Posts by Michael H.F. Wilkinson 4255 publicly visible posts • joined 24 Apr 2007
Both can seem to have eternal (un)life. You might think they are long dead, and all that is left is dust along the wayside or in the courtroom as the case may be, but all it takes is a drop of blood (or money) in the right place, and...
LOOK WHO'S BACK!!
I really wonder when we will see a stake driven home properly
Many problems in image analysis (and physics) can be cast into sparse matrix form, so I will certainly take a look, especially for the gigapixel and terapixel images we are getting. I also wonder whether many graph-based methods could benefit, especially if the average number of edges per vertex is low. Many of those algorithms may well be suitable for this sparse-matrix architecture.
For my MSc thesis I worked on part of the design of an asteroseismometer, which used Doppler shifting of absorption lines in stellar spectra. In those days that approach produces a much stronger signal than a photometric approach (measuring the slight brightness fluctuations caused by the changes in temperature) used in the this study. Interesting to see the advances in precision in current instruments (I gather the data used were from the Keppler mission). Great work from those astroboffins
Good point about the coding skills. The danger of not needing good coding skills means that people with poor programming skills think they can do it. As I always tell my students coding means you can cast a logical solution to a problem into the correct syntax for a given language, programming means you can actually derive the logical solution to the problem in the first place. I suspect many more problems in code derive from sloppy thinking than from the odd syntax error.
Trust me, I have read more on chaos theory than most (though there are certainly deeper thinkers on the topic), I even gave a lecture or two, and have written a paper or two about the subject. Gleick's book is nicely accessible, but there are more rigorous works (Ott, Sauer, and Yorke, Coping with Chaos (1994) contains a good collection of papers, I found, but that is a bit outdated now, perhaps).
One of the first things to do in any system of non-linear equations is to see if positive Lyapunov exponents are expected (sometimes in a simplified version of the full set), and the system might therefore be expected to be chaotic. In the case of my intestinal micro-flora simulations the (simplified) system showed no chaotic behaviour whatsoever. That makes life a lot easier.
The problems with floating point inaccuracies coupled to chaotic behaviour is well-known to any scientist worthy of using HPC systems for numerical simulation (and, yes, there are many that aren't worthy). The Shadowing Lemma has a thing or two to say about this, in particular, that you may not be tracking the path form the initial state, but there exists a path close enough to the simulation run for an arbitrary length of time, starting from some (unknown) perturbed version of the initial state. The simulation trajectory "shadows" a trajectory of the true system. Having said that, a true chaotic system with one or more positive Lyapunov exponents cannot safely be simulated with just one run. Typically you have to run a huge ensemble of runs with different starting points for each set of parameter settings before you can say anything about the model system's behaviour. Only then can you start making sensible comparisons with experimental results, or real life. Should real life observations differ from the model significantly, your understanding of the problem as captured in the model is inadequate, or the simulation method is flawed.
Results from computer models should always be scrutinized carefully (just like any other result in science, really). It is essential that all source code (including version information), parameters, and hardware information should be made public.
In many cases, I find that failure of simulations is often the most interesting part. People postulate a mechanism to explain some phenomenon, and might leave it at that: a just-so story. By drawing up a computer simulation you can test ideas more rigorously, and you sometimes find that the proposed mechanism cannot produce the observed effect. Once you have ruled out numerical instability or just programming errors, you can then safely state that the proposed mechanism does not explain observed behaviour, and that therefore the theory requires some adaptation.
Likewise, leaving out parts of a system because they are too hard to model, or you first want to start with a simple model, and add complexity later (always a good approach) can also give important information. I have done some modelling of bacterial communities in the intestine (insert "your model is shit" joke here), and found that quite a few properties can be modelled well enough fairly simply. For example, even without modelling an immune system, ratios of aerobic vs anaerobic bacteria could be reproduced very well simply from decent estimates of the amounts of nutrients and oxygen entering the system. Practically no other parameter had ANY impact on that ratio (ODE/PDE solvers used, hardware used, time steps used etc.). Not that surprising to biologists perhaps, but medical researchers had assumed the immune system controlled the intestinal microflora. It might do so, but apparently it is not necessary to control the aerobe/anaerobe ratio.
By contrast, if your simulation explains things nicely, you have not learnt that much, only that your mechanism is plausible. Proof requires a lot more than mere simulation
It is indeed important to separate the situation at schools from the situation at universities. Our CS curriculum contains many courses where the foundations of computer science are taught, including the maths behind it, such as discrete structures, languages and automata, program correctness, besides courses in imperative programming, object-oriented programming, functional programming, parallel programming, software engineering, networks and computer architecture. etc. The emphasis is very much on what is happening under the hood. Prospective students often ask us what programming languages we teach, and we invariably answer that that is really unimportant. We teach programming paradigms, and the ability to learn new languages. Once you know how to program in one structured imperative language, learning another is largely a matter of learning syntax. What is far more important is learning how to cast a problem into imperative-programming (or OO, or functional) terms.
Many schools here in the Netherlands do not teach CS, and those that do, often struggle to find good, qualified teachers. We also find that for those students who did follow CS at school, their maths grade is a far better predictor of success in CS at university than their CS grade at school. The CS taught at school does not really prepare them for CS at university. This is why we organise outreach events for school kids, to show them what CS is really all about. We recently had a contest for school children in which they had to solve a series of problems (such as cracking a Caesar Shift code) by designing Turing machines for the task. This levelled the playing field, because none had ever done this, and they cannot cut and paste solutions from anywhere. It also teaches them a structured approach to solving problems. The day was a great success, and they thoroughly enjoyed the challenge. We will make the course material and web-tools available to schools, and are looking into other ideas.
Having ballistic missiles fail at launch has a strong "shoot yourself in the foot" feeling to it, although the effects may not be restricted to a single foot.
Any suggestions for the programming language(s) they use?
Fortunately, in case there was evidently more of a damp squib effect, so the icon is not entirely appropriate.
There are many open access journals already, where the author pays for a publication, which should contribute to the costs of running such a site. Fees vary wildly, I must say. Many classical journals are also offering the option of open access, and often allow the author to have a copy on his or her website, typically with a statement that this has been made available only for research and education purposes, and with clear copyright statement.
Making data sets (and code) publicly available is also a trend we already see in science. Very handy for those who which to replicate results.
I think the move by the EU makes sense.
"Share and Enjoy!!"
I personally would much rather be served by a human than a humanoid robot which generates an intolerable air of smugness whenever it is about to sell you something.
A human would (OK: might) know when to be chirpy and cheerful, and when just to be businesslike. There are situation when chirpy and cheerful might earn a robot a reprogramming with a big axe it will never forget
He flew as a navigator in Lockheed Venturas over Africa. He experienced some real turbulence during that time, with the plane lurching or dropping like a brick suddenly as they were flying a much lighter plane than any Boeing or Airbus you care to mention through the weather, rather than over most of it.
Beluga whales are far from ungainly, they are actually very graceful in motion, if not quite as acrobatic as dolphins (their distant cousins). Their vocalizations earned them the name "sea canaries"
Regarding the sexiest plane ever? People have proposed some great contenders (XB70, TSR2, SR71, Vulcan, Spitfire and Me262 are all in the super-model category), but I do have a particular soft spot for the Wooden Wonder: the De Havilland Mosquito.
The story somehow reminds me of the time I spent reading the small print of the insurance policy my parents took out on a small sailing dinghy (a 12 foot affair hardly counts as a yacht) they had bought. I was rather amused to see that the insurance explicitly stated that if the craft was used in acts of war, they would not cover any resulting damage. I immediately had this mental image of the little craft capsizing under the weight of 21" torpedo tube installed anywhere on deck.
Nuclear explosions (even if the little dinghy wasn't actively involved in military actions) were also not covered.
The reason we often do not want truly random numbers is that we want to be able to generate the same sequence again with the same seed (or set of seeds). If we have truly random bits, and want to use that for encryption, we must pass the whole sequence to the receiver through a secure channel (i.e. we are using a one-time pad). Very secure if we can pass the pad safely to the receiver.
In the past linear congruential RNGs (shudder) were often combined (it's even Numerical Recipes) by taking the "better" higher order bytes of two different ones and concatenating them, because the lower order bits were not very random at all (the least significant goes 0,1,0,1,0,1,0,1,0,1,0,1,0,1,.... if you have got it right, otherwise it goes 0,0,0,0,0,... or 1,1,1,1,1,...). This shows that the basic idea is certainly not new, and for certain (toy) problems it may well suffice. What apparently is new is a proof of method to combine two reliably (still have to read the paper properly). Proof is a thing that carries rather more weight in science than just a nice (and plausible, or even obvious) idea.
I gather John Cleese was an option at one stage:
"Right! Now turn left!"
I bet he could handle swearing back at the driver as well. Battery sergeant major 'Shut up' Williams could do that nicely too:
"I'll make you turn left so fast YOUR FEET WON'T TOUCH THE GROUND!!!"
Megaphone not really needed, I suppose
I didn't get that pop-up. Looks like GWX control panel is working.
Win 10 may be better on various fronts, but I resent the way they are trying to force feed us the OS, as much as I resent the data slurping and forced upgrades embedded in it. I have several data capture applications that work under Win 7 and 8.X. When capturing a long sequence of data (like the 176 GB from the Mercury transit), I do not want this interrupted by some upgrade MS foists upon me. I do tend to keep my system updated, but I also do want some degree of control.
Couldn't we make an even bigger balloon by filling it with all the hot gases produced by politicians of all nations?
On the other hand, maybe it wouldn't get off the ground, just as most ideas vented by said politicians.
More seriously: good work from NASA getting the balloon launched after all.
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