Catch up on stories from the past week (and beyond) at the Slashdot story archive


Forgot your password?
Compare cell phone plans using Wirefly's innovative plan comparison tool ×

Comment Re:Prepare to be (Score 1) 363

Sure, here you go, since you couldn't be bothered to check the link:

We will demonstrate that the Einstein Field Equations reduce to Newton's Law of Gravity in the case of a weak field and slow-motion of a particle (v less than the speed of light, c).

As we have already seen, Newton's Gravitation Law can be written as

(1) 2 = 4G, Equation (16) in Newton's Law of Gravity.

In free-fall, a particle satisfies,

(2) d2x/dt2 = F/m= –, Equations (4),(15) in Newton's Law of Gravity.

In tensor notation, this is written as,

(3) ,ii 4G

(4) d2x/dt2 –,i

Corresponding to (1) in General Relativity is the Einstein Field Equations, which can be written in the trace-reverse form,

(5) R = (8G/c4)[T – ½Tg]

And corresponding to (2), is the geodesic equation,

(6) d2x/d2 = –(dx/d)(dx/d)

Our task is to show that (5) will reduce to (1) in low gravity, low velocity.

The first approximation we make is that the particle is moving at velocity near zero,

(7)(dx/d) (dt/d,0,0,0)

The only non-zero term for the 's in (6) will be =i, = =0. The equation becomes,

(8) d2x/d2 = –i00(dx0/d)(dx0/d)


(9) d2x/d2 = –i00(dt/d)(dt/d), (x0 =t)

But by the chain rule of calculus,

(10) d2x/d2 = (dt/d)(dt/d)d2x/dt2


(11) d2x/dt2 = –i00

Equation (4) and (11) yields,

(12) ,i = i00

Using the Christoffel symbols of the second kind ( Torsion =0 in GR),

(13) = ½ g(g, + g, – g,)

Again setting =i, = =0, equation (12) becomes,

(14) ,i = ½ gi(g0,0 + g0,0 – g00,)

Since the time derivative of the metric is zero (Einstein Equivalence Principle), the only surviving term is = i. Then,

(15) ,i = ½ gii(– g00,i)= (–½g00,i)

A solution is,

(16) g00 = – c2 – 2

Turning to the Einstein Field Equations (5), and from (16), we only need the time-time equation, = = 0

(17) R00 = (8G/c4)[T00 – ½Tg00]


(18) T00 = c4 and T = g00T00 = (–1/c2)c4= –c2

Substituting (18) into (17), we get,

(19) R00 = 4G

Now to get to Newton's Law, equation (1), we need to work on the left-hand side of equation (19).

For that we use the definition of the Ricci tensor,

(20) R = /x – /x + –

From (19), = = 0,

(21) R00 = 00/x – 0/x0 + 00 – 00

In low gravity, the square of the 's is zero, and its time derivative is also zero. The only surviving term is,

(22) R00 = i00/xi i00,i

But from (12), repeating below,

(23) i00 = ,i

Take the derivative, and then use (23)

(24) i00,i = ,ii = R00

Using (19),

(25) ,ii 2 = 4G

And that is Newton's Law of Gravity (1).

Comment why the EM drive is bullocks (Score 0) 363

A reactionless drive, and the likelihood of it existing, can be compared to a perpetuum mobile. And there is a reason why patents are not granted for perpetuum mobile: it's because they don't work, and can't work.

The further discussion in this, thus, is whether one indeed 'need' to test it, seen the fact there is a limited budget, time and money, of scientific research. And if one deems it is, then why should you then also not claim the same for any perpetuum mobile or other crack-pot idea that pops up, to be researched?

It then is often argued 'because three tests have confirmed it', but that is exactly the crux of the matter: they didn't. There is as much proof or evidence or confirmation for that microwave-oven to be a reactionless device, then there is for any perpetuum mobile to be truly working, or for the 'thrust' to be due to dragon-magic.

My suggestion is: that those who are so convinced of it actually working start a kickstarter, and pay for it that way. Then, everyone is happy: the EM-believers, the sceptics, and the taxpayers.

Most physicists worth their salt realise how EXTREMELY (and I'm using an euphemism here) unlikely this is, and that's exactly why the scientific community at large doesn't jump on it: they have better things to do than waste their time and money on such a thing.

The inventor claims it can be explained by conventional (Newtonian) physics. That is utterly impossible. Newtonian laws (nor GR, *nor* classical QM for that matter) allow for such a thing.

At least White realised this, which is why he invented the "pushing against the virtual plasma" as an 'explanation'. Only problem is, there is no such virtual plasma, at least not in any known theoretical framework (or even an unknown but consistent one), and I didn't see White put one forward neither. So, basically, it has the same validity as saying 'it's pushing against dragon-magic'. It explains *nothing* at all, and thus can't be considered a valid explanation for the force measured neither.

Basically, for all intents and purposes, it IS a reaction-less drive, because it adheres to all the hallmarks of one. Claiming it is not doesn't change that fact. It would be akin to someone claiming the apple he throws and comes down is not due to gravity, but due to dragon-magic. Now, one might *CLAIM* it' not due to gravity, but it doesn't change the fact it IS still due to gravity. And until one comes up with proof or at least a consistent theory which is better than the explanation of gravity (aka, a falsifiable explanation of dragon-magic) such claims change nothing and are nonsensical statements.

To the defence of Tajmar, White, and maybe even the inventor: I don't think it are *deliberate* falsehoods. Contrary to say, Rossi with his scam of the E-cats. I don't think they are scamming or consciously deluding people. I've seen the presentation of White, and noted the critical stance of Tajmar: they don't seem to be deliberately confusing people into believing this story. Certainly not the latter. White might have been a bit too speculative, and the scoop-searching press and EM-fanfappers do the rest.

That said, one can rationally argue, or at least ask the question: with a limited budget for scientific research, if it's warranted to put taxpayers' money in such an extremely unlikely claim - without any theory nor decent experimental data to back it up. Even if it's only 500K, it could be better spend on some other scientific projects.

Or, alternatively, the fanfappers could make a kickstarter for it. I've asked this 6 times already, but: where *is* that kickstarter? What's keeping them from doing one? If everyone of the EM-believers would put their money where their mouth is, it would already have gotten 500K. Everyone happy.

I love new things. Only, I don't *believe* every new thing that comes along, until proof of it has been delivered.

And, as usual, you are reversing the burden of proof. And you know it. I have told you numerous, numerous times in the past, so you ARE aware of this: it's for the ones MAKING the claim, to prove it exists, not for others to prove it doesn't exist.

That's exactly why I give the example of a magical dragon in my garage.

Now, what is the initial claim here? That the EM-drive is a reactionless drive who pushes itself forward by bouncing microwaves around INSIDE itself, in a closed compartment.

Well, then: the burden of proof is on those making that claim. It's as simple as that.

And did they prove that? No, they proved they measured a tiny force, which could be caused (and is far more likely) by a myriad of things, such as artefacts and measure errors.

Am I claiming it IS an artefact? No, I'm saying it's a trillion times more likely to be an artefact or error than it is that we've discovered new physics that invalidates all what we know and have observed for the last 400 years. It's for THEM to prove it's due to the EM-drive working as they claim it does, and not something which is vastly more likely to be the case, such as artefacts/errors.

Seems rather rational and logical to me.

The problem with White's 'explanation' is that it can not be. There is no virtual plasma to push anything off, and the quantum Vacuum can not act like a propellent. It's simply not possible, within QM. Some people also mistakenly think that QM defies the law of conservation of momentum. IT DOESN'T. Not only classical physics, but also QM adheres to CoM.

So, there is NO way one can explain things that way.

What white is saying, basically, is that that it works by tooth-fairy magic. That is to say: that statement has the EXACT same worth as what he is saying for an explanation.

Hence, nothing has been explained at all, and you still have that CoM violation, which is impossible. Ergo, it can't be working as it is claimed that it does (as a reactionless drive).

Btw, not too many people seem aware of this, but the EM drive would also violate the law of conservation of energy, since you could make an over unity device with it, and thus free energy (aka, a perpetuum mobile).

Now, what about 'new physics'? Why couldn't completely new physics not explain everything, and be responsible for the 'thrust'?

This is because IF the CoM principle could be violated (and by the mere resonance of microwaves, no less), it would mean that fundamental laws vary depending on localisation. This in turn would mean, the speed of light varies, the strong nuclear force would change, etc., and thus whole swats of matter would spontaneously disintegrate into atomic and subatomic particles and exotic matter, and flood the universe . This, however, we have not observed, not even once, for the last 400 years. Hence, the extreme unlikelihood of such a claim.

As said, any new laws would still need to adhere to all previous predictions and observations. Since we never observed any of the consequences of such a thing, it is EXTREMELY unlikely to be true. About as unlikely as that we'll discover tooth-fairy magic holds the universe in check.

What a lot of people do not seem to get, is that any new physics will not suddenly make all our previous observations and predictions invalid, and THUS any new physics will incorporate the old physics (just as newtonian physics was incorporated in that of Einstein). Only, it will do a better job at it, especially in domains where our current laws falter, like inside a singularity.

What it WON'T do, however, is being antithetic to it, and claiming a violation of CoM, even under normal conditions (like in a mirowave-oven), is suddenly no problem anymore.

Comment Re:Prepare to be (Score 1) 363

Nonsense. Name one of the basic natural laws that has been 'broken'.

And no, Newtons' theory of gravity wasn't broken by that of Einstein:

I'm saying this in front, because it the typical thing people who are not to knowledgeable about the topic come up with. No, it's not been 'broken'. It is *incorporated* into GR as a special case, but within its domain and a given framework, it remains valid.

Comment Re:Sixty Years Ago... (Score 1) 55

Is Skylon the only possible system using atmospheric air?

People seem to be a bit too much focussed on Skylon, although I admit I'm interested in it too, from a technical/technological standpoint.

The *principle* of the matter I just explained stands, though: that which you avoid taking with you as weight (the oxidiser) means you do not have to carry that weight, plus, you don't need to carry the weight of the fuel needed to carry the weight of the oxidiser, plus you do not need the weight of the fuel to lift the weight of the fuel that is needed to lift of the oxidiser, etc. There is a considerable reduction in weight, thus, and - as said - that matters for the useful payload, not for the reduction of *cost of the fuel* as some posts imply.

Whether or not the reduction of weight is always offset by the additional complexity and added weight (of the wings and engines, for instance), remains to be seen, but I would wager that totally depends on what system you use. Especially with non-SSTO systems (the Falcon is non SSTO too, after all) there are myriads of options one could use, and I doubt they'll all be 'worse' than the current Falcon-oxidiser-carying-system in regard of dry mass.

That said, if you're talking economically, and especially if you're talking specifically of Skylon, I think the reusable Falcon is far more viable. But just like all his competitors thought re-usability of a rocket was not (cost/benefit) worthwhile, and Elon Musk is (going to) proving them wrong, it might be that someone smart comes along, also with new ideas how to tackle things, and get affordable air-using systems going, which would be a new 'disruptive technology'. After all, the advantages - at least in principle - I explained in my former post remain valid and are physical sound. It's just a matter of finding a good approach that outpaces current systems.

Comment Re:Sixty Years Ago... (Score 1) 55

That said, the point of a SSTO with airbreathing is not the *cost* of the fuel, but the weight-reduction itself, since the added weight that has been avoided in this way can be used for payload.

Even Elon Musk said it's not about the cost of the fuel, but the fact it impacts the useful weight of the payload. ;-)

It's possible, thus, that for LEO an airbreathing spaceplane is more suited than a rocket, especially if that rocket isn't fully SSTO. You say it doesn't save 'much', but the oxidiser on a typical rocket makes out about a third of the total mass... I would call that pretty much.

Comment The problem with ReactOS (Score 4, Interesting) 145

Does not lay within it's technical capabilities and prowess, but with it's management skills and ability to create a community.

For years I've been a fan of ReactOS, actively participating and contributing to the community, albeit not in the form of a coder. Which, in the (still) current ReactOS mentality, is the only thing that counts for the brass, apparently. Testing ReactOS on a dedicated real-life machine (thus, not in VM), debugging, translating, making info-pages on their wikipedia: nothing really matters to the 'elite' of ReactOS. They just consider you some sort of fodder, and the moment you ask for a bit more transparency (on their financial side, for instance), or try to address the complete lack of community involvement, they bork (and bark) at you - and worse. Time and again I've tried to explain this in the past, that a successful project is NOT merely depended on the technical/coder side of things, but also how you establish a community, and try to involve people in your project. There were some half-baked trials at it, but those were mere lipservice, where it was always a consideration of a top-down approach, not a bottom-up way of seeing things.

Ergo; in the 20 years of their project, they have not succeeded in gathering 1/100th of the community that other projects like Linux have. Again and again I've pointed this out to them, but to no avail. They just refuse to see it, and only want things done strictly to their wishes, with their attitude of finding no other (real) importance to the project than 'code'. Which, granted, is an important part, but which will NOT get you a lively, engaging, and growing community - which is an absolute necessity for any open source project to know success. Eventually, since I kept hitting that nail, they got annoyed (not: they realised the error of their ways) and kicked me out too, the so many-th person who was a tester/sponsor/translator/helper of ReactOS they managed to drive away.

Whatever; so I went away, which is what they wanted. But what did it help them, then? Nothing at all. They view constructive criticism as a threat, not as an opportunity to better themselves and the project. They don't value anything someone does outside of their little constraints, and outside their preconceived notions as to what they see as important, yet have the mouth full of 'community-involvement'. Whatever you do, how many years you may have sponsored or helped out as a non-coder, you just are not counted as having done anything worthwhile. They have no inkling of an idea how to get a thriving community where you allow the bottom-up approach as well.

It's a sad thing to say, but the whole thing is run by people with overblown ego's, trying to protect their little turf and egotistical whims, ignoring anyone else, and being autistically elitist in wanting to decide virtually everything to the minute detail. Its only a community-project in name, but not in essence. And that's why, even after 20 years of operation, their project is just a small-scale project who - in comparison with other projects - has almost nothing to show for. They make some small technical progress year after year, that's true, but they fail to realise how much their ego-driven top-down approach has muffled the project to achieve the grandeur and support it could have had, had the top been more prone to some input from the people actually supporting it.

I've poured hundreds of dollars in it, and spend years on it helping them in my own way, only to kicked out when I pointed out their mistakes and lack of transparency. I still lie, the project, but I can not, in good conscience, support (most of) the people running that program/project. they've done it with me, as they've done it with dozens of people: it's no surprise, thus, that they're still at the small scale they are, without any community to speak off. They can't get any traction, because they kill off everything that would get them traction. There are some good coders under them, to be sure, but almost all of them lack the managing skills to actually create a project where people feel engaged in, and want to continue to support. How many people, running, debugging, sponsoring the project didn't they drive away, just as they drove me away? It's painful to see people shoot in their own feet, but that's the way it is. They just don't care.

Comment Re:Laws of nature, not Man's laws of physics (Score 1) 347

The point is, a microwave-oven is well understood, and has well-understood physics. It's not an extreme situation, like in a singularity of a black hole. And it's in those circumstances (the very large or the very small, or the very energy dense) that our current laws break down.

IF the resonance in a microwave oven really could break CoM and CoE, we would have seen the dramatic consequences of such a thing (and its underlying law) ages ago.

Also: our basic laws aren't wrong in the strict sense. (Though this comes into the domain of semantics, I guess). They're incomplete, yes, but in their own domain, they describe reality with an astonishing accuracy. If any *new* physics will be discovered or developed, it will incorporate ALL that our current physics have demonstrated and have been observed, it won't negate it.

Comment Re:Umm no.... (Score 1) 347

There have been a few tests with lightsails, such as...well, the Lightsail (of the planetary society). I happen to have sponsored that on their kickstarter, since I deemed it worthwhile. They'll send an updated version with the heavy falcon.

Of course, most of these experiments were meant to use photons from our sun, not from laserbeams. And most were near-earth tests, nothing like a flight to Jupiter. I'm all for testing it out small-scale first, with an solar-system interplanetary StarChip (PlanetChip? ;-) ).

But well, for arguments' sake, I think we can agree that, while unlikely, it *could* be possible we (depending on our age, and/or longevity progress) see a thing like the StarChip reach that starsystem. They've raised most of the problems themselves on their site, but all of them are technical and engineering problems, nothing that is a real showstopper, let alone that it would violate a law of nature. So I think we'll get there, at least in principle. And, well, in our lifetime... I deem it unlikely, but as said, it's *arguably* possible.

I agree with you on the beryllium copper shield (well, actually, they said coating); it doesn't seem very practical in resolving the issue, and depending on the thickness it would have a dramatic effect on the weight (and thus, thrust/weight ratio) - which seems weird, seen their focus on ultra-light apparatus. But I'm no expert (yes, an unheard-off confession on slashdot ;-)) on shielding, so I wouldn't know the benefits of such a coating, and how thick it would need to be to be effective.

That said, I disagree with your stance .2c would be unattainable. At least, on the principle of the matter (talking outside the 'in our lifetime'-argument, thus). In fact, I remember reading that, at least theoretically, a lightsail with good (sail)surface/(useful)mass ratio powered by large amounts of powerful lasers, and - most importantly - which are *continuous* working for years or decades, could attain speeds of up to .6c. The latter being the most important, since the speed it gets from the photons is cumulative, and thus the longer it gets pushed forward, the more its relative speed will increase. Of course, the destructive power of interstellar dust-particles would augment too, so there are other considerations to be made as well. But I don't think there is anything prohibiting reaching .6c in principle, at least.

The same article also said the more mass it has, the more exponentially difficult, aka: more energy it would need (which is why the StarChip is so focussed on the ultra-small too), and anything that transports humans would be quite massive, and need gigantic lightsails and dito lasers pointing at it.

So, yeah, not going to happen any time soon.

Still, I'm all for NASA or whomever to at least test a system like StarChip out in our solarsystem. At least it wouldn't be wasted like on an EM-drive.

Comment Re:Laws of nature, not Man's laws of physics (Score 2, Insightful) 347

I would refute this.

The laws of physics are not 'made' by men - at least not in the sense of 'made up', it's based on what nature tells us it is. If nature had shown us something else, our physical laws would be something else as well. If you want to argue that our knowledge is not perfect, I'll grant you that. In fact, this has been known to science for quite some while.

But what most lay people do not seem to understand, is that, while our current laws aren't perfect, they're astonishingly accurate nevertheless and *anything new* (aka, new physics) would NOT contradict what we already observed for the last 400 years. Any new physics, thus, would not go *against* our current physics, but would merely improve upon it, specifically in extreme situations (like in the singularity of a black hole), where our current laws break down.

It would NOT suddenly allow for CoM and CoE to be broken, like the EM device would. Because if a microwave-oven would be able to brake CoM, we would *ALREADY HAVE OBSERVED* the consequences of such a thing. A microwave hardly is an extreme situation where our laws break down, after all. And if that's all that it takes to break CoM and CoE, we would already have seen the consequences in the universe around us. This is because IF the CoM principle could be violated (and by mere resonance of microwaves, no less), it would mean that fundamental laws vary depending on localisation. This in turn would mean, the speed of light varies, the strong nuclear force would change, etc., and thus whole swats of matter would spontaneously disintegrate into atomic and subatomic particles and exotic matter, and flood the universe . This, however, we have not observed, not even once, for the last 400 years. Hence, the extreme unlikelihood of such a claim.

As said, any new laws would still need to adhere to all previous predictions and observations. Since we never observed any of the consequences of such a thing, it is EXTREMELY unlikely to be true. About as unlikely as that we'll discover tooth-fairy magic holds the universe in check.

That's why I think people thinking a microwave-oven (which the EM device basically is) is going to get us to the stars, are, indeed, extremely funny. :-)

Well, sometimes they're pretty annoying too, granted. That's because they're fanatical in their ignorance, and are not prone to any arguments whatsoever. So after a while it gets tiring.

Comment Re:Umm no.... (Score 1) 347

I would agree speed is an important parameter and indicator of being 'better' or 'higher performance'. It could also be argued, however, other things may be regarded as improvements too.

Efficiency is also such a possible indicator. "Who cares?" is not an argument in determining that.

Let's say you have airplanes that go equally fast, but one uses half as much fuel than the other (which also means less pollution, more potential carrying-capacity, etc.).. now which one is 'superior'?

If all things are equal for the rest, it clearly is the more efficient one.

Ego, more than one 'measure-stick' can be used, as the parent poster said. Speed is one of the more important ones though, as you indicated.

Comment Re:Umm no.... (Score 2) 347

Hope, faith and dreams may be of importance to the mental state of an individual, but it has no place in science, though - at least, not in the actual implementation, methodology and results of it.

In fact; hope, faith and dreams are often in the way of reaching a scientific conclusion.

And it's not with hope and faith that we managed to save millions with modern medicines, but by the fruits of science. We saved more lives in the past 300 years with science, than in the 3000 years before that with 'faith'.

Comment Re: Umm no.... (Score 1) 347

With the caveat that a difference must be made between saying something is impossible based on technological limits or difficulties, and saying the same about things that break the basic laws of nature (physical laws). The former is likely to be shown incorrect, the latter isn't.

After all; keep an open mind, but not so open your brain falls out.

Comment Re:Umm no.... (Score 1) 347

Being a nerd doesn't mean being pedantic. Though granted, on slashdot you may find pedantic nerds as well. ;-)
Let's face it here: you actually *knew* he was talking about 'heavier than air flight', right? As anyone with a bit of reading comprehension would understand. As any nerd with even a slither of knowledge would understand, because any nerd worth his salt knows fully well the Montgolfier brothers didn't fly for the first time in a balloon in 1916 (in the middle of WWI, thus). And since you seem pretty intelligent - seen your second paragraph - I'm sure you understood that as well. Those who didn't, aren't nerds, they're dumbasses, which you find plentiful on slashdot too these days, granted. But catering to the dumbasses makes little sense, so my guess is you were being a bit pedantic about it.

"I see a huge difference between the state of the art in aviation in 1916 vs 1903."

As do I. As does the parent poster, since that was part of his argument.

"Yes you actually can say something is impossible we have limits based on the physical universe."

Which is why I said: "It's a whole other story for things that go against the basic laws of physics, though, (such as the FTL, or 'devices' like the EM-drive, which is pure bullocks)." The parent poster didn't argument with things going against basic laws of physics, though. Let's, thus, give him the benefit of the doubt he was talking about technical difficulties.

I'll grant you that enough idiots are on slashdot that think the two are similar, and come up with things like 'the EM drive' (in fact, didn't I see such a post already?), and it's also true those same idiots quote things that people once (presumably) said were impossible, while talking about technical obstacles, instead of inherent (physical law) obstacles. And many make the faulty jump to say: "well, since that person said *that* (technological) was impossible, but he was shown wrong, it follows that *this* (physical law) which is said to be impossible, will turn out to be possible too!" (As, for instance, the first, original poster of this thread (anonymous coward), was implying, me thinks).

Those are idiots failing to see the difference, though. But I wouldn't call them nerds, since those lack even a passing knowledge of the subject at hand, nor, in fact, normal reasoning capacity.

"Tech reaches a level of maturity and then it slows."

This is largely true. But after some time of stagnation, new tech comes up which is better than the old, and replaces it. With the caveat that 'better' can also mean 'more economically' in our times, not merely technically better (though it often is, since at least on some fronts it must be superior for it to be also economically better).

If you're argumenting that normal chemical rockets are not going to reach any star soon, and certainly not in our lifetime; hey, I'm with you on that. But other things are feasible, even with current or near-term tech. You *did* look at the link I provided, I hope? The (unmanned) 'StarChip' they're proposing is technological feasible, and *could* get to the nearest star, arguably within our lifetime.

As said, I do agree with your last paragraph, that it's pretty damn unlikely we'll ever see a manned interstellar rocket land there, though. Well, unless longevity-research knows some major breakthroughs, mayhaps. ;-)

Slashdot Top Deals

ASCII a stupid question, you get an EBCDIC answer.