Leaving aside the fact that light has momentum and therefore is sufficiently "physical" a propellant for this example, and the fact that this thing produces orders of magnitudes more thrust than a few Watts worth of photons could impart, you're still missing a really key problem:

You can impart momentum on a mirror by shining a flashlight on it, but you can't impart momentum on a sealed box by having a lit flashlight *inside* it!

The EmDrive uses a sealed cavity. There's nowhere for any propellant to come out, even if there were any!

The order of magnitude isn't actually the weird part. It's the "without propellant" part, or the "but the back is closed" (this thing produces infinite orders of magnitude more force than you'd get out of a photon drive with nowhere for the photons to escape, yet the microwaves are emitted into a sealed chamber).

Basically, it appears to be reactionless. All other propulsion systems are reaction based. When walking, your feet push on the ground; ground goes one way and you go the other. When sailing, your sails push on the wind (bending it to go in a direction more behind you than it otherwise would) and your boat goes in the opposite direction from where the wind goes. When flying, your engines (or an animal's wings) push the air out behind you; air goes backward and you go forward. With a chemical rocket, burning gases come out of the rocket going one way, and the rocket goes the other way. With an ion drive, particles (usually of a gas, like xenon) are ionized and then shoot towards a magnetic field (with their ionization canceled out before passing through the field, so they don't get pulled back by the field); particles go one way, spacecraft goes the other. With a light drive, photons go out one end of the ship (and yes, they have momentum even though the conventional Newtonion function for momentum, mass times velocity, suggests that massless photons shouldn't have any momentum) and each one going backwards imparts and equal (tiny!) amount of momentum in the other direction to the emitter or reflector that they were directed backward from.

In each case, the momentum of the stuff being pushed one direction equals the momentum imparted to the thing doing the pushing. Then we get this thing. With the EmDrive, nothing comes out of the drive. There's nothing (that we can detect) going out the back of the drive. No momentum in one direction to impart momentum in the other. Despite this, the drive tries to go forwards. This is a really, seriously, mysterious result.

I get that this is Slashdot and almost nobody reads TFAs, but seriously, the last time this thing was discussed there were plenty of comments pointing out that it had already been replicated in three different labs around the world... *last year*! True, I don't know if the "... in a vacuum" result has been replicated yet (though at least one lab has offered to do so) but considering that the results in the vacuum were consistent with the atmospheric results (and also considering the care that was taken to ensure that the result wasn't being caused by the atmosphere anyhow, like comparing the operational device with a dummy load that still generates the same heat, or turning the device around) I don't think that the error in our expectations is due to vacuum-vs.-atmosphere, so the other experiments are useful examples of the same effect.

100KW is the theoretical energy that you might be able to make a deep-space craft out of with this drive. The power it's been tested with so far is three orders of magnitude lower.

BUT it certainly stands to reason from our observations of the universe that some frequencies of EM are better suited to this purpose than others, as well as various drive configurations

BS. That most certainly does not "stand to reason". Higher-energy photons have more momentum, not less, yet this thing uses microwaves (much lower energy than visible light) and gets orders of magnitude more thrust than could be explained by the quite-well-understood thrust from EM radiation. Besides, why would there be a net thrust in one direction? The microwaves should escape the cavity in all directions, not just out the back, if they're escaping at all. A light drive has to be open at the back, or the photons would bounce off the rear wall and counter the thrust they imparted to the ship by bouncing off the reflector around the emitter.

First of all, you need to be careful of your pronouns. "They" the inventor of the device is mostly just saying it can be used to replace satellite thrusters, which would be a huge weight saving (no need for maneuvering or stationkeeping fuel). "They" NASA are saying that, *if* it scales up the way their current model says (yes, they have one), then after a lot of refinement and with a nuclear reactor powering it, this thing could produce hundreds of Newtons of thrust at a scale that would be feasible for spacecraft propulsion. Some people have worked out, based on the predicted thrust and the likely mass of such a craft generating that thrust, that it can be used to reach Alpha Centauri in under a century. The first of these is an obvious use case for anything that can generate a tiny thrust for a long time. The second is a straightforward application of the current-best (though probably still in need of major refinement) model of thrust detected to power input, which has been measured. The model is a guess, but the fact that more power = more thrust has been demonstrated; it is, as you say, a concrete fact. The third thing - the starship drive - is simply once again a straightforward application of the results of the model. Nobody is saying that the EmDrive makes starships possible, just that if we *had* a starship, and if it was propelled by an EmDrive that corresponds to the current model of power output, then it could reach Alpha Centauri in N years. That's simple mathematics; you can do the same for chemical rockets or solar sails or any other form of propulsion that works in vacuum.

Second, I don't know why you're calling the idea of scaling up an observed result "delusional". The first Wright Flyer could barely get one person off the ground for a few seconds, flew slower than a horse could run, and was so fragile that it was shattered by a gust of wind after only four flights. But, once you've demonstrated that it *works* - that heavier-than-air flight is actually possible (which should have been obvious to everybody, given that birds and bats and insects exist, but plenty of people thought humans would never achieve it) - then scaling that up to WW2 bombers was pretty straightforward: more-powerful-for-their-weight engines (the Wright brothers had to design their own engine; the existing ones at the time didn't have enough power to weight ratio), stronger-for-their-weight materials, lots of refinements to the design (the Wright brothers pioneered the use of wind tunnels and used them to fix several significant errors in the equations that govern aspects of flight like lift, but their first Flyer was still a very primitive design with many compromises or outright flaws), and other simple, iterative improvements. Breaking the sound barrier was a bigger challenge than getting across the ocean in one flight, in terms of theoretical challenges. All this from a craft that could barely get one guy a few hundred feet down a beach.

If somebody had watched the Wright Flyer and said "one day, people will be able to fly to Europe from the US in less than a day" would you have called them "totally delusional" too?

No offense, but even an AP physics class could point out that this doesn't make any sense. Yes, the perpendicular (sideways) moments (instances of thrust imparted by bouncing particles) would cancel out, but the thrusts at the normal to the back plate (that is, inline) would also cancel out. The inline component of the thrust has to exactly equal the inline-but-opposite component of the thrust caused by the bounce. You don't get to start out the set of all emitted photons with a net velocity in one direction.

If you think about it in terms of where the particles end up, it's pretty obvious this doesn't work: if there's always more thrust on the front side of the chamber (towards the thruster) than on the back, then that means the photons would all end up at the back of the chamber. For that to happen, the back of the chamber must have absorbed their perpendicular components towards itself, or the particles would have bounced back to the front of the chamber. But that would produce a thrust pushing the back plate away, which (since it's attached to the whole assembly) would counter the forward thrust.

Also, your idea just flat-out doesn't make sense: the sloped section is the forward part of the thruster (the cone points in the direction of travel). The back plate perpendicular to the direction of thrust. By what you're saying, the full component of the particle's bounce-imparted momentum would be acting *against* forward thrust, while only part of the thrust in the other direction would. The net thrust would be opposite the observed direction.

Yep. An ion engine also can't be closed at the back the way the EmDrive is; the emitted particles (ions) would hit it and bounce, canceling out the thrust they provided.

That's an excellent example. Gravity is still, in some ways, a mysteroius answer. We know a lot about it, and we can use what we know to make a lot of predictions, but we also know there's a lot that we don't know, and some of our theories about its properties are better termed guesses than predictions.

Now, obviously the EmDrive is far more mysterious than gravity, both because it's much more conceptually novel and unexplored, and because we can really easily detect there's *something* causing the effect called gravity while only a handful of labs around the world are equipped to test the thrust of an EmDrive. I'm not attempting to equate the two. But, as you say, the actual mechanism of gravity has never been observed directly, and the theories about it are still unconfirmed. Similarly, the EmDrive offers some (much less mature) theories as to its operation, but nobody has actually been able to confirm or deny those theories.

Of course, the EmDrive itself hasn't been confirmed yet, at least not to the degree that makes it practical for anything real-world. We have repeatable experiments saying that emitting microwaves into a specially-shaped resonant cavity causes a *tiny* thrust, and we've accounted for some of the likely errors in the experiment (atmosphere, whether the same result happens with a dummy load that doesn't generate the microwaves, etc.), but as of writing this, we have no direct evidence that it scales to useful sizes.

It's a half-sleeve on each arm, which is distinguished from having a single full-sleeve. :)

You've been marked as a troll, but I don't really think that's fair. Not everyone wants a tattoo or understands the tattoos that other people get.

I know these things are going to be on my arms for the rest of my life. And when I wake up in the morning and look at them, I think, "these are the arms I should've been born with".

First of all, you have to understand that not all tattoos are created equal. I paid $150/hr for mine. I looked for a long time for an artist whose style I liked, and when we sat down and did them, it took a really long time. All tattoos fade and bleed a bit, but good artists will know how to handle that a little. But hey, when I'm 80, I'm gonna be a little faded and fuzzy around the edges too.

My sleeves are thematic--I was born in the year of the Snake, in a fire cycle. I already had the words for 'fire' and 'snake' tattooed each on one wrist. My right arm is a red and black snake wrapped around bamboo with clouds, and my left arm is a blue, slick snake on a backdrop of flames and smoke. (Their mouths are closed, and they look quite happy--I firmly believe you need to be able to talk or fight your way out of any tattoo you have, and I dislike aggressive snakes for myself, because I'm not going to want to fight my way out of anything.)

Anyway, my tattoos are just a way for me to feel closer to part of my culture. They're a pretty bit of art that I get to carry around with me. They look good with the clothes I wear, like any accessory. That's what I wanted out of them, and that's what I got.

Other people have other reasons, but those are mine. :)

A small monohull is also a lot cheaper than a house, at least in most of the developed world. Even a fairly nice "yacht" (which just means "private non-business vessel") is probably affordable if you can sell your house. My parents have lived aboard for 13 years now, and their 48' (14.5m) catamaran cost significantly less than their house near Seattle.

It's actually really annoying when people assume that yachties must be rolling in dough. Most have very little income, so even though the lifestyle is cheaper than living ashore there's not a lot of disposable income.

4-digit UID, just under 40, two half-sleeves. :)

My arms are worth...uh, about 7x what an Apple watch Sport costs (alternately, I could afford one steel Apple watch for each arm for what I paid for each of my sleeves). $200 isn't much tattoo. In fact, that was 1h20m at my artist's rates.

It's not about capacitance. The watch shines different coloured light through the skin and monitors colour changes to figure certain things out. Ink is going to absorb or reflect that light in a way that the watch isn't calibrated to handle. Ink isn't melanin, so darker skinned people won't have the same problems.

My sleeves look a lot better than an Apple watch ever could, but I may just barely have enough open skin to wear one if I wanted to.

Interesting idea, but the hardware spec for that device is so lacking in basic facilities that it will probably be a non-starter for a lot of people.