If it seems too good to be true ...

At the end of July news sites offered many breathless articles about a new space drive. And three weeks later? Not so much.

This, from Wired on July 31, was typical: "Nasa validates 'impossible' space drive." Or this, from ExtremeTech on August 1, "NASA tests ‘impossible’ no-fuel quantum space engine – and it actually works."

The (supposed) space drive at issue bounces microwaves around a specially shaped chamber, and in the process is said (somehow) to produce a net thrust in one direction. The nature of the impossibility? That the drive -- if it works as advertised -- violates conservation of momentum.

Physics has gone a long time with every bit of evidence showing momentum is conserved. Always.

Space drive: old school

Consider a traditional, chemical rocket. It relies upon Newton's third law of motion: that every action produces an equal but opposite reaction. The action: exhaust gasses streaming out behind the rocket. The reaction: the rocket itself moves forward.

Newer, ion thrusters forgo chemistry but exploit the same action/reaction mechanism. That is, they use electromagnetic fields to propel Xenon ions. In both cases, matter ("reaction mass") is expelled from the spacecraft. Momentum is conserved.

If the "emdrive" or the related variant, the "Cannae drive," does work, it would mean spacecraft could operate solely with solar energy, not expelling any reaction mass. If a satellite or space probe need not carry and expend fuel, it could be much smaller and/or operate much longer.

Hunting the wily neutrino

I am reminded of the even more breathless reports, back in 2011, that neutrinos had been clocked traveling faster than light. Only that "discovery" turned out to be in error, an anomaly caused by a loose cable within the experimental apparatus.

But NASA reported that the "space drive" worked. NASA! Surely they took precautions, checked and rechecked everything. Well, no -- because (notwithstanding the headlines) NASA didn't perform the study. A few people at NASA did the study.

How does the "impossible" drive work? We read:

"Test results indicate that the RF resonant cavity thruster design, which is unique as an electric propulsion device, 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."

Only what is the "quantum vacuum virtual plasma?" Not a standard term in physics.

Might these inventers have discovered some new, conservation-of-momentum-defying physics? That would certainly be interesting. But (in Carl Sagan's words, not mine), extraordinary claims require extraordinary evidence. 

The thrust claimed for the new drive was tiny: about ten micro-newtons. (In customary units, that's 0.000036 ounces of force.) Such a small value makes problematical that the test of this supposed space drive wasn't done in a vacuum. Perhaps the tiny effect will be traceable to an errant air current.

Even if that tiny thrust can be replicated in a vacuum chamber, attributing the result to new physics should be the last resort. Consider, for example, that physical objects of nonuniform composition or shape radiate heat asymmetrically, producing a tiny thrust --

Billions of miles (and still going)

As appears to be the resolution of the longstanding Pioneer Anomaly. As exciting as a long-range-only, theretofore unobserved fine-tuning to our understanding of gravity would have been, nothing of the sort was needed to explain Pioneer 10 and 11 probes following what were slightly unexpected courses as they take leave of the Solar System. 

To what cause, then, shall we attribute both probes' anomalous courses? Waste heat from radioisotope thermoelectric generators suffices to produce enough thrust to explain the anomaly. Here's how:

Heat, aka the random jitter of molecules, results in "thermal radiation": aka, warm objects emit photons. (That's how night vision goggles work, by exploiting the infrared emissions of objects not observable in the wavelengths to which our eyes are sensitive.)  Photons, though massless, carry momentum in proportion to their frequency. When those emissions are asymmetric, there's a tiny net force vector ...

Nor am I the only one skeptical about the "impossible drive." Within days of the first, uncritical reports, skeptical responses began to pop up, as in (from International Business Times on August 4), "NASA ‘Impossible’ Space Engine: A Healthy Dose Of Skepticism Is Needed For Latest Science Miracle."

NASA did not conduct the study; a research team within NASA did. So the findings are not quite as authoritative as they appear to be, notes io9. This differentiation has posed a problem in the past, most recently in March, when news circulated that NASA did a study predicting when society would end. NASA funded the climate model used in the study but was not involved in the research.

As noted (also on August 4th) by The Daily Beast, in "Dear NASA: Fuel-Free Rocket Thruster Is Literally Too Good to Be True" there's another Really Suspect Aspect to the test reports:

They also found the same results when they tested their “control” apparatus, designed to give zero thrust. 

That sure sounds like one messed-up experiment.

On August 6th, Discovery weighed in with "Did NASA Validate an “Impossible” Space Drive? In a Word, No."

“... this is about as plausible as powering a spaceship by having the crew push on it from the inside.”

Has this wondrous new drive been disproven? Not yet. Not definitively. Wouldn't it be great if this tech proves out? Absolutely.

But I will be astonished if more rigorous testing actually shows that this drive does work and that conservation of momentum has a loophole.