dark matter, but they are "dark" enough and very abundant). It catches some neutrinos from the sun, change their direction a little, and then the neutrinos exit the chamber unnoticed. The change in the neutrinos direction gives the momentum to the thruster. I think that this doesn't break any theoretical law in an obvious way. The problem is that the dark mater is "dark" because it's very difficult to measure and to interact in any way. It's very difficult to change the direction of neutrinos, but at least it's an engineers' problem, not a physicists' problem. (In particular, neutrinos can travel through the earth.) The easier explanation is a measurement error.
Like I said, there would have to be some breakthrough physics here, like what "vacuum" actually means, or how dark dark matter is, etc.
I'm just pointing out that there are a number of logically possible solutions that don't involve violating the conservation of momentum, which may or may not be physically possible.
I believe that the dp=dE/c requirement for thrust isn't valid with static fields: simply consider an electromagnet a large distance D away from a permanent magnet. For a very small energy cost (P<<F * c), you can generate a magnetic field to propel yourself forward. The opposing magnet won't move for D/c seconds, so you're transporting momentum at a much lower cost.* , which I believe is what lukev refers as "other logical possibilities" -- the car is another example of this.
Of course, in free space magnetic field decays as 1/D^3, so eventually you will be better off spending F * c power to generate a force F. I do not disagree with "The device must emit something" though, since obviously momentum has to be carried by some particle, although I'm not a physicist to attest the specifics precisely.
* - I believe in this case the energy cost is so low because you're "storing" energy in the field, energy you're spending acts more like a trigger. As a thought experiment consider a stream of photons carrying a large amount of power P through a cross section. Now suppose your "device" is a mirror aligned parallel to the stream. Now for an arbitrarily small energy cost, you can align the mirror to the stream and cause the photons to produce a force P/c, "triggering" the system to relocate energy.
> For a very small energy cost (P<<F c), you can generate a magnetic field to propel yourself forward. The opposing magnet won't move for D/c seconds, so you're transporting momentum at a much lower cost.*
The mechanical moment, i.e. the moment of the object you can see (for example magnets, electrons, ...), is not conserved. The total moment is conserved, i.e. the moment of the objects and the moment of the fields, in this case the electromagnetic field. The electromagnetic field is quantized, and the particles are the photons. So in this case the photons travel from the electromagnet to the static magnet, and they carry away some moment and energy, so the moment is conserved locally.
It's important to consider the moment of the fields, because some of the photons don't go to the other magnet, but go to other directions and are never absorbed, the thecnical name is "radiation". If you don't consider it, then the system will lost some moment and some energy.
> As a thought experiment consider a stream of photons carrying a large amount of power P through a cross section. Now suppose your "device" is a mirror aligned parallel to the stream.
This is a solar sail. The solar sail is a real device. But I think that this experiment is not trying to build a complicated version of a solar sail. To test a solar sail, you should put the device in a strong light. I suppose that the test chamber is in the dark.
I presume you're referring to the third law of motion, but if so, you've mischaracterized it. Nothing in the laws of motion state that engines must "emit" something. My car drives along quite efficiently without relying on any emission of mass. So does a sailboat.
Of course, these are still third-law compliant. My car exerts force against the earth (slowing it slightly), the sailboat takes energy from the wind.
To be third-law compliant, this alleged "reactionless" drive would need to apply force against something. But ejecting matter or energy to gain moment in the opposite direction is only one way to do that.
The biggest red flag about this is that there's no physical explanation. It's true that an explanation would imply some new physics. But it wouldn't be breaking the existing physical laws of motion, it would just be discovering some new thing to apply force against (virtual particles, or dark matter/energy, or whatever).