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Comment by Niten | Hacker News Reader

It's good to see that someone finally got around to testing this in a vacuum. Here's the paper mentioned in this story:

http://arc.aiaa.org/doi/abs/10.2514/6.2015-4083

High levels of skepticism are still warranted, as much as I would love for this to pan out. Every time this gets tested—presumably more and more carefully—the measured force gets smaller. The original test at Chinese Northwestern Polytechnical University measured 720 mN; this latest test in a vacuum chamber, only 20 μN. This is so tiny that a test anomaly would seem a much more plausible explanation than a violation of the conservation of momentum.

Of course the ultimate test would be to put one of these in space and try accelerating something with it, in the same way that the ultimate test of a perpetual motion machine would be to make one perform macro-scale net work.

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From the abstract: "We identified the magnetic interaction of the power feeding lines going to and from the liquid metal contacts as the most important possible side-effect that is not characterized yet. Our test campaign cannot confirm or refute the claims of the EMDrive..."

Given that this is a 700W microwave oven putting out a tiny amount of thrust, and that a positive finding would invalidate a basic conservation law, my money is on those wires.

On the other hand, the current theory of the EmDrive predicted exactly that amount of thrust for the given configuration.

The current theory of the EMDrive is complete, absolute bunk. Whatever an EMDrive is doing, can in no way be described by the incorrect relativistic reasoning used by Shawyer.

Smaller effect size as studies improve is the classic sign of a non-event. ESP and the like went through similar cycles - impressive early studies followed by a series of more rigorous studies with diminishing effects until people got bored and moved on to the next big thing.

In this case different tests have occurred at different power levels. Has anyone normalized the results? Of course with only a couple replications its early for a meta study.

In this case the tests weren't even normalized, this is one of the 1st tests that was conducted in vacuum under shielding, many of the earlier tests were not even thermally or electromagnetically shielded, considering the tiny amounts of thrust generated by both the "predictions" and the tests pressure difference due to thermal variance as well as magnetic forces could explain it just as well.

I also have a strong gut feeling that this some none issue but it never hurts to test it the US, Russia, China and to lesser Extent the UK, France and other European powers have all conducted studies into ESP, the cost of such testing is ridiculously low compared to the potential discovery.

When it costs near to nothing to explore a phenomenon that can change how we understand physics it's worth exploring no matter how ridiculous it is.

Someone compiled here a list of experiments: http://forum.nasaspaceflight.com/index.php?topic=36313.msg13...

I think that the best metric is to compare this with the maximal Force/Power ratio that is consistent with a "massless" thruster and special relativity, (i.e. a laser beam, photon thruster, ...). In this case the Force/Power ratio is at most 1/c, that is 0.0033 uN/W = 0.0033 mN/KW.

The results in the table vary from 3x to 200000x the theoretical maximum if we assume that this doesn't break the conservation of momentum, special relativity and quantum mechanics.

"Every time this gets tested—presumably more and more carefully—the measured force gets smaller."

Uh oh. That's a classic sign of a bogus effect. It's like physicists at Stanford trying to replicate the Pons-Fleischmann cold fusion experiment. At first, they were enthusiastic, and expected big results. They started out with the apparatus surrounded with radiation alarms in case it generated dangerous amounts of neutrons. After a while, they discovered that the effect was at most twice background radiation. Since humans have water in them and water is a neutron reflector, just moving around the equipment could produce effects at that level. Then they moved the equipment to the inside of a cube of lead bricks, to eliminate external neutrons. No more neutrons, background or otherwise.

No it is because the Chinese test dumped kilowatts of energy into the test item but the NASA test used only miniscule power. The guy you replied to presented the facts in a misleading way.

I'm slightly more skeptical about EmDrive than optimistic, however, you are right: if someone has to distort the truth in order to make a point it might be time that they question their standpoint on a matter. This looks like a lot of opinions about science. Which is a logical contradiction. An opinion has absolutely no sway in determining whether or not the device works. Opinions have nothing to do with science.

The device will either be shown to work or not to work with additional experiments and nothing anyone says regarding how it can or can't work will change that.

The new test was at slightly lower power but within an order of magnitude of the original test: 700 W versus 2.5 kW. The measured effect, on the other hand, has decreased by more than four orders of magnitude.

I haven't studied the theory behind the device, but it's hardly a distortion to characterize such a disproportionate drop as a declining effect size in the face of additional testing.

Like you, I don't claim to have any idea whether this ultimately pans out. As I said, the ultimate test is to put one in space and see what it does. Until that (or at least a more promising lab test) happens, all we have is speculation. But my point is that more skepticism is warranted than was implied by the Wired headline and by some of the comments in this thread.

The Chinese test used kilowatts rather than watts of power:

http://www.you-tech.it/index.php/content/download/12369/1124...

All the more possible for small extra forces to slip through the cracks from magnetic fields or whatever.

Maybe so, but it makes it meaningless to compare the measured forces and claim that they've decreased in later tests.

To be clear, the Chinese test used 2500 watts, and this latest test used 700 watts. That is a decrease, but it's within a factor of 4.

On the other hand, the decrease in effect size is closer to a factor of 40,000.

The difference in measure thrust is due to different test articles and testing done in different labs. The 720 mN result was from China and they dumped a ton of energy into it. The smaller 20 μN result was from NASA's testing which used far less power, so the resulting thrust measure was also smaller.

You're incorrect, 20 μN was measured in the Dresden test at 700W.

Why the assumption that if it works it must violate a conservation law? That's the last thing I would suggest after exhausting every other possibility, even weird ones like space time distortion thst would still operate within known conservation laws.

The whole reason the EmDrive is supposedly so revolutionary is that it's claimed to produce thrust without requiring any propellant. If conservation of momentum isn't going to be violated, something with mass-energy has to be carrying that momentum away from the spacecraft, and if it's not matter, it has to be radiation. But the claimed efficiency is about 5 orders of magnitude higher than radiation pressure could possibly produce.

Explanations aside, if in the slight chance it's workably better than ion thrusters for certain applications, that's great. If it turns out to be snake oil or a hoax, that would not be a huge surprise either.

An ISP of infinity would be quite a lot better than ion thrusters, yes, and that's what they're suggesting.

Ion thrusters still require expendable fuel as well as an electrical power supply, if you can cut out the need for expendable fuel that would mean allot for space travel especially to the outer solar system and beyond.

It's not an ISP of infinity.

You're forgetting mass-energy equivalence. <x> energy / <y> thrust is a specific impulse, once you work in the factor of c^2 (that being the minimum amount of mass you need to burn to get the energy to produce the thrust.)

In this case, for instance, they are saying they think it produced 20 uN with an input of 700w. That works out to an effective specific impulse of 2.5x10^9m/s as an upper limit (yes, this is tachyonic. One of the problems with this drive is that it throws conservation of energy and momentum out the window...), assuming we could convert mass to energy directly. (If you're using D-T fusion, it's a specific impulse of ~9.7x10^6m/s instead as an upper limit, assuming 339.72 TJ/kg. Other reactions are lower.)

If you're assuming solar panels or some other external power source, then the only real difference between this and a laser is that this produces a constant factor more thrust per energy input. Sure, it's reactionless - but, effectively, so is a neutrino source, for instance. All that means is that you don't need to be as careful as to which direction you burn in.

There are independent replications, which IMO mostly rule out hoaxes -- or at least greatly reduce the likelihood to below other explanations. It's tough to create an independently reproducible hoax.

Right now my guess is either measurement artifact or some form of obscure but conventional effect like EM interaction with the surround environment. That's most likely.

If those can be ruled out, then we're in new physics territory.

I'm not suggesting it's explained by current physics. What I'm saying is that if it works, I'd consider new physical explanations that maximally preserve conservation laws first -- sort of for Ockham's Razor reasons.

What would it take to get one of these into space?

from the looks of how testing is going, a good old fashioned rocket.

Well yes, I don't expect a tiny fraction of a Newton of force to get them into orbit, but I meant more as a way to test whether the anomalous force is usable for spaceflight.

After all, it will either sit there and do nothing or it will move.

No one's ever claimed that the EmDrive could launch payloads to orbit, to my knowledge. That's not what it's for. Once you're already in space, a drive that can produce 0.001G indefinitely will get you going way faster than a rocket that can produce 10G for a few minutes before its fuel runs out.

Solar sails and (to an extent) ion thrusters work on the same principle.

At least $4000 per kilogram, based a quick google.

I'm sure someone will put the cash together eventually if the dirtside experiments look promising, but there's no sense in risking the money until we're pretty sure.

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