Archive September 2009

All your favourite science blogs Marcus Wilson Sep 30

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The amazing people at the Science Media Centre in Wellington have put together , all your favourite NZ science blogs in one easy to access site. Physicsstop is there, along with a host of others. The only downside is that with such a great collection of items to read, when am I going to find time to do any work?

I hate thunderstorms Marcus Wilson Sep 30

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The speed of sound in air is about 330 metres per second (which means it takes  three seconds to go one kilometre). So count the seconds between the lightning and the thunder, divide by three, and you have approximately your range from the lightning in kilometres. (Divide by five for miles). So, the lightning that struck about 4pm yesterday, which took at most I’d say quarter of a second between sight and sound, would be 0.25 divided by 3 which comes to awfully close.

I can’t say I’ve ever heard a bomb going off, but I reckon that sound must have been like it.

For more on my opinion of thunderstorms, click here

Climate engineering Marcus Wilson Sep 28

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So PhysicsWorld has done a nice article on some of the ‘engineering’ solutions that might be available for tackling global warming.

Generally they are pretty ambitious global-scale plans to turn down the thermostat a bit, given the premise that either carbon dioxide emissions will not fall sufficiently or that, even if they did, the earth would still be too hot and something else will be needed.   They fall into a few categories. First, there is the CO2-vac. Suck up that excess carbon dioxide using whatever technology you can make work. Secondly there is control of the earth’s surface. This might mean things like planting crops / forests that are a little more reflective to sunlight than current crops. And thirdly, there is control of the amount of sunlight that hits earth.

The greenhouse effect Marcus Wilson Sep 25

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I’ve been reading in PhysicsWorld about some grand ideas for controlling the earth’s climate by engineering on a global scale. Some sound pretty fanciful, though some might be just plausible. But before I get there (which will probably be another entry) I think it’s worthwhile reminding you what the greenhouse effect actually is. As in, why is it a greenhouse gets hot, and what has this got to do with the atmosphere?

Let’s take the greenhouse first of all. Just a few pieces of glass enclosing your tomato plants. Why is it so hot in there? The sun emits a lot of radiation. Because it’s so hot (around 5500 degrees celcius or so) a substantial fraction of its energy it emits at wavelengths in the visible spectrum. When this light arrives at the surface of the earth (let’s ignore the atmosphere for now) it travels through the glass of our greenhouse and hits the ground / tomatos or whatever is in there, where some of it is absorbed. This generates heat, which heats up the material inside.

However, as we know, hot things radiate the heat again. But, unlike the sun, which radiates hugely in the visible spectrum, things at say 20 or 30 Celcius radiate mostly at infra-red wavelengths. And at infra-red, unlike visible, glass is opaque. So the radiated heat can’t escape. (It can enter, but not leave, because it tries to leave at a different wavelength from what it arrives). Contrast this with the case of no greenhouse – the light from the sun falls on the ground – the ground heats up, but this heat can now escape back upwards as it is radiated. It doesn’t get so hot.

Now let’s go to the atmosphere. Water vapour and carbon dioxide molecules (amongst others) in the atmosphere do a similar thing to glass. They let visible light through, but absorb infra-red light. The sunlight makes it to the surface, but the heat radiated from the surface gets trapped. Same mechanism as the greenhouse, hence the name greenhouse effect.

NB The different properties of materials at infra-red and visible wavelengths are often overlooked. Paint is a good example – I often hear people remark that radiators should be painted black to maximise the amount they radiate (black bodies radiate well.) The argument doesn’t hold because pretty well any colour paint you buy is close to ‘black’ if you look at it in infra-red – i.e. it absorbs (and hence radiates) infra-red radiation (heat) very well. There is no need to repaint your radiators.

Fishics Marcus Wilson Sep 24

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Eco-systems are of course very complex things – the success of one species is linked to the success of another, which is linked to another, and all of which are linked to outside factors such as climate etc etc.  Now there is direct evidence of another degree of complexity in the ocean eco-system, namely that fish (and other swimmy things) have a significant role to play in mixing the ocean.

The hypothesis is not new – it was postulated by Charles Darwin (grandson of his more famous namesake) in the 1950s – but now experimental evidence has been found. Katija and Dabiri (Nature 460, 624-626) have measured the effect of jellyfish migrating upwards from deeper, cold water, to closer to the surface where the water is warmer. As they move, they drag with them the cold water. This is denser than the warm water nearer the surface, and so, once it leaves the vicinity of the jellyfish, it will fall, creating a circulation that mixes the water.

The researchers argue that this mixing is as significant as some other forms of ocean mixing, and shows that we must add jellyfish (and, the researchers point out, other fish that migrate in depth) to the list of creatures that influence the environment in a big way.

That’s it! The solution to climate change – genetically engineer a species of fish to do more or less upward / downward migration (whichever is appropriate for influencing the ocean in the right way), and we control the surface temperature of the oceans! No more silly than putting a giant sunshade in orbit.

All those strange physics symbols Marcus Wilson Sep 22

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If ever you thought reading a physics textbook was like reading a page of Tolkein, this one is for you. (Thanks due to University of Nottingham). Each symbol has a short video behind it.

Risky things Marcus Wilson Sep 21

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I love the headline at the end of last week ‘Wellington quake risk halves’. As if you can wake up one morning and find that the chances of an earthquake happening today are suddenly half of what they were yesterday just because someone says so.  What next – someone decreeing that summer will last 12 months of a year and so magically it does?

Of course, what is really meant (and to be fair the second paragraph of the article on gets it right) -our best estimate of the risk ‘of the big one’ is now half what was previously thought.

To be honest, I don’t know a lot about predicting earthquakes, but what I do suspect is that a major problem the scientists have is that there isn’t a great deal of data to go on. Really big earthquakes don’t happen all that often in any given place. Hence the difficulty in knowing what the risk really is.

There is a whole branch of physics devoted to things that occur randomly. Some random processes are rather easier to work with that others. For example, radioactive decay. Now, if I have an atom of (say) Caesium 137, I don’t know when it will decay, but I can be very specific about its chances. I can say that the chance of it still being a Cs 137 atom in 30.1 years time is exactly half. Why? Because people have measured the rate at which decay events happen to Cs 137 atoms. There are a lot of atoms around (contrast the case with earthquakes) which makes it easy to construct very accurate predictions. It doesn’t help me predicting the exact day that my atom will decay on, but what I can say is that if I had a million atoms, and waited 30.1 years (the half-life of Cs 137) I would have pretty well half a million left.

Radioactive decay is an easy example because the underlying process is what we describe as ‘Poisson’ – which leads to a very simple mathematical description. Other random processes are not always so nice. A good deal of research in this area is driven by insurance companies, and for good reason – they are the ones that need to properly assess and quantify the risk of something, because they have to put money on it. The Earthquake Commission will be very interested in the Wellington fault-line study, because it may well affect how much insurance levy they feel the need to collect from us NZ residents.

As I said, I’m not an earthquake expert, but I can hazard a guess that a big earthquake in Wellington isn’t going to be great for the economy, whether you live in Wellington or Kaitaia.

The end of the week… Marcus Wilson Sep 18

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Have you ever had one of those days when you have worked flat out all day and seem to have accomplished nothing?

I think that’s today.  My desk looks like a tornado has been through the office. Now, I wonder, statsitically speaking, how many tornados I’d need to come through before one picked up all the loose bits of paper and kindly deposited them in the right places in the filing cabinet.   I live in hope.

I’m off to enjoy a coffee.

Scholarship physics questions Marcus Wilson Sep 17

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I’ve just been putting together a presentation for final year school children on the NZ scholarship physics exam.   NZ Scholarship is awarded to the top 3% or so of students in a particular subject in a particular year, and there is some big money up for grabs.

But the exam questions for scholarship are hard.  Really.  I find them hard (remember I lecture physics at university – I should be able to do school physics stuff).   I can open an exam paper and think …err…how on earth am I meant to do that? If that’s my response to a physics question, then I can only think that it will be the response of most people sitting the exam.

But what makes the exam hard (generally) is that the physics problems come in unfamiliar contexts, with unfamiliar wrapping paper on them. There is an infamous question from a few years ago about phugoid oscillations on a model aircraft. What the phugoid are phugoid oscillations? I bet the percentage of the population who know that is small indeed. But, think more carefully, and one realises that phugoid oscillations (go look them up – I’m not going to explain) are just an example of an extremely broad class of motion in physics called simple harmonic motion. (Simple harmonic motion is what a swinging pendulum does – moves one way, then the other). Once the student realises this, the question is not so daunting after all.

It’s often true with real-life science problems too. Once we unpackage the problem, and get to the bottom of the issue, often we find that the science is not all that complicated. However, solving the problem might not necessarily be so easy.

Geometric algebra Marcus Wilson Sep 16

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Here’s a lovely quote that students will empathize with:

“A recent study on the use of vectors by introductory physics students summarized the conclusions in two words: “vector avoidance”. This state of mind tends to propagate through the physics curriculum. In some 25 years of graduate physics teaching, I have noted that perhaps a third of the students seem incapable of reasoning with vectors as abstract elements of a linear space…I have come to regard this concept of a vector as a kind of conceptual virus, because it impedes development of a more general and powerful concept of a vector…”

David Hestenes, Oersted Medal Lecture 2002: Reforming the Mathematical Language of Physics. Reference made to E. Redish and G. Shama, AAPT Announcer vol 27, p98 (1997)

Very, very true. But help is at hand, in the form of geometric algebra. It is not unrelated to the complex algebra I’ve talked about, but has an advantage in that it is all firmly based in reality. Geometry is real. Objects are three dimensional things, with volume, surfaces, edges and vertices (corners). We should (and do) have a method of dealing with all these entities properly.

Geometric algebra has been around for a while, but popularised recently by the likes of David Hestenes and Anthony Lasenby, amongst others. I’m searching the net for a really simple introduction to it (something suitable for a blog) but haven’t got there yet. I would call it ‘vectors done properly’. As a physicist, I find geometric algebra a simpler and way more powerful method of dealing with geometrical objects such as vectors (and these come up everywhere in physics) than the more traditional approaches taught at school and university. So why isn’t it taught?

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