Wired.co.uk's piece last week about Nasa's test of a new type of space drive triggered a tsunami of responses online. Many were understandably sceptical, others were unsure how it would advance space travel. In fact, the paper produced on the day gave much more detail than the advance abstract we linked to then. The actual paper reveals details of tests in early 2014 as well as those in summer 2013 -- and the results are even more astounding.
Here we answer many of your questions, quibbles and criticisms.
1. Isn't such a tiny force likely to be experimental error?
The equipment can measure forces of less than ten micronewtons, and the thrust was several times that high.
The test rig is carefully designed to remove any possible sources of error. Even the lapping of waves in the Gulf of Mexico 25 miles away every three to four seconds would have showed up on the sensors, so the apparatus was floated pneumatically to avoid any influence. The apparatus is completely sealed, with power and signals going through liquid metal contacts to prevent any force being transmitted through cables.
Similar consideration was given to any other possible factors that could influence the result, for example shielding everything from electromagnetic effects. There may be a gap somewhere, but the Nasa experimenters appear to have been scrupulous.
2. Thrust was also measured from the 'Null Drive', doesn't that mean the experiment failed?
Lots of commenters jumped on this, assuming incorrectly that this was a control test and that thrust was measured when there was no drive.
In fact, the 'Null Drive' was a modified version of the Cannae Drive, a flying-saucer-shaped device with slots engraved in one face only. The underlying theory is that the slots create a force imbalance in resonating microwaves; the 'Null Drive' was unslotted, but still produced thrust when filled with microwaves. This may challenge the theory -- it is probably no coincidence that Cannae inventor Guido Fetta is patenting a new version which works differently -- but not the results.
The true 'null test' was when a load was used with no resonant cavity, and as expected this produced no thrust:
"Finally, a 50 ohm RF resistive load was used in place of the test article to verify no significant systemic effects that would cause apparent or real torsion pendulum displacements. The RF load was energised twice at an amplifier output power of approximately 28 watts and no significant pendulum arm displacements were observed."
Equally significantly, reversing the orientation of the drive reversed the thrust.
3. They didn't do it in a vacuum, so how do we know the result is valid in space?
While the original abstract says that tests were run "within a stainless steel vacuum chamber with the door closed but at ambient atmospheric pressure", the full report describes tests in which turbo vacuum pumps were used to evacuate the test chamber to a pressure of five millionths of a Torr, or about a hundred-millionth of normal atmospheric pressure.
4. Why didn't they test Shawyer's EmDrive design as well as the Cannae drive?
It turns out that in January this year they did test the EmDrive design.
The test results for this were also positive, and in fact their tapered-cavity drive, derived from the Chinese drive which is in turn based on Shawyer's EmDrive, produced 91 micronewtons of thrust for 17 watts of power, compared to the 40 micronewtons of thrust from 28 watts for the Cannae drive.
5. Even if it works, how can such a small thrust push a spacecraft?
The thrust was low because this is a very low-powered apparatus. The Chinese have demonstrated a system using kilowatts rather than watts of power that produces a push of 720 millinewtons. This is enough to lift a couple of ounces, making it competitive with modern space drives. The difference is that this drive doesn't require any propellant, which usually takes up a lot of launch weight and places a limit on how long other drives can operate for.
The Nasa paper says "the expected thrust to power for initial flight applications is expected to be in the 0.4 newton per kilowatt electric (N/kWe) range, which is about seven times higher than the current state of the art Hall thruster in use on orbit today."
6. How does this get us to Mars?
The small but steady push of the EmDrive is a winner for space missions, gradually accelerating spacecraft to high speed.
The Nasa paper projects a 'conservative' manned mission to Mars from Earth orbit, with a 90-ton spacecraft driven by the new technology. Using a 2-megawatt nuclear power source, it can develop 800 newtons (180 pounds) of thrust. The entire mission would take eight months, including a 70-day stay on Mars.
This compares with Nasa's plans using conventional technology which takes six months just to get there, and requires several hundred tons to be put into Earth's orbit to start with. You also have to stay there for at least 18 months while you wait for the planets to align again for the journey back. The new drive provides enough thrust to overcome the gravitational attraction of the Sun at these distances, which makes manoeuvring much easier.
A less conservative projection has an advanced drive developing ten times as much thrust for the same power -- this cuts the transit time to Mars to 28 days, and can generally fly around the solar system at will, a true Nasa dream machine.
7. What's this about hoverboards and flying cars?
A superconducting version of the EmDrive, would, in principle, generate thousands of times more thrust. And because it does not require energy just to hold things up (just as a chair does not require power to keep you off the ground), in theory you could have a hoverboard which does not require energy to float in the air.
You'll have to provide the lateral thrust yourself though, or expend energy pushing the thing along by other means --- and in any case, superconducting electronics are rather bulky and expensive, so the super-EmDrive is likely to be a few years away.
8. Surely a single result by one lab is likely to be an error?
The Nasa work builds on previous results by Roger Shawyer in Britain and Prof Yang Juan at Northwestern Polytechnical University in Xi'an as well as Guido Fetta's work at Cannae. This is more of a confirmation.
9. Why isn't there a simple explanation of how it's supposed to work without violating the laws of physics?
Different research groups all seem to have their own theories -- Shawyer's is based on relativity, the Chinese one is based on Maxwell's Law and Nasa is now talking about pushing against "quantum vacuum virtual particles" and saying that this is "similar to the way a naval submarine interacts with the water which surrounds it." The Nasa report deliberately avoids any theoretical discussion on this point, with good reason.
None of these explanations has gone unchallenged by theoreticians, and it might be fair to say that there is no accepted explanation as to how a close system of resonating microwaves can produce a thrust. There is no accepted theoretical explanation of how high-temperature superconductors work either, but because the effect has been replicated so many times, nobody doubts that it happens.
If the new drive results continue to be replicated, then theory may have to catch up.
10. What happens next?
The next stage will be more tests and more validation. An improved version of the tapered drive based on the EmDrive has been designed, and this will be built and sent out to other facilities so they can confirm the initials results.
The current plan is for IV&V (Independent Verification and Validation) tests at the Glenn Research Center using their low thrust torsion pendulum, similar to the one used, followed by another one at the Jet Propulsion Laboratory (JPL) using their low thrust torsion pendulum. The Johns Hopkins University Applied Physics Laboratory may also test the device using a different type of apparatus known as a Cavendish Balance.
After that, the sky's the limit. Or perhaps it isn't.