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The End of the Viva

Posted in Biographical, Education with tags , , , , , on June 13, 2012 by telescoper

I’m stuck at home today, waiting for UPS to come and collect a defective printer. Any time between 9am and 7pm, they said. Very helpful. Anyway, I’ve got plenty to do while I’m here, catching up on STFC Astronomy Grant Panel business that I’ve been too busy to attend to. Also, this week’s Private Eye has just arrived in the post, so I’ll take a break at some point to do the crossword by Cyclops. It’s a lovely day. Pity I can’t sit in the garden. I’d miss the doorbell when the carrier arrives.

Anyway, the past two days have been largely given over to the business of examinations in the School of Physics & Astronomy at Cardiff University. The External Examiners spent a big slice of Monday doing viva voce examinations of selected candidates; not just those on borderlines, but also some others for “calibration”. I wasn’t involved in them this year, but have taken part in the past as External in various places. Obviously these examinations are very stressful for the students, and also quite difficult to conduct fairly, but sometimes provide useful insights in the cases where a student’s marks put them on a knife-edge between two degree classifications (or even between pass and fail).

Anyway, in its infinite wisdom Cardiff University has decided to scrap the viva voce examination after next year. From 2014 onwards we’ll just have to apply a formula to deal with borderline cases; the algorithm involves counting how many modules were passed at the higher level, etc. Actually, I probably agree with this for the purposes of classifying degrees. Twenty minutes’ questioning under stress can hardly be expected to yield much objective  information about a candidate’s knowledge of the subject that dozens of written papers and other assessments. Often, in my experience, students (especially the shy ones) are so nervous that the shutters come down almost straight away.  I would  prefer a system which is algorithmic as possible, so everyone knows what the rules are, rather than relying on subjective judgements.

As external, I always found the viva examinations a useful way of getting feedback from the students on their course which can be fed back – either usefully or not – to the department. In losing the viva  for drawing up the classification lists, I hope that we can find another way for the externals to talk to students in some other context to get some feedback about the course. Perhaps they could attend for project talks, or something like that?

Yesterday, the entire Board of Examiners (including Externals) gathered to go through all the individual cases and draw up the Honours List. I was delighted when I saw all the consolidated marks in advance of the meeting, to see how well how many of our students had done. There were one or two difficult cases, but in the end we produce the lists. As I went back to my office, students were already gathering in the corridor by the noticeboard where it is always placed as soon as the definitive final version has been prepared, shortly after the meeting closed.

Soon I heard whoops of joy and laughter and had a quick look to see the students congratulating one another. As always on such occasions, I was tempted to go along and chat to a few of them but, as always, I resisted doing so. It’s a time for them, the students, not us, the staff.

Anyway, congratulations to all those who had good news yesterday!

I hope your hangovers aren’t too bad…

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Teaching (about) Physics

Posted in Education with tags , , , on June 10, 2012 by telescoper

So the academic year nears its end. This week we have the dreaded meetings of the Examination Boards, complete with External Examiners, ordeal by viva voce for selected students, and finally the lists go up announcing success (or otherwise) for this year’s finalists. It’s all a lot of work – and I’m sure also extremely stressful for the students waiting for their results.

If it’s any consolation for any students reading this post, I can assure you that there’s no lack of stress on this side of the fence either. I always feel a sense of dread opening the packets of examination scripts, and this year was no different. Have I set the exam too hard? Will the marks be a fair reflection of the students’ ability? Have they learned anything at all from the hours I spent droning on? These questions are all the more apt for a third-year class, since these are the papers that really count in determining the final outcome of their course. When the lists go up later this week, one’s delight at the sight of happy (or relieved) faces is always tempered by sadness when things have obviously gone wrong.

Coincidentally, I noticed the other day that a former student from the School of Physics and Astronomy at Cardiff University posted an item on her blog giving her view of her degree. It’s a very frank assessment of her own opinion of the course she took, including a list of her  three favourite courses. None of the ones I lectured are amongst them, by the way, in case you think I’m mentioning it for egocentric reasons. Indeed, I’m pretty confident that I’m one of the lecturers she didn’t like at all!

The main thing is that, for better or worse, our course involves an enormous amount of contact time with academic staff.  In the new fee regime students will pay the same £9K for a science course as they would for the Arts and Humanities:

See, doing a Physics and Astronomy degree, I had about 20 contact hours. With lab time. so in one month I had out stripped the BA people for an entire academic year. So in the 12 weeks of one semester, I have had more contact time than they will get in their entire degree. Worth it?

As for whether we make the best use of the time we devote to teaching, that’s a different matter. We have in fact recently overhauled the entire curriculum so we’ll see whether that has the desired effect. One can’t please all of the people all of the time, so we’ve tried to introduce new teaching methods – e.g. fewer lectures, more problems classes – to try to engage better with more students. Only time will tell whether it works.

Anyway, although it’s not one of the topics of her post, Harriet’s blog brought something from the back of my mind where it usually lurks ready to trouble me when I start to think about teaching physics. The point is that most of us involved in teaching physics at University level think that what we should be doing is training people to be professional physicists. That means teaching them to do physics the way it is actually done by people who do research. That means that, especially in Astronomy, students have to grapple with strange unit systems, peculiar terminology and quite a lot of maths. Those aren’t put into our courses in order to torment students – they’re there in the curriculum because they’re there in the world of (astro)physics research. It would be dishonest for us to pretend we were training physicists if we made out that it was all easier than it actually is.

What I mean to say is that I don’t think it should be our job to present physics in a way that’s different from (specifically easier than) the way it is  done at the coalface, in the world of scientific research.  What we should be doing is giving students the skills and confidence to solve the difficult problems a scientist can expect to confront in that situation. To be honest I don’t think we do that particularly well either, but that’s the aim. And that’s why our courses are mainly taught by people who actually do physics and why we claim our teaching is research-led.

That’s an oversimplification, of course. Especially in earlier years, much of the undergraduate curriculum – Newtonian Mechanics, Electromagnetism, Quantum Mechanics, etc  – is not “frontier” stuff so probably doesn’t require an active researcher to teach it. On the other hand, none of that is exactly easy so anyone who is going to teach it competently needs to have mastered it themselves. And in later years, the more specialist material and projects certainly require an active research environment.

Anyway, the point is that  in the new fee regime science courses will attract the same level of funding as courses in, e.g. English Literature. But a course in Physics requires physicists to teach it, while a course in literature does not require a team of successful novelists. Given the fact that the way we teach physics is more expensive by a very large margin, should we be rethinking our approach to the basic physics degree, and leave all the fancy research-led stuff to Masters courses?

Should we really be trying to teach all our students how to do physics? Or should we just be teaching them about physics?

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The Transparent Dishonesty of the Research Excellence Framework

Posted in Open Access, Science Politics with tags , , , , , , on May 30, 2012 by telescoper

Some of my colleagues in the School of Physics & Astronomy recently attended a briefing session about the  forthcoming Research Excellence Framework. This, together with the post I reblogged earlier this morning, suggested that I should re-hash an article I wrote some time ago about the arithmetic of the REF, and how it will clearly not do what it says on the tin.

The first thing is the scale of the task facing members of the panel undertaking the assessment. Every research active member of staff in every University in the UK is requested to submit four research publications (“outputs”) to the panel, and we are told that each of these will be read by at least two panel members. The Physics panel comprises 20 members.

As a rough guess I’d say that the UK has about 40 Physics departments, and the average number of research-active staff in each is probably about 40. That gives about 1600 individuals for the REF. Actually the number of category A staff submitted to the 2008 RAE was 1,685.57 FTE (Full-Time Equivalent), pretty close to this figure. At 4 outputs per person that gives 6400 papers to be read. We’re told that each will be read by at least two members of the panel, so that gives an overall job size of 12800 paper-readings. There are 20 members of the panel, so that means that between 29th November 2013 (the deadline for submissions) and the announcement of the results in December 2014 each member of the panel will have to have read 640 research papers. That’s an average of about two a day. Every day. Weekends included.

Now we are told the panel will use their expert judgment to decide which outputs belong to the following categories:

  • 4*  World Leading
  • 3* Internationally Excellent
  • 2* Internationally Recognized
  • 1* Nationally Recognized
  • U   Unclassified

There is an expectation that the so-called QR  funding allocated as a result of the 2013 REF will be heavily weighted towards 4*, with perhaps a small allocation to 3* and probably nothing at all for lower grades. In other words “Internationally recognized” research will probably be deemed completely worthless by HEFCE. Will the papers belonging to the category “Not really understood by the panel member” suffer the same fate?

The panel members will apparently know enough about every single one of the papers they are going to read in order to place them  into one of the above categories, especially the crucial ones “world-leading” or “internationally excellent”, both of which are obviously defined in a completely transparent and objective manner. Not.

We are told that after forming this judgement based on their expertise the panel members will “check” the citation information for the papers. This will be done using the SCOPUS service provided (no doubt at considerable cost) by   Elsevier, which by sheer coincidence also happens to be a purveyor of ridiculously overpriced academic journals. No doubt Elsevier are  on a nice little earner peddling meaningless data for the HECFE bean-counters, but I haven’t any confidence that it will add much value to the assessment process.

There have been high-profile statements to the effect that the REF will take no account of where the relevant “outputs”  are published, including a recent pronouncement by David Willetts. On the face of it, that would suggest that a paper published in the spirit of Open Access in a free archive would not be disadvantaged. However, I very much doubt that will be the case.

I think if you look at the volume of work facing the REF panel members it’s pretty clear that citation statistics will be much more important for the Physics panel than we’ve been led to believe. The panel simply won’t have the time or the breadth of understanding to do an in-depth assessment of every paper, so will inevitably in many cases be led by bibliometric information. The fact that SCOPUS doesn’t cover the arXiv means that citation information will be entirely missing from papers just published there.

The involvement of  a company like Elsevier in this system just demonstrates the extent to which the machinery of research assessment is driven by the academic publishing industry. The REF is now pretty much the only reason why we have to use traditional journals. It would be better for research, better for public accountability and better economically if we all published our research free of charge in open archives. It wouldn’t be good for academic publishing houses, however, so they’re naturally very keen to keep things just the way they are. The saddest thing is that we’re all so cowed by the system that we see no alternative but to participate in this scam.

Incidentally we were told before the 2008 Research Assessment Exercise that citation data would emphatically not be used;  we were also told afterwards that citation data had been used by the Physics panel. That’s just one of the reasons why I’m very sceptical about the veracity of some of the pronouncements coming out from the REF establishment. Who knows what they actually do behind closed doors?  All the documentation is shredded after the results are published. Who can trust such a system?

To put it bluntly, the apparatus of research assessment has done what most bureaucracies eventually do; it has become  entirely self-serving. It is imposing increasingly  ridiculous administrative burdens on researchers, inventing increasingly  arbitrary assessment criteria and wasting increasing amounts of money on red tape which should actually be going to fund research.

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EPSRC Blues

Posted in Science Politics with tags , , , , , on May 15, 2012 by telescoper

I woke up this morning to find via Twitter an interesting blog post about a demonstration in London against the policies of the Engineering and Physical Sciences Research Council (EPSRC).

For those of you not up with the ins and outs of the UK science funding regime, EPSRC is the agency that funds the more mainstream areas of physics (as well as chemistry, engineering and some mathematics) while the more exotic bits (particle physics, nuclear physics and astronomy) are the responsibility of the Science and Technology Facilities Council (STFC). The current protest seems to be lead by a number of eminent chemists, including Prof. Sir Harry Kroto, Prof. Sir John Cadogan and Prof. Anthony Barrett.

Almost five years ago – was it really so long? – owing to a mixture of funding cuts and incompetent management, STFC was born into a financial crisis that made many of us doing astronomy and particle physics wish that we also were protected by the friendly hands of EPSRC rather than left out in the cold as we felt we were at STFC. Things have slowly improved at STFC, which now has an executive team that actually seems to listen to its community as well as speaking the language that Whitehall wants to hear. Funding is still tight, but STFC is a noticeably happier ship now than it was it first launched.

In the meantime, any envy we might have had about our colleagues in, e.g., condensed matter physics being safer in the EPSRC stable has now well and truly evaporated. Their strategy, “Shaping Capability“, expressed in dreadful management-speak, involves the imposition of arbitrary priorities such as the restriction of fellowship applications to certain areas chosen by The Management. Worse, its new funding rules attempt to target funding at commercially-driven research. Dark clouds are gathering in the “blue skies” under which UK science has hitherto flourished.

The unresponsive top-down character of EPSRC has strengthened under the leadership of David Delpy who must have been made in the same factory as Keith Mason, former Chief Executive of STFC, whose diplomatic skills were similarly remarkable by their absence.

For some reason, this reminds me of the following quote from Smiley’s People

In my time, Peter Guillam, I’ve seen Whitehall skirts go up and come down again. I’ve listened to all the excellent argument for doing nothing, and reaped the consequent frightful harvest. I’ve watched people hop up and down and call it progress. I’ve seen good men go to the wall and the idiots get promoted with a dazzling regularity.

I’ve argued before that I think EPSRC’s approach is fundamentally wrong. When taxpayers’ money used is used to generate immediate commercial returns, it ends up in the pockets of entrepreneurs when the research succeeds and, if it doesn’t, the grant has effectively been wasted. Commercial Impact should not be a factor in awarding public funding, because it is perfectly suited as a criterion for attracting private funding. This is why we have a national fiscal policy: the only justification for levying taxation is to fund projects which will not yield short-term economic returns. There is no reason to spend public money on commercial projects: we need to justify pure research by a non-economic valuation.

This morning EPSRC have issued a press release calling upon scientists to work together ahead of the forthcoming comprehensive spending review. It doesn’t mention the demonstration, or other manifestations of unrest within the EPSRC community, but instead re-asserts the need for its so-called strategy, with a clear message not to rock the boat ahead of the next Comprehensive Spending Review.

I’ve heard that argument many times in the context of STFC during its crisis period. I firmly believe that rocking the boat in that case helped it get off the rocks. It remains to be seen whether the EPSRC protest, which is currently rather small, will gather enough momentum to make a difference. It all depends on what fraction of EPSRC scientists have actually signed up to the Delpy Agenda. Is the new campaign representative of the views of the EPSRC community? No doubt many research groups will be prospering under the new regime, at least in the short term. Time alone will tell what the long-term impact of short-termism will be.

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Rigid Body Sings

Posted in Poetry, The Universe and Stuff with tags , , , , on May 14, 2012 by telescoper

Gin a body meet a body
Flyin’ thro the air,
Gin a body hit a body,
Will it fly? And where?

Ilka impact has its measure
Ne’er a’ ane hae I
Yet a’ the lads they measure me,
Or, at least, they try.

Gin a body meet a body
Altogether free,
How they travel afterwards
We do not always see.

Ilka problem has its method
By analytics high;
For me, I ken na ane o’ them,
But what the waur am I?

by James Clerk Maxwell (1831-1879)

P.S. This poet is of course much better known as a physicist, but this is a nice little parody of Robert Burns’ Comin’ through the Rye in authentic Scots.

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Dielectric Breakdown

Posted in The Universe and Stuff with tags , , on May 9, 2012 by telescoper

This is the season when our  second-year students are picking the projects they want to do in their third year, as are those third-year students intending to carry on for Year 4 of the MPhys programme. I’ve been chatting to quite a few students about this particular project so thought I’d do a quick post here.


One of the third-year projects I’ve got in the current catalogue for next year concerns a computer model of dielectric breakdown based on the idea of diffusion-limited aggregation. This is  a neat model that allows the students to simulate pretty patterns like the one shown on the left.

The mathematics of it was first presented in Niemeyer, L., Pietronero, L., Wiesmann, H., “Fractal dimension of dielectric breakdown,” Physical Review Letters 52 (1984), 1033-1036.

Dielectric breakdown happens when a sufficiently large voltage is applied across a material that doesn’t normally conduct electricity, the classic example being a lightning strike. Here’s another example, which I find particularly electrifying…

Classical physics can be fun, you know!

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B2FH

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

I spent a pleasant evening yesterday at a public lecture arranged by Cardiff Scientific Society and given by Professor Mike Edmunds, former Head of the School of Physics & Astronomy at Cardiff University and now Emeritus Professor here. The subject of his talk was Origin of the Chemical Elements, a subject Mike has worked on for many years. Here’s the abstract of his talk:

When the Universe was 300,000 years old, the only chemical elements with significant abundance were hydrogen, helium and a small amount of lithium. All the atoms of all the other elements in the Periodic Table have been synthesised during the 13.7 billion years since that time. Research in physics and astronomy over the last 64 years has allowed us to identify the nuclear processes involved, including the importance of the humble neutron in the manufacture of the heavier elements. We now have a good picture of the astronomical sites where elements such as the carbon, nitrogen, oxygen and iron in our bodies were made, including violent supernova explosions. It is a picture that appears almost, but not quite, complete.

That last sentence is tempting fate a bit, but it’s fair comment! The lecture, which I had the pleasure of chairing, was both entertaining and informative, and very warmly received by the large audience in the Reardon Smith Lecture Theatre (in the National Museum of Wales).

Inevitably in a talk on this subject, the subject came up of the classic work of Burbidge, Burbidge, Fowler and Hoyle in 1957 (a paper usually referred to as B2FH after the initials of its authors). It’s such an important contribution, in fact, that it has its own wikipedia page…

This reminded me that one of the interesting astronomical things I’ve acquired over the years is a preprint of the B2FH paper. Younger readers will probably not be aware of preprints – we all used to post them in large numbers to (potentially) interested colleagues before publication to get comments – because in the age of the internet people don’t really bother to make them any more.

Anyway, here’s a snap of it.

It’s a hefty piece of work, and an important piece of astronomical history. In years to come perhaps it may even acquire some financial value. Who knows?

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Teaching Physics

Posted in Education, The Universe and Stuff with tags , , on March 22, 2012 by telescoper

More on this weeks’ theme, from the inestimable xkcd…

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Failed Physics Teaching Analogies

Posted in Education, The Universe and Stuff with tags , , , , , , , on March 18, 2012 by telescoper

Last week I deputized for a colleague who was skiving off away at an important meeting so, for the first time ever in my current job, I actually got to give a proper lecture on cosmology. As the only out-and-out specialist in cosmology research in the School of Physics and Astronomy at Cardiff, I’ve always thought it a bit strange that I’ve never been asked to teach this subject to undergraduates, but there you are. Ours not to reason why, etc. Anyway, the lecture I gave was about the cosmic microwave background, and since I have taught cosmology elsewhere in the past it was quite easy to cobble something together.

As a lecturer you find, over the years, that various analogies come to mind that you think will help students understand the physical concepts underpinning what’s going on, and that you hope will complement the way they are developed in a more mathematical language. Sometimes these seem to work well during the lecture, but only afterwards do you find out they didn’t really serve their intended purpose. Sadly it also  sometimes turns out that they can also confuse rather than enlighten…

For instance, the two key ideas behind the production of the cosmic microwave background are recombination and the consequent decoupling of matter and radiation. In the early stages of the Big Bang there was a hot plasma consisting mainly of protons and electrons in an intense radiation field. Since it  was extremely hot back then  the plasma was more-or-less  fully ionized, which is to say that the equilibrium for the formation of neutral hydrogen atoms via

p+e^{-} \rightarrow H+ \gamma

lay firmly to the left hand side. The free electrons scatter radiation very efficiently via Compton  scattering

\gamma +e^{-} \rightarrow \gamma + e^{-}

thus establishing thermal equilibrium between the matter and the radiation field. In effect, the plasma is opaque so that the radiation field acquires an accurate black-body spectrum (as observed). As long as the rate of collisions between electrons and photons remains large the radiation temperature adjusts to that of the matter and equilibrium is preserved because matter and radiation are in good thermal contact.

Eventually, however, the temperature falls to a point at which electrons begin to bind with protons to form hydrogen atoms. When this happens the efficiency of scattering falls dramatically and as a consequence the matter and radiation temperatures are no longer coupled together, i.e. decoupling occurs; collisions can longer keep everything in thermal equilibrium. The matter in the Universe then becomes transparent, and the radiation field propagates freely as a kind of relic of the time that it was last in thermal equilibrium. We see that radiation now, heavily redshifted, as the cosmic microwave background.

So far, so good, but I’ve always thought that everyday analogies are useful to explain physics like this so I thought of the following. When people are young and energetic, they interact very effectively with everyone around them and that process allows them to keep in touch with all the latest trends in clothing, music, books, and so on. As you get older you don’t get about so much , and may even get married (which is just like recombination, in that it dramatically  reduces your cross-section for interaction with the outside world). Changing trends begin to pass you buy and eventually you become a relic, surrounded by records and books you acquired in the past when you were less introverted, and wearing clothes that went out of fashion years ago.

I’ve used this analogy in the past and students generally find it quite amusing even if it has modest explanatory value. I wasn’t best pleased, however, when a few years ago I set an examination question which asked the students to explain the processes of recombination and decoupling. One answer said “Decoupling explains Prof. Coles’ terrible fashion sense”. Grrr.

An even worse example happened when I was teaching particle physics some time ago. I had to explain neutrino oscillations, a process in which neutrinos (which have three distinct flavour states, associated with the electron, mu and tau leptons) can change flavour as they propagate. It’s quite a weird thing to spring on students who previously thought that lepton number was always conserved so I decided to start with an analogy based on more familiar physics.

A charged fermion such as an electron (or in fact anything that has a magnetic moment, which would include, e.g. the neutron)  has spin and, according to standard quantum mechanics, the component of this in any direction can  can be described in terms of two basis states, say |\uparrow> and |\downarrow> for spin in the z direction. In general, however, the spin state will be a superposition of these, e.g.

\frac{1}{\sqrt{2}} \left( |\uparrow> + |\downarrow>\right)

In this example, as long as the particle is travelling through empty space, the probability of finding it with spin “up” is  50%, as is the probability of finding it in the spin “down” state. Once a measurement is made, the state collapses into a definite “up” or “down” wherein it remains until something else is done to it.

If, on the other hand, the particle  is travelling through a region where there is a  magnetic field the “spin-up” and “spin-down” states can acquire different energies owing to the interaction between the spin and the magnetic field. This is important because it means the bits of the wave function describing the up and down states evolve at different rates, and this  has measurable consequences: measurements made at different positions yield different probabilities of finding the spin pointing in different directions. In effect, the spin vector of the  particle performs  a sort of oscillation, similar to the classical phenomenon called  precession.

The mathematical description of neutrino oscillations is very similar to this, except it’s not the spin part of the wavefunction being affected by an external field that breaks the symmetry between “up” and “down”. Instead the flavour part of the wavefunction is “precessing” because the flavour states don’t coincide with the eigenstates of the Hamiltonian that describes the neutrinos’ evolution. However, it does require that different neutrino types have intrinsically different energies  (which, in turn, means that the neutrinos must have different masses), in quite  a similar way similar to the spin-precession example.

Although this isn’t a perfect analogy I thought it was a good way of getting across the basic idea. Unfortunately, however, when I subsequently asked an examination question about neutrino oscillations I got a significant number of answers that said “neutrino oscillations happen when a neutrino travels through a magnetic field….”. Sigh. Neutrinos don’t interact with  magnetic fields, you see…

Anyhow, I’m sure there’s more than one reader out there who has had a similar experience with an analogy that wasn’t perhaps as instructive as hoped. Feel free to share through the comments box…

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Big Bang Acoustics

Posted in The Universe and Stuff with tags , , , , , , on March 12, 2012 by telescoper

It’s National Science and Engineering Week this week and as part of the programme of events in Cardiff we have an open evening at the School of Physics & Astronomy tonight. This will comprise a series of public talks followed by an observing session using the School’s Observatory. I’m actually giving a (short) talk myself, which means it will be a long day, so I’m going to save time by recycling the following from an old blog post on the subject of my talk.

As you probably know the Big Bang theory involves the assumption that the entire Universe – not only the matter and energy but also space-time itself – had its origins in a single event a finite time in the past and it has been expanding ever since. The earliest mathematical models of what we now call the  Big Bang were derived independently by Alexander Friedman and George Lemaître in the 1920s. The term “Big Bang” was later coined by Fred Hoyle as a derogatory description of an idea he couldn’t stomach, but the phrase caught on. Strictly speaking, though, the Big Bang was a misnomer.

Friedman and Lemaître had made mathematical models of universes that obeyed the Cosmological Principle, i.e. in which the matter was distributed in a completely uniform manner throughout space. Sound consists of oscillating fluctuations in the pressure and density of the medium through which it travels. These are longitudinal “acoustic” waves that involve successive compressions and rarefactions of matter, in other words departures from the purely homogeneous state required by the Cosmological Principle. The Friedman-Lemaitre models contained no sound waves so they did not really describe a Big Bang at all, let alone how loud it was.

However, as I have blogged about before, newer versions of the Big Bang theory do contain a mechanism for generating sound waves in the early Universe and, even more importantly, these waves have now been detected and their properties measured.

The above image shows the variations in temperature of the cosmic microwave background as charted by the Wilkinson Microwave Anisotropy Probe about a decade years ago. The average temperature of the sky is about 2.73 K but there are variations across the sky that have an rms value of about 0.08 milliKelvin. This corresponds to a fractional variation of a few parts in a hundred thousand relative to the mean temperature. It doesn’t sound like much, but this is evidence for the existence of primordial acoustic waves and therefore of a Big Bang with a genuine “Bang” to it.

A full description of what causes these temperature fluctuations would be very complicated but, roughly speaking, the variation in temperature you see in the CMB corresponds directly to variations in density and pressure arising from sound waves.

So how loud was it?

The waves we are dealing with have wavelengths up to about 200,000 light years and the human ear can only actually hear sound waves with wavelengths up to about 17 metres. In any case the Universe was far too hot and dense for there to have been anyone around listening to the cacophony at the time. In some sense, therefore, it wouldn’t have been loud at all because our ears can’t have heard anything.

Setting aside these rather pedantic objections – I’m never one to allow dull realism to get in the way of a good story- we can get a reasonable value for the loudness in terms of the familiar language of decibels. This defines the level of sound (L) logarithmically in terms of the rms pressure level of the sound wave Prms relative to some reference pressure level Pref

L=20 log10[Prms/Pref]

(the 20 appears because of the fact that the energy carried goes as the square of the amplitude of the wave; in terms of energy there would be a factor 10).

There is no absolute scale for loudness because this expression involves the specification of the reference pressure. We have to set this level by analogy with everyday experience. For sound waves in air this is taken to be about 20 microPascals, or about 2×10-10 times the ambient atmospheric air pressure which is about 100,000 Pa.  This reference is chosen because the limit of audibility for most people corresponds to pressure variations of this order and these consequently have L=0 dB. It seems reasonable to set the reference pressure of the early Universe to be about the same fraction of the ambient pressure then, i.e.

Pref~2×10-10 Pamb

The physics of how primordial variations in pressure translate into observed fluctuations in the CMB temperature is quite complicated, and the actual sound of the Big Bang contains a mixture of wavelengths with slightly different amplitudes so it all gets a bit messy if you want to do it exactly, but it’s quite easy to get a rough estimate. We simply take the rms pressure variation to be the same fraction of ambient pressure as the averaged temperature variation are compared to the average CMB temperature,  i.e.

Prms~ a few ×10-5Pamb

If we do this, scaling both pressures in logarithm in the equation in proportion to the ambient pressure, the ambient pressure cancels out in the ratio, which turns out to be a few times 10-5

With our definition of the decibel level we find that waves corresponding to variations of one part in a hundred thousand of the reference level  give roughly L=100dB while part in ten thousand gives about L=120dB. The sound of the Big Bang therefore peaks at levels just a bit less than  120 dB. As you can see in the Figure to the left, this is close to the threshold of pain,  but it’s perhaps not as loud as you might have guessed in response to the initial question. Many rock concerts are actually louder than the Big Bang, so I suspect any metalheads in the audience will be distinctly unimpressed.

A useful yardstick is the amplitude  at which the fluctuations in pressure are comparable to the mean pressure. This would give a factor of about 1010 in the logarithm and is pretty much the limit that sound waves can propagate without distortion. These would have L≈190 dB. It is estimated that the 1883 Krakatoa eruption produced a sound level of about 180 dB at a range of 100 miles. By comparison the Big Bang was little more than a whimper.

PS. If you would like to read more about the actual sound of the Big Bang, have a look at John Cramer’s webpages. You can also download simulations of the actual sound. If you listen to them you will hear that it’s more of  a “Roar” than a “Bang” because the sound waves don’t actually originate at a single well-defined event but are excited incoherently all over the Universe.

PPS. If you would like to hear a series of increasingly sophisticated computer simulations showing how our idea of the sounds accompanying the start of the Universe has evolved over the past few years, please take a look at the following video. It’s amazing how crude the 1995 version seems, compared with that describing the new era of precision cosmology.

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