If humanity ever wants to truly explore the outer reaches of our solar system or travel to other stars, we're going to have to build a better engine for our spacecraft. In fact, this engine will have to be so good that it will be capable of generating thrust without consuming propellant. That's right: If we ever want a four-hour trip to the moon or a two-month trip to Mars, we're going to need a rocket engine that doesn't need rocket fuel.
This is the idea behind the radio frequency resonant cavity thruster—otherwise known as EmDrive—a theoretical reactionless engine powered by turning electricity into microwaves and bouncing them around a closed metal funnel.
The EmDrive has been dismissed as "impossible" on more than one occasion because it appears to violate Newton's Third Law of Motion, which states that for every action there is an equal and opposite reaction. But if a leaked NASA paper posted online last Saturday is to be believed, then not only are NASA scientists pursuing an EmDrive, but they actually made one that works.
As detailed in the paper, a team of NASA physicists led by Harold "Sonny" White and Paul March—both leading figures in exotic propulsion systems—was able to generate thrust in a "tapered RF test article" (read: EmDrive prototype) during a series of tests at NASA's Eagleworks Labs at Johnson Space Center in the fall of 2015.
In essence, the NASA EmDrive in the paper consists of a closed copper cone, the inside of which is being bombarded with microwaves. To test the EmDrive, the researchers powered it with 40, 60, and 80 watts and found that it generated up to 58, 128, and 119 micronewtons of thrust, respectively. Given that this "anomaly" was still observed by White and his colleagues after accounting for error, this suggests that the results of the experiment show an EmDrive is indeed possible.
Based on these results, the researchers estimated that their contraption would be capable of generating approximately 1.2 millinewtons of thrust per kilowatt were they to scale up the power input. To put this in perspective, the Hall thruster—one of the most powerful in development and powered by ejecting plasma—generates about 60 millinewtons of thrust per kilowatt.
"The issue involved here is whether the experiment is seeing something real or not," Jim Woodward, a physicist at California State-Fullerton, told Motherboard. "I know Paul [March] does clean work and to be honest, I suspect there may really be something there. But the result they're seeing can't actually be explained in terms of the theory they're proposing. So the question is: what is causing it?"
Per Newton, if you want to propel a rocket through space, you're going to have to eject some material in the opposite direction of the rocket's travel. But an EmDrive appears to generate a reaction without any action—so what gives?
A number of theories have been floated attempting to explain this ostensible violation of the bedrock of physics. White has been a proponent of the quantum vacuum explanation, which posits that the EmDrive is able to generate thrust by acting on virtual particle pairs that are generated by fluctuations in the quantum vacuum (in this theory, these vacuum fluctuations are created by the electromagnetic field generated by the EmDrive). In essence, the microwaves would be 'pushing off' of these virtual particles within the EmDrive cavity to generate the thrust that has been observed by White and his colleagues in EmDrive experiments.
Another leading explanation is that the EmDrive's thrust is generated by radiation pressure, a position held by its inventor Roger Shawyer. On this view, when the microwave radiation enters the copper cavity, the radiation pushes against the walls of the EmDrive and generates thrust.
Yet, according to Woodward, both of these theories are unlikely to be correct for a simple reason: Physics doesn't allow them.
By way of example, Woodward likened explaining the results seen at NASA purely in terms of microwave pressure to arguing that you can accelerate a car by getting in the driver's seat and pushing on the windshield.
"Can any disposition of microwaves inside the cavity produce thrust?" said Woodward. "There's a simple answer to that question: No, it cannot. Conservation of momentum dictates that any purely electromagnetic system that is enclosed cannot produce thrust. This is for both quantum theory and classical electrodynamics. It's physically impossible."
This is what led White to his quantum-vacuum theory: If the microwaves can't push against the inside walls in the copper cone to generate thrust, then they must be pushing off of something else within the cone—like virtual particle/anti-particle pairs that exist in the quantum vacuum. The problem is that while the existence of the quantum vacuum has been experimentally proven to exist, physicists are in pretty strong agreement that it can't be used as a medium to generate thrust.
Although White's quantum vacuum theory is referenced as the theoretical explanation of the thrust observed in both the recently leaked NASA paper and the first NASA EmDrive paper from 2014, it has been debunked time and time again by leading physicists within the community.
EmDrive inventor Roger Shawyer demonstrates why he thinks radiation pressure explains the EmDrive thrust
"A theoretical construct related to the quantum vacuum is nonsense," said Woodward. "There is no experimental evidence that suggests anything remotely like [White's theory] is a reasonable way to view the vacuum. It's something that no serious quantum field theorist takes seriously as far as I know."
Instead, Woodward thinks that the results of NASA's EmDrive experiments can be explained without having to overturn the basic laws of physics.
"The Mach effect is the only possible way to explain this," said Woodward. "If they are looking at a real anomalous signal, and you want to try to explain it with physics that is not nonsense, then that seems to be the only way to approach this."
As the first person to theorize about Mach effects in 1990, Woodward should know. According to Woodward's Mach effect theory, when a body of mass is accelerated, some of the force applied to that body does not result in kinetic energy but is stored as potential energy in the body. While the acceleration is changing the internal energy of the body changes as well, which manifests itself as a change in the resting mass of that body.
The accelerating body is essentially being squished between the force being applied in the direction of its acceleration and the pushback from the rest of the material in the universe via a gravitational field. These two oppositional forces change the internal energy of the accelerating body and no physical laws are being broken: Momentum is conserved, but small amounts of it are being temporarily "stored" in the accelerating body.
According to Woodward, this mass fluctuation effect could account for the thrust observed at the NASA Eagleworks lab. If the microwaves propagating in the EmDrive cone are applying force to the material that the cone is made of, then you might explain the thrust generated this way by the Mach effect outlined by Woodward.
Woodward has spent the last two decades building devices which have successfully demonstrated the Mach effect, although his theory is still far from mainstream within the physics community. Still, it is gaining ground as more successful experiments confirm his theory. March, for one, has worked with Woodward on a number of successful Mach effect experiments over the last decade, and, at September's Exotic Propulsion conference in Estes Park, Colorado, three physicists besides Woodward claimed to have experimentally reproduced Woodward's Mach effect results.
"In my view, it's the only physics that I know and trust to not involve wishful thinking and magical fields," said Woodward.
For now, all we know is that the paper leaked on Saturday is in fact legitimate—it has been peer reviewed and accepted for publication in the AIAA's Journal of Propulsion and Power, where it is slated to appear in the December issue. What is unclear is how much the peer-reviewed version will have changed from the early release version leaked online.
According to Woodward, who saw a copy of the paper shortly after it had been accepted for peer review, the main difference between the accepted copy and the 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.
What remains to be seen is whether others can use Woodward's Mach effect theory to reproduce the results seen in the NASA Eagleworks lab and thereby offer a compelling explanation for what White and his colleagues observed.
"Absent a convincing physical explanation that doesn't fly in the face of well-known principles of physics, you should hold in abeyance any judgment as to whether or not [the NASA results are] real," said Woodward. "This stuff is much harder to do and get right than almost anybody who has not actually been in the trenches doing it really appreciates. The odds against anything being real in this business are very high."