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.

Plants in circular motion - Physics Stop

Oct 05, 2015

In our first-year physics lab we have the following horticultural experiment.  Here we have some bulbs growing on a rotating turntable. The array of five pots is placed on the turntable so that the centre pot is at the centre of the turntable; the left- and right-hand pots are at the perimeter.The turntable is rotating at about half-a-revolution a second. What happens as the plants grow? Actually, this is a demonstration of centrifugal force just as much as it is a horticultural experiment. First the biology bit. Plants grow, pretty-much, towards the light. I’m sure someone will tell me the mechanism by which this happens, but for now I’ll just state that as true. In this case, however, what is towards the light is constantly changing. The plants get equally illuminated from all sides. So we can take light out … Read More

If gravity increased… - Physics Stop

Sep 25, 2015

A colleague remarked to me yesterday, as we were trudging up the two flights of stairs from the tea-room to the third floor, “I’m sure they turn up the force of gravity in this building each year.” I feel like that too, sometimes. However, I suspect it has more to do with the aging process that any changes in physical constants. But…what if…? What would life be like if gravity were say, double what it is now. How different would life be? There are many ways of tackling that question. So I’ll phrase it in this way. Imagine that there’s another universe in which a similar-sized and atmosphered Earth exists, but where the force of gravity is double what it is in this universe. What might life look like on that Earth? First, I need to say that the ‘Earth’ … Read More

Clothes racks are not the reason for mouldy homes - Physics Stop

Sep 21, 2015

I read the ‘Rental Nightmare‘ article on last night. Some of the stories are horrific indeed, and I’m reasonably confident that the writer has deliberately sought out the worst situations rather than the most common situations. But one cannot deny that a great deal of housing in New Zealand is sub-standard. In housing-deficient Auckland, in particular, families are forced into cold, damp homes because there is nowhere else they can afford.  I’ve been in NZ eleven years now, and I have still to wrap my head around why this is. It seems that up to the 1970’s, houses were designed with three underlying assumptions: 1. New Zealand has a warm climate, 2. New Zealand has a dry climate, 3. Everyone needs a large, detached house. While the first is arguably true for parts of the country for … Read More

Axis labels – accurate but not at all obvious - Physics Stop

Sep 14, 2015

I had a conversation with a class this morning regarding the labelling of axes on graphs.In particular, how we should indicate the units. Most quantities we deal with in physics carry units. A speed might be 35 km/h, a distance might be 16.8 mm, a pressure could be 28 kPa. Saying that a speed is 35 is wrong and meaningless. 35 km/h is rather different to 35 m/s or 35 inches/day.  If we are plotting two quantities, for example to find a relationship between them, we need to indicate what units the axes are in. A typical way to do that would be to include the unit in brackets after the label. So if we are plotting a distance, we might label our axis ‘distance (m)’, with the ‘(m)’ indicating that the numbers on the plot are in … Read More

A small maths lesson for our prime minister - Physics Stop

Sep 03, 2015

This isn’t physics but I do feel strongly about it. John Key today is reported by the New Zealand Herald today as saying, with regard to the refugee crisis: :  It’s a global problem. I accept everyone needs to take their fair share of responsibility but actually as a government we have been doing quite a lot over recent years and I have every confidence we’ll do more in the future. Has Mr Key done his maths on what is New Zealand’s fair share of responsibility? Because it’s rather more than the yearly 750 quota that I’ve heard reported as being New Zealand’s current contribution. Just how many refugees have fled or are fleeing Syria alone. My understanding is that it’s the majority of the population. OK, so I’m not a demographer, but I think 10 … Read More

How to get entry into a physics degree (but not necessarily physics) - Physics Stop

Sep 03, 2015

So, I’m now back from a lovely holiday in the UK, following a not-so-lovely period of being sick. Quite possibly I can also get back to blogging. Among the great many emails awaiting for me yesterday were a few about school physics and university physics. They were coming from different sources for different reasons, but there was a co-incidental unifying theme which went along the lines of ‘how well does school physics prepare students for university physics?, and how well does university physics prepare students for a career in science?’.  First, the school-to-university transition. Here’s a quick little piece by Peter Coles in  Times Higher Education, drawing from an Institute of Physics report on the male-female balance in ‘A’-level physics. A key point he makes is that efforts by universities to recruit more women … Read More

Pluto as you’ve not seen it before - Physics Stop

Aug 03, 2015

Most of us have never seen Pluto. Most of us never will. Neptune is more plausible.I remember as a student looking for Neptune with the Northumberland Telescope in Cambridge. We were doing a 'planets' night - in which it was theoretically possible to tick off all the planets (save Pluto, which was still a planet back then) in one night. A hunt for Neptune had some historical significance - James Challis failed to find Nepturne with the same telescope in 1846. As I recall, we failed to find it too. We might have seen it, but without taking a photograph and coming back to the same area of the sky the next night, we weren't going to be certain. But I remember the view of Mercury just after sunset being spectacular - that's a planet that doesn't often show itself, being so close to the sun. 

Anyway, back to the 'Trans Neptunian Object known as Pluto'. The New Horizon's website has some fantastic pictures. Here's my favourite so far - a 'time-lapse' film of our view of Pluto through the years. The latest images are unimaginally detailed compared with what I remember seeing in the popular astronomy books when I was a child. 



Calculating pi with darts - Physics Stop

Jul 16, 2015

I love this one. Really, it's maths not physics, but there is a bit of experimental physics creeping in at the fringes when the experimenters realize that the first method is biased. The second method is much better designed. 

Regrettably, pi-day (March 14th, 2015, or 3.14.15) only works if you use the US system of recording dates.  But fear not,  e-day (2nd July 2018, or  February 7th, 2018 if you're American) isn't so far away...

A light puzzle - Physics Stop

Jul 13, 2015

Here's a puzzling photograph that Hans Bachor showed me at the end of the NZ Institute of Physics conference last week. It comes from his public lecture on lasers a week ago. And we don't have the answer to it, so maybe you can enlighten us (pun intended). 


The photo is of a demonstration of total internal reflection with a laser. Hans is holding a container of water, which has a small hole at the bottom. Consequently there is a jet of water emerging. A laser is held up to the container, and with careful orientation it can be made to shine down the stream of water. The light follows the water, due to total internal reflection at the boundary between the water and the air (rather like a fibre-optic). Actually, it's not TOTAL internal reflection - if it were we wouldn't see the light escaping from the stream of water, but a great proportion of it is contained within the water stream. 

Now, in this case, Hans didn't quite get the hole the right size and shape. Consequently the stream breaks up into discrete droplets, which you can see in the photograph. Now, here's the puzzle. Look at the droplets and you can see that a couple of them are shining green - i.e. they appear to have laser light in them. 

But how does that work? Light moves so much faster than water one can consider the water to be 'frozen' in space as far as the light is concerned. While the laser light will happily travel along the water stream, when the stream breaks up into drops there is no total internal reflection anymore. The drops should not be glowing. Perhaps the light is jumping from drop to drop to drop. Unlikely - each drop will scatter the light considerably so that very little will jump from one drop to the next - let alone across many drops. 

As you think about this, you should bear in mind the conditions the photograph is taken over. It's a flash photograph, but it's likely that the shutter is open for longer than the flash illuminates the scence. This might (or might not) be significant, since the flash will capture the position of the water stream, but the shutter will still be letting in light from the laser even after the flash has stopped. So the capturing of the 'green' laser light in the photograph is not completely synchronized with the capturing of the rest of the image. 

Our best hypothesis is that the light that is that drops are illuminated directly by light that is emerging from the end of the stream - that is, the light leaves the stream, travels though the air, and hits a drop. In the spirit of Eugenia Etkina's ISLE approach then, are there other hypotheses and what experiments can we formulate to test them?

NZIP2015 Highlights - Physics Stop

Jul 07, 2015

So the NZ Institute of Physics conference is in full swing. I have a bit of a break between the end of the last session and tonight's conference dinner, so there's time to give some highlights so far. 

Well, first, the low-light: Like the rest of my family and half of Hamilton I've had a horrible cold. On Sunday morning I was wondering whether I'd be able to make any of the conference. But I've managed to hold things together and now I've stopped sneezing I'm rather less infectious than I was at the weekend. So I've been able to get to some of the sessions. 

So what's been going on? We've heard from Hans Bachor that after decades of international scientific research into getting lasers to work, the world's first funding application for using lasers was for a 'death ray'. Fortunately, applications have grown well beyond this one (which is still, thankfully, not in place) and far beyond the ideas of the original researchers (i.e. 'blue sky' research can have real value). We've seen edible fibre-optics (basically jelly), and heard from Jenni Adams about the ICE CUBE detector at the South Pole for detecting high-energy neutrinos. 

The speed talk session last night gave us a rapid-fire mix-and-match bag of physics research from across the country - from Kannan Ridings' simulations of the melting of metal nanowire's through to Inga Smith's (unanswered) question of why do so few women do physics?  

But the real highlight for me has been Eugenia Etkina's inspiring talk yesterday and workshop this morning, on physics laboratory experiments. The basic idea here is that experimental science is done by experts in a particular way (and she has evidence for this), including a cycle of observation, hypothesis, experimental design, prediction-making, experimental testing, then judgement. Experiments  by experts are done for particular reasons - either to observe, to test, or to apply. Give a group of scientists a practical problem and they will tackle it in a very systematic way, that usually allows them to get to the bottom of what's happening. Give the same problem to first-year university students, and it's a mess of hypothesis, tesing, judgement, observation all rolled into one. So it then makes sense for us to give students opportunities to carry out the same scientific processes as real scientists. Too often we give them a series of instructions to follow. This isn't how real science works. It simply doesn't help them learn science. 

At the end of her talk, Eugenia asked a very simple but really telling question. "How do you know that Newton's third law is true?" My initial answer, to be honest, was: "because the text-books say so". Not the answer of a scientist.  Thinking about it a bit more, I can say "because that's what I experience...if I hit something hard it hurts...i.e. if I exert a large force on something it exerts a large force on me". But here's (roughly) what one of Eugenia's students said when given the same question:

"I have carried out many independent tests of this law and have not found a single case where it is violated." Now, that is the response of a real scientist.