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A new year, a new semester

I always enjoy the start of the second semester. There’s usually a great atmosphere around the college – after long weeks of quiet, it’s great to see the students back and all the restaurants, shops and canteens back open. The students themselves always seem to be in good form too. I suspect it’s the prospect of starting afresh with new modules, one of the benefits of semesterisation.

I’m particularly enjoying the start of term this year as I managed to finish a hefty piece of research before the teaching semester got under way. I’ve been working steadily on the project, a review of a key paper published by Einstein in 1917, since June 1st, so it’s nice to have it off my desk for a while. Of course, the paper will come back in due course with corrections and suggestions from the referees, but I usually enjoy that part of the process.

In the meantime, I’d forgotten how much I enjoy teaching, especially in the absence of a great cloud of research to be done in the evenings. One of the courses I’m teaching this semester is a history of the atomic hypothesis. It’s fascinating to study how the idea emerged from different roots: philosophical considerations in ancient Greece, considerations of chemical reactions in the 18th and 19th century , and considerations of statistical mechanics in the 19th century. The big problem  was how to test the hypothesis: at least until a brilliant young patent clerk suggested that the motion of small particles suspended in water might betray the presence of millions of water molecules.  Einstein’s formula was put to the test by the French physicist Jean Perrin in 1908, and it is one of Einstein’s great triumphs that by 1910, most scientists no longer talked of the ‘atomic hypothesis’, but of ‘atoms’.


In 1905, a young Albert Einstein developed a formula describing the motion of particles  suspended in a liquid, based on the hypothesis that the liquid was made up of millions of molecules. In 1908, the French physicist Jean Perrin demonstrated that the motion of such particles matched Einstein’s formula, giving strong support for the atomic hypothesis.  

For more on Perrin’s exeriment see here


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End of the second semester

It’s hard to believe we have almost reached the end of the second teaching semester. I’m always a bit sorry to see the end of classes, but I accept that it’s important that students are given time to reflect on what they have learnt. With that in mind, I don’t quite understand why exams start in early May rather than June.

As regards research, I can now get back to putting the finishing touches to a review paper I have been trying to finish for months. Mind you, thanks to the open-plan layout of offices in our college,  there will be more – not less – noise and distraction for the next few months as staff are no longer in class. Whoever came up with the idea that open-plan offices are a good idea for academics?

On top of finishing off my various teaching modules, I agreed to give a research seminar this week. The general theory of relativity, Einstein’s greatest contribution to science, is a hundred years old this month and I couldn’t resist an invitation to give a brief history of the theory, together with a summary of the observational evidence supporting many strange predictions of the theory – from black holes to the expanding universe, from the ‘big bang’ to gravitational waves. The talk took quite a bit of prep, but I think it went well and there were plenty of questions afterwards – a nice way to finish off the teaching semester.


The slides for the talk are here. Now the excitement is over and it’s back to the lonely business of writing research papers…


I gave a similar talk in University College Dublin yesterday. A tiring trip, but it’s always very satisfying to give a repeat performance.

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Skiing in Zermatt with the Ski Club of Great Britain

This week I’m back in Zermatt, Switzerland, one of my  all-time favourite ski resorts.I’m having a great week’s skiing with the Ski Club of Great Britain, a unique organisation that allows members to be guided around the mountain by Club Leaders who find all the best pistes and restaurants – not to mention a ready-made gang of cronies to hang out with after the lifts close. It really is a unique service they offer, perfect for the skier who arrives out solo.

Apres-ski with SKGB

The snow has been fine all week both on- and off-piste, if a bit icy in the mornings and a bit slushy in the afternoons. below you can see the well-known Gornergrat Observatory, right at the top of one of the main runs – can someone please organise a cosmology conference here ?

Gornergrat station and observatory above Zermatt with the Matterhorn in the background

Zermatt village is as gorgeous as ever, the archetypal Swiss ski resort, with superb restaurants, no cars and unbelievable views. I am often tempted to come down off the mountain early, just to stroll down the Hauptsrasse and look at the sights, but it hasn’t happened yet.

Zermatt Hauptstrasse

It’s not always the most relaxing of holidays, of course. First there’s the organisational challenge of getting oneself to the meeting point at 9 am sharp each morning with skis, poles, boots, hat, gloves, goggles, shades, transceiver, rucksack etc. I also had problems adjusting to the altitude for the first few days, plus my fancy skiboots have been killing me all week, not unusual when I haven’t used them in a while. I’ve had to become reconciled to the fact that off-piste is now pretty much beyond my fitness level. ..but none of that stuff matters much when you get up top!


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Centenary conference on the history of general relativity

In December, I attended a wonderful conference celebrating the centenary of the general theory of relativity, hosted by the Max Planck Institute for the History of Science in Berlin. The meeting, which took place in Berlin’s splendid Harnack Haus, was a  feast for anyone with an interest in Einstein’s theories or indeed the history of 20th century science.


Harnack Haus in Berlin

There were many talks by historians I have long admired, such as Helge Kragh,  Jurgen Renn, Jean EisenstaedtHannoch Gutfreund , Daniel KennefickChris Smeek and Dennis Lehmkuhl, to name a few. Topics covered included the genesis of general relativity in the 1910s, the low watermark of GR in the period 1940-1960, the history of gravitational waves, the renaissance of GR in the 1960s, the history of gravitational lensing, the history of the black hole information paradox and the history of relativistic cosmology. As regards the latter, I was delighted to give a talk on our recent work concerning Einstein’s cosmology. The program for the conference can be found here and videos of all the talks will soon be available . You can download the slides for my own talk here.


The conference room in Harnack Haus

Best of all, the history conference took place immediately after a conference on general relativity in the same venue,  organised by the Max Planck Institute for Gravitational Physics. Many delegates chose to attend both conferences , a double feast. It included  talks by noted researchers such as Rai Weiss, Paul Steinhardt, Joeseph Polchinski, Eric Adelberger, David Gross and Alexander Blum. Topics covered included black holes, gravitational waves, quantum gravity, the cosmological constant and tests of the equivalence principle. Many of the talks, although technical, took a historical approach: you can find the program here. I was particularly chuffed that Paul Steinhardt discussed my own group’s work on Einstein’s cosmology.


All in all, a superb week in Berlin, where it all started. I found the combination of a conference on physics with a conference on the history of physics very satisfying. It  fits very nicely with my conviction that the study of the history of science isn’t really a branch of historical study in the normal sense –  it’s more the study of the evolution of science. .


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Einstein’s blackboard and the Friedman-Einstein model of the cosmos

The Einstein biographer Andrew Robinson, author of Einstein: A Hundred Years of Relativity , recently reminded me of the saga of Einstein’s blackboard. The blackboard, a well-known exhibit at the Oxford Museum for the History of Science, was used by Einstein in the second of three lectures he gave at Oxford University in 1931.


An image of the blackboard used in Einstein’s 2nd Rhodes lecture at Oxford in April 1931 (reproduced by permission of the Hebrew University of Jerusalem). The analysis is taken from Einstein’s 1931 model of the cosmos.

I came across Einstein’s blackboard during the course of our first Einstein study, a translation and analysis of Einstein’s 1931 paper on cosmology. Although the paper is not very well known in the English-speaking world, it is a work of historical importance, as it constituted the first scientific publication in which Einstein formally rejected his static model of the universe and embraced the possibility of a cosmos of time-varying radius. In the paper, Einstein adopts Alexander Friedman’s 1922 analysis of relativistic cosmic models of time-varying radius and positive curvature, but sets the cosmological constant to zero, predicting a cosmos that expands and contracts over time (the model is sometimes known as the Friedman-Einstein model and should not be confused with the later Einstein-deSitter model of the cosmos). With the use of Edwin Hubble’s redshift/distance graph for the spiral nebulae, Einstein extracts estimates from his analysis of ρ ~ 10-26 g/cm3 , P ~ 108 light-years and t ~ 1010 years for the density of matter, the radius of the cosmos and the timespan of the cosmic expansion respectively. However, our analysis of the paper indicated that Einstein’s estimates contain a systematic numerical error.

Before submitting our paper on this to a journal, I discovered to my great surprise that Einstein’s 1931 paper is neatly summarized on the Oxford blackboard (see above). Although the blackboard is quite well known as an intriguing museum exhibit, it seems no-one had made the connection with a published paper. Even better, one extra line on the blackboard, not included in Einstein’s published paper, makes clear the source of the numerical errors in the paper.

The analysis is quite easy to follow: for a cosmos of radius P, the quantity D is defined on the top line of the blackboard as D= (1/c). (1/P).(dP/dt): essentially the Hubble constant divided by the speed of light. From his earlier analysis, Einstein has developed two independent relations from the Friedman equations that relate D to the radius and density of the cosmos respectively:  D~ 1/P2  and D2  ~ 1/3 , shown as equations (1a) and (2a) on the blackboard. Using the contemporaneous Hubble constant of 500 kms-1Mpc-1, he thus extracts estimates of cosmic density, radius and timespan of expansion respectively,  displayed in the last three lines on the blackboard. However, these estimates contain errors as noted above, and the fourth line on the blackboard (not shown in Einstein’s published paper), makes the source of his error clear. Where Einstein obtains a value of 10-53 cm-2 for the quantity D2, simple calculation shows that this quantity should have been D2 ~ 10-55 cm-2 (or 10 -51 m-2). It appears that Einstein stumbled in converting the Hubble constant to his customary cgs units, resulting in a density of matter that was too high by a factor of a hundred, a cosmic radius that was too low by a factor of ten, and a timespan for the expansion that was too high by a factor of ten (although the units of measurement are not specifically stated for the density estimate, cgs units are implied by the other calculations).

Thus Einstein’s blackboard helped us to solve the riddle of the anomalous estimates of his 1931 paper! Our paper on this made the cover of the European Physical Journal and you can read more about Einstein’s blackboard on this blog here and on wikipedia here.


All of the above was interesting and good fun. However, I should say that our translation and analysis of Einstein’s 1931 paper yielded another, rather more serious result – namely that the 1931 paper is NOT a cyclic model, although it is often cited as the first cyclic model of the expanding universe. Einstein specifically rules out this possibility, pointing out that the model breaks down at the endpoints of the single ‘cycle’.


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A celebration of Sir Fred Hoyle at the Royal Astronomical Society

The birth centenary of the noted British astrophysicist Sir Fred Hoyle was celebrated on Friday at the Royal Astronomical Society with a one-day meeting of talks describing Sir Fred’s many contributions to 20th century physics. While he is chiefly remembered in some quarters as the physicist who was ‘wrong on the big bang’, Sir Fred in fact made a number of seminal contributions to modern physics in several fields. Indeed, it was a treat to witness former collaborators and students recall his contribution to stellar evolution, stellar nucleosynthesis, astrobiology and cosmology, to name but a few.

I hadn’t been to the RAS before although I was elected a Fellow a few years ago, and I was stunned by its fantastic location in central London. It is housed in the famous Burlington House on Piccadilly, sharing the premises and courtyard with the Linnean Society, the Geological Society and the Royal Academy of Arts, a stone’s throw from Piccadilly Circus. As luck would have it, the Royal Academy are currently hosting a show of the work of the Chinese artist Weiwei, and his striking ‘Tree Sculpture’ filled the Burlington courtyard.

Burlington house on Piccadilly, housing five learned London societies, including the Royal Academy of Arts

Weiwei’s exhibit ‘Tree Sculpture’ in the Burlington courtyard. The RAS is located on the west wing of the courtyard.

The meeting opened with an introduction by Lord Martin Rees, who gave a comprehensive overview of Sir Fred’s works in a short, a ten-minute talk. This was followed by a description of Hoyle’s contribution to accretion physics by Professor Andrew Fabian of the Institute for Astronomy (IOA) in Cambridge, and talks on Hoyle’s work on nucleosynthesis by Professor Lynden-Bell (IOA) and Professor Malcolm Longair, Director of the Cavendish laboratory.

Professor Jayant Narlikar of the Inter-University Centre for Astronomy and Astrophysics (Pune, India) then gave the talk ‘Fred’s theories and ideas about gravity’. This was a rare treat – as a long term collaborator, Jayant made several important contributions to the development of Hoyle’s steady-state model of the universe (including the development of a new version of the model based on the principle of least action, and the development of the later ‘quasi-steady-state’ model), so it was most interesting to hear his take on the genesis of the steady-state cosmologies of Hoyle and Bondi and Gold.

Hoyle and Narlikar in 1966

Another treat was a talk by Professor Chandra Wickramasinghe of the University of Buckingham, ‘Fred Hoyle and the foundation of astrobiology’. This presented interesting insights into Chandra’s long collaboration with Sir Fred, from their early work on interstellar dust to their famous hypothesis that life on earth was seeded by comets. The latter work essentially founded the modern field of astrobiology, although they are not always credited for this.

Jayant’s talk was followed by a talk on Hoyle’s cosmology by Professor John Barrow of the Department of Applied Mathematics and Theoretical Physics at Cambridge. John gave a superb overview of Hoyle’s steady-state model of the universe, and of the battle royale between Hoyle’s theory group and Martin Ryle’s astronomy group at Cambridge during the 1950s. Most members of the audience were familiar with this story, but John brought out many points that are not well known – not least that the widespread skepticism concerning Hoyle’s hypothesis of the continuous creation of matter was something of a moot point. As demonstrated by Bill McCrea in 1951, a viable steady-state model can be constructed without this assumption. John also reminded the audience that today’s models of cosmic inflation are effectively steady-state cosmologies, and, if the eternal inflationary scenario is right, it is possible that the observed universe is a locally evolving patch in a global ensemble that is in a steady state!

Eternal inflation could give rise to evolving universes embedded in a global steady-state ensemble

I touched on both these points in my own talk ‘Steady-state cosmologies in context‘, although my main aim was to remind the audience that Hoyle’s steady-state model was a reasonable hypothesis at the time – and that the notion of a steady-state universe surfaced in cosmology on many occasions, from Arrhenius to Nernst, from Holmes to MacMillan. Further, not many people know that soon after the emergence of the first evidence for an expanding universe, several physicists considered the possibility of an expanding universe that remains in a steady state due to a continuous replenishment of matter, from Tolman to Einstein, from Schroedinger to Mimura. That said, my main focus was to discuss Einstein’s abandoned attempt at a steady-state model , an unpublished work discovered 2 years ago. (You can find more on Einstein’s attempt at a steady=state model here and here).

Between the two talks on Sir Fred’s cosmology, Nicola Hoyle gave a fascinating description of her personal recollections of her grandfather. It included many intriguing photos and pieces of information I hadn’t seen before. The meeting ended with a talk by Professor John Faulkner of the University of California at Santa Cruz on Sir Fred’s contribution to our understanding of stellar structure and evolution, after which we all trooped off to the beautiful library of Geological Society on the other side of the square for coffee.

All in all, a superb conference in memory of a superb physicist. The meeting was organized by Simon Mitton of Cambridge University. You can find the programme here and slides for my own talk on the ‘Seminars’ page of this blog.

After the meeting, speakers were treated to dinner at the famous RAS Dining Club, and I rolled back to my hotel through the gardens of Buckingham Palace. On Saturday, I took a hop-on hop-off open bus tour of London and it was a great success. In particular, I caught a young string orchestra rehearsing Correlli and Vivaldi concerti in St Martin in the Fields, and a superbly talented string quartet playing Brahms’s Hungarian Dances in Covent Garden, not to mention a relaxing stroll  along the Embankment in the afternoon sun. I’d forgotten what London can be like on a Saturday afternoon, must visit the RAS more often!

The best way to see London


There is a detailed summary of the meeting in this month’s issue of Astronomy and Geophysics


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A day out (and a solar eclipse) at Maynooth University

I  had a most enjoyable day on Friday at the mathematical physics department of Maynooth University, or NUI Maynooth, to give it its proper title. I was there to attend an international masterclass in particle physics. This project, a few years old, is a superb science outreach initiative associated with CERN, the European Centre for Particle Physics particle in Geneva, home of the famous Large Hadron Collider (LHC). If you live on planet earth, you will probably have heard that a famous particle known as the Higgs boson was recently discovered at the LHC. The idea behind the masterclasses is to give secondary school students the opportunity to “become a particle physicists for a day” by performing measurements on real data from CERN.


The day got off to a great start with a lecture on “Quarks, leptons and the forces of nature” by Dr. Paul Watts, a theoretical physicist at Maynooth. An excellent introduction to the world pf particle physics, I was amused by Paul’s cautious answer to a question on the chances of finding supersymmetric particles at the LHC. What the students didn’t know was that Paul studied under the late Bruno Zumino, a world expert on supersymmetry, and one of the pioneers of the theory. Paul’s seminar was followed by another lecture, “Particle Physics Experiments and the Little Bang” , an excellent a talk on the detection of particles at the LHC by Dr Jonivar Skullerud, another physicist at Maynooth. In between the two lectures. we all trooped outside in the hope of seeing something of today’s solar eclipse . I was not hopeful, given that the sky was heavily overcast until about 9.30. Lo and behold, the skies cleared in time and we all got a ringside view of the event through glasses supplied by Maynooth physics department! Now that’s how you impress visitors to the college… images IMG_20150320_101922

Viewing the eclipse

After lunch we had the workshop proper. Each student was assigned a computer on which software had been installed that allowed them to analyse particle events from the ALICE detector at the LHC (lead ion collisions). Basically, the program allowed the students to measure the momentum and energy of decay products of particles from the tracks produced in collisions, allowing them to calculate the mass of the parent particle and thus identify it. As so often, I was impressed how quickly the students got the hang of the program – having missed the introduction thanks to a meeting, I was by far the slowest in the room. We all then submitted our results, only to find a large discrepancy between the total number of particles we detected and the number predicted by theory! We then all returned to the conference room, and uploaded our results to the control room at the LHC. It was fun comparing our data live with other groups around Europe and discussing the results. Much hilarity greeted the fact that many of the other groups got very different results, and the explanation for that (but what many groups really wanted to know was  whether we got a good look at the eclipse in Ireland). IMG_20150320_154423

Uploading our results via a conference call with the contol room at the LHC, CERN

All in all, a wonderful way for students to get a glimpse of life in the world of the LHC, to meet active particle physics researchers, and to link up with students from other countries. See here for the day’s program.


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