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.

What’s the catch? - Physics Stop

Nov 05, 2009

Last weekend Alison Campbell and I took a trip to New Plymouth to do a day session with final year school students to help them prepare for their physics and biology scholarship exams. (Alison did the Biology half, I did the physics). I do hope the students got something useful out of it. Doing this kind of thing certainly teaches me things too - it really gets me focused on the essential concepts behind physics, and being able to communicate them in simple terms.   (Talking about communication - I head down to Wellington very shortly to give a cafe-scientifique-style talk at Te Papa  on The Large Hadron Collider - it's a little bit daunting to be honest - don't quite know which politicians may sneak in the back of the audience...)

The endless pace of technology - Physics Stop

Nov 03, 2009

I discovered at coffee time this morning that one of my work colleagues has never seen a record. That's record, as in the black vinyl disc with grooves. This isn't a child, it's an adult old enough to have a degree. From this I conclude that 1. The pace of technology is faster than I appreciated or 2. I'm older than I thought.

Come to think of it, I haven't seen a record for about the last twenty years.

Beyond cornflakes - Physics Stop

Nov 02, 2009

This is something that Aimee Whitcroft at the Science Media Centre in Wellington drew my attention to - thanks Aimee.

Most of us who have ever eaten breakfast cereal will probably be familiar with the phenomenon whereby the larger flakes of whatever-your-favourite-breakfast-is tend to be at the top of the packet, whereas the smaller flakes tend to accumulate at the bottom. This segregation of particle sizes is pretty common in physical processes. So it comes as no surprise that, when you stuff small beads of two different sizes into a container, and disturb them by rotating the container, the beads separate out in size, to give regions where large beads predominate, and regions where small beads predominate. That phenomenon has been known for seventy years - being first demonstrated by Yositsi Oyama.

But, what has been discovered  recently  by Ralf Stannarius and Frank Rietz is that, when you stuff even more beads into the container, and rotate it, the segregation patterns are not stable; instead you get rolling patterns reminiscent of convection currents (similar to the circulating currents in, say, a pot of water being heated from beneath). What is most intruiging is that there is as yet no full explanation of the phenomenon - the physical models that exist just don't seem to be adequate for what is happening. You can look at the movie or, for those more physics-inclined, read the associated publication in Physical Review Letters.

That is what makes science so fun. Basically, with equipment you can put together in your own shed, you can demonstrate a phenomenon that is beyond our current understanding. We by no means know everything there is to know even about simple mechanical systems, as this example demonstrates. It begs the question as to what other effects are there that are just waiting to be discovered. So start experimenting.

The3is in three final - Physics Stop

Oct 30, 2009

I attended the final of the The3is in Three competition on Wednesday night. It was a really entertaining evening; compere Te Radar was in great form, as were the eight finalists  (N.B. I know that those of you who are not from NZ won't have the slightest idea who Te Radar is, but I'm sure Google could solve that for you.) Each contestant had three minutes to present their PhD research, with the aid of one powerpoint slide and whatever over-exaggerated hand movements they wished to employ.

Overall, it was simply impressive how well the research topics were communicated. Even those that you would think were so technical that it would take more like three hours to do it. There were three science finalists, looking at topics of dealing with waste products from meat processing, tracing the origin of sediment in river estuaries, and understanding the survival mechanisms of a harmful bacteria. As a scientist, I hoped that one of those would win, but, alas, the prize went to an analysis of the last words uttered by Shakespearean characters as they met with unorthodox demises.

I wonder whether I could put my PhD thesis (a few years old now) into three minutes. It's a tough ask;  it rejoices under the title of 'Auxiliary field quantum Monte Carlo calculations for exotic jellium'. If you want to know more, I'm sure the University of Bristol library could fish you out a copy from its archives. But if the students from Wednesday night could manage it with their work, it's got to be possible.

Using words is OK - Physics Stop

Oct 29, 2009

I'm in the thick of marking exam papers. In physics, a lot of what a student does is mathematically based, so a fair bit of any exam is going to contain calculations of things. But don't think that it is compulsory to make your answer totally incomprehensible.

Many of the exam answers I see from students look like the result of a twisted experiment involving Sudoku and Scrabble. Letters and numbers are strewn around the page in a fairly random manner, occasionally with an equals sign that may or may not be in the right place. Units are always conspicous by their absence. Such a scrawl is really really hard to mark. Your reasoning, in your head, might be perfect, but unless you can get it down onto the paper in a comprehensible manner it might not be getting you much credit.

So please, put in a few well chosen words. For example, you can say 'taking moments about point O gives...', or 'using conservation of energy we have...' rather than launching straight into the equation. Remember, if the examiner can't work out what on earth you are doing, your chances of getting credit for it are on a par with those of New Zealand winning the rugby world cup.

Anti-gravity - Physics Stop

Oct 28, 2009

There are some lovely physics demonstrations that get repeatedly wheeled-out for things like Open Day and visits from school groups. Things like holding a spinning bike wheel on a rotating chair (flip it over and you start rotating - conservation of angular momentum) and levitating a piece of superconductor above a magnet at liquid nitrogen temperatures.  But one thing that I've yet to get my hands on to demonstrate is Cavorite.

I think it's an excellent suggestion, but with two minor drawbacks. One, it comes with significant health and safety concerns, and two, it is fictional.

The material Cavorite makes an appearance in H.G. Wells' novel "The First Men in the Moon". Its basic characteristic is that it is impervious to gravity - that is, if you put a sheet of Cavorite on the floor, anything you put on top of the Cavorite will be weightless.  Now, you can imagine that would be great fun, but it plays havoc with air pressure which is why one needs an appropriate risk management strategy before children can be allowed near it.

In the novel, Dr Cavor uses it to power a space ship to the moon - first of all he uses it to control air pressure to blast his ship off from the earth, then by opening and closing shutters of Cavorite he allows the moon's gravitational attraction to pull  the ship towards its destination, but shuts off that of the earth.

A fun concept for a novel, but more seriously we can ask whether it is at all plausible that Cavorite could exist. For example, it is possible to shield the electrostatic force - simply putting a sheet of electrically conductive material between two charges will mean that neither can 'see' each other (there are issues though - both charges will be attracted to the sheet)  - so why not gravity? Now, in some ways electrostatics is similar to gravity - both obey the inverse square law for example, but in other ways it is not. There are positive and negative charges, but there is not positive and negative mass. Gravity is always attractive. From what we understand from General Relativity, a mass distorts space-time, which will be felt by another mass. To create Cavorite, we have somehow to put a rip through space-time. Can it be done?

It is perhaps just possible that the Higgs Boson can shed some light on this. I don't think anyone seriously expects its discovery  (if it happens) and subsequent analysis will help us create Cavorite (would we want to?)  but it will probably help us to understand just what mass really is. And that is of great interest to physics. Roll on Large Hadron Collider switch-on.

Magnets attract, right? - Physics Stop

Oct 27, 2009

MVC-445S.JPGHere's an example of some physics that doesn't quite seem to work out.

Magnets attract iron. Yes? So what happens when you place a drop of ferrofluid (which is basically an oil whose molecules have been laced with iron atoms) on the surface of water and lower a maget towards it.   The oil will flow on top of the water and accumulate under the magnet, since that is the closest it can get.

Well, no. As the picture shows, what happens is the fluid accumulates in a ring, which is devoid of fluid near the centre - the very place that is closest to the magnet.

Why? It's because each molecule of fluid doesn't just feel the magnetic field from the magnet - it feels the field due to each other molecule of fluid. And under the magnet this field acts to rip the drop apart - each molecule repels the other molecules, so the drop cannot remain intact there. It's another example of where the whole is not equal to the sum of its parts - sure, every oil molecule, if it were the only one, would head underneath the magnet, but put them together and the interactions between them are important, and we end up with none there.

Often in physics this kind of interaction can result in chaos - but in the case of the ferrofluid, we get a nice ring structure. The full explanation is a little complicated - but for those interested it's in BJ Ackerson and AN Novy, American Journal of Physics vol 69(5) p614 (2001).

Virtual field trips - Physics Stop

Oct 23, 2009

If you think it unfair that your children get to go on lots of exciting school trips that you never went on this is for you - virtual field trips throughout New Zealand on the LEARNZ website.

Sent to me by NZ Institute of Physics - thanks guys.

Monopoles, Dipoles, Quadrupoles and the like - Physics Stop

Oct 21, 2009

The alternate stretching and squashing casued by a gravitational wave is an example of a quadrupole oscillation. This is another word that probably means very little to most readers, and, unless you like maths, Wikipedia isn't going to help you, so I'll explain.

 Let's start with a monopole. You get a monopole when you put 'stuff' somewhere.  Here, 'stuff' can mean almost anything you like - mass, electric charge, nematodes...(cafe scientifique last night was about nematodes, or 'roundworms' - such is their world domination that they deserve to be used to illustrate these physics ideas...) So talking about the number of nematodes in a centimetre cubed of soil would be a describing their distribution in a monopole form.

Dipoles are the next step here. The most obvious example of a dipole is a bar magnet. As well as having a certain strength, it has a North Pole and a South Pole.  So it is directional. A bar magnet pointing horizontally is not the same as one pointing vertically, at least not in this description. Magnetic dipoles are easily described by vectors - with a length (the strength of the dipole) and a direction (from south to north). We could add to our monopole description of our nematodes a dipole description too. In this case, we could describe in what way the nematodes were travelling (on average) in a dipole manner. For example, they could on average be moving westwards at a speed of half a millimetre a second - so they have a strength (half a millimetre a second) and a direction (west).

Now for the quadrupole. This is less obvious, and I'm not sure I can do it with nematodes. The quadrupole moment can relate preferences in more than one direction.  Imagine two roads, each with equal traffic flows in both directions, intersecting at a roundabout. More cars want to travel straight on than turn. What tends to happen, is that, say, the east-west flow will dominate for a while (the flow stops the north-south traffic entering the roundabout), then a north-south car muscles in and stops the east-west, and the north-south dominates.  There is no dipole moment (as many cars flow north to south as south to north, and east to west as west to east, but there is a quadrupole  - at a given moment in time one route (e.g. north-south) will tend to dominate over the other one.

In a gravitational wave we get a regular swapping between the dominating directions - e.g. first the up-down direction dominates the left-right (and we stretch and get thin); then the left-right dominates over the up-down (and we shrink and get fat).

We can build up more multipoles too - e.g. the next one is the octupole; and so it goes on. But physically it gets rather complicated to describe, and, for most things, the monopole, dipole, quadrupole series will cover what we need.