After years of speculation, a maverick research team at NASA’s Johnson Space Center has reached a milestone that many experts thought was impossible. This week, the team formally published their experimental evidence for an electromagnetic propulsion system that could power a spacecraft through the void—without using any kind of propellant.

According to the team, the electromagnetic drive, or EmDrive, converts electricity into thrust simply by bouncing around microwaves in a closed cavity. In theory, such a lightweight engine could one day send a spacecraft to Mars in just 70 days. (Find out why Elon Musk thinks a million people could live on Mars by the 2060s.)

The long-standing catch is that the EmDrive seemingly defies the laws of classical physics, so even if it’s doing what the team claims, scientists still aren’t sure how the thing actually works. Previous reports about the engine have been met with heaping doses of skepticism, with many physicists relegating the EmDrive to the world of pseudoscience.

Now, though, the latest study has passed a level of scrutiny by independent scientists that suggests the EmDrive really does work. Is this the beginning of a revolution in space travel—or just another false start for the “impossible” spaceship engine?

What’s an EmDrive?

First proposed nearly 20 years ago by British scientist Roger Shawyer, this incarnation of the EmDrive has been developed and tested by engineers at NASA’s Advanced Propulsion Physics Research Laboratory, informally known as Eagleworks.

Put simply, the Eagleworks EmDrive generates thrust by bouncing around electromagnetic energy (in this case, microwave photons) in a closed, cone-shaped chamber. As those photons collide with the chamber’s walls, they somehow propel the device forward, despite the fact that nothing is released from the chamber. By contrast, ion drives now in use on some NASA spacecraft create thrust by ionizing a propellant, often xenon gas, and shooting out beams of charged atoms.

What this means, if the EmDrive withstands further scrutiny, is that future vehicles could hurtle through space without needing to carry literal tons of propellant. In space travel, staying light is crucial for fast and cost-effective trips over long distances.

Why does this engine break the laws of physics?

Way back in 1687, Sir Isaac Newton published three laws of motion that formed the foundation for classical mechanics. Over the intervening three centuries, those laws have been tested and verified over and over again. (Also see “Isaac Newton’s Lost Alchemy Recipe Rediscovered.”)

The trouble is, the EmDrive violates Newton’s third law, which states that for every action, there is an equal and opposing reaction. This principle explains, for instance, why a canoe glides forward when someone paddles. The force applied as the paddle moves through the water propels the canoe in the opposite direction. It’s also why jet engines generate thrust: As the engine expels hot gases backward, the plane moves forward.

Weirdly, the EmDrive doesn’t expel anything at all, and that doesn’t make sense in light of Newton’s third law or another tenet of classical mechanics, the conservation of momentum. If the EmDrive moves forward without expelling anything out the back, then there’s no opposing force to explain the thrust. It’s a bit like arguing that a person inside a car could propel it forward by repeatedly hitting the steering wheel, or that the crew of a spaceship could fly the craft to their destination simply by pushing on the walls.

Has anyone tried to test it before?

In 2014, the Eagleworks group made waves when it announced the results of early tests suggesting the EM engine actually worked. Since then, the group has tested the EmDrive in increasingly more stringent conditions, including the latest experiments.

Other groups have also developed and tested various incarnations of the EmDrive. In addition to experiments conducted by U.S., European, and Chinese academics, there’s a community of DIY EmDrivers who are busy making and testing their own impossible physics engines. But no one has been able to say conclusively that such a drive has worked as described. (Let’s be real: Physicists don’t like seemingly miraculous inventions.)

So what’s different now?

Now, the NASA team behind the EmDrive has published the results of their experiments in a peer-reviewed journal. While peer review doesn’t guarantee that a finding or observation is valid, it does indicate that at least a few independent scientists looked over the experimental setup, results, and interpretation and found it all to be reasonable.

In this paper, the team describes how they tested the EmDrive in a near vacuum, similar to what it would encounter in space. Scientists placed the engine on a device called a torsion pendulum, fired it up, and determined how much thrust it generated based on how much it moved. Turns out, the EmDrive is capable of producing 1.2 millinewtons per kilowatt of energy, according to the authors’ estimates.

That’s not a lot of thrust compared to more traditional engines, but it’s far from insignificant considering the completely fuel-free setup. And to put that in perspective, light sails and other related technologies—which are propelled by the push of photons—only generate a fraction of that thrust, between 3.33 and 6.67 micronewtons per kilowatt.

Before now, one of the major criticisms about the EmDrive is that it warmed up while activated, which some scientists suggested could heat the surrounding air and generate thrust. Testing the device in a vacuum resolved some of that criticism, though there are still loads of caveats that need addressing.

OK. How is that possible?

First things first: It’s still unclear that the EmDrive truly generates thrust, a claim that will require further verification. But people are already tossing around ideas for how the drive might work.

The Eagleworks team that tested the EmDrive thinks the microwave photons push against “quantum vacuum virtual plasma,” or a roiling sea of particles that flit in and out of existence at the quantum level. The trouble is, there’s no evidence that quantum vacuum virtual plasma is even a real thing, says Caltech physicist Sean Carroll. Quantum vacuums exist, he says, but they don’t generate a plasma that’s available for pushing against.

In their paper, the Eagleworks team invokes an idea called pilot-wave theory to describe how the quantum vacuum could be used to generate thrust, while noting that such interpretations are “not the dominant view of physics today.”

Mike McCulloch, a physicist at the University of Plymouth, argues that the EmDrive is evidence of a new theory of inertia that involves something called Unruh radiation, a sort of heat experienced by accelerating objects. In his telling, since the wide and narrow ends of the EmDrive’s cone permit different wavelengths of Unruh radiation, the inertia of the photons inside the cavity must change as they bounce back and forth—which must produce thrust in order to conserve momentum.

But McCulloch’s model assumes that Unruh radiation is real—it hasn’t been experimentally confirmed—and also suggests that the speed of light varies within the EmDrive’s cavity, which violates Einstein’s theory of special relativity, according to Rochester Institute of Technology physicist Brian Koberlein.

Could this still be bunk?

For sure. There’s a long history of findings that seemingly defy the laws of physics (faster-than-light neutrinos, anyone?) that were ultimately shown to be casualties of faulty experimentation.