Split a piece of wood and I am there; lift a stone and you will find me.
All very well, but it's hard to convert people to your religion with broken twigs and cracked rocks. Stained glass and gold ornaments on the other hands really wow the average pleb. And hey, when you're going about saving people souls, who's to say you should look the part! It's all for a good cause right?
Why does everybody have to sue for everything that a company does or doesn't do?
Because our legal system is broken. Very, very broken.
I for one have lost faith in modern jurisprudence. The simplest of cases take years to resolve. Big corporations routinely beat unmonied opponents into submission. Bar associations have complete monopolies over the legal services. Rhetoric and hysteria dominate court decisions, and the sway that the media have over judges and juries is such that in a lot of cases, in effect, the justice of the mob prevails.
I live in a country(Ireland) where if you are imprisoned under a law that is later found to be unconstitutional, you don't automatically gain your freedom when it is struck down. I'm sure there's a Paddy joke or two to be made on this one, but if you think this isn't the kind of legal system your country is inexorably heading towards, you're more optimistic than I am.
People are abusing our legal system because it is in a state of decadence, or always has been. Maybe back in the day when only lords and ladies took cases to court, the pressure wasn't too great and the barge could still hold water. But in the era of ambulance chasers and "no win, no fee" legal lotteries, with everyone and his mother being able to take cases, leaks are springing up everywhere. Lawsuits like this one are merely a symptom of a much more serious disease.
...a new test suite for comparing server efficiency that Nelson believes will be similar to his own benchmarks that measure server power usage directly from the wall plug.
OK, I'm intrigued. What kind of fudge do the current efficiency tests consist of? Measuring generated heat with a thermometer?
Before, people used to say "I don't like ads in my browser" as an excuse for not using it. Then when it became free, it was "I use lots of GreaseMonkey scripts", despite the fact that you can use most GM scripts in Opera too.
I have had one consistent objection to Opera, despite its technical superiority.
Opera is a closed source application made by a private company.
That alone is enough for me not to use it. Frankly I'm hard pressed to tell you which I'm more adverse to. Closed source or the private company. The two together make for an completely unpalatable mix.
Closed source is not to be trusted. Private companies are not to be trusted. Opera is not to be trusted. They may be on the straight and narrow now, but their methods and philosophy can and probably will change at the drop of a hat, contract or lawsuit. Some may think I'm being paranoid. I think they're being naive.
Ignoring the >1 case, which ain't ever going to work, I'd seriously doubt it is the CFL condition that's really causing it. What are your boundary conditions?
I've set the function to zero at the boundary points for the 1D and 2D waves. I don't believe these are the problem. If c*dt/dx is not exactly one, then, as I have coded it, the hyperbolic characteristics of the equation will not pass directly through the grid points. I believe this is the cause of the error when c*dt/dx That's a really disheartening statement. If you're considering making your living in this field, learning the basic theory is not a waste of time. If you find mathematics tedious, it's likely that numerical simulations are not for you. 90% of anything is crud, and this applies as well to mathematics textbooks as it does to anything else. I have therefore a less than 10% chance of coming across a good textbook, and this is exacerbated by the fact that it generally takes a few hours to discern how good the book is. I think "lucky dip" is actually a pretty generous description.
I've already done the basic theory of finite differences and finite elements, but this isn't of much use to me for the current problem I now face.
Out of curiosity, does this problem have a direct application, or just solving it out of academic interest?
My grant application proclaims the usefulness of my work to all and sundry, from the bade in the crib to the heights of international space programs. I however am engaged in the process, as they say nowadays, "for the love of the game".
I'm mostly interested in the cauchy problem, or the initial value kind. Forcing terms probably won't be singular, but will be short pulses. The coefficients will not only be variable, but will be themselves singular, as I'll have discontinuities between the various mediums.
Computing integrals did occur to me, in the context of using Green's functions. However, many of the Green's function integrals are naturally divergent and have to be massaged in some way in order to converge and I'd like to avoid that approach.
In what ways does it not work well for you? It doesn't converge, takes too long to converge? What is the problem?
Standard level centered differencing for the wave equation has instabilities, related to the CFL condition. Unless c*dt/dx is exactly equal to one. Greater than one and the solution is unstable. Less than one and the solution invariably develops about a 5% error per time unit. Variable wave speed means I can't get a stable mesh without drastically customizing it based on the specifics of every speed profile. I don't want to go into this level of model nursing.
I haven't yet tried an implicit, rather than explicit, finite difference scheme because coding it would take a considerable amount of time and I have no guarantee that the exact same problem won't arise all over again despite the investment. Textbooks are of little help as most, if they deal with the second order wave equation at all, generally stick like glue to explicit methods. Conditions about stability, if they are mentioned, are usually phrased in several pages of tedious and theoretical derivation, so it's not possible to simply look up the "assumptions" in most cases.
There wasn't a lot one can say about a problem in the space of a Slashdot summary, so that's why it's fairly patchy. I'll take this opportunity to more fully expound my problem.
I work on sonar/seismic/radar inversion problems. Essentially the problem of mapping terrain or subterrain by measuring scattered sound or radio signals, e.g. with synthetic aperture radar. One thing I seriously lack at the moment is a good wave simulation that I can simply play around with to get a feel for both wave mechanics itself and for the equations and techniques of the field.
Analytical, asymptotic and ray tracing methods to approximate the wave equation are all very well, but at some point I feel I need to see a full solution, or a good approximation to one. I also need a method of simulating emitted sonar and radar pulses, their interaction with "obstacles" or features they encounter, and the returned or scattered signals from this interaction. I need a way of doing this with highly irregular scattering obstacles, both in terms of geometry and wave speed.
What I would most like to get is a model of wave propagation in a simulated 3D domain with highly irregular boundaries and speeds, something that would defy most analytical approaches. My goal is to try and simulate actual subterranean features via fractals and other techniques, and use the numerical wave equation simulation to get a good simulation of what real life returned signals would look like. I need a good simulation because, as you would expect, the inversion algorithms that map out the terrain from the returned signals, can be very sensitive to variations in the signals they receive.
Are you doing the time harmonic case (3-D Helmholtz) or an unsteady case?
I'll be working in the unsteady case as I have reservations about transforming to the Helmholtz equation, not least of which is the necessity of taking the fourier transform of the source signal. I'm trying to get as exact a solution as possible.
You don't say what goes wrong with finite difference codes...
The ones I have tried suffer from the problems related to the CFL condition. To sum it up -if c*dt/dx is not exactly equal to one, problems arise. Greater than one and the method is unstable(horribleness). Equal to one and things are peachy. Unfortunately, less than one and the method, though stable, seems to suffer from either a numerical or some other more subtle type of instability. I'm not a numerical analysit, nor do I have time to probe further. This rules out these methods as c will be variable in any practical problem I use the code on. I'm also worried about other types of potential pitfalls; caustics, shocks, infinities, etc.
In many cases you can use multiple-scales/WKB approaches, but that depends on how the wave speed varies (relative to the wavelength).
Which is exactly why I don't want to use those methods, or any method that requires me to nurse or otherwise "prep" the method before use. I intend to throw multiple simulated terrains at the method and I'd like it to perform well across all ranges. I was hoping that in this day and age such a solution existed, but I'm aware I may be asking for the impossible.
I posted the question because I was tired of unsuccessfully Googling and unwilling to waste more time playing lucky dip with tedious textbook monographs. The reason I've posted this question on Slashdot is because the comments on many a science story suggest that a lot of professional scientists do post comments here. I'm holding out that the question may catch the eye of a meteorologist or radio modeling specialist who has worked on such a problem, and who has precisely the right technique, program and visualization method I'm looking for. Here's hoping.
Libertarians of the first kind are invariably classists, or elitists who believe that people have "their place" in society, usually with them at the top and everyone "unworthy" at the bottom. They object not to government, but to democratic government that gives power to all the people, and not just "the right people".
Most Randian literature exalts the virtues of "independent" persons, who "inherit" their virtue from their "lineage" and upbringing. It's essentially a call for an aristocracy or oligarchy, usually hereditary, with the rest of us knowing our place.
I mean, if the bastards actually want fascism, they should just come right out and say so.
It's common practice for retail store to check receipts and bag contents at the exit.
I have never experienced this. Anywhere. The only time I was ever (politely) asked could my bags be searched was the week the shop got a new scanner in and it was periodically getting false positives from items purchased elsewhere.
But then again I don't live in the US.
If I was legitimately asked out of the blue if my bags could be searched, I would probably give the person on duty the benefit of the doubt. However, if I was being asked every single time I tried to walk out of the shop, or if they actually had a policy of searching everyone (they don't _actually_ go this far do they?), then I wouldn't even bother walking into that shop anymore, no matter how big the savings were.
...Who the hell is twitter? I'm beginning to think this little spat is itself some kind of astroturfing.
Is this a Roland Piquepaille repeat incident, or a Beatles-Beatles one? Is this something new. Is this a bunch of rejected posters playing sour grapes or actually something we should give a damn about? Is this whole thing an elaborate troll?
I read this site a lot, and this is the first I've heard of "The Great twitter Affair". Explain yourselves sirs.
Slashdot Mirror
Do... do the scones have raisins in them?
All very well, but it's hard to convert people to your religion with broken twigs and cracked rocks. Stained glass and gold ornaments on the other hands really wow the average pleb. And hey, when you're going about saving people souls, who's to say you should look the part! It's all for a good cause right?
Because our legal system is broken. Very, very broken.
I for one have lost faith in modern jurisprudence. The simplest of cases take years to resolve. Big corporations routinely beat unmonied opponents into submission. Bar associations have complete monopolies over the legal services. Rhetoric and hysteria dominate court decisions, and the sway that the media have over judges and juries is such that in a lot of cases, in effect, the justice of the mob prevails.
I live in a country(Ireland) where if you are imprisoned under a law that is later found to be unconstitutional, you don't automatically gain your freedom when it is struck down. I'm sure there's a Paddy joke or two to be made on this one, but if you think this isn't the kind of legal system your country is inexorably heading towards, you're more optimistic than I am.
People are abusing our legal system because it is in a state of decadence, or always has been. Maybe back in the day when only lords and ladies took cases to court, the pressure wasn't too great and the barge could still hold water. But in the era of ambulance chasers and "no win, no fee" legal lotteries, with everyone and his mother being able to take cases, leaks are springing up everywhere. Lawsuits like this one are merely a symptom of a much more serious disease.
I blame the lawyers.
I have had one consistent objection to Opera, despite its technical superiority.
Opera is a closed source application made by a private company.
That alone is enough for me not to use it. Frankly I'm hard pressed to tell you which I'm more adverse to. Closed source or the private company. The two together make for an completely unpalatable mix.
Closed source is not to be trusted. Private companies are not to be trusted. Opera is not to be trusted. They may be on the straight and narrow now, but their methods and philosophy can and probably will change at the drop of a hat, contract or lawsuit. Some may think I'm being paranoid. I think they're being naive.
That's what she said!
I've set the function to zero at the boundary points for the 1D and 2D waves. I don't believe these are the problem. If c*dt/dx is not exactly one, then, as I have coded it, the hyperbolic characteristics of the equation will not pass directly through the grid points. I believe this is the cause of the error when c*dt/dx That's a really disheartening statement. If you're considering making your living in this field, learning the basic theory is not a waste of time. If you find mathematics tedious, it's likely that numerical simulations are not for you.
90% of anything is crud, and this applies as well to mathematics textbooks as it does to anything else. I have therefore a less than 10% chance of coming across a good textbook, and this is exacerbated by the fact that it generally takes a few hours to discern how good the book is. I think "lucky dip" is actually a pretty generous description.
I've already done the basic theory of finite differences and finite elements, but this isn't of much use to me for the current problem I now face.
I'm mostly interested in the cauchy problem, or the initial value kind. Forcing terms probably won't be singular, but will be short pulses. The coefficients will not only be variable, but will be themselves singular, as I'll have discontinuities between the various mediums.
Computing integrals did occur to me, in the context of using Green's functions. However, many of the Green's function integrals are naturally divergent and have to be massaged in some way in order to converge and I'd like to avoid that approach.
Standard level centered differencing for the wave equation has instabilities, related to the CFL condition. Unless c*dt/dx is exactly equal to one. Greater than one and the solution is unstable. Less than one and the solution invariably develops about a 5% error per time unit. Variable wave speed means I can't get a stable mesh without drastically customizing it based on the specifics of every speed profile. I don't want to go into this level of model nursing.
I haven't yet tried an implicit, rather than explicit, finite difference scheme because coding it would take a considerable amount of time and I have no guarantee that the exact same problem won't arise all over again despite the investment. Textbooks are of little help as most, if they deal with the second order wave equation at all, generally stick like glue to explicit methods. Conditions about stability, if they are mentioned, are usually phrased in several pages of tedious and theoretical derivation, so it's not possible to simply look up the "assumptions" in most cases.
I work on sonar/seismic/radar inversion problems. Essentially the problem of mapping terrain or subterrain by measuring scattered sound or radio signals, e.g. with synthetic aperture radar. One thing I seriously lack at the moment is a good wave simulation that I can simply play around with to get a feel for both wave mechanics itself and for the equations and techniques of the field.
Analytical, asymptotic and ray tracing methods to approximate the wave equation are all very well, but at some point I feel I need to see a full solution, or a good approximation to one. I also need a method of simulating emitted sonar and radar pulses, their interaction with "obstacles" or features they encounter, and the returned or scattered signals from this interaction. I need a way of doing this with highly irregular scattering obstacles, both in terms of geometry and wave speed.
What I would most like to get is a model of wave propagation in a simulated 3D domain with highly irregular boundaries and speeds, something that would defy most analytical approaches. My goal is to try and simulate actual subterranean features via fractals and other techniques, and use the numerical wave equation simulation to get a good simulation of what real life returned signals would look like. I need a good simulation because, as you would expect, the inversion algorithms that map out the terrain from the returned signals, can be very sensitive to variations in the signals they receive.
I'll be working in the unsteady case as I have reservations about transforming to the Helmholtz equation, not least of which is the necessity of taking the fourier transform of the source signal. I'm trying to get as exact a solution as possible.
The ones I have tried suffer from the problems related to the CFL condition. To sum it up -if c*dt/dx is not exactly equal to one, problems arise. Greater than one and the method is unstable(horribleness). Equal to one and things are peachy. Unfortunately, less than one and the method, though stable, seems to suffer from either a numerical or some other more subtle type of instability. I'm not a numerical analysit, nor do I have time to probe further. This rules out these methods as c will be variable in any practical problem I use the code on. I'm also worried about other types of potential pitfalls; caustics, shocks, infinities, etc.
Which is exactly why I don't want to use those methods, or any method that requires me to nurse or otherwise "prep" the method before use. I intend to throw multiple simulated terrains at the method and I'd like it to perform well across all ranges. I was hoping that in this day and age such a solution existed, but I'm aware I may be asking for the impossible.
I posted the question because I was tired of unsuccessfully Googling and unwilling to waste more time playing lucky dip with tedious textbook monographs. The reason I've posted this question on Slashdot is because the comments on many a science story suggest that a lot of professional scientists do post comments here. I'm holding out that the question may catch the eye of a meteorologist or radio modeling specialist who has worked on such a problem, and who has precisely the right technique, program and visualization method I'm looking for. Here's hoping.
There's been a lot of good suggestions so
Libertarians of the first kind are invariably classists, or elitists who believe that people have "their place" in society, usually with them at the top and everyone "unworthy" at the bottom. They object not to government, but to democratic government that gives power to all the people, and not just "the right people".
Most Randian literature exalts the virtues of "independent" persons, who "inherit" their virtue from their "lineage" and upbringing. It's essentially a call for an aristocracy or oligarchy, usually hereditary, with the rest of us knowing our place.
I mean, if the bastards actually want fascism, they should just come right out and say so.
I have never experienced this. Anywhere. The only time I was ever (politely) asked could my bags be searched was the week the shop got a new scanner in and it was periodically getting false positives from items purchased elsewhere.
But then again I don't live in the US.
If I was legitimately asked out of the blue if my bags could be searched, I would probably give the person on duty the benefit of the doubt. However, if I was being asked every single time I tried to walk out of the shop, or if they actually had a policy of searching everyone (they don't _actually_ go this far do they?), then I wouldn't even bother walking into that shop anymore, no matter how big the savings were.
...Who the hell is twitter? I'm beginning to think this little spat is itself some kind of astroturfing.
Is this a Roland Piquepaille repeat incident, or a Beatles-Beatles one? Is this something new. Is this a bunch of rejected posters playing sour grapes or actually something we should give a damn about? Is this whole thing an elaborate troll?
I read this site a lot, and this is the first I've heard of "The Great twitter Affair". Explain yourselves sirs.
Well, you've made some interesting points, but, as a firm believer in the self determination of nations, I'm going to have to say:
Fuck You And Your Groveling Apology For Colonialism.
I'd also like to say that this has been a stimulating intellectual discourse.
What your son should do: "Hi Dad! I'm telling Mom there's a log of every site you've visited."
Ah, well, you see... in actual fact... it turns out... that is to say... oh dear...
void republician_retort(point accusation_of_fascism){
o n_of_fascism))){
_ of_fascism));
_ discredit(accusation_of_fascism))){
f ascism)
if(exists(democracts.spurious_similarity(accusati
play_up(democracts.spurious_similarity(accusation
}
else{
play_down(accusation_of_fascism);
}
if(exists(democrats.main_candidate.opportunity_to
democrats.main_candidate.discredit(accusation_of_
}
fox_news.discredit_democrats();
}
Oh, how I wish that were true.