Want to read Slashdot from your mobile device? Point it at m.slashdot.org and keep reading!

 



Forgot your password?
typodupeerror

Comment: Re:Did a paid shill write this summary? (Score 2) 163

by cbhacking (#49604681) Attached to: NASA Gets Its Marching Orders: Look Up! Look Out!

The sheer mass of ignorance in that post is staggering. For example, Darwin's theory of evolution by natural selection wasn't even formed in the Galapagos, much less before visiting. It came years after his return to England, though it was in large part informed by his observations on the voyage. He wasn't trying to disprove anything, so far as I know, though as a botanist with theological training but low (for his time) personal piety he may have questioned the theological explanations already.

Also, merely observing things and making theories about them after the fact is, at best, a part of the scientific process. The critical step is using theories to make predictions, and then testing those predictions. Calling that "attempting to disprove a theory" is bad science as well, since it implies a bias against a result; one should simply test whether the predictions are upheld by experimental results (or, where experiments aren't practical, further observation of the environment, preferably a new and untainted example) or the predictions fail. One then refines (or replaces) the theory, based on this new data, and makes new predictions.

Comment: Re:I want this to be true, but... (Score 1) 428

by cbhacking (#49599515) Attached to: New Test Supports NASA's Controversial EM Drive

Probably (assuming there's any leakage at all), but then you've just got a thruster that is a *less efficient* light drive. As in, it's like a normal photon drive, but a lot of the photons it produces don't actually produce thrust. That would lead to the opposite affect of what we're seeing here, which is a drive that appears to only produce photons yet gets hundreds to thousands of times as much thrust as a photon drive would at that power.

Comment: Plenty of them here too (Score 1) 214

by NormalVisual (#49598893) Attached to: Want 30 Job Offers a Month? It's Not As Great As You Think
I usually get one or two of these spammy mails every day. My favorite was one I got several years ago, which described an interesting opportunity, but I wasn't (and still am not) willing to relocate. So, I wrote back and asked the guy if the gig could be done via telecommuting. His response was "yes, but they require that you work from the office. Will that work for you?". I wrote back asking if he knew what "telecommuting" meant, and never heard back.

Comment: Re:Conservation of momentum (Score 3, Informative) 428

by cbhacking (#49597813) Attached to: New Test Supports NASA's Controversial EM Drive

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!

Comment: Re:I want this to be true, but... (Score 1) 428

by cbhacking (#49597779) Attached to: New Test Supports NASA's Controversial EM Drive

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.

Comment: Already been reproduced... a year ago (Score 1) 428

by cbhacking (#49597699) Attached to: New Test Supports NASA's Controversial EM Drive

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.

Comment: Re:I want this to be true, but... (Score 5, Informative) 428

by cbhacking (#49597669) Attached to: New Test Supports NASA's Controversial EM Drive

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.

Comment: Re:This again? (Score 2) 428

by cbhacking (#49597613) Attached to: New Test Supports NASA's Controversial EM Drive

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?

Comment: Re:This again? (Score 1) 428

by cbhacking (#49597369) Attached to: New Test Supports NASA's Controversial EM Drive

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.

Comment: Re:This again? (Score 1) 428

by cbhacking (#49597183) Attached to: New Test Supports NASA's Controversial EM Drive

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.

Comment: Error in headline (Score 5, Insightful) 291

The paper was not rejected because of one reviewer. It's standard to have THREE reviewers, this is one guy out of three. Additionally, it's the editor's call whether to accept or reject it. Typically that's based on the reviewers recommendation. However, the editor could and should have ignored that one reviewer and accepted it anyway. Actually, the AE should have deleted the review and said to the authors "Sorry, the third reviewer never turned in his review, sending it out for a different reviewer." The AE could have accepted it even if all three reviewers had insightful criticisms of the paper and said it was horrible.

In other words, the rejection for publication could have nothing to do with that one review, it was not rejected due to that review, it was rejected by the editor who showed poor judgement in accepting the sexist review.

The first rule of intelligent tinkering is to save all the parts. -- Paul Erlich

Working...