It's one of the most intriguing stories of the year: NASA's version of the 'impossible' EM Drive appears to produce thrust, violating Newton's third law and hence our current understanding of the physics that govern the Universe.
But just because NASA's space drive test passed peer-review last month, doesn't mean it actually works, says Brice Cassenti, an expert in advanced propulsion systems at the University of Connecticut. In fact, due to the array of errors that could have affected the experiment, he says the only way we can actually know the truth is to test the EM Drive in space.
"[M]any scientists and engineers feel the thrust measurements reported for the EM Drive are due to experimental error," Cassenti said in a recent interview.
"Adding to this is the fact that those who believe the results are valid do not yet have an experimentally or a theoretically plausible proven physical explanation. I personally believe that there is a mundane explanation for the results."
For those of you who are out of the loop, after months of speculation and rumours, NASA's Eagleworks Laboratory team finally had their EM Drive test results accepted for publication in the American Institute of Aeronautics and Astronautics' (AIAA) peer-reviewed Journal of Propulsion and Power last month.
It was a big moment, because it took this peculiar technology out of the realm of pseudoscience, and forced the scientific community to take it seriously - arguably the first time since British inventor Roger Shawyer come up with the initial designs back in 1999.
But things remain incredibly complicated, because while the NASA team reported that their EM Drive produced a thrust of 1.2 millinewtons per kilowatt in a vacuum - a very, very small, but noticeable, amount - no one, not at NASA, nor elsewhere in the scientific community, has come up with a plausible explanation for how they did it.
This is a problem, because the EM Drive should not produce any thrust at all, according to Newton's third law, "For each action there's an equal and opposite reaction."
According to this law, for a system to produce thrust, it has to push something out the other way. The EM Drive doesn't do this, and yet somehow produces thrust.
The principle of conservation of momentum is so fundamental, that to violate it would mean invalidating much of the basis of physics as we know it, and until now, has never been challenged to such an extent.
And as Cassenti explains, Newton's third law does break down at very high speeds, but that's not what's going on here.
"If the results are valid, it definitely points to new physics," he says. "Newton's laws have already been shown not to apply at high relative speeds (where special relativity applies), in large gravitational fields, and with very small scale molecules. But Newton is still mostly right."
So what's his argument against the EM Drive?
Let's start with an easy one. A number of experts have suggested that electric currents used to activate the EM Drive could be heating up components inside it, and they expend during the experiments, causing a tiny amount of motion that would appear as a force.
"It is very difficult to remove such effects, although the authors of the journal article tried to remove not only these thermal effects but also many other possible sources for experimental errors," Cassenti explains, adding that testing the drive in space should eliminate many of these potential errors, due to it being a natural, not artificially rendered, vacuum.
"It is extremely difficult to know for sure that all of the possible sources for errors have been removed. The only sure method is to have a hypothesis (or theory) that can be tested independently."
Despite the fact that the results have been peer-reviewed, they have not been independently tested or replicated. The experiment might have peer-review backing, but that just means that independent experts have scrutinised the experimental design and considered it to be reasonable.
"A reviewer of the journal paper that I spoke with before the paper was submitted does not believe the results point to any new physics," says Cassenti. "But that person felt the results are puzzling enough to publish."
The second big argument for skepticism when it comes to NASA's EM Drive has to do with quantum vacuum fluctuations, where the 'virtual particles' that inhabit a vacuum are made momentarily real, accelerated like a plasma to produce thrust, and then disappear again.
That might sound crazy, but as physicist Chris Lee explains for Ars Technica, that principle actually has a strong backing in physics - it's even thought to happen at the event horizon of black holes. But it doesn't make sense in the context of an EM Drive cavity.
In a paper published by the NASA team back in 2014, the team wrote that the drive, "is producing a force that is not attributable to any classical electromagnetic phenomenon and therefore is potentially demonstrating an interaction with the quantum vacuum virtual plasma".
This explanation has proven wildly unpopular among other physicists, as Sean Carroll from Caltech told Phil Plait at Discover, "There is no such thing as a 'quantum vacuum virtual plasma', so that should be a tip-off right there. There is a quantum vacuum, but it is nothing like a plasma."
John Baez, an expert in quantum gravity from the University of California, Riverside, even went so far as to call the EM Drive "bullshit" in 2014, before later softening that to "baloney".
The quantum vacuum virtual plasma explanation is conspicuously absent from NASA's recently peer-reviewed paper, according to Daniel Oberhaus at Motherboard.
Physicist Jim Woodward from California State-Fullerton reportedly saw a copy of the paper shortly after it had been accepted for peer-review, and told Oberhaur the main difference between the accepted copy and a leaked early release is that the latter has way more theory trying to explain the results.
"Supposedly the AIAA would only accept the paper if White and his colleagues ditched the quantum vacuum theory and just published the results of their research without trying to explain it," Oberhaus reports.
Of course, this is all "he said, she said", and it's easy to criticise the EM Drive when you're not under pressure to offer up a solution.
But among all the excitement of NASA's EM Drive results, many physicists are recommending caution until the results can be replicated - because while a number of teams have seen similar levels of success with different iterations of an EM Drive using different experimental designs, it would be a big deal if these particular results could be replicated.
As Cassenti said in his recent interview:
"[O]ver my professional life, I have seen several of these exciting experimental or theoretical results reported in peer-reviewed literature. So far, only the reality of black holes has come through. So, based on my experience, the probability of this holding up under further analysis and testing appears slim. But it's not zero."
Watch this space.