This rocket engine uses no fuel and breaks a law of physics. But a Nasa test says it works anyway

Nasa scientists have been daydreaming about a new kind of rocket engine that could carry astronauts to Mars in 70 days without burning any fuel. Now, in a new paper published in the peer-reviewed Journal of Propulsion and Power, they say that it might really work.

The paper, written by astrophysicists at Nasa’s Eagleworks Laboratories, tested a electromagnetic propulsion system, or “EM drive,” that generates a small amount of thrust simply by bouncing microwaves around a cone-shaped copper chamber. No propellant goes in, no exhaust comes out, and yet, somehow, the engine can make things move.

If you think that sounds too good to be true - it just might be. This “impossible” fuel-less engine appears to violate one of the fundamental laws of physics.

Rocket or jet engines work thanks to Newton’s third law of motion, that for every action there is an equal and opposite reaction. It is the principle that explains why pushing against a wall will send an ice skater zooming in the opposite direction. It also explains how jet engines work: As hot gases are expelled out the back of the plane, they produce a thrusting force that moves the plane forward.

But the EM drive doesn’t work that way. Its thrust seems to come from the impact of photons on the walls of the copper cavity. That would be like moving a car forward by just banging against the windshield.

And yet, according to the new paper, it works. The Eagleworks scientists report that their machine generated 1.2 millinewtons of thrust per kilowatt of electricity pumped in (that electricity could come from solar panels in a hypothetical spaceship). That’s a fraction of thrust produced by the lightweight ion drives now used in many Nasa spacecraft, National Geographic noted, but it’s a lot more than the few micronewtons per kilowatt produced by light sails, a proven technology that generates thrust using radiation from the sun.

The idea for an EM drive was first published a decade ago by British engineer Roger Shawyer. He argued that the drive isn’t really “reactionless” - instead, he argued, the thrust comes from radiation pressure. Microwaves inside the cavity create an imbalance of radiation that pushes against the walls and generates thrust.