Explaining the Impossible Thruster

I don’t understand the things you do, and you therefore may represent an interaction with the quantum vacuum virtual plasma.

I’m slightly late on this, but via xkcd I see that NASA has confirmed a supposedly impossible thruster design.

For some quick background, the thruster in question works by pumping microwaves into a cavity where they resonate.  The cavity itself is shaped or otherwise constructed so that (it is claimed) the photon pressure on one end is greater than the photon pressure on the other end.  No microwaves escape, but the differential force applied between the plates is the thrust.  And the idea is “impossible” of course because the design doesn’t “push” on anything… Kind of like when you were a kid and you jumped in a wagon because you wanted to ride it and tried to get it to move by shifting your body weight (didn’t anyone else try this?), it didn’t work then, so why does a slightly fancier version of the same idea seem to work now?

I like this question because it allows me to stretch my too-long-unused physics/mad-scientist mental-muscles.

So, the first candidate explanation I have will be the easiest to test:  some of the microwaves are leaking.   Contrary to the media reports, propellant-less thruster designs are easy to do.  Turn on your laser pointer, and it produces thrust.   That thrust, of course, is just caused by the photon recoil–really pretty standard physics.   If the container you’re using leaks photons preferentially in one direction you’ll get an anomalous thrust reading.   Although it would work, shining lasers in space would make for a rather pathetic engine design.

This possibility is made more plausible by the fact that the three groups reporting results on this device  all find very different results for thrust produced… something like a factor of 10,000 between the strongest and weakest results.   More than that, it’s the least trusted sources claiming the greatest thrust from their engine design.   Could it simply be that the Chinese group (who found the largest effect) didn’t check how much photon’s were leaking?

Having said all that, the second candidate explanation, that is the one which xkcd is lampooning, isn’t as crazy as it sounds.  First, it’s always important to remember that “conservation of momentum” doesn’t exist as an immutable law of physics.  Instead, Noether’s Theorem tells us that  what we call “conservation of momentum” is really the result of a symmetry found in nature.   Particularly, symmetry with respect to translation:  position x is the same as position x + dx as far as the laws of physics goes.

So break that symmetry: x is the same as x + dx only for certain values of dx.   That would mean that on a certain scale (translations smaller than the dx, say)  the physics has a preferred dimension.   That may sound a little weird, but really it’s not: a ladder has the exact same symmetry,  it’s only invariant to translations which are multiples of the step length.  So keep in mind that when people say “pushing against the quantum vacuum”, that a step-ladder analogy may be appropriate.

The question is why… exactly how does a specially shaped cavity create a metaphorical step-ladder?   Honestly, it’s a little surprising, but it occurs to me that there is a well known effect which can explain it:  the Casimir effect.   Basically, if you set up two parallel metal plates, the photon’s vacuum energy between the plates is different than the vacuum energy of empty space around the plates.  That generates a force between the plates.   That effect has been seen in the lab.

Conversely, by making the plates assymetric, the vacuum state is distorted in the neighborhood of each plate slightly differently so that in equilibrium there is a smoothly varying value of the vacuum energy as you move from one plate to another.   That’s the step-ladder and it just means that near one plate virtual particles pop up at a slightly higher frequency than at the other one.  The analog of the ladder-climber, then, is the added microwave energy.  Those photons interact with the virtual photons of the vacuum and with the right configuration will “push” the Casimir-virtual photons out of their equilibrium spatial distribution–and that’s your thrust.

I think that must be what people have in mind and I do find the explanation surprisingly plausible.   Plausible but still far fetched.  But it’s also exciting… it’s not just because it’s propellant-free (that’s easy), but also because it’s a closed system.  Hell, use this engine to hover and it’s indistinguishable from anti-gravity.

Still, I’m skeptical.   Although the above (second) explanation is my own, I’m inclined to think the first (boring) explanation is far more likely.