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April 2014 Archives

As part of some 'NetSafe' training, I was shown this lovely xkcd cartoon .


Password entropy. That's a good way of putting it. The statistical mechanics definition of entropy would be k ln W, where k is Boltzmann's constant, W is the number of permutations possible, and ln is 'the natural logarithm'. Higher entropy, means more possibilities. Simply put, the longer the word is, the harder it is for a machine to guess. One doesn't need terribly long sequences before the possibilities become immense indeed. 

So then, how come just about everywhere where I need to use a password* requires me to include non-alphabetic characters, and, sometimes, non-numeric characters too, making it very hard to remember my passwords. Plus, we have passwords on so many different systems nowadays (if I felt I had the time I'd compile a list of how many different systems I have accounts and passwords on - I reckon it's of the order of one hundred) that it is inevitable that we start writing down passwords in an easy to access place - thus almost completely negating the point of them. 

Computer scientists, we need a better system...

*This includes the University of Waikato, whose site I am using for composing this blog entry. It demands a mix of letters, numbers and other symbols and for the password not to be based on a word. And these IT people who control the site are the ones pointing me towards the xkcd cartoon above...Sorry to get at you guys, but the discrepancy is rather obvious. 

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Back to blogging, after a nice holiday in Taranaki dodging the rain showers (and, as it turned out, the volcano, which we never even got a glimpse of) and a frantic week of lab work while the undergraduates were away. 

Both were very interesting, but it's the lab work I'll talk about here. 

Something that I've learned over the years is that if something looks dodgy, it probably is. Obviously, when doing experimental work, we don't know what results we are going to get. (If we did, we wouldn't bother doing the research). It is true that sometimes results can surprise us. Sometimes this is the start of a discovery of a new phenomenon, which will make the experimenter famous. But more usually, much more usually, it's because you've stuffed something up in your method or analysis. If your data just looks wrong, it probably is.

We had this with our conductivity measurements in the lab two weeks ago. We were using a moderately high-tech (approx 10k NZD or so) piece of equipment to measure electrical impedance of our samples of biological tissue. The results were odd - we had an unexpected jump in conductivity as we changed frequency. It took a while to track down what the problem was. First, I talked to a colleague who used to work for the company that made the equipment we were using. He hadn't seen anything like it before, and offered a few suggestions as to what we might do to track down what was going on. There was the suggestion that it might even be a calibration failure in one of the machine's internal circuits. 

We did a few tests, and were still puzzled. We tested progressively simpler and simpler things, trying to isolate the problem. It was a good exercise in troubleshooting, really, and it took a while. We ended up with testing the impedance of just a single 10 ohm resistor. We didn't expect this to be an issue. But, when the machine told us that it's impedance was 8 ohms for frequencies lower than 121 Hz and 6 ohms for frequencies above 121 Hz we knew something was terribly wrong somewhere. Then the machine refused to work altogether. At this point the thought of ten thousand New Zealand dollars going up in smoke in front of our eyes did cross my mind, but only momentarily, since suddenly it kicked into life again and started reading 10 ohms. Then I just touched the front and it was 8 ohms. A bit more experimenting quickly narrowed down the problem to a dodgy lead.

That was all it was. One of our coaxial cables had a dodgy connection on it. We replaced the lead, and suddenly the results are all perfectly believeable again. Only two days of work for two people completely wasted by a cable worth only a few dollars.

The moral of the story: It pays to do some really simple tests of the equipment every time you use it. Don't just blindly trust the readout on the machine. Check it's working first. I recall now the words of one of our (now retired) technicians here - "If you have a perplexing problem with something electronic, it's a fair bet it's simply a dodgy connection." How true. 

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I've had a few difficulties in some discussions with students recently. It comes down to this: "How do I explain something that is so blatantly obvious it doesn't need explanation?" 

The problem really is that a particular concept can be obvious to me, but not obvious to a student. The danger is then that, in a lecture, I just assume that a student is implicitly happy with such a concept and I plough on forward without any hint of explanation. The consequence is a bemused student who just doesn't get what I'm talking about. 

It is really, really hard for a lecturer on two counts. First, you have to recognize the fact that something might not be so obvious to a student. That's pretty tough when it's second nature to you. Secondly, you have to find a way of explaining the (to you) blatantly obvious. 

Here's an example. I was asked by a student why velocity was equal to rate of change of position. He didn't get it. My initial response was "But that's just the definition of velocity - it IS the rate of change of position - what's there to get?". But that didn't cut it for the student. I had to dig deeper to see what the stumbling block was. It turns out that he had a vague understanding of velocity from his everyday experience, but not one he could put down in precise, physical terms. He was also (as many students are) uncomfortable with using vectors. Therefore, he couldn't see how to match a rate-of-change of position, described in terms of vector calculus, with his everyday concept of what velocity was. When I said "They're the same thing by definition" - that didn't help one bit. Why are they the same? 

Often, when someone is not grasping the blatantly obvious, there's some underlying block in their thinking. In teaching and learning literature, we talk about threshold concepts - ones that are really difficult to get, but once grasped, transform the way that someone thinks. Once someone's crossed that threshold, it might well be blatantly obvious. But beforehand, it certainly isn't. Teaching a threshold concept is very, very hard indeed, especially if you yourself crossed that threshold many years ago. Often these things are best taught by those who have only just 'got it' - i.e. students teaching students, or peer-instruction. 


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A couple of weeks ago I had the misfortune to be on a bus which had an accident. I wasn't hurt, because I was safely seated, which is more than I can say for one unfortunate passenger who was still on his way to his seat at the time. It wasn't a high-speed event - I'd guess we were doing about 10 km/h. We had just pulled away from a bus stop, when a car that had been parked a few metres in front of the bus decided to pull out into the road right in front of us. The driver hits the breaks hard, and, as a result, the fellow passenger ends up in a heap on the floor at the front of the bus. 

While the cause of the crash I would say rests firmly with the driver of the car that pulled out, that's little comfort to the poor guy with blood dripping from a wound on his head, down the back of his shirt, which is probably now dyed a nice shade of maroon. Standing on buses is pretty dangerous, even at low speed. I do think the driver should have waited till everyone was seated before pulling away. 

So, from a physics perspective, what happened? One can explain this in two ways. There's the 'inertial' approach, as explained by the witness on the side of the road: The bus stopped, but the guy standing, who has inertia, carried on. Then there's my viewpoint, from inside the bus. Everything experiences a sudden acceleration forward. This causes the passenger to lose his balance, and down he goes. 

This forward acceleration, from the perspective of the person on the bus, is called an apparent force. It arises because the frame-of-reference, the bus, isn't an inertial frame. That is, it's accelerating (or, in this case, decelerating). It's called 'apparent' because the person on the side of the road wouldn't see it in this way; it only becomes apparent if the observer is in the accelerating frame of reference. It might be termed an 'apparent' force, but for the person on the bus it's a very real push forwards, one that splatted him on the floor and would have given the bus cleaners a more interesting job that usual.  It's the same kind of thing as centrifugal force (yes, the 'f' word), which one experiences when going round corners. To the person in the object that is doing the moving, the force is a very real thing (ask the racing car driver). But to everyone else, it doesn't actually exist. 

Apparent forces are pretty hard to teach (I've just been doing it), but I think the key is really to emphasize that they are there only to the observer who is in the accelerating frame. 

What happened to the passenger? Against the advice of everyone around him, including me, he refused to be taken to a medical centre, which was only a few hundred metres from the place of the incident, and insisted on carrying on the journey to his destination. Possibly if he'd been able to see the back of his head he might have thought differently. One shudders to think of the consequences at 50 km/h. Seat belts in buses? Yes please. 





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