Marcus Wilson

Dr Marcus Wilson is a lecturer in the Engineering Department at Waikato University and author of the Physics Stop blog. His current research involves modelling of the electrical behaviour of the human brain during natural sleep, focussing particularly on the transitions between sleep states. Previous research interests include infra-red physics and signature control (stealth) and quantum Monte Carlo methods. He graduated from Cambridge University in 1992 (BA Hons) and completed his PhD at Bristol University in 1995.

Physicsstop back in business - Physics Stop

Feb 07, 2017

I have been rather conscious of my looonnnnggggg absence from the blogosphere. That really is down to other commitments getting in the way, and then falling out of the habit of blogging.  Hopefully this will be a restart. I have a good opportunity here – I have just started a period of study leave (what used to be called Sabbatical in the old days) and arrived this week in Perth, where I’m visiting the University of Western Australia. I’ll be here for nine weeks – a fantastic chance not to be interrupted by people knocking on my door (and, yes, to develop some research ideas too, I should add). So, first stop, naturally enough when you have a four-year old, is the local playground. And what a playground it is too. It’s been set-up to blend in with the trees … Read More

Video: laminar flow, microorganisms and mangroves - Physics Stop

Jun 21, 2016

This movie is a demonstration of laminar flow. My colleague Julia Mullarney used it last week in our Osborne lectures to high-school students to demonstrate what turbulent flow ISN’T. Video showing Laminar Flow and demonstrating fluid flowing in layers. This apparatus was developed by John DeMoss and Kevin Cahill of the Department of Physics & Astronomy, University of New Mexico.   Basically, laminar flow is time-reversal invariant. This implies a few things, but, notably here that if you reverse the processes involved you get back to where you started with. Motion of micro-organisms This is the problem that micro-organisms face when they move. Any motion that has time-reversal symmetry (like a swimmer kicking their legs, or a scallop shell opening and closing) will get them nowhere. The solution for the micro-organism is to rotate a flagellum (or … Read More

Probability madness - Physics Stop

May 10, 2016

Probability crops up in many places in physics, not least quantum mechanics and statistical mechanics where we are only sure of things in an average or ‘statistical’ sense. Dealing with probabilities can be a headache for many students. They are also a headache for many in everyday life. There are numerous occasions where we need to estimate the likelihood of something occurring and we can get it very wrong indeed. What is the probability of encountering a monster traffic jam on the way to Auckland Airport – just how early should I leave for that flight?  What are the chances that my visitor will actually turn up on time – or at all?  Bookmakers make their living on estimating probabilities so it’s always amusing when they get it wrong (I have deep issues with gambling). According to the odds First, … Read More

The universal joint - Physics Stop

Mar 10, 2016

…No, it isn’t something everyone smokes…  But it is common in machine mechanisms. The universal joint is a neat way of turning rotation in one plane into rotation in another. A common use is on driveshafts where you want the direction of the shaft to bend. There’s a neat animation on Wikipedia of how the thing moves. Despite seeing them in action (including in our teaching lab) I’m always amazed that it works. Let’s think about it. There are four pins on the joint – two for one shaft, and two for the other. They have to stay in the same geometry (namely at the four corners of a square) as both shafts rotate. That doesn’t seem possible. Three points are what defines a plane – put in a fourth and surely it’s not, in general, going to … Read More

The storm surge - Physics Stop

Feb 26, 2016

I shudder to think what it must have been like in the path of Cyclone Winston. It is hard to conceive of winds 230 km/h sustained for minutes at a time. I remember vividly what is now known as the Great Storm of 1987 (an extra-tropical cyclone) which pulverised south-east England on 15/16 October 1987. There were (according to Wikipedia – ahem!)  gusts close to 200 km/h recorded in Sussex (where I lived), but there were possibly higher ones than these – the anemometers failed. I spent the night listening to trees falling one by one around our house. Opposite the house was (and still is) a very tall Wellingtonia – one of the earliest specimens of this tree planted in the UK – and if that had fallen on us there wouldn’t have been much house … Read More

The problem with science communication - Physics Stop

Feb 24, 2016

Yesterday I was part of a very interesting workshop on Science in Society, in Auckland. There was a plethora of good examples of science communication discussed – including forest restoration on the East Coast, biological control of pests in vineyards in Canterbury and improvement of health outcomes for Native Americans in Montana. For me, it was clear that there were some resounding messages coming through about science communicaton. 1. It needs to be driven by the community. Here, community could mean a town or village, a marae, an industry group, a school – any group of people with an interest in achieving something. The participation of the scientist is as a partner, often as a junior partner. In other words, the community takes the lead. The scientist(s) doesn’t go out and say “Right, now I am going … Read More

Gravitational Waves - Physics Stop

Feb 12, 2016

The big breaking physics news is the detection of gravitational waves. These waves are distortions in space-time, caused by a large mass doing something spectacular (two colliding black holes in this case)  that propagate across the universe and create tiny changes in space when they reach us. The commentary here describes what goes on. Essentially, things change their length/width. When a gravitational wave passes through my office (say ceiling to floor) one can imagine the length of the office increasing slightly, coupled with a decrease in the width of the office, followed by the reverse – a decrease in the length and an increase in the width.  But its not just that the bricks that make up the room vibrate (e.g. as in a seismic wave) – its the whole of space that does it. These waves … Read More

Trusting your life to your own physics calculations - Physics Stop

Jan 30, 2016

Alison Campbell alerted me to the following: Physicist Andreas Wahl shoots himself with a gun underwater – and proves a point about drag force.  For the record – I won’t be repeating this. Physics or no physics, the guy is crazy. BUT, what I have done, is a quick post-hoc analysis from the safety of my own office. There’s a little bit of maths involved, but the gist of it is this. The drag force on an object (in turbulent conditions – which this most certainly is), is given by the equation c rho A v2 where c is the ‘drag coefficient’, rho is the density of the fluid in which the object moves, A its cross-sectional area and v its speed. If we equate this to the objects mass times acceleration (Newton’s second law) we get an expression … Read More

The world’s most beautiful equation - Physics Stop

Jan 25, 2016

Don’t miss the BBC poll on what is the world’s most beautiful equation. Are you a fan of Einstein’s field equation, or does the Riemann zeta-function hold you in raptures? There’s some great commentary on the twelve candidates here.  How did I vote? Well, that would be telling, but the fact that my very first publication is titled Auxiliary-field quantum Monte Carlo calculations for the relativistic electron gas [read it here! – at least if you have access to the Journal of Physics: Condensed Matter] might give the physicist readers a few clues.    … Read More

Don’t confuse accuracy with precision - Physics Stop

Jan 22, 2016

Going back to my last post, our fancy balance proclaims that it weighs objects from 0 to 200 g with a precision of 0.001 g (that’s one milligram).   And it does – put an object on and the balance gives you an attractive-looking number on its prominent display reading 184.139 g, or something similar. It is precise to 1 milligram. It’s not reading 184.138 g, neither is it reading 184.140 g, it is reading 184.139 g. So does that mean our test object has a mass of 184.139 g? Unfortunately not. Just because the balance gives us that number that precisely, it doesn’t mean that it is that accurate. University lecturers always have a good giggle when some poor unsuspecting first-year student records an answer to a wildly inappropriate number of significant figures – for example, … Read More