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.

Climate change - Physics Stop

Dec 10, 2009

I feel that, as a physicist, I should be making some reasonable and informed comment on the Copenhagen summit. After all, climate is immensely physicsy. We have fluid flow, conduction, convection and radiation of heat, interaction of electromagnetic radiation with electrons in molecules, scattering of light by small particles, solar activity (on second thoughts, scrub that one, WAY too controversial). Actually, all of these I've mentioned in my blog over the past year, in some form or other (follow the links if you don't belive me.) And then there's the issue of whether fish mix the oceans of not.

But, to be honest, the Copenhagen summit appears a white elephant to me. Don't misunderstand me, I'm not at all suggesting that climate change isn't something that we should tackle. Rather, the key is to actually get on and do something about it. Talking about it is OK, but what matters is action. My recollection was that climate change was talked about seriously at the Rio Earth Summit in 1992. What has happened since then? No, I don't mean the Kyoto protocol, and legally binding or non-binding targets etc, I mean by how much have global emissions actually reduced. You can set what targets and agreements you like, but it is the action that solves any problem.

To be honest, I don't know the answer to that question (how much have global CO2 emissions changed since 1992) - the internet is no help to me here. I'm sure someone can tell me. But I don't think it's downwards.

So here's my point - Copenhagen is all well and good, but as a burning current story it is just not doing it for me. So I shall leave the comment to people who have more idea what they are talking about, like Gareth Renowden.

Small Transistors - Physics Stop

Dec 09, 2009

How often do you get attracted to an article somewhere because of its outrageous headline, and then discover on reading the article that its headline, if not an outright lie, doesn't quite represent what the article is actually about?

This is one that got my attention earlier this week on  The headline "Scientists build 'single-atom transistor' " certainly promises a lot. Getting ever-decreasingly small transistors is what has driven computer and electronics technology forward at such a blistering pace. But there is (probably?) a fundamental limit - when a transistor is the size of an atom it can't be made any smaller. A single atom transistor (or just a few atoms transistor) is one of the holy grails of nanotechnolgy. So are we there now?

Hmmm. No, not really. The clue was in the '...' quotation marks around 'single-atom transistor'. What is really meant is that researchers have studied the effect of single phosphorous donor atoms in silicon. That is not the same as building a single-atom transistor - although it might exhibit measurable effects due to single phosphorous atoms, the device itself is much bigger than a single atom. After a bit of trawling with search engines I recovered the original paper on which this headline is based. Unsurprisingly, at no point in it do the researchers claim to have build a single-atom transistor. Kuan Yen Tan et al, Nano Lett., December 1, 2009 (Letter), DOI: 10.1021/nl901635j.

Moral: In case you didn't know, don't believe everything you read.

A boost to my vocabulary - Physics Stop

Dec 07, 2009

I learnt a new word recently, courtesy of PhysicsWorld. Yoctosecond. That's a period of time equal to ten to the power of minus twenty-four of a second. Or 0.000 000 000 000 000 000 000 001 seconds. As in "I'm just popping out - I'll only be a yoctosecond."

Physicists (or anyone else for that matter) haven't needed to delve into this short a time-frame before. There are femtosecond lasers (ten to the power minus 15 seconds) but this is still a long way off a yoctosecond.

But it's thought that the quark-gluon plasma soupy thing that the ALICE experiment at the Large Hadron Collider will look at could emit light pulses that last just a few yoctoseconds.

Waves in a plasma - Physics Stop

Dec 04, 2009

This post follows from a comment I had yesterday from Robert McCormick on the version of PhysicsStop. (Unfortunately the mapping of PhysicsStop onto the sciblogs website doesn't combine the comments - so if you read my blog through The University of Waikato website you won't have got his comment, so I attach the link below) 

Plasmas provide a wonderful medium for waves. A plasma, in simple terms, is a gas (often very high temperature) consisting of charged particles (e.g. electrons and positively-charged molecules). As well as bouncing around like your normal gas molecules will do, the particles in the plasma feel the electric and magnetic fields that each other create, leading to a great array of possible ways for waves to travel. So plasmas can carry sound waves, electromagnetic waves and bizarre combinations of  both.

I studied a little plasma theory while I was an undergraduate and found in fascinating. I can't say I've had much to do with plasmas since, but the theoretical techniques I learnt about for analyzing waves in plasmas have proved really useful for other physical situations, such as waves of neuronal activity in the brain. It's an example of how the same underlying phenomena can manifest themselves in very different physical systems. And it's an example of how to answer the undergraduate student how says 'Why am I studying this stuff - I'm never going to use it'.

Rainbows - Physics Stop

Dec 03, 2009

The sun poked out from behind the clouds on my way in to work this morning just long enough to produce a beautiful rainbow for a few seconds. Not sure when we'll next see it.

I guess most of us studied rainbows at school, but I'll throw in a couple of physics words with this comment that you may not have heard of. The light from the sun is, as we know, made up of a whole range of different colours. Each point in the spectrum is made up of light with a different wavelength (the distance between peaks of the wave); red is longer wavelength (I say long, but I mean about 700 nanometres, where one nanometre is a millionth of a millimetre); violet is shorter, about 400 nanometres.

Now, the key to making a spectrum (e.g. a rainbow) from white light is a dispersive material. Water is one example, as is glass (e.g. your prism). Physicists describe something as dispersive if it has different properties at different wavelengths. That means the red light interacts with it differently to the blue, so they get split up. One of the key things physicists like to know about waves of some kind moving through a medium (e.g. light in air, sound in water, waves on a violin string) is the dispersion relation. This describes the relationship between wavelength and frequency (the number of oscillations a second).  A non-dispersive medium has wavelength times frequency equal to a constant (the wave velocity). But, what they forget to tell you at school, is that, in general, this isn't true. And it's materials where it isn't true that are way more interesting to work with.

No dispersion, no rainbows. That simple.

Rocket Science and Spam - Physics Stop

Dec 01, 2009

Well, congratulations to Rocket Lab with the launch of their Atea-1 rocket. (Watch the movie). Hopefully this will go some small way to convincing the key players in the New Zealand economy that we can and should do more (a lot more) than just agriculture and tourism.

And spamming. What an unfortunate distinction for a country to hold. Spam can, in the extreme, cripple people's ability to work with email - it's not just the flood of things that you have to delete everyday, it's also those vital emails that never make it to you because the spam blocker mistakes them for spam. I am very pleased that Mr Atkinson, who must be one of NZs most unwanted exports,  has a very large fine to pay.




Orbits - Physics Stop

Nov 30, 2009

Going back to my comments on the Karman line (100 km about the earth's surface), I think it's worth commenting a bit 'being in orbit' means. We are familiar with the fact that if we drop something it accelerates downwards and hits the ground. If we throw something away from us, it will still accelerate downwards and hit the ground, but this time at some distance from us. If we throw it hard enough (and I mean, really hard), it will accelerate downwards, but, because the earth curves, by the time it has fallen the earth has dropped away too, so that it is still the same distance above the earth. It's then in orbit, and will come back and hit you on the head (obstructions and atmosphere being absent).

Put more precisely, a circular orbit results when the acceleration due to gravity matches the centripetal acceleration required to move the object in a circle.

Orbits don't have to be circular. Kepler worked out that the planets orbit the sun in ellipses, with the sun at one focus. Newton worked out that, if the sun exerted a force on a planet inversely proportional to the distance between the sun and the planet squared, then an elliptical orbit would necessarily result. (Well, actually the orbit could be circular, elliptical, parabolic or hyperbolic - these are all conic sections).  

One curious result of orbit theory is that, the closer a particle is to the focus of its orbit (e.g. the closer a planet is to the sun; Mercury as opposed to Neptune - alas, Pluto no longer counts) the faster it goes. The same applies to satellites orbiting the earth as well. Satellites that are in lower orbit move faster.  These satellites have to contend with more atmosphere as well (remember the edge of space is not distinct, there are still a few air molecules up there). When a satellite hits an air molecule, it loses energy. It drops to a lower orbit (less potential energy) and so it speeds up. We don't normally think of something that is gaining speed as losing energy, but, in the case of a satellite, that is the case.

To counteract air resistance, satellites have to be given little boosts of energy to keep them in orbit. Without it, they will progressively spiral inwards, until they burn up in the earth's atmosphere, or, sometimes, hit the earth itself.

Dazed and confused - Physics Stop

Nov 27, 2009

Physicists love units. The best way to wind up a physicist is to tell him you were driving at 100 down the road.  One hundred what?  Just hope you don't get pulled over by a traffic cop with a physics degree or he'll ticket you for leaving your unit off, even if you were within the speed limit.

A unit is usually a very meaningful thing. One hundred kilometres an hour means, at that constant speed (and I mean speed not velocity, direction is irrelevant) you will travel one hundred kilometres in one hour.  An freefall acceleration of ten metres per second, per second (or 10 metres per second squared) means every second something is in free fall it gains ten metres per second of velocity. Easy.

Not quite.  In the area of stochastic physics, which I have worked with for several years, we have units that are metres per square-root second. What on earth is a square-root second?  It still confuses me no end. Sure, I can work with it, and write computer programmes to use equations that involve it, but quite grasping it in my mind is something I haven't succeeded in doing.

I know I haven't succeeded yet because I find it so hard to describe what's happening to my summer students.

(N.B. It's related to the random walk ('drunkard's walk) problem - where the root mean square distance you travel is proportional to the square root of time.)

However, not understanding a concept does not exclude you from using it.  I remember years ago working with Lagrange's method of undetermined multipliers, (N.B. don't click on this link unless you have a solid grasp of calculus) and being able to use it to work out problems, but not really having a clue as to WHY it worked. No text book seemed to help me. Then I remember one winter in Bedford walking to a bus stop and having a sudden flash of inspiration. At last I got it. It didn't help solving problems, but it made the process that much less mysterious.

Maybe, just maybe, one day the same thing will happen with square-root units.

How to get publicity - Physics Stop

Nov 26, 2009

For those of you wondering (and several have asked) how I managed to get a third of a page in the main section of last Sunday's Sunday Star Times, the answer is simple.  Tell the press about whatever story you want them to know about.  Some journalists can be pretty good at digging up stories, but it helps them no end if you give them one, rather than hoping that they find it.  So, whatever you want to publicise, go and tell the media about it.

(The article, thankfully minus the photos, is available on, click here.) The cafe scientifique in question went really well - Alison Campbell and I did a session on some of those bizarre science stories that crop up from time to time. We didn't get onto the zombies, but, in case you didn't know, or need a reminder, that was one of the more 'interesting' bits of scientific research published this year - click on the link. And as for the fruit bats, Alison handled it very well indeed.