Category Archives: Particle physics

October 9, 2012 · 8:18 pm

The God particle at Trinity College

On Monday evening, I gave a public lecture on the Higgs boson at Trinity College Dublin. The talk was organised by Astronomy Ireland and I think it was quite a success; 200 tickets were sold and quite a few people had to be turned away.

In the Joly lecture theatre at Trinity College Dublin

How to explain the basics of particle physics to a public audience? As always, I presented the material as a short history of discovery: from the atom to the nucleus,  from protons and neutrons to Gell-mann’s quarks. I also included some theory on the fundamental interactions, right up to the Standard Model,  electro-weak unification and the role of the Higgs field in electro-weak symmetry breaking. Not for the first time, I came away with the impression that the Standard Model isn’t as intimidating for the uninitiated as you might expect. As for physics beyond the Standard Model, the audience seemed to take the hypothesis of grand unification in their stride, and the connection between particle experiments and the early universe struck a chord, as always.

The results  It was a pleasure to present the fantastic results of the ATLAS and CMS teams, first announced at CERN last July. Giving such talks is a lot easier now that the data are publicly available in two beautiful papers on the ArXiv here and here. I gave an overview of the main findings in the context of previous experiments at CERN and at the Tevatron,  and I think the audience got a feel for the historic importance of the result. Certainly, there were plenty of questions afterwards, which continued in the pub afterwards.

The famous bumps ( excess decay events) seen by both ATLAS and CMS at around 125 GeV in the di-photon decay channel

Combined signal (all decay channels) for both ATLAS and CMS

So what about that title? Yes, I did agree to the title ‘The God particle at last’? I am aware that most physicists have a major problem with the moniker; it is sensationalist, inaccurate and incurs a completely gratuitous connection with religion. (Some religious folk consider it blasphemous,  while others misunderstand the term as evidence for their beliefs).

A poster for the talk; naughty

All of this is true, yet I must admit I’ve got to like the nickname; it is catchy and just mysterious enough to cause one to think. I imagine a tired lawyer catching sight of the poster as she walks home after work;  ‘God particle’ might cause a moment of reflection, where ‘Higgs boson’ will not. At least the former expression contains the word ‘particle’, giving the reader some chance to guess the subject. Of course the ‘God’ part is hubris, but is hubris so bad if it gets people thinking about science? Also, I disagree with commentators who insist that the Higgs is ‘no more important than any other particle’. Since all massive particles are thought to interact with the Higgs field, finding the particle associated with that quantum field is of great importance.

So is it found?  CERN Director General Rolf Heuer stated in Dublin, “As far as the layman is concerned with have it. As far as the physicist is concerned, we have to characterize it”. Such characterization has been going on since July. Without question, a new particle of integer spin (boson) and mass 125 +- 0.5 GeV has been discovered. So far, the branching ratios (the ratio of various decay channels to lighter particles) match the prediction of a Standard Model Higgs boson very well. So it looks and smells like a Higgs, and we are all getting used to the idea of the Higgs field as reality rather than hypothesis. (That said, there is still the possibility of spin 1 or 2 for the new particle, but this is not very likely).

All in all, a very enjoyable evening. The slides and poster I used for the talk are available here.  No doubt, some Trinity professors may have been none too pleased to see ‘God particle’ posters in the Hamilton building. Me, I’ve decided I can live with the name if that’s what it takes to get the public excited about particle physics…

Update

Some bloke called Zephyr is upset and accuses me of misleading the public (comments). His point is that I refer to the Higgs as a particle, instead of a quantum field. There is a valid point here; what were once thought of as elementary ‘particles’ of matter are now considered to be manifestations of quantum fields. However, in the business of communicating physics to the public, each physicist must find their own balance between what is accurate and what is comprehensible. My own experience is that people grasp the idea of the Standard Model reasonably well if it’s told as a story of particle discovery (phenomenology). A small amount on quantum theory is ok, but too much soon leaves ’em bewildered. For this reason, I much prefer books like Particle physics: A Very Brief Introduction by Frank Close to books like Higgs: The Invention and Discovery of a Particle  (Jim Baggot)

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July 21, 2012 · 6:28 pm

O’Raifeartaigh Conference in Munich

I’m in Munich this weekend, at a physics conference in honour of my late father. The 2012 O’Raifeartaigh Conference is taking place in Munich’s Ludwig Maximilians Universität (LMU) and there are speakers here from Harvard, MIT, Stanford, the University of Tokyo, the Niels Bohr Institute (DK), the Eugene Wigner Institute (HN) and the Dublin Institute for Advanced Studies.

It sounds rather grand, but such memorial conferences are a good way for researchers who work in related fields to meet and present their latest work to each other. Many of the speakers worked with Dad at one stage or another and I think he would be very pleased to be remembered in this way. There are also some really sharp young scholars here and he would have liked that too. It’s the third memorial conference in Lochlainn’s memory, see here for the programme and other details.

Munich itself is fantastic – the university is right in the middle of the city and the neighbourhood is full of bookshops, coffee-houses, museums and beer gardens. The teaching term is not yet finished in Germany so there are students everywhere (don’t tell Minister Quinn!). In fact, I have never seen so many bicycles and bookshops in one place. The conference talks are in the University’s Arnold –Sommerfeld Centre for Theoretical Physics and the building has a Museum for Modern Art on one side and a music conservatoire or Musik Hochshule down the block. I could get used to this.

LMU University Munich (Main Entrance)

Lochlainn’s work concerned the use of mathematical symmetry methods to describe the physics of the elementary particles. Throughout his career at the Dublin Institute for Advanced Studies, he was considered a leading expert in the field. He is probably best known for his contributions to a radical theory known as ‘supersymmetry’, a theory that is currently being tested at the Large Hadron Collider at CERN. You can read more on his career by clicking on the tab Lochlainn on the top of the page.

There are some great talks here although some are are far beyond the comprehension of yours truly (an experimentalist). As always, I’m impressed by the style of presentation in theoretical physics; there are no polite powerpoint lectures here, but chalk-and-blackboard sessions with searching questions from the audience every few minutes. ‘‘Does that function even have a ground state?’, a speaker was asked within the first two minutes of his talk. ‘‘Well, it doesn’t in anti-deSitter space, but I hope to convince you that it does in deSitter space”, was the response. Answers to the frequent questions are tackled at the board until everyone in the room is satisfied. No-one gets away with anything here, from the youngest postdoc to the most eminent physicist. I think it’s a style of presentation that helps both lecturer and audience and I wish the humanities would adopt it – my pet hate is listening politely to a philosopher or historian for an hour before one gets to question a statement made in the first three minutes.

I gave a short talk myself on Friday. This was a ‘life-in-science’ presentation where I used pictures of people and places that influenced Lochlainn during his career: from his early work on general relativity with JL Synge  at the Dublin Institute for Advances Studies to his work on quantum field theory with Walter Heitler at the University of Zurich, from his use of group theory to prove his famous no-go theorem at Syracuse University in New York State to his work on the history of gauge theory at L’Institut des Hautes Etudes in Paris. I was worried I might have got some things wrong (e.g. “No, that work was completely incidental!’’), but thankfully it didn’t happen. In fact, I think the audience enjoyed the presentation as many of them had known the people and places mentioned at firsthand. You can find the photos and slides I used here.

Update

The conference is over today so Mum and I took an open bus tour of Munich. I find this a great way to get to know any city and it didn’t disappoint. Munich may not be as large as Berlin or Hamburg, but it is the capital of Bavaria and is an extremely impressive city. I’m amazed by the huge number of parks, wide boulevards and splendid buildings – clearly, it was did not suffer as much as so many other German cities from bombing in the war. This is one of the great privileges of being an academic – you get to see the most interesting places, all in the line of work.

The ‘heroes’ monument on Leopoldstrasse

And finally

On the way back to the hotel, I was intrigued to see a huge banner draped over the main university entrance; the legend’ STRINGS 2012′ is leaving the whole city in no doubt that a major conference on string theory is about to take place here! Such a civilised country..

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July 8, 2012 · 9:19 pm

Hawking, Walton and O’Raifeartaigh

I was surprised and delighted by the photograph below, prominently displayed in this week’s Irish Times magazine. In the accompanying article, journalist Arminta Wallace makes the point that the central figure in the photo is recognizable anywhere in the world, and challenges the reader to name the two Irish scientists flanking him (they are identified later in the piece).

This photo appeared in Saturday’s Irish Times under the caption Science Superstars

The scientist on Hawking’s right is the Irish physicist Ernest Walton, famous for splitting the atomic nucleus in 1932. The Cockroft-Walton experiment was the first successful accelerator experiment (and the first demonstration of E = mc2) and led to a well-deserved Nobel prize. As the prototype of all ‘atom-smashing’ experiments, Walton’s work is extremely relevant to this week’s discovery of the Higgs boson at the Large Hadron Collider (LHC).

The scientist on the left is my late father, Lochlainn O’Raifeartaigh. A senior professor in the School of Theoretical Physics at the Dublin Institute of Advanced Studies (DIAS), Lochlainn was a well known theorist in the field of elementary particle physics. The photo was taken at a conference at DIAS in 1983. I think it’s quite nice – it is not at all staged and one has the impression that the three physicists are enjoying a rare meeting. One sad aspect of the photo is that, even twenty years ago, there is already a marked deterioration in Hawking’s condition. That said, he has outlived the other scientists in the picture…

What would the trio have discussed? What do a leading particle theorist, a cosmologist and a Nobel experimentalist talk about over coffee? My guess is the newly-minted theory of cosmic inflation might have come up. Inflation is a theory that concerns the behaviour of the entire universe in the first fraction of a second, but it borrows heavily from ideas in particle physics. Hence it represents a convergence of cosmology ( the study of the universe at large) with particle physics (the study of the world of the extremely small). Given that the theory had only recently been posited, it’s highly likely that it was discussed by the trio with some excitement. (Of course Walton was an experimentalist but he had a lifelong interest in theory; it is often forgotten that he had a first class degree in mathematics as well as physics and he attended many conferences at the Institute over the years).

Ms Wallace draws a nice connection between the photo and the upcoming Dublin City of Science Festival. There is also a connection with science’s latest triumph, the discovery of a Higgs-like particle. First, Walton’s pioneering accelerator work laid the foundations for today’s experiments at the LHC (see above). Second,  Lochlainn made several important contributions to a theory now known as ‘supersymmetry’.  Supersymmetry is currently being put to the test at the LHC, as experimenters search for the ‘supersymmetric’ particles predicted by the theory. Thus the work of both Irish physicists remains relevant today.

You can read the Irish Times article here and more on Lochlainn’s work here. By coincidence, Lochlainn’s work will be celebrated at an international conference on theoretical physics in Munich next week.

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July 4, 2012 · 12:53 pm

Discovery of the Higgs vs the discovery of the atom

Most people on the planet will hear sometime today that scientists at CERN, the particle physics laboratory in Switzerland, have announced the discovery of a new particle, almost certainly the Higgs boson. ‘Discovery’ is shorthand for 99% confidence level, so this is a great result, coming from two independent experiments at CERN. But what does it all mean?

Below is a script I used for interviews on tv (RTE 1 Six One News) and radio (RE 1 Drivetime); you can see the tv interview here

Q: How important is the discovery, what does it compare with?

It’s not unexpected, but it’s very important. I think it is quite similar to the discovery of the first experimental evidence for atoms by Jean Perrin in 1908 (following a suggestion by the young Einstein). Scientists had long suspected that matter is composed of tiny entities known of atoms but they had never been observed directly. Perrin demonstrated their existence by showing that the random motion of tiny grains of gum in water could be explained in terms of the collisions of the particles with the atoms of the liquid.

Q:What exactly is a Higgs boson, is it like an atom?

We now know that the atom consists of a minute nucleus, with tiny, sub-atomic particles called electrons orbiting the nucleus. The nucleus itself contains other sub-atomic particles of matter called proton and neutrons, themselves made up of even smaller entities called quarks. The full list of the elementary particles of matter is described by the ‘Standard Model of Particle Physics’, the modern theory of the structure of the atom and the forces that hold it together. The Higgs particle doesn’t live inside the nucleus, it is a ‘messenger particle’ predicted by the Standard Model; while all other particles predicted by the model have been detected in experiments in particle accelerators, the Higgs has remained outstanding until now.

Q: And that’s why it’s so important?

Not only that. The Higgs is also of central importance in our understanding of the atom. According to the Standard Model, particles acquire mass as a result of their interaction with the Higgs – or to be specific, their interaction with a certain type of quantum field named the Higgs field (after theoretician Peter Higgs of Edinburgh University). The Higgs particle is simply the ‘messenger particle’ associated with this field.

Q: Why is it sometimes called the God particle?

Most physicists dislike the name, but it is somewhat apt since the field associated with the Higgs particle is thought to endow all other particles with mass. Another reason is that the particle has become something of a Holy Grail in particle physics because it has proved remarkably hard to find over five decades. The discovery of the Higgs boson is an important confirmation that our view of the fundamental structure of matter is on the right track.

Q: How was the particle observed?

At the LHC, two beams of protons are slammed into each other at extremely high energy. Exotic particles are created out of the energy of collision, just as predicted by Einstein (E = mc2). These unstable bits of matter quickly decay into other particles, including Higgs bosons. The Higgs particles themselves then decay into lighter particles in a number of different ways or ‘decay channels’. These particles are detected at the giant particle detectors attached to the beam at CERN – two independent detectors  (ATLAS and CMS) have detected two different decay channels of the Higgs, hence the excitement.

Q: How definite are the results?

Each group is quoting a sigma level of 5, corresponding to 99% certainty. This certainty reflects that a new particle has been found with mass 125 GeV, consistent with a Higgs. However, further work is required to determine whether the particle has other properties consistent with a Higgs.

Q: What comes after the Higgs?

The Higgs particle closes one chapter, but opens another.This is because the Standard Model is known to be incomplete. The properties of the new particle should give great insights into new physics beyond the Standard Model. For example, evidence of more than one type of Higgs particle would be a strong hint of the existence of a whole new family of particles known as supersymmetric particles. The detection of these particles is an important test for unified field theories, theories that suggest that the four fundamental forces of nature once comprised a single force in the infant universe. Indeed, the next round of experiments should give us many important insights into the very early universe because the high-energy conditions resemble those that existed when our universe was very young.

Q: Does the Higgs have a technological application?

No. However, the technologies developed in particle experiments find important application in society. A good example is the use of accelerators in modern medicine. Another is the world-wide web, a software platform first developed at CERN in order to allow scientists to share collision data. The latest innovation is the GRID, the networking of thousands of computers worldwide in order to facilitate the analysis of huge amounts of data emerging from the LHC. Today’s result is a great triumph for the GRID, it is quite amazing that the data was analyzed so fast.

Q: To wrap up; an exciting discovery?

Huge. Expected, but huge. Compares with the discovery of the atom, or putting a man on the moon. The morale of the story is that scientists are like the Mounties – they always get there in the end.

There is a good summary of today’s result in the Guardian here

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July 3, 2012 · 9:57 pm

How about Higgs particle instead of Higgs boson?

Like so many of us physicists, Micheal has a problem with the name ‘God particle’. Scientists have a healthy dislike of hubris (not to mention the needless antagonizing of religious-minded folk) and I am inclined to agree with a ‘do-er’, e.g. a researcher from CMS. Happy Higgs day Micheal, you and yours have done us proud!

Yet as someone who spends a lot of time attempting to engage the public’ s interest in science, I think there are several points worth examining here:

1. The name ‘Higgs boson’ isn’t great either, at least when dealing with the public. It is a classic case of over-specialization, as one immediately has to explain what a ‘boson’ is. Surely ‘Higgs particle’ would be better, as the audience immediately gets a pointer to the area of science under discussion, namely the world of the elementary particles (and whoever heard of the electron fermion?)

2. There is also the problem of priority; as every physicist knows, Professor Higgs was not the only theorist involved in the development of what is now known as the Higgs field (and he predicted the field, not the particle, as he often points out). Many theorists played a part in developing the theory, something that will create something of a Nobel headache – the name won’t help!

3, I still think the moniker ‘God particle’ has some good features; at least it contains the word ‘particle’,  and it is reasonably apt given that (i) the particle is an outstanding piece of the Standard Model (ii) it has an associated field that plays a crucial role in the acquisition of mass and (iii) it has proved remarkably hard to pin down. To put it another way, I suspect the moniker has been helpful in getting across the importance of the particle; without the nickname, I suspect it would have been harder to sustain the media’s attention in the search (how many members of the public were aware of the long search for the top quark?)

4. Could it that the hubris, which we physicists find so annoying, is exactly what it takes to get the public interested? Perhaps science journalists know more than we give them credit for.

Finally, there is the problem of religion/theology. Granted, there are some amongst the devout who take grave offence. Actually, I have never heard a serious theologian criticize or applaud the moniker – they understand the concept of a nickname. Those who can’t see past this may not be worth appeasing.

One obvious comparison here is the nickname ‘big bang’. However, cosmologists hate this moniker for a different reason;it is technically misleading because the theory says nothing about a bang (the name was originally coined by Hoyle as reductio ad absurdum). Yet the expression has been enormously useful at getting across a crude version of the theory. I would much prefer the expression ‘ evolving universe’, but I wonder would the theory have captured the imagination of the public to the same extent. Truth is, I suppose we’ll never know…in the meantime, I think I’ll compromise with ‘Higgs particle’ if I’m interviewed tomorrow!

Update

In the comments section, Sean raises an important point I should have mentioned. A second disadvantage of the term ‘God particle’ is that it encourages those who are inclined to see their particular God in everything. At a time when science is under attack from ultra-conservative religious all over the world (note in particular the attacks on evolution, big bang cosmology and climate science in the US), it is a huge mistake to encourage this sort of sloppy writing. I agree absolutely so, again, I think ‘Higgs particle’ is a reasonable compromise

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May 30, 2012 · 5:44 pm

Book review: Neutrino

This week I’m reading ‘Neutrino’ by Frank Close, published by Oxford University Press in 2010.  It’s a real treat; a short, succinct account of the history of neutrino physics, from Pauli’s ‘difficult hypothesis’ of a particle that might never be detectable to Fermi’s theory of beta decay, from the successful detection of neutrinos by Cowan and Reines to the famous conflict of theory and experiment in the case of solar neutrinos.

The story is brought up to date with a superb overview of modern advances such as the detection of supernova neutrinos, the discovery of neutrino oscillation, the resolution of the puzzle of the missing solar neutrinos and the great promise of neutrino astronomy.

Neutrino (Frank Close , OUP)

I must say I think books like this do an enormous service to physics. Like John Gribbin and Paul Davies, Close is an expert in his field who presents his material as a simple, chronological story that is extremely readable. Neutrino physics is also a worthy topic as it is one of the few areas of major progress in particle physics in recent years. It’s often forgotten that the discovery of neutrino oscillation caused the first re-writing of the Standard Model for decades.

I particularly enjoyed the historical sections on Pauli and Fermi as the relative contributions of the two are often confused. I also enjoyed the description of the astonishing work of Bruno Pontecorvo and the early experiments of Ray Davis. As for John Bachall, I came away from the book feeling that he was very badly treated by the physics community; even when his calculations of solar neutrino flux were finally vindicated (having endured suspicion for many years) he was denied a Nobel prize, hard to understand why.

The book was published before this year’s ‘faster-than-light’ neutrino controversy. I’m glad for this as I always felt that story was a bit of a distraction. That said, I wish I had read Close’s book before the many talks I gave on neutrinos this year!

Update

There is a very important international conference on neutrinos happening this week in Kyoto, Japan, see here for details of Neutrino 2012.

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March 29, 2012 · 10:07 am

The Irish Times and the God particle

Today, The Irish Times has an article of mine on its weekly science page. In the piece, I describe the tentative results from CERN and Fermilab on the famous Higgs boson, amidst some explanatory background on particle physics. I put some thought into the piece, but I suspect what will be remembered is the headline ‘Nearer, my God particle, to thee’. This was not the title I submitted, to put it mildly.

I have no particular problem with the nickname ‘God particle’ for the Higgs boson (unlike many of my colleagues). I admit the moniker is both catchy and reasonably apt as the Higgs field is thought to endow all other particles with mass. It is also appropriate because the Higgs is an important keystone of our model of particle physics, yet it has proved remarkably elusive – so something of a Holy Grail.

However, I’m not comfortable with the Irish Times headline. The hymn ‘Nearer, my God, to Thee’ has a lot of resonance for people who have lost loved ones (think Titanic). A pun based on such a hymn isn’t very clever in my view; it manages to trivialise both science and religion, all in my name.

This keeps happening to me. I put time and thought into expressing science clearly, and what eventually appears does so under a headline I dislike. Journalist friends tell me not to be precious but I think language is important.

This morning, I suspect my name is mud in the coffee room of every physics department in Ireland. As for the humanities, we can expect some outraged letters to the editor from professors of theology or philosophy – to the delight of The Irish Times. Sigh.

The article is here.

Caution: silly puns trivialise both science and religion and may cause offence

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January 10, 2012 · 5:39 pm

Peter Watkins and Z bosons at Trinity College Dublin

last night, I attended a terrific lecture on recent developments at CERN’s Large Hadron Collider, hosted by Astronomy Ireland at Trinity College Dublin. The lecture was presented by Professor Peter Watkins, a former leader of the particle physics group at University of Birmingham and a member of the ATLAS collaboration at the LHC. Professor Watkins was a member of one of the experimental teams that discovered the Z boson at the LEP at CERN in 1983. He is also very well-known for his work in bringing particle physics to the public and is the author of ‘The story of the W and Z‘, one of my favourite books on particle physics.

I try to go to as many of these public lectures as I can, in order to see how others present physics to the public. In this case, the lecture was superb, very easy to understand yet at quite a high level. It was loosely divided into five sections;

– an introduction to the building blocks of matter

–  a description of what the LHC is looking for

– a description of experimental setup of the LHC and the ATLAS detector

– a description of the methods of searching for particles

– a discussion of recent discoveries at the LHC

The first section gave a brief introduction to the standard model of particle physics. However, rather than present the audience with a list of quarks and leptons, Peter described our view of ordinary matter in terms of up and down quarks, electrons and neutrinos. Only after this did he mention the higher generations, an approach that worked really well. On the next slide, he gave a description of the fundamental forces, explaining along the way how electricity and magnetism were unified into the unified framework of electromagnetism many years ago, and how the latter interaction was more recently unified with the weak force to form the electroweak interaction. There followed a very nice discussion of the force-carrying particles, and the subsequent search for the W and Z bosons. This section finished with an overview of the role of the Higgs field in determining the mass of the particles – about as succinct an introduction to particle physics as I’ve seen!

The second section of the talk described what the LHC will search for; from the Higgs boson to supersymmetric particles, from investigations of the slight asymmetry in matter and antimatter decay to candidates for dark matter. Professor Watkins was also careful to explain that the LHC may yield great surprises, from missing energy that might constitute evidence of hidden dimensions to possible hints of new forces.

An experimental overview of the LHC and the ATLAS detector was presented in the third part of the talk. The technical challenges of LHC operation were clearly laid out, from the need for ultra-low temperatures to the problem of establishing an ultra-high vacuum on this scale, from issues with beam focusing to problems with superconducting magnets. This section included a great overview of the ATLAS detector, with each component described carefully.

The ATLAS detector, not the LHC as many newspapers seem to think

The fourth section of the talk was most unusual, where Peter gave a clear description of how the existence of elusive particles is inferred from those beautiful patterns on computer monitors.  Starting with E2 = p2c2 + m2c4, he gave a few examples where measurements of momentum and energy in the detector lead to an estimate of the mass of the parent particle. This section included a great description of the search for the Higgs via the ZZ and photon-photon decay channels.

In the last part of the talk, the speaker gave a clear description of recent work at the LHC. Touching briefly on the initial accident of 2008, he explained how ATLAS and CMS have gradually been closing the window on mass ranges for the Higgs (including earlier data from LEP). He had a nice surprise for many in the audience when he mentioned that ATLAS has already discovered its first new particle – a new state of the chi-b particle . The lecture finished with a discussion of the famous ‘bump’ in the ATLAS data at 126 GeV announced two weeks ago, and the possible significance of the discovery.

Hints of a higgs in the ATLAS measurements ? (Dec 2011)

I found this a superb lecture overall. The speaker outlined difficult concepts extremely clearly and gave a great description of how concepts emerge, rather than presenting ‘facts’ as fixed dogma. The audience certainly thought so too and there were dozens of questions afterwards. As always with Astronomy Ireland lectures, the discussion continued in the pub across the road. At one point, Peter explained that part of the current excitement is due to where the bump is; if the 126 GeV result stands, this relatively low mass for the Higgs may be compatible with extensions to the standard model such as supersymmetry….a good time to be a particle physicist!

Update

Rumours are circulating that the CMS bump has not disappeared on further analysis, but is converging on the ATLAS result, exciting times

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November 21, 2011 · 8:37 pm

Neutrinos and string theory at Trinity College Dublin

I gave a few more talks on the Gran Sasso neutrino experiments last week, in Waterford on Wednesday and in Trinity College Dublin on Saturday. I really enjoy giving these talks; it’s not often one gets an excuse to present the theory of relativity to the hapless public. Journalists talk about the ‘hook’ – well this is a hook from heaven. I even got a 20-minute interview on Ireland’s premier radio show Today with Pat Kenny . You can find the podcast and the slides I used for the lecture here.

There was a real buzz in the air at Saturday’s lecture, thanks to the latest results from OPERA. As you probably know, the team announced on Friday that the superluminal result has passed its first major test: a repeat experiment using a much shorter proton pulse. This time they used pulses only 3 nanoseconds long, separated by by gaps ten time larger. This is vastly shorter than before (10 microseconds) and obviates the statistical approach used for matching transmitted and received pulses used in the original experiment. Like most physicists, I am still pretty certain the result will eventually turn out to be an anomaly, but I certainly hope it survives for another few months! See here for the new OPERA paper.

The lecture was hosted by Astronomy Ireland,a very interested audience that always turn out in droves. The theatre was jammers, quite a few audience members had to stand throughout. As always, I particularly enjoyed the questions and answers afterwards. It’s also fun to be come home; as a postgraduate student, I spent many long years in the magnetic resonance lab next door!

David Moore of Astronomy Ireland presents me with something (?)

Afterwards, some of us all legged it over to another Trinity lecture theatre, to hear the annual statutory lecture of the School of Theoretical Physics of the Dublin Institute for Advanced Studies. This year, the speaker was well-known string theorist Cumrun Vafa from Harvard. Titled ‘Geometric Physics’, the lecture was an excellent introduction to string theory today.

String theorist Cumrun Vafa from Harvard

And after all that, there was a reception to celebrate the fact that Werner Nahm, the director of the DIAS School, was recently made a fellow of the Royal Society. What a weekend

Update

On Sunday, Werner gave a fascinating talk on ancient astronomy at the Dublin Institute of Advanced Studies. After the seminar, many of us remained in the fading light in that famous seminar room, discussing the neutrino result and other experiments at CERN. As so often, I was struck by the depth and detail of knowledge the theorists had of particle experiments. I also enjoyed the way the discussion wandered into German for a while, then seamlessly back to English – only at DIAS!

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September 23, 2011 · 5:10 pm

Faster than the speed of light

So. A respected experimental group, doing respected work, the OPERA neutrino experiment at Grand Sasso in Italy, have reported a startling result; they have measured a velocity for neutrinos that is in excess of the speed of light (a fractional increase of about of 1 in 100,000). The result is getting a huge amount of publicity because it appears to be in conflict with Einstein’s theory of relativity. ‘Einstein wrong‘ always makes headlines. I’m certainly getting a lot of calls and emails on the subject, not least because I had an article on relativity in Thursday’s Irish Times (see here).

In the OPERA experiment, a beam of neutrinos travels underground from CERN travel to Gran Sasso in Italy

The OPERA paper has been posted on the ArXiv here. Most physicists (including the participants) are calling the result an ‘anomaly’ and expect to find a hidden error, for two reasons

1. Thousands and thousands of experiments on elementary particles suggest that the speed of light represents a natural speed limit for material bodies, no matter how much energy you whack them with

2. There are deep mathematical reasons for believing that the speed of light in vacuum represents an absolute limit, from arguments of symmetry to the principle of least action. Basically, all sorts of mathematics suggests that the speed of photons- massless particles –  is the highest speed achievable. In addition, the principle underlies a great deal of observed physics, far beyond the remit of relativity.

So what is going on?

Science is a skeptical activity and scientists are slow to throw out a successful theory at the first sign of trouble -especially a theory as successful and as central as special relativity. Most scientists adopt a ‘wait and see’ approach when an experiment like this is reported.

For example, we know a great deal more about relativity than we do about neutrinos. It is only a few years since it was discovered that neutrinos have mass, and the phenomenon of neutrino oscillation – the transformation of one type of neutrino to another – is still not well understood. So it is possible that this experiment is an artefact of some unknown neutrino process.

A more prosaic possibility is that there is a systematic error in the extremely precise time/distance measurements necessary for the experiment. For example, the time of flight of the neutrinos is measured using a sophisticated version of GPS – perhaps there is a hitherto undetected error lurking in this method that is affecting the measurement. A few years ago, it was discovered that the moon has an effect on the curvature of the LHC tunnel, as does the TGV arriving at Geneva – these effects only show up because of the unprecedented precision involved in the experiments.

Finally, it is always possible that this result may turn out to be a real effect. In this case, we could be looking at some exciting new physics; not a violation of relativity, but the first evidence of hidden dimensions. String theorists have long mooted the possibility that the three familiar three dimensions of space may be accompanied by other dimensions, tiny ones that are curled up so that they are undetectable at normal energies. In principle, a particle that takes a shortcut through such a dimension could arrive early! This may sound like a rather fantastic explanation, but it is possible that an experiment at the unprecedented energy and precision of OPERA could see this effect for the first time. Certainly, it would not contradict any previous theory or experiment.

So an exciting wait, but my money is on a systematic error in the measurement of distance or time

Technical note

I keep hearing in the media that ‘relativity forbids travelling at speeds faster than the speed of light in vacuum.’ Actually, it doesn’t, as Einstein was fond of pointing out. Special relativity suggests that it is impossible for  body to be accelerated from subluminal to superluminal speed. Thus particles that travel faster than light are possible in principle so long as they always travel at that speed (known as tachyons). However, such behaviour has implications for time (it would run backwards) and for causality, and is therefore thought unlikely. Also, no such particles have been observed  in five decades of experimentation in particle physics .

Last weekend, I was quoted (well misquoted) in The Irish Times, making the last point above; you can read it here, it’s quite a good article.

Update

If it is a systematic error, what could it be?  Looking at the paper, my own guess is that it is significant that the group do not measure the time-of-flight of individual neutrinos, but massive bunches of the particles. Essentially they measure the beginning and end of a bunch, and apply statistics to get the mean time. A messy enough procedure, considering the accuracy required..

Update II

I have a letter on the experiment in The Irish Times today, you can read it here

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