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The post below made reference to the theory of supersymmetry and this weblog is long overdue a post on the subject. However, as supersymmetry is proposed as an extension of the Standard Model (SM) of particle physics, we’d better have a few words about the SM first…

As we said before, one of the big discoveries of 20th century physics is that there exist only four independent forces or interactions. These are gravity, electromagnetism (the unification of electricity and magnetism achieved by Maxwell in the 19th century), the strong nuclear force (that holds the protons and neutrons together in the nucleus), and the weak nuclear force (responsible for nuclear decay and radioactivity).

Physicists have long suspected that the four fundamental forces are not truly independent, but deeply connected. The idea is that at the tremendous energies of the Big Bang, a single superforce existed, which gradually split off into the four seperate entities we see today as the universe cooled. This idea received a great boost in the 1970s, when Salaam, Weinberg and Glashow established a strong theoretical connection between the electromagnetic and the weak nuclear interactions, using the methods of gauge symmetry. The theory predicted the existence of new particles (W and Z bosons), which were subsequently discovered in high-energy experiments at CERN in the 1980s…ever since we talk about the electro-weak interaction as a single entity.

Shortly before this, the first comprehensive theory of the strong nuclear force had also emerged – the key idea being Gellman’s prediction that the nuclear particles (protons and neutrons) are in fact made up of quarks, and the strong nuclear force is really an interquark force. This was verified by scattering experiments at Stanford in 1979, and the theory of the strong interaction is now known as quantum chromodynamics

Putting the two theories together gave rise to the Standard Model – a model that has been fantastically accurate at predicting the masses and properties of all particles discovered so far. However, the model contains several shortcomings

– there is no real unification between the electro-weak and strong interactions, they are treated in parallel

– gravity doesn’t appear at all

These shortcomings led to new theories that attempted to unify the strong nuclear force with the electro-weak interaction (known as Grand Unified Theories), and even more ambitious attempts to unify all three with gravity (Theories of Everything). To accomplish either of these, some new mathematical approaches would be needed….see next instalment…

Update

I forgot to mention another shortcoming of the Standard Model – namely that one particle, necessary to the model, has never been observed (thanks, tankers!). The Higgs boson plays a central role in the SM as the Higgs field gives the mechanism for other particles to acquire the masses we observe. Unfortunately, no evidence of the Higgs particle has been seen in accelerator experiments so far. Most theoreticians are convinced this is simply because we need higher energies than currently available to create it (i.e. it has a large mass), and expect to see evidence of Higgs bosons in the next round of accelerator experiments due to begin at the new accelerator in CERN next year – the Large Hadron Collider.

The alternative is that we’ll see something quite different, which would be even more interesting!

I learned at lunchtime on Monday that Professor Tim Sumner of Imperial College was booked to give a talk in Trinity College that very evening on the search for Dark Matter (DM). Prof Sumner is one of the project directors of the well-known UK Zeplin DM experiment, so I jumped in my car and drove up to Dublin. It’s not every day you get to hear a lecture from a player at that level….

It was certainly worth the drive, it was a cracking lecture. The seminar was organised by Astronomy Ireland, so there were quite a few non-professionals in the audience (I didn’t spot many staff from Trinity Maths or Physics, perhaps the talk wasn’t terribly well advertised). Of course, there’s something slightly ironic about an astronomical society hosting a seminar on Dark Matter as you’re not to likely to see DM through a telescope, but good for AI ! In the event, Tim gave a thorough overview of the whole area before describing current experiments to detect DM.

Recall that Dark Matter is thought to account over 2/3 of the matter of the universe (not to be confused with dark energy). Although we can’t see it, we’re pretty sure it exists because of its gravitational effect on the matter that we can see. I said in a previous post that the phenomenon was first suggested by Fritz Zwicky, but according to Tim, the suggestion first came from a scientist whose name I didn’t catch (Oert?).

The seminar was divided in four parts –

I. Indirect evidence of DM from gravitation effects

II. Indirect evidence of DM from cosmological models

III. DM candidates

IV. Current DM experiments

In part I, Tim gave a comprehensive account of the gravitational evidence, explaining the discrepancy between the expected velocity of stars and galaxies to that measured, working from smaller scales to the largest e.g. local stellar dynamics, galaxy rotation, galaxy cluster dynamics, X-ray halos, gravitational lensing and cluster streaming. I was only aware of a few of these so this was very interesting.

Calculations for galaxy rotation (curve A) and experimental points (curve B)

There was also a brief discussion of the alternative explanation, that our laws of gravity (both Newtonian and Einsteinian) need to be modified (MOND) and why this idea has lost ground recently

Part II concerned the role of DM in analysis of the cosmic microwave backgound (CMB). Tim explained the challenge to relate the temperature perturbations seen in the CMB to galaxy formation, and how all current models rely heavily on the postulate of DM…he also explained how the postulate is necessary to provide enough gravity to explain the geomety of the universe as observed.

Part III concerned the various candidates for DM. Such particles are expected to be weakly interacting (otherwise we would see them) and probably massive – i.e. weakly interacting massive particles or WIMPs. Tim then explained that the most likely candidates are thought to be certain supersymmetric particles. (As we saw before, the theory of supersymmetry (SUSY) arises out of attempts to unify three of the fundamental forces – the theory postulates that every normal particle has a heavier supersymmetric partner). Anyway, it turns out the most likely candidiate for DM is the neutralino, the lightest SUSY particle which cannot decay further.

In part IV, Tim described current experiments. He gave a full description of the recent galactic bullet cluster phenomenon, and was very positive about their results. He also mentioned the DAMA-LIBRA experiment, but was a lot less positive about this. The problem seems to be that their technique is less, not more, sensitive than other experiments, none of which have detected similar results. He confirmed that many in the community are sceptical that the DAMA result is really DM-related at all. Tim then finished with a brief overview of his own group’s attempt to detect WIMPS by their nuclear interactions in underground detectors in a mine over 1km deep, the Zeplin III experiment. There is a very good overview of the Zeplin experiment here .

The photomultiplier tubes of the ZEPLIN III detector

In summary, this was a super overview of the search for Dark Matter. There is always something to learn in such seminars, and things I particularly liked were

1. The lecturer took the time for a thorough overview of the whole area

2. There was time for a description of the experiments of other groups

3. There was great emphasis on the ‘double-whammy”. For many years, many scientists have scoffed at the idea of SUSY particles, as none have so far been seen in our particle detectors. Others have scoffed at the idea of Dark Matter, seeing it as a fudge. If DM turns out to be made up of SUSY particles, that solves both conundrums beautifully – and confirms supersymmetry as the way forward in unified field theory. It would also represent another step in the fantastic convergence of particle physics and cosmology, two of the most fundamental areas of physics.

4. There were plenty of questions afterwards – always interesting. In my case, I asked Tim about mass constraints put on SUSY particles by recent experiments in particle physics (accelerators). In fact, one of his slides showed that the ZEPLIN results so far are in agreement with accelerator experiments, ie. suggest candidate particles lying well within the ‘mass window’ provided by accelerator studies…the key slide was basically an updated version of the slide shown below – the predicted red curve (labelled Zeplin III) is now a reality (note that the vertcial line at 60 GeV is the lower mass limit set by accelerator experiments).

The above is written from my own notes at the talk, I may have missed a few points. Astronomy Ireland will provide a webcast and a DVD of the talk on their website and there is a very good overview of the worldwide search for DM here

Update:

I just read on the Cosmic V ariance blog that the GLAST satellite has just successfully launched (see earlier post on GLAST). Among other things GLAST will look for DM, by looking for gamma-rays produced by DM annihilation…there is a very nice discussion of this on their blog. I meant to ask Prof Sumner about the prospect of success of DM detection by this method but I forgot..

What are you doing for the summer? Like most academics, I’m asked this question regularly, by people envious of our holidays. I sometimes think they’re more interested in my holidays than I am myself.

But what will I do? I used to head back to my alma mater Trinity College as soon as term ended, doing experimental work in the magnetic resonance lab. These days, I find myself doing more and more writing about science, and less and less labwork. Truth is, I always liked writing papers more than getting the results…

This summer, I intend to make a start on a short book on particle physics, aimed at the layman – The Story of Atoms. I’ve noticed that while there are lots of good introductory books on cosmology, there are fewer such books on particle physics. Also, I’ve always had an interest in the area and l teach an introductory course in high-energy physics. Of course, particle physics probably doesn’t have quite the popular appeal of cosmology – but there’s enough convergence between the two fields to draw in plenty of readers. Plus, it’d be great to get a simple introductory book on particle physics out in time for expected dramatic results at CERN sometime next year. (I’m sure no-one else has thought of this – Ed).

Apparently one needs an outline, chapter headings and at least one full chapter to get a publisher interested. I think I’ll use the summer break to get the structure organised and bang out the first chapter, ready to send off to a few publishers by the time term starts up again.

‘Course I won’t spend the entire summer on it – all work and no play makes Albert a very dull boy. I intend to travel, and hole up somewhere where I can surf in the mornings and work in the afternoons (and socialize in the evenings). Anywhere really, so long as it’s outside Ireland, for God’s sake.

Garret Lisi, the surfer dude with the exceptionally simple theory of everything, has already been in touch with a list of suitable surf spots in California as long as your arm – thanks Garrett!

Mind you, I suspect what Garrett considers ‘suitable’ is probably life-threatening.

Tip – try not to land on the board when you wipeout

So that’s the summer plan.

1. Get started on a pop science book that will eventually make me rich and famous

2. Get back surfing

3. Meet someone nice. You’d be amazed how many academics are single, it’s frightening. All I ask is that a girl can surf and handle complex equations…

Good luck with all that – Ed

Incredibly, the cold fusion controversy is with us again. Physics World, normally a reputable source of news in physics, have a posting by Jon Cartwright on their weblog concerning claims that Japanese physicist Yoshiaki Arata of Osaka University may have demonstrated cold fusion.

To understand how startling – and controversial- such a claim is, you only have to call things by their proper names. ‘Cold fusion’ is media-speak for nuclear fusion at low energy, a process most physicists consider pretty much a contradiction in terms (it’s very difficult to achieve nuclear fusion even at extremely high temperatures and energies, with certain well-understood exceptions).

The dream of ‘cold fusion’ first hit the news in 1989, when chemists Fleischmann and Pons claimed to have observed a dramatic, unexplained heating effect in a chemical reaction, and attributed it to nuclear fusion processes ocurring at normal temperatures. The discovery made headlines around the world, because it offered the dream of a clean, cheap energy source on a small scale (nuclear fusion is a very different process from nuclear fission). However, the whole field was controversial from the very start.

Most physicists felt the jump from an unexplained heating effect to the assumption of nuclear fusion was highly speculative. Secondly, the effect was publicized (and funding received) long before the results were published in recognized journals, one of the first times this happened. Worst of all, when physics labs around the world rushed to reproduce the results, no discernible heating effect was found. The end result was a withdrawal of funding and a great career blow to the experimenters…and prompted a serious debate on the importance of peer review before going to the press!

Fusion in a beaker – the Fleischmann apparatus

It’s probably too early to say, but the current Japanese story bears many resemblances to the Fleischmann fiasco – a great deal of talk in the press (now web), a paucity of peer-reviewed results, and a great deal of copy written by non-physicists. In particular, I notice that most descriptions of the experiment focus once more on the benefits of ‘fusion energy’, (cheap, clean energy etc) with only a few lines concerning the skepticism of mainstream scientists.. (see this thread on the Richard Dawkins website for example)

There is also the question of biased opinion. For example, Cartwright’s article states ‘I also received a detailed account from Jed Rothwell, who is editor of the US site LENR (Low Energy Nuclear Reactions) and who has long thought that cold-fusion research shows promise’. Hmm. Not exactly an unbiased opinion, then. Indeed, a glance at the LENR website suggests that the above is not likely to represent the mainstream view…This is exactly the sort of press that caused such a problem the first time around…

Today and yesterday, I’ve been attending a conference on service teaching, hosted by the maths lecturers of our college. The conference is supported by the National Digital Learning Repository and the Irish Mathematical Society.

Service teaching refers to the teaching of students who are not majoring in mathematics (IT students physicists, engineers etc). It was an interesting conference, with a good few talks from colleagues in other Institutes of Technology. Not many IoTs have degrees in pure maths, so most maths teaching in the sector is service by definition.

Almost all contributors made reference to the problems 3rd level students have with maths. (There are many reasons for this, from the increase in college attendence among the general population, to low entry points, to the dumbing down of society, etc). The conference was mainly concerned with practical strategies to aid students, although Dr George McClelland talked of a large research programme into the teaching of maths and science at the University of Limerick.

A common theme was the introduction of extra support in the form of ‘drop-in’ maths centers – at least three speakers spoke of such centres in their institutions. It seems many students hate to approach lecturers in their office, but find it helpful to have a dedicated help center, with a different lecturer on hand to get them over a particular hump. Once over it, many first-years never look back. Small tutorial groups in a similar setting were found to be similarily beneficial.

This is a very good idea, if a little resource heavy. One speaker, Dr Diarmaid O’ Se of IT Carlow, found that the ‘drop-in’ idea worked better when modified to appointment by email. At the other end of the scale, Prof Tony Croft spoke of a very comprehensive support operation in Loughborough University (UK), with a large drop-in centre manned by permanent staff with very good resources, an initiative that has proved extremely popular with students and spread to several UK universities.

There were good tips concerning teaching methods in maths – Dr Neil Challis of Sheffield Hallam University (UK), had some great ideas on motivation for mathematics through technology. He showed how the simple measurement of physical data ( movement, sound etc) in maths class could help students relate to basic mathematical functions. Another great idea was to get the audience (or students) to participate in the representation of mathematical functions using semaphore !

The function y = -x

My favourite talk was one on the teaching of circuit analysis by Donncha O hEallaithe of Galway-Mayo Institute of Technology. The talk concerned the use of phasors in the analysis of AC circuits, and why students are usually told everything except why! I was one of these students… I could never see the connection between AC current (or voltage) and complex numbers – did this mean AC current wasn’t real?

Donncha explained that students are rarely told that it is simply a matter of representation. Since an ac voltage Vsin (ωt) appears across a circuit element as Vsin (ωt + Ф), the variables are the amplitude V and the phase angle Ф, which we can represent using vectors. However, since vector division is messy, it makes more sense to handle the amplitude and phase angle using the same 2D representation as complex numbers. And then translate back when you’re done. No imaginary current. Tra la!

No-one told me this when I was a student. (I suspect they did – Ed ).

Hoosier (below) is a bit confused between Dark Matter and Dark Energy, and unconvinced by the whole shebang. This is very common, so let’s have a post on it..

Dark matter is thought to account for 20% of all the matter/energy of the universe. Although we can’t see it, we’re pretty sure it exists, because its gravitational effect on visible matter can be seen. Put differently, we don’t insist that all existing matter must be ‘visible’ (i.e. emit or reflect electromagnetic radiation). Instead , we include the possibility that some matter may be seen only by its gravitational effect on neighbouring matter. The idea was first postulated by Fritz Zwicky in the 1930s – today, the known motion of certain spiral galaxies suggests that dark matter makes up 22% of all matter/energy, while ordinary (visible) matter makes up only 4% .

Of course, like the MOND crowd suggest, there is always the possibility is that our laws of gravity (both Newtonian and Einsteinian) are simply wrong. But most physicists consider this unlikely, as the predictions of our theory of gravity match observation in so many other instances…

Dark energy is a lot more speculative, and a lot more recent. It’s simply the name we give to whatever is causing the expansion of the universe to speed up (since 1998, it has been known that the expansion rate is currently increasing). The physical cause for dark energy is thought to be some sort of vacuum energy, but nobody’s sure yet. (From the point of view of theory, the phenomenon suggests that Einstein’s equations need an extra term, known technically as the ‘positive cosmological constant’.)

Putting the two together, cosmologists postulate that ordinary matter, dark matter and dark energy all add up to the critical density required for the geometry of the universe to be flat (which is what observation suggests). In other words, the current model of the universe can be summed up by

Density ord matter (4%) + Dens dark matter (22%) + Dens dark energy (74%) = 100%

More

The strongest evidence yet for dark matter was reported last summer. In the passage of one galaxy through another, one might expect the dark matter of one galaxy to interact differently than its ordinary matter, and researchers at the University of Arizona are pretty sure this is exactly they saw.

Galaxy collision seen by the CHANDRA space telescope

It is also reported here and here that another group, the DAMA-LIBRA collaboration, have observed seperate evidence of dark matter, but this claim is more controversial.

Yipee. Wow. Gosh.

I was informed yesterday that Physics World are going to feature an article of mine in their July issue! I had thought that my recent experience of a public talk on science and religion (see ‘The Big Bang and the Mind of God’ post below) might make a suitable article for their quirky backpage (Lateral Thoughts), and it seems they think so too…

Physics World is the flagship publication of the Institute of Physics. It’s a physics magazine of very high standard, easily my favourite (it’s a bit like a European version of the American Physics Today, but better). PW regularly has excellent, comprehensive articles on every area of physics research today, written by world-class researchers.

One snag – the dreaded words “we’ve made a few small changes”. In fact, the copy-editor made quite a lot of changes, especially at the beginning. To me, it doesn’t read like my voice at all. It’s something I’ll never understand, the compulsion of editors to change submitted prose around. What writer wants their carefully chosen words changed? Besides, all too often, the ‘edited’ version conveys a slightly different meaning to that originally intended…

So I’m now engaged in a process of trying to reach a compromise. I spent hours today trying to incoporate the changes I can live with, and sent the result back. Hopefully, we can each agreement.

Sigh. One day I’ll have my own magazine column somewhere!

The Lisi story below is a good hook for a post on the theory of everything…so here goes.

One of the big discoveries of 20th century physics is that there exist only four independent forces or interactions. These are gravity (known since Newton), electromagnetism (the unification of electricity and magnetism achieved by Maxwell in the 19th century), the strong nuclear force (that holds the protons together in the nucleus), and the weak nuclear force (responsible for nuclear decay and radioactivity).

Einstein always suspected that these interactions were not truly independent and spent most of the latter part of his life trying to achieve a unified theory that could describe both gravity and electromagnetism (a program that became known as unified field theory, initiated by Kaluza and Klein). Einstein failed in this program, not least because we now know that gravity is the hardest nut to crack (we have no satisfactory quantum theory of gravity, while all the others interactions can be described in terms of quantum theory).

Nowadays, unified field theory works from the oposite direction. Using the methods of gauge symmetry, theoreticians in the 1970s established a strong connection between the electromagnetic and the weak nuclear interactions. The theory predicted the existence of unkown particles (W and Z bosons), which were subsequently discovered in high-energy experiments at CERN in the 1980s…ever since we talk about the electro-weak interaction as a single entity.

One of the giant particle detectors at CERN

This success of electro-weak unification resulted in furious attempts to extend the unification program to include the strong interaction (a program known as Grand Unified Theory) . However, the GUT program soon ran into serious trouble, with a clutch of ‘no-go’ theorems showing that such unification was mathematically unsound (see the O’ Raifeartaigh theorem and the Coleman-Mandula theorem). Various novel ideas to circumvent this problem gradually emerged in the 1970s, the most promising of which is probably the theory of supersymmetry. Anyway, there now are strong hints of connections between the electro-weak and the strong interactions at high energies. Most ambitious of all is the prospect of a unified theory that also includes gravity i.e. that describes all four interactions in a single framework – the so-called theory of everything.

All the above is really boils down to the simple idea of a single super-force existing at the tremendous energies of the Big Bang, which gradually split off into the four seperate entities we see today as the universe cooled…pretty neat eh? The problem is that the mathematics of such a theory of everything (TOE) remains elusive – the leading candidate is string theory – yep, the famous string theory that is controversial because it is so mathematically abstract that it makes almost no predicitions that can ever be verified/falsified by experiment…..but that’s a separate story!

Suffice it to say that Einstein’s famous quest for a theory that incorporates a description of the elementary particles and all their interactions, now continues under the title Theories of Everything, and is still the Holy Grail of theoretical physics. As regards Garrett Lisi’s paper, part of the unification program inviolves the description of all the elementary particles using the mathematical theory of groups. (For example, Gellman’s classification of the known particles in the 1960s using group theory led to the prediction of a deeper layer of matter making up most particles – the quarks, later detected experimentally). Lisi’s paper purports to show that a particular mathematical group, the E8 group, may offer a very useful way of decribing all of today’s known particles, in a very simple framework…hence the interest. Plus, he’s an excellent surfer!

Over at Not Even Wrong, Peter Woit has a reference to a new article on the Garrett Lisi affair, this time in Outside Magazine. Lisi is the physicist who received a great deal of media attention earler this year when he posted a paper on the ArXiv server on a new classification of elementary particles based on the group E8, superbly titled ‘ An exceptionally simple theory of everything’. The paper was picked up by science magazines and newpapers all over the world, with prominent articles in outlets like the New Scientist, and The Telegraph, all of whom made a great deal of the fact that Lisi is an untenured academic, who spends much of his time surfing and snowboarding…

The Outside Magazine article is a good place to start if you’re unfamiliar with this story. Given his lifestyle, I imagine Garret probably appreciated this article more than any of the others. I didn’t know what to make of the Lisi story at first, but I’m glad it’s resurfacing, it’s a bit of light relief in our dull lives. ..

A few points strike me

1. Every time journos draw a comparison with Einstein, Lisi patiently points out that the lack of tenure is the only similarity – so it’s not his fault they keep making this comparison

2. I don’t see the problem with the surfer angle – surely it makes a welcome change from the usual media view of scientists. Besides, if Lisi is not a full-time academic, it simply means he probably has more time to think than the rest of us, not having to deal with endless admin, emails, proposals, teaching etc!

3. I’m delighted to see group theory get some attention – few outside the field have the slightest idea of the importance of group theory in particle physics. It seems some experts think that the whole E8xE8 thing may turns out to be a fairly trivial classification, but I enjoyed Lisi’s paper no end. It’s interesting that Dad’s book on group theory (chap 10) makes it clear that the E8 group had long been of interest to the supersymmetry gang, for reasons I don’t begin to understand. Technical stuff aside, the whole story is reminiscent of Gellmann’s eigthfold way, no bad thing.

Go Garrett!

All in all, I think scientists are inclined to react strongly against media attention, especially if we feel there are more deserving cases. We need to get over this, because such stories probably do far more for the public perception of physics than any number of well-intentioned school visits!

Update:

I probably should have explained what a Theory of Everything (TOE) is – it’s a theory that incorporates a description of the elementary particles and all their interactions (as opposed to a Grand Unified Theory, which is a theory that unifies three of the fundamental forces). If gravity is included in a unified framework like this, it’s called a Theory of Everything, because that’s all the forces we know. Unfortunately, it has proven very difficult to incorporate gravity with the other three interactions, not least because we don’t have a quantum theory of gravity (all the others are quantum theories). In short, Einstein’s famous quest for a unified field theory now continues under the title ToE, and is still the Holy Grail of theoretical physics.

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This month’s issue of Physics World has a super article on the GLAST satellite, the new gamma ray detector to be launched by NASA this month. Unlike in optical astronomy, gamma ray teleccopes study the universe at the highest energies of the electromagnetic spectrum. It’s a fascinating area of physics that has really exploded (oops, sorry) in recent years.

For many years, the high energy gamma ray bursts (GRBs) seen in the sky were a big mystery -nobody was sure what caused them. We now know that the universe contains a rich variety of gamma-ray emitters, including pulsars, supernova remnants, and coolest of all, supermassive black holes. (The Sun also produces gamma rays by accelerating charged particles in solar flares and coronal mass ejections, and our galaxy glows brilliantly with gamma rays due to interactions of high-energy cosmic rays with interstellar gas). One of the key reasons to extend our observations of celestial gamma rays is to look for signatures of as-yet-unknown fundamental physical processes.

The launch of the Gamma-ray Large Area Space Telescope (GLAST) will open this high-energy world to exploration, and follows on from the spectacular success of the ESA’s INTEGRAL satellite. Just as with the cosmic microwave background, there is a need for satellite measurements to get around the problem of absorbtion in the earth’s atmosphere (albeit in a vastly different energy range). With GLAST, astronomers will have an awesome tool to study how black holes can accelerate jets of gas outward at fantastic speeds.

Physicists will also be able to study subatomic particles at energies far greater than those seen in ground-based particle accelerators. And cosmologists will gain valuable information about the birth and early evolution of the Universe. Wow.

You can read more about the GLAST launch on the official NASA website here.