Oct 14 2009

Wordless Wednesday

Oct 09 2009

Superconductivity

This is a little post about superconductivity, lecturing and liquid nitrogen.

The lecture I remember most clearly was when I first demonstrated the Meissner effect in a superconductor. You can buy a little kit to help with this. It contains a little powerful magnet, a disk of a high temperature superconductor and a polystyrene dish. Put superconductor in dish, add liquid nitrogen to dish, wait for bubbling to subside then drop small magnet onto superconductor and this happens:

(A video is better, see here)
The little magnet just sits there, suspended above the superconductor, if you give it a prod it’ll spin around on it’s axis. It’s magic! Now the first time I did this was live in a lecture theatre in front of fifty students. I’d not had a chance to try it out in advance, and I must admit I was a bit underwhelmed by the equipment provided. So I did the tippy-out-the-liquid-nitrogen and wotnot, and my first words thereafter were “Bloody hell – it works!” – the students seemed impressed too. Much poking of the little magnet with the plastic tweezers was done, and we also splashed around the liquid nitrogen for more fun. I did the demonstration the following year, but it wasn’t the same without my genuine surprise and excitement.

Lecturing is a bit of performance (quite literally), I struggled with the format because I found it hard to get meaningful feedback from a large group of students. If you do it passionately and enthusiastically it comes across to the students, but that’s difficult to sustain for lecture after lecture. If you get it spot on, it’s brilliant but usually its just a chore (for both student and lecturer).

Just to explain a little more about superconductors: a superconductor is a material which conducts electricity perfectly – it’s resistance is zero (not just small, zero). A light bulb, an electric fire or kettle would be utterly useless with a superconducting element, the electric current would flow through it without emitting any light or heat. Heike Kamerlingh Onnes discovered superconductivity in 1911 (having first worked out how to liquify helium to cool his samples). More recently a bunch of so-called high temperature superconductors have been discovered, the weird thing is these materials are ceramics – they don’t conduct at all at room temperature and yet cool them down to liquid nitrogen temperature (-196degrees centigrade) and they conduct really well. As I’ve mentioned in earlier blog posts, superconductors are used for the making of big magnets and there are also some applications in very sensitive detectors. In principle they would be great for electrical power transmission, but the requirement to cool everything down to at least liquid nitrogen temperatures has meant they’ve not been economically viable.

Laboratory scientists take liquid nitrogen for granted but it’s an utterly alien material, like furiously boiling water but at the same time deep-bitingly cold. It hisses as it’s poured into a new vessel, wreathed in clouds of condensing water vapour. Liquid nitrogen splashed on a laboratory floor will chase dust bunnies around with distinct droplets of fiercely boiling liquid, like tiny hovercraft. The droplets vanish without a trace.

Oct 07 2009

Wordless Wednesday

Oct 03 2009

Schrodinger’s flippin’ cat!

There comes a time in a blogs life when a bit of a rant is called for, here’s mine or at least the first one. To be honest it’s a fairly discrete, civilised rant – because that’s the sort of chap I am. It’s about cliches in science.

Quite some years ago, Stephen Jay Gould wrote an essay entitled “The case of the creeping fox-terrier clone”, published in “Bully for Brontosaurus“. In it he describes how he was writing a piece on evolution using the time-worn example of the horse, and in particular an animal named hyracotherium also known as eohippus or “The Dawn Horse”. The problem for Professor Gould was that he found himself on the point of typing that eohippus was “the size of a fox-terrier”, the thing is he had no idea how big a fox-terrier was! That’s right, Prof Gould (who I think writes very nicely) was about to commit a cliche to paper, and rather admirably he stopped and had a bit of a think instead. Now the reason he was about to write this was that he’d read it many times before, it’s a very standard story in evolution. He wasn’t alone, many writers have written how “eohippus was the size of a fox-terrier”, and doubtless many of them had no idea how big a fox-terrier was. Many readers have, no doubt, read those words, nodded sagely to themselves and said “All is well, I know that eohippus was the size of a fox-terrier”. It’s not really the cliche that’s the problem, the problem is that we’ve gone through the motions of communicating an idea, but sort of failed. Just in case it was bothering you, a fox-terrier is about the same size as eohippus, or roughly 40cm at the shoulder ;-)  I reckon that’s about the same size as a large lamb.

This isn’t an isolated example, science writing (and education) is riddled with cliche, not just cliche in word, but cliche in thought. My own bugbear is Schrödingers cat, of whom surely everyone must have heard. Erwin Schrödinger was one of the fathers of quantum mechanics.

IM IN UR QUANTUM BOX � MAYBE.
 (I have a bit of a weakness for lolcats)

Briefly, Schrödingers “thought experiment” is as follows: take one quantum mechanical system (a radioactively decaying material is common), one cat, one diabolical system to kill the cat based on a random event from the quantum mechanical system and one opaque, cat-proof box. Combine ingredients and wait…now open the box. The argument put is that prior to opening the box the cat is in an uncertain state between dead and alive (which is true of the quantum system, atoms in the radioactive material could be said to be decayed and undecayed simultaneously). 

However, Schrödinger prefaces this thought experiment thusly: “One can even set up quite ridiculous cases.” Schrödinger didn’t think his cat was genuinely in some weird half-way house between dead and alive he was quite clear that it was very definitely one or the other and the problem was that for systems obeying quantum mechanical rules this wasn’t the case. That’s the useful point in this thought experiment: “There’s something weird that goes on between the quantum and the classical and we don’t know what it is”. Yet time after time you see this experiment described without the critical proviso. People go away with the false impression that undead cats exist!

oh dear I can feel my self getting a bit incoherent now… special relativity, I’ve taught special relativity, it’s genuinely a marvelous intellectual leap that solved a couple of serious problems in physics. It has some real world applications (understanding my old friend the synchrotron, GPS satellites, lifetimes for relativistic muons in the atmosphere etc). But the text book examples we give to students are rather worn, nope, “worn” is the wrong word. “flippin’ ridiculous” gets a bit closer. Here’s one:

“You have a 10 meter long ladder, and a 5 meter long shed. How fast must the ladder enter the shed in order for it to appear to fit inside to a stationary observer?”

I can tell you the answer: it’s “really fast” – some large fraction of the the speed of light. To put it another way, a ladder travelling at the requiste speed could travel the length of the equator in something under quarter of a second, that’s probably a little faster than your reaction time and I’m sure you have an intuitive feel for the length of the equator. My point here is that (1) You’re going to struggle to get your ladder going that fast (2) if that ladder’s going past you that fast, the absolute last thing on your mind is going to be “ooo…look, the 10m ladder is fitting into the 5m shed”. If your shed is in a vacuum then you won’t get killed by massive plasma shockwave, but how many sheds have you seen in a vacuum? For part 2 of this experiment one may find some halfwit has placed a concrete block at the back of the shed to check the ladder really is fitting into the shed by bringing the ladder to an instant standstill inside the shed. Once again, when ladder hits concrete whether ladder fits into shed is the least of your worries. Assuming that you were in a vacuum, your ladder/concrete collision is going to release “absolutely loads” of energy – fusion bomb scale. There you go, I’ve lost it completely now. Special relativity teaching is full of everyday objects (trains and rulers are typical) traveling at implausible speeds, and it really winds me up!

Don’t get me started on “Alice and Bob“, the quantum cryptographers and if one more string theorist tells me that all the extra dimensions are “curled up very small”, there’s going to be some hurtin’.

And relax… I feel better now that I’ve written it down.

Sep 29 2009

An introduction to the big and shiny

So off to university I skipped, with paternal instructions to “blossom”. I did BSc Chemical Physics at Bristol University. This is a fine course for the indecisive, such as myself who couldn’t decide whether they were chemists or physicists. There were only 16 students in my year, we could all fit in one lift in the physics building. The chemical physics degree involves most of the first year chemistry and physics courses, but in later years you drop organic chemistry, particle physics and astrophysics to make space to do courses in the rest of chemistry and physics. This training means I’m less bemused by molecules than the majority of physicists.

It was at Bristol that I was introduced to big, shiny machines for doing science: for my final year project I worked on Extended X-ray Absorption Fine Structure (EXAFS) at the Daresbury Synchrotron Radiation Source (SRS).

I was using EXAFS to discover something about the structure of molecules in solution, and the arrangement of a glass around metal ions. This is typical of the sorts of study done at synchrotron sources: finding exactly where the atoms in a particular material. Where the atoms are helps you understand the properties of the material: what reactions does it catalyse, how well does it conduct, how strong is it, and so on.

The synchrotron radiation source is a device for producing huge quantities of light all the way across the spectrum from x-rays to ultra-violet light, passing through the visible spectrum on the way. To do this you make electrons go round and round in circles really fast inside an evacuated tube, as they go around they emit electromagnetic radiation: here’s a practical application of relativity – the electrons are traveling at a substantial fraction of the speed of light, so to calculate (and control) their behaviour you need to include relativistic effects. X-rays are actually emitted when the electrons pass through the intriguingly labelled wigglers and undulators – sets of cunningly arranged magnets around the evacuated tube.

The SRS was a pretty big machine – 30 metres in diameter, it closed at the end of 2008 to be replaced by Diamond which is substantially larger. You can see SRS, along with Diamond (near Harwell) and the ESRF in Grenoble, which is even bigger still, on this Google Map. The ring shape of SRS is just about apparent, Diamond and ESRF are very obviously rings.

I’ve worked in several big science facilities (all of them either neutron or synchrotron sources), they have  some common features. Separate instruments cluster around either a ring shaped source or a central point – a beam line (usually an evacuated metal tube) runs from the source to your experiment. The instruments have been built for different consortia by a small team and then used by scientists from around the world. The instruments are scattered around warehouse-sized sheds, filled with a wide range of machinery (which generally contribute to rather high background noise). The control stations for each instrument may be in portacabins, or small corrals. For neutron and x-ray facilities there are a lot of really big concrete blocks around to block harmful radiation.

The deal with working on these instruments is that they run 24 hours a day, and you apply for time on them. If your application is successful then you get to use the apparatus for a block of time, typically a couple of days or so. Some users are lucky: their experiments take days or hours to run, so they can pop off to the pub between runs. I’ve always been stuck doing experiments which last 20-30 minutes per shot. Normally you try to get a team of three or four people to work in shifts, through the night you survive on machine coffee, cantankerous vending machines and possibly loud music. You try to do data analysis as you go, to make sure nothing is going wrong and to help pick which samples to run next. These are one-off research machines, so the data analysis process may be a bit convoluted, and not very user-friendly.

It turns out I have some pictures from my visits to Daresbury (as part of my small obsessive streak I digitized my old photo collection a few years ago)

This is me looking really very boyish at the control station for the EXAFS machine, that’s the back of Sue’s head. Sue was a PhD student working in the same lab as me.

I’m pretty sure this is the EXAFS station I worked on, John is pointing at a polysytrene vessel containing liquid nitrogen (not quite sure why). John was another member of the research group.

This isn’t the equipment I was using but see how shiny it is, how many bolts hold it together and how many unexplained protruberances it has. Who can but fall in love with such things?

All in all it was a rather exciting introduction to the world of research science. My rather small contribution to the measurement of cobalt ions in glasses (which were subtle shades of blue and pink) led to my first paper[1] in the scientific literature!

Reference
1. Harrison, C.C., X.C. Li, I. Hopkinson, S.E. Stratford, and A.G. Orpen, Ultraviolet-Visible-Near-Infrared and EXAFS Study of Co-Ii Coordination Chemistry in Post-Doped Silica Sol-Gel Glasses. Journal of the Chemical Society-Faraday Transactions, 1993. 89(22): p. 4115-4122.