# Physics Stop

## A computer isn’t a replacement for your brainMarcus WilsonApr 30

As part of one of our research projects, one of my students has just acquired a set of tiny electrodes, set into plastic in a grid-like pattern. We’ll use this array to measure the electrical conductivity of various fluids. We don’t need 60 electrodes, about 4 would do nicely, but the particular company concerned makes the electrode arrays like this, so that’s what we’ve got. My student asked the reasonable question of whether the presence of the unused electrodes would significantly change the electric fields set up by the 4 that we would be using – i.e. would it significantly affect our measurements of conductivity.

Well, a metal electrode, even one not connected to anything, isn’t fluid, so yes, there would be some effect, but how big? Is it one we need to worry about?  I suggested he use a piece of computer software we have to ‘model’ this situation – let the computer find what the electric fields look like with all the redundant electrodes present and see.

Easier said than done.  Poor student devoted last Saturday to the task, which was unsuccessful.  The software seems to be playing up. So I said that I’d have a good look at his work and see if I could work out what was going on.  So yesterday morning, I basically pulled apart the computer model he had made and reconstructed it again, making sure everything was correct. And, bingo, the software ran, solved the problem, and gave me some lovely pictorial representations of the electric fields around these tiny electrodes.

Problem solved? Well, no.  Because at this point I start applying my brain to the problem, and ask whether the solution (as pretty as it is) looks physically reasonable.  What do I mean by that?  Well, I know that electric field lines travel from high potential to low potential. They hit good conductors at normal incidence to the conductor. Where there is a field in a conductor, currents are going to flow. And so on. And looking carefully at the solution the computer gave me, I could see that in one aspect it didn’t look right.  All my extra electrodes seemed to have magically earthed themselves. Why? I have no idea. I’ve trawled through the model and can’t find anywhere where I’ve told the computer that these things are earthed. I just don’t know at the moment what’s going on.

But, here is my point (at last, I hear you say).  If I just assumed the output from the computer programme was ‘right’, I’d have been making a big mistake. A computer programme, no matter how much it costs, always has to be run in parallel with your brain. You’ve got to ask yourself whether the output looks physically reasonable before you go and do anything with it, especially things that could prove expensive if you get it wrong. I’m glad I took the time to do this, before giving my already overworked student duff results.

## Making sense of those numbersMarcus WilsonApr 28

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A couple of weeks ago I had a cholesterol test.  (That involves taking a blood sample, and I was relieved that this time I didn’t faint.)  I collected my results from the doctor’s surgery earlier this week. The nurse handed me a piece of paper, with lots of numbers on, and provided me the reassuring comment "yeah, those look fine."

Anything more specific?  No.  What exactly does X mmol/L of triglyceride actually mean? I assume it means I had X millimoles of triglyceride compounds per litre of sampled blood, but what is normal for someone my age? What is considered acceptable? Does this mean there are issues with my diet? So it was left for me to sift through hundreds of trashy ask-the-doctor websites till I found some reasonable stuff on what cholesterol tests tested for, how variable the results were, and what that actually meant in terms of health, diet choices, etc.  And, yes, after suitable education, I came to the same conclusion as the nurse: "yeah, those look fine."

Without some kind of informed comment, scientific data can be pretty meaningless to the average guy on the street (Just as my cholesterol numbers were initially meaningless to me.) Whose job is it to provide that comment? Last night, we had a great Cafe Scientifique discussion with Aimee Whitcroft (Science Media Centre – sciblogs followers will know her I’m sure), about how science journalism works in New Zealand. Often journalists (particularly in NZ where there are not enough of them to specialise deeply) would love to be able to put that informed comment into their writing, but are prevented from doing so by lack of understanding themselves. That is where they have to talk to scientists who know the area. And scientists should be prepared to make such comments.  Not every member of the public has the skill, the patience, or the scientific discernment to trawl through the literature and find these things out for themselves.  Remember, data doesn’t equal information.

## A quarter of a thousandMarcus WilsonApr 26

According to Movable Type, this is entry number 250 for PhysicsStop.  A quarter of the way to one thousand entries. Has anyone read them all?  Now, according to the statistics I get to see every month, the single most looked at entry by far is this one, on The3is in Three.  Why is it so popular? I have no idea.

In recent times, the second most looked-at entry has been the joint Seccombe/Wilson family effort on Fallstreak cloud

## Another exponential decay exampleMarcus WilsonApr 24

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In the last couple of weeks, my wife has been having a go at making sourdough bread. One of the defining characteristics of this bread is that it doesn’t use yeast – at least, not directly. The idea is, to start it off  rising, you leave it outside for a while, and allow lots of useful microbes to land on it. These then start multiplying, producing carbon dioxide, and allow the bread to rise. Presumably any nasties that land on it get walloped by the baking process (Anyway, I haven’t suffered any ill effects from eating it yet).

But – here’s the clever bit, once you’ve got one lot of dough you can reuse it. With your first batch of dough you make about 3/4 of it into bread, and keep the other quarter till tomorrow. Then, tomorrow, you add in more flour, liquid etc, bake 3/4 of it, but keep a quarter for the next day. And so on. Once you’ve got a nice population of microorganisms in the dough they’ll keep multiplying overnight, and so you can carry out this process indefinitely. I’m told that there are sourdoughs that are 200 years old – that is this quartering process has been going on for a very very long time. (I haven’t checked this out myself, so I might be getting some details wrong here, but hopefully you get the idea).

Now, here’s a question – how much of the original dough is left after 200 years of daily quartering? The answer follows along the much discussed homeopathic lines – nothing.  The first day you take 1/4 of what you had, and keep it. The second day, you take 1/4 of that. After N days, you have 1/(4 to the power N) of the original left. With N just 5, (say about a week), you are left with 1/1000 of the original mixture. With N=10, you are left with a thosandth of a thousandth, or a millionth of the original. It doesn’t take long to end up with virtually nothing of the original left. If your starter dough contained every atom in the universe, it would only take about 140 days before there wasn’t a single atom left.  So after 200 years, the chances of there being even a single molecule from your first day’s dough left in the bread is for all practical purposes zero. In which case, can you really say that the dough is 200 years old?

It’s just another example of exponential decay, which permeates through so much of physics.

## Can you believe this guy?Marcus WilsonApr 22

I stumbled upon this snippet on Yahoo this morning. "Professor proves Germany will win World Cup".  Of course the reference is to the only World Cup that matters, the one in South Africa later this year, not the pretend one happening here in NZ next year.

I can only think that something has been lost from translation from ‘science’ to ‘media speak’, and then from German media speak to English media speak. For a start, the ‘formula’ used is based, according to the article, on trigonometry.  I’m not sure where trigonometry comes into it – it would make some sense though if the author meant probability. The argument is basically that, if you look back at past world cup winners, Germany wins the world cup on average every fourth or fifth. There have been 4 unsuccessful world cup campaigns for Germany since their victory in 1990 (yes, I remember the agonising semifinal where England’s Stuart Pearce and Chris Waddle stuffed up their penalty shots – I think it was them, anyway – you always tend to remember the players that didn’t live up to expectations). Therefore Germany are due to win this time in South Africa.

What dumb application of probability is that? It doesn’t work like that, though it is a pretty strong misconception held by many, many people. I remember playing a board game a few years ago with a group of about 8 adults, which involved a dice.  At one point we had a rather long run where the number six just wouldn’t come up, and that sparked a discussion about whether it would be more likely that 1 in 6 that the number six would appear on the next throw.  Interestingly, only one other in the group would support me when I said the probability was still 1 in 6.  I had a tough time explaining, to mostly well educated people, that the dice doesn’t have a memory.

I hope that our German physics professor who is confident of a German victory was being a bit tounge-in-cheek with his ‘formula’.  I would hope that there aren’t any physics teachers out there who really believe that if a probability of one fifth event hasn’t happened in the last four goes, it will happen for sure next time.

Of course, as any England fan knows, the fact that Germany will win the world cup is a dismal inevitability. They always do…

## Some cardboard picturesMarcus WilsonApr 20

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Following a request for some pics…

## Cardboard CityMarcus WilsonApr 20

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Last weekend saw the ‘Cardboard City’ event at church. Basically, it’s a fun weekend for children. The main event is the construction of ‘houses’ from bits of cardboard, sticky tape and string, in the church hall. Then the children get to sleep in their box houses overnight (supervised by foolhardy and sleep-deprived volunteers like myself). So in the weeks beforehand we accumulated numerous large cardboard boxes (e.g. the ones that fridges and armchairs come packed in – what a consumer society we have…), piled them up (flat of course) in the church hall and had everyone raid them for what they needed.

There were some pretty impressive constructions, I have to say – you couldn’t say the children we had lacked ambition. I reckon one group had a ten-kilo roof on their house – I know because I had to help put it on. As a physicist, constructing things from cardboard is a great showcase for how strong some materials are.  The humble cardboard box is made from cardboard that is essentially a sandwich – two flat papery layers with a corrugated paper structure between the layers. The corrugated structure is hard to bend perpendicular to the corrugations, giving exceptional rigidity in a material that has a very low density.

To really get rigidity, you’d stick two sheets together, with corrugations running opposite ways. An alternative is to roll the cardboard into long tubes (tricky – they turn out more like prisms, but it doesn’t matter) – it then takes a lot to bend each tube. The overall result is that, with a bit of effort, over a whole afternoon, you can construct some fantastic buildings with just boxes and tape.

Of course the real fun is in destroying them afterwards, and then the real effort is in tidying up the mess and disposing of fifty mangled fridge boxes.

## No such thing as a free lunch?Marcus WilsonApr 16

One of the things I’ve been looking at this week is working out some travel details for a conference I’ll be going to in Wanaka in August. Kiwis will immediately be suspicious – the word that forms a natural triplet with ‘Wanaka’ and ‘August’ is ‘skiing’.  And furthermore the sessions are early morning and late afternoon and evening, leaving the middle of the day conveniently free for ‘other’ activities.  In my case it’s more likely to be hiring a bicycle and heading up along Lake Hawea, than skiing (I had a go once, and I think I got away with it – apologies to John Cleese), but it will still be a little bit of a holiday.

And my employer pays for this trip?  Well, as much as conferences might seem to be holidays, they do actually serve a really useful (and some might say necessary) purpose for the scientist. First, you get to hear some really interesting talks that broaden your knowledge of a particular area - or open up completely new ones to you. You also get the opportunity to ask questions directly to the speakers.

But, probably the greatest use is that you get to develop useful contacts.  Much of science research is collaborative – it involves two or more teams of individuals at different organizations. By bringing different skill sets and experience to the same problem, a collaboration can often achieve more that just an individual can. Conferences are a great way to develop collaborations – be it formal or informal – you can see where other people have skills and spot areas where they can help you, or where you can help them. And this bit isn’t done during the talks – it’s done during the coffee breaks and poster sessions and that conference dinner which is beginning to look a bargain at just 90 dollars a ticket…

## Physics is everywhereMarcus WilsonApr 14

One of the problems with being a physicist is that I start seeing physics happening everywhere I look.  Here’s an example. Last week I was sitting in a cafe on Dunedin’s George Street enjoying a coffee with my wife, when the moment was utterly ruined by a completely inconsiderate couple on the other side of the street choosing to demonstrate a physics phenomenon for me.

Their crime?  Walking arm-in-arm down the street exactly in step with each other.  I mean exactly in step, for an entire block.  They were unwittingly demonstrating the phenomenon of synchronization of coupled systems.  Basically, if you take two things that oscillate at nearly the same frequency, and link them together in some way, you’ll get the two oscillations synchronizing.  Huygens described it with his clocks, and there are lots of other examples (one oft-mentioned one is the synchronization of menstrual cycles in women who live together – not that I have experience of that one).  Walking is a form of oscillation (swing your left leg, then swing your right leg, and swing your left again), and it has a defined frequency (number of left-swings per second – or half the total number of swings per second, because one cycle consists of a left leg swing and a right leg swing ). If someone measured the frequency for each member of the couple individually, I speculate that the two frequencies would be very similar, but not quite the same. If they separated, they would slowly get out of step.  But, have them walk together, and the swing of one left leg influences the swing of the other left leg, and they end up walking in perfect synchrony.

I wonder if those who are experts in other fields suffer the same problem - of seeing their field of expertise in action everywhere they look?