Archive for April, 2012

Godzilla versus the Olympics

Posted in Uncategorized with tags , on April 30, 2012 by telescoper

Breaking News. Alarming footage just released by MI5 reveals the true nature of the threat to the forthcoming 2012 Olympic games and explains why it is necessary to station missile batteries in London’s East End.

A New Baryon on the Block

Posted in The Universe and Stuff with tags , , , , , on April 29, 2012 by telescoper

I just chanced upon the news that a new particle has been discovered at the Large Hadron Collider. This is probably old hat for people who work at CERN, but for those of us following along in their wake it definitely belongs to the category of things marked Quite Interesting.

The new particle is a baryon, which means that it consists of three quarks. These quarks are held together by the colour force (which I refuse to spell the American way); baryonic states exist by virtue of the colours of constituent quarks being a red-green-blue mixture that is colourless.

Quarks are fermions with spin 1/2. The new particle has spin 3/2 which contrasts with the most familiar baryons, the proton and the neutron, which also consist of three quarks but which have spin 1/2. The difference can be understood from basic quantum mechanics: spins have to be added like vectors, so the three individual quark spins can be added to produce total spin 3/2 or 1/2.

The most familiar spin 3/2 baryons are made from the lightest quarks (the up, down and strange) as shown in the diagram below:

The top row contains no strange quarks, only up and down. In fact the Δ0 and Δ+ contain exactly the same quark compositions as the proton and the neutron (udd and uud respectively), but differ in spin. The next row down contains one strange quark (e.g. uds) , the one below two (e.g uss), and the particle at the bottom is a very famous one called the Ω which is entirely strange (sss). For reasons I’ve never really understood, a strange quark carries a strangeness quantum number S=-1 (why not +1?) and the electrical charge is labelled by q in the diagram.

There are six quark flavours altogether so one can construct further baryonic states by substituting various combinations of heavier quarks (c,b and t) in the basic configurations shown above. There are also excited states with greater orbital energy; all the particles shown above have quarks in the lowest state of orbital angular momentum (L=O). There is then a potential plethora of baryonic particles,  but because all are unstable you need higher and higher energies to bring them into existence. Bring on the LHC.

The new particle is called the Ξb*, and it consists of a combination of up, strange and bottom quarks that required collision energies of 7 TeV to make it. The nomenclature reflects the fact that this chap looks a bit like the particles in the third row of the figure, but with one strange quark replaced by a much more massive bottom quark; this one has zero electrical charge because the charges on the u, s and b are +2/3, -1/3 and -1/3 respectively.

Anyway, here’s the graph that represents the detection of the new baryon on the block:

Only 21 events, mind you, but still pretty convincing. For technical details, see the arXiv preprint here.

Whether you really think of this as a new particle depends on how fundamental you think a particle should be. All six quark species have been experimentally detected and in a sense those are the real particles. Things like the Ξb* are merely combinations of these states. You probably wouldn’t say that an excited state of the hydrogen atom (say with the electron in the 2s energy level) is actually a different particle from the ground state so why do different permutations of the same quarks warrant distinct names?

The answer to this I guess is the fact that the mass of an excited hydrogen atom differs from the ground state by only a tiny amount; electronic energy levels correspond to electron-volt scales compared to the 1000 MeV or so that is the rest-mass energy of the nucleus. It’s all very different when you’re talking about energy levels of quarks in baryonic particles. In such situations the binding energies of the quarks are comparable to, or even larger than, their rest masses because the colour force is very strong and the quarks are whirling around inside baryons  with correspondingly enormous energies. When two creatures have enormously different masses, it’s difficult to force yourself to think of them as different manifestations of the same beast!

Anyway, the naming of this particle isn’t really the important thing. A rose by any other name would smell as sweet. What matters is that existence of this new quark state provides another example of a test of our understanding of quark-quark interactions based on the theory of quantum chromodynamics. You might say that it passed with flying colours…

Here’s That Rainy Day ..

Posted in Jazz with tags , , on April 28, 2012 by telescoper

If yesterday’s post made you wonder how difficult it is to turn a piece of sheet music into sound using a piano keyboard, then perhaps today’s will make you wonder how a pianist like Bill Evans managed to create music as beautiful as this without any score at all! This is Here’s that Rainy Day from the 1968 album Bill Evans Alone. Miles Davis said of Bill Evans “He plays the piano the way it should be played”. I, for one, won’t argue with that.


The Piano in Question

Posted in Music with tags , , on April 27, 2012 by telescoper

Here’s a trip down memory lane for me. While I was at school I was captivated by the BBC TV series, directed and introduced by Jonathan Miller, called the Body in Question. This episode, first broadcast in 1978, shows Dr Miller at the piano with Dudley Moore, his old friend from Beyond the Fringe. They’re exploring the mysterious process by which pianists manage to put their fingers on the right keys without apparently consciously thinking about the mechanical operations involved or even looking at the keyboard. Practice seems to program the hands so that the translation from sheet music to sound becomes second nature, but to those without the ability to effect the transformation (like myself), the process still seems almost miraculous.

A Little Bit of Nuclear..

Posted in Cute Problems with tags , , on April 26, 2012 by telescoper

It’s been a while since I posted any cute physics problems, so here’s a little one to amuse you this rainy Thursday morning.

In the following the notation A(a,b)B means the reaction a+A→b+B. The atomic number of Oxygen is 8 and that of Fluorine is 9.

The Q-value (i.e. energy release) of the reaction 19O(p,n)19F is 4.036 MeV, but the minimum energy of a neutron which, incident on a carbon tetrafluoride target, can induce the reaction 19F(n,p)19O is 4.248 MeV. Account for the difference between these two values.

April Rain Song

Posted in Poetry with tags , , on April 26, 2012 by telescoper

Let the rain kiss you.
Let the rain beat upon your head with silver liquid drops.
Let the rain sing you a lullaby.

The rain makes still pools on the sidewalk.
The rain makes running pools in the gutter.
The rain plays a little sleep-song on our roof at night—

And I love the rain.

by Langston Hughes (1902-1967)


Posted in Uncategorized with tags on April 25, 2012 by telescoper

COBE and after…

Posted in Biographical, The Universe and Stuff with tags , , , on April 24, 2012 by telescoper

An item on the BBC website yesterday reminds me that it is twenty years since the announcement, in April 1992, of the discovery of temperature variations across the sky in the cosmic microwave background radiation by the Cosmic Background Explorer (COBE). Was it really so long ago?

At the time the announcement was made as I actually in the USA. In fact,  I was at the University of Kansas for about a month working on this paper with Adrian Melott and Sergei Shandarin, which eventually came out early in 1993. I remember it very well because we started the project, did all the calculations and wrote up the paper within the short time I was there. Oh what it is to be a postdoc, having only research to think about and none of the other distractions that come with more senior positions.

Anyway, the COBE announcement hit the news while I was there and it got a lot of press coverage. I even did a TV interview myself, for a local cable news channel. Nor surprisingly, they were pretty clueless about the physics of the cosmic microwave background; what had drawn them to the story was George Smoot’s comment that seeing the pattern of fluctuations was “like seeing the face of God”. They were disappointed when I answered their questions about God with “I don’t know, I’m an atheist”.

The Face of God?

I didn’t know at the time that the way the announcement of the COBE discovery was handled had caused such ructions. Apparently George Smoot let his enthusiasm get the better of him, broke ranks with the rest of the COBE team, and did his own press conference which led to accusations that he was trying to steal the limelight and a big falling-out between Smoot and other members of the team, especially John Mather. It’s unfortunate that this cast a shadow over what was undoubtedly one of the most important science discoveries of the twentieth century. Without COBE there would have been no WMAP and no Planck, and our understanding of the early Universe and the formation of galaxies and large-scale structure would still be in the dark ages.

As a lowly postdoc at the time, living a hand-to-mouth existence on short-term contracts, I didn’t realise that I would still be working in cosmology twenty years later, let alone become a Professor.  Nor could I have predicted how much cosmology would change over the next two decades. Most of all, though, I never even imagined that I’d find myself travelling to Stockholm as a guest of the Nobel Foundation to attend the ceremony and banquet at which the 2006 Nobel Prize for Physics was awarded to George Smoot and John Mather for the COBE discovery. It was a wonderful one-in-a-lifetime experience, made all the nicer because Smoot and Mather seemed to have made peace at last.

Where were you when the COBE results came out?

Controversy brewing at ESA?

Posted in Science Politics with tags , , , , on April 23, 2012 by telescoper

Interesting stuff over at the e-astronomer relating to ESA’s handling of the process of selecting its next L-class mission. The plot thickens.

The e-Astronomer

So the Athena folk are somewhat miffed at being pipped by Juice. (This metaphor doesn’t seem quite right ? Ed.) But what about Horse Number Three ? Aren’t the NGO folk doing a Grand Petition ? Nope. It seems their tactic is a semi-formal complaint about inadeqacies in the process : an email letter direct to Gimenez. I am not sure how widely it has been circulated, but I understand it is stern stuff, bringing up issues of inappropriate revisions of costings and risk factors, and inadequately resolved conflicts of interest. Feel free to comment if you have clear knowledge, but please (a) do not leak things that are confidential, and (b) keep coments about process and not about individuals.

Its not really clear what competition means when a very small number of items is under consideration, and moreoever each item represents one community-segment, each of which ESA wishes to…

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On the Dearth of Dark Matter in the Solar Neighbourhood

Posted in Astrohype, The Universe and Stuff with tags , , , , , , , , on April 22, 2012 by telescoper

I’m a bit late getting onto the topic of dark matter in the Solar Neighbourhood, but it has been generating quite a lot of news, blogposts and other discussion recently so I thought I’d have a bash this morning. The result in question is a paper on the arXiv by Moni Bidin et al. which has the following abstract:

We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane, by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.

As far as I’m aware, Oort (1932, 1960) was the first to perform an analysis of the vertical equilibrium of the stellar distribution in the solar neighbourhood. He argued that there is more mass in the galactic disk than can be accounted for by star counts. A reanalysis of this problem by Bahcall (1984) argued for the presence of a dark “disk” of a scale height of about 700 pc. This was called into question by Bienaymé et al. (1987), and by Kuijken & Gilmore in 1989. In a later analysis based on a sample of stars with HIPPARCOS distances and Coravel radial velocities, within 125 pc of the Sun. Crézé et al. (1998) found that there is no evidence for dark matter in the disk of the Milky Way, claiming that all the matter is accounted for by adding up the contributions of gas, young stars and old stars.

The lack of evidence for dark matter in the Solar Neighbourhood is not therefore a particularly new finding; there’s never been any strong evidence that it is present in significant quantities out in the suburbs of the Milky Way where we reside. Indeed, I remember a big bust-up about this at a Royal Society meeting I attended in 1985 as a fledgling graduate student. Interesting that it’s still so controversial 27 years later.

Of course the result doesn’t mean that the dark matter isn’t there. It just means that its effect is too small compared to that of the luminous matter, i.e. stars, for it to be detected. We know that the luminous matter has to be concentrated more centrally than the dark matter, so it’s possible that the dark component is there, but does not have a significant effect on stellar motions near the Sun.

The latest, and probably most accurate, study has again found no evidence for dark matter in the vicinity of the Sun. If true, this may mean that attempts to detect dark matter particles using experiments on Earth are unlikely to be successful.

The team in question used the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory, along with other telescopes, to map the positions and motions of more than 400 stars with distances up to 13000 light-years from the Sun. From these new data they have estimated the mass of material in a volume four times larger than ever considered before but found that everything is well explained by the gravitational effects of stars, dust and gas with no need for a dark matter component.

The reason for postulating the existence of large quantities of dark matter in spiral galaxies like the Milky Way is the motion of material in the outer parts, far from the Solar Neighbourhood (which is a mere 30,000 light years from Galactic Centre). These measurements are clearly inconsistent with the distribution of visible matter if our understanding of gravity is correct. So either there’s some invisible matter that gravitates or we need to reconsider our theories of gravitation. The dark matter explanation also fits with circumstantial evidence from other contexts (e.g. galaxy clusters), so is favoured by most astronomers. In the standard theory the Milky Way is surrounded by am extended halo of dark matter which is much less concentrated than the luminous material by virtue of it not being able to dissipate energy because it consists of particles that only interact weakly and can’t radiate. Luminous matter therefore outweighs dark matter in the cores of galaxies, but the situation is reversed in the outskirts. In between there should be some contribution from dark matter, but since it could be relatively modest it is difficult to estimate.

The study by Moni Bidin et al. makes a number of questionable assumptions about the shape of the Milky Way halo – they take it to be smooth and spherical – and the distribution of velocities within it is taken to have a very simple form. These may well turn out to be untrue. In any case the measurements they needed are extremely difficult to make, so they’ll need to be checked by other teams. It’s quite possible that this controversy won’t be actually resolved until the European Space Agency’s forthcoming GAIA mission.

So my take on this is that it’s a very interesting challenge to the orthodox theory, but the dark matter interpretation is far from dead because it’s not obvious to me that these observations would have uncovered it even if it is there. Moreover, there are alternative analyses (e.g. this one) which find a significant amount of dark matter using an alternative modelling method which seems to be more robust. (I’m grateful to Andrew Pontzen for pointing that out to me.)

Anyway, this all just goes to show that absence of evidence is not necessarily evidence of absence…