Friday, 25 January 2013

Joking, then to Essex (via Woking)

Last night I took part in Guildford Bright Club.  It's a kind of academic outreach activity in the form of stand-up comedy.  The idea is that university researchers, be they professors who find it hard to keep their eyes open during talks or new PhD students, with eyes blinking in wonder, the Bright Club format provides a place for them to engage with the sort of audience who thinks that going to a comedy night is perhaps more fun than going to a more formal public evening lecture.

I had misgivings about the name Bright Club, as being a bit superior (we're the bright ones), until I realised that it was a play on Fight Club, and my misgivings abated a bit.  It's a fun idea, and the events have a paid-for comedian to compere and some musical accompaniment.

Last night there were six researchers in a set of two halves, with comedy music to round off each half.  I was billed right at the end, and carefully sipped a single beer the whole evening so as not to have an inappropriate kind of Dutch courage that the stupid joke I suddenly though of might be funny.

I think it went okay.  I got some laughs.  I sort of lapsed into a stupid pastiche nerdy physicist voice at the end, which I hand't really meant or wanted to, but all-in-all it went well, I think.  I was allowed a free drink for performing, though was quite disappointed that it didn't extend to a free cocktail.  A shame, since we were in a cocktail bar.  Still, I guess I know my worth now; more than a glass of water but less than a martini.

On the other hand, today, I, without even the reward of an alcopop, went to Theydon Bois, near the far northeastern end of the Central Line to talk to a University of the Third Age (i.e. retired people interested in general intellectual things) group about nuclear physics.  I gave a version of the talk about nuclear applications that i give to general audiences, so was slightly abashed to discover that the core of the group is formed from retired (or semi-retired) scientists from Queen Mary University.  It turned out too that I was in a double bill with a member of the UA1 experiment that discovered the W and Z bosons, who is, as well as an ex-UA1 member, also a U3A member.  When he presented pictures of Feynman diagrams, I wondered if I hadn't pitched my talk a little low.

I was hoping that someone at the U3A meeting would be able to teach me something in exchange for the undoubted words of wisdom that I was giving them.  So I appealed at the end of my talk that if anyone could teach me how to tie a bow tie I would be most grateful, as I was going to a black tie event that evening.  One kind chap came up and showed me him tying it on himself.  He did it quite well, but with a little difficulty, and admitted that he was no expert and would have trouble doing it on me.  I should have stood there and tried, following his instructions over and over, but the call back to answer questions helped me wimp out of this display.

So after the talk, I headed off to the Royal Institution to the Friday Evening Discourse in which Jim Al-Khalili was talking.  Jim was kind enough to invite me along as his guest not only to the talk, but to a drinks reception beforehand.  I tried my usual trick of asking the people at the door if they could help me tie my tie, but they couldn't and I turned up the black tie reception with my black bow tie crossed over in a sort of rakish cravat affair.  It was deemed pretty cool.  The net effect is in the picture for you to see.  I enjoyed Jim's talk, and particularly that he acknowledged me at the end as "Professor" Paul Stevenson, despite the fact that lack that lofty status. I was sitting next to our faculty dean, whom Jim had also invited, and I'm sure he'll slowly get to think of me as a professor, and will be shocked into promotive action when he discovers I'm not.

Anyway, I'm on the train home, as you can see from the picture, and about to arrive at GLD, so I will bid you goodnight

Tuesday, 22 January 2013

Boundary conditions, marking, music and the prize crossword.

Hello.  Here are a few disjointed things:

I don't exactly intend to turn this blog in to a pastiche of any other famous physics blogs but still, I do want to mention that I sent off the solution to the Saturday Independent crossword today to see if I might win the prize for solving it.  Not that I particularly lack dictionaries, which are the prize, but more for the kudos.  The funny thing about this Saturday's crossword was that it didn't have the usual π rotation symmetry.  This was quite disturbing.  I expect there was a clever reason why not.  Indeed, there was a sort-of anarchic theme to the crossword, which I didn't really understand, so I probably missed something, though I think I got all the clues correct.

I had a nice email from one of my PhD students to point out that we had a paper published today, on boundary conditions in solutions of the time-dependent Schroedinger equation.  I'll try to make a proper post about that soon to explain what it all means.

On Thursday I'll be appearing at Guildford Bright Club doing a stand-up comedy set.  Thanks to a quick run-through yesterday, I know I have plenty of preparation still to do, but I think it'll be pretty good, so come to Bar Des Arts in Guildford this Thursday to check it out if you're free

Finally, I don't exactly intend to turn this blog in to a pastiche of any other famous physics blogs but still, I was listening to Radio 3 while doing some marking earlier, and came across a bizarrely wonderful piece of organ music.  There's a recording of it by the composer, which is a joy to watch, so I attach it here:

Monday, 21 January 2013

I am a Material Girl

When I was at STFC headquarters last week, I was pleased to be recognised there as someone who helps advertise UK advances in nuclear physics.  I was a little crestfallen when asked if I'd seen the UK paper in Physical Review Letters that had come out the day before, and about how it was an exciting story about nuclear astrophysics.   The paper referred to was an excellent paper by the nuclear astrophysics group at York, but I had published a paper in the same journal on the same day, that seemed to have gone unnoticed.  Sob.  Still, go York group.  I post more because I've just listened to the lead author on that York paper, Alison Laird, speak on Radio 4's Material World programme.  Go check it out here!

Sunday, 20 January 2013

The size of lead

Overlap of the different quantum orbits of
the extra neutrons in lead isotopes with the
proton orbits
Sorry to have been a bit silent since the new year.  The day after my last post, I went for a week-long holiday, and since I've been back blogging has not quite made the top of the list of things to do.  Still, following mention of a paper I recently wrote with my student and his co-supervisor, I've been meaning to write a bit more about the physics of what the paper contains.  I think the concepts involved are mostly quite simple, and so I hope I can do a reasonable job of explaining what it's about.

Nuclear physics is the study of the really tiny nuclei that are at the centre of every atom, and atoms are what make up all material.  Atoms are small enough that the conclusive evidence that they exist only came around 100 years thanks to advances in physics.  Nuclei, though, are far smaller.  The usual analogy is that if the atom is the size of a cathedral, then a nucleus is sized like a fly buzzing round inside.  Despite the tiny size, though, we can measure the size of a nucleus and see that some nuclei are different sizes to some others.

A nucleus can be charaterised by the number of protons and neutrons it contains, as these two more basic objects are the sole constituents of nuclei.  The number of protons tells you what chemical element the nucleus corresponds to and the number of neutrons tells you what isotope of that chemical element you have (see Elizabeth Williams' excellent recent explanation for more of the basics).

It turns out that if you keep adding neutrons or protons to a nucleus, you expect it to grow in a steady way (quite unlike the way atoms grow when you add electrons), so that a nucleus with more protons and neutrons will be larger than one with fewer with a very regular kind of pattern.  By and large, this is true.  In fact, I've just presented a rather circular argument - it's expected more because it has been observed to be generally true, rather than us having guessed in before-hand.

One particular anomaly in nuclear sizes comes in isotopes of lead nuclei.  All lead nuclei have 82 protons, but can exist with a number of neutrons ranging from around 100 to at least a little over 130.  At least, this range has been seen in experiments, though many of the combinations are short-lived an radioactive.  If you keep adding neutrons to lighter lead isotopes, and measure the size of the proton distribution, which is much easier than measuring the size of the neutron distribution, then you observe that the protons get pulled out a bit by the extra neutrons, and the radius of the proton matter distribution increases, and it does it in a steady way, all the way up to 126 neutrons.  As soon as you go over 126 neutrons the size starts increasing much more rapidly.

Various reasons have been given for why this is so.  Some are in terms of particular nuclear models which either do or don't correctly reproduce this "anomalous kink effect," as it has been termed.  Other reasons use a description based on the fact that quantum mechanics dictates that neutrons and protons have to exist in certain shells or orbits (roughly analogous to planetary orbits) and that different models predict different radii for different shells.  What we have suggested, in our new paper, is that although their might be some truth in the previous explanations, the key factor seems to be not the size of the orbits themselves, but how much the orbits make the extra neutrons interact with the existing protons.  It turns out that tighter orbits, with smaller radii, can generate more of a pull on the protons and make their density distribution larger, than larger orbits that don't interact so much with the protons.

On the level of some of the advanced and complex ways now available to theoretical nuclear physics, it is on the basic side.  Sometimes the simple explanations turn out to be correct.  We at least hope that this sheds some light on the puzzle.

Goddard, P., Stevenson, P., & Rios, A. (2013). Charge Radius Isotope Shift Across the N=126 Shell Gap Physical Review Letters, 110 (3) DOI: 10.1103/PhysRevLett.110.032503