This space engine breaks a law of physics. But a NASA test says it works anyway.

This space engine breaks a law of physics. But a NASA test says it works anyway.
By Sarah Kaplan
Nov 22 2016
https://www.washingtonpost.com/news/speaking-of-science/wp/2016/11/22/this-rocket-engine-breaks-a-law-of-physics-but-a-nasa-test-says-it-works-anyway/

NASA scientists have been daydreaming about a new kind of 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 news sounds too good to be true, you’ve got good instincts — it just might be. This “impossible” fuel-less engine appears to violate one of the fundamental laws of physics.

Say what?

Hark back to your high school science classroom. Avert your eyes from the unfortunate hair styles and acne, if necessary, and try to focus on what’s written
on the blackboard: For every action, there is an equal and opposite
reaction.

That’s Newton’s third law of motion. It’s 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 that works?

According to the new paper, yes. 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.

Where did this idea come from?

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.

The idea was hyped in headlines and splashed across the cover of New Scientist magazine, but most scientists were, and still are, extremely skeptical. There’s
no theoretical explanation for how such an engine might work, and not
all the possible sources of experimental error have been eliminated.

[snip]

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