Tuesday, December 11, 2012

More new physics prizes

The news today in physics, reported by the New York Times and Peter Woit, is that Yuri Milner, the Russian billionaire physics enthusiast, has announced a new set of prizes for fundamental physics. I wrote a little about the previous award of prizes here. These prizes are somewhat less generous than those announced over the summer (which were worth \$3 million each); this time only Stephen Hawking receives that amount, and the others get a smaller share. Woit typically notes that almost all the non-experimental recipients have some connection to string theory.

But the other interesting thing about the new prizes is that in this round, some experimentalists have actually been included. Of course all of them are involved with the LHC at CERN: no surprise there, given Milner's personal interests in physics.

What I do find rather noteworthy, however, is the way in which the prizes have been divided. $1 million is to be divided between the current and former ATLAS spokespersons Fabiola Gianotti and Peter Jenni, and another million between CMS current and ex-spokepersons Joe Incandela, Michel Della Negra and Tejinder Virdee. Now I'm not very knowledgeable about the organisational structure or the division of labour in these experiments, but each experiment has roughly 3000 scientists working on it and everything I had read about them in relation to speculation about a possible Nobel award suggested that there was no real way to single out the contribution of any individuals out of these thousands as being especially more worthy than any other.

It does appear that Milner has found a way to do this though: pick the spokesperson elected by the experiment. Now I wonder what the reaction to this is at ATLAS and CMS. Is this regarded as fair? Do the spokespersons really make a bigger contribution to the discovery of the Higgs? I really don't know but I'd be interested to learn what people think. I know some of the readers of this blog are part of the ATLAS and CMS collaborations — if any of you have an opinion, please do share it through the comment box!

Update: Via Shaun in the comments, there is interesting news about what the current spokespersons intend to do with their money. Also, Tommaso Dorigo gives his opinion on the matter here; I have to say my views are rather more in line with those expressed in the comment by Bernhard on that blog.

Less serious update: An anonymous friend in the ATLAS group says "I can't believe Fabiola has had her choice of font vindicated in this way."

Sunday, December 2, 2012

Things to Read, 2nd December

Time for another collection of worthwhile links from elsewhere on the internet.

Science links:
  • Jester at Résonaances has been producing a series of very good informative posts from the frontlines of particle physics: particularly worth reading are this summary of the state of play with the "detection" of dark matter by the Fermi telescope, this explanation on what is and isn't contained in the new Higgs analyses from the LHC, including intriguing information on why some data hasn't yet been updated, and this take on the relevance of the LHCb measurements of Bs meson decay for supersymmetry (which has been the subject of a lot of hype in the popular press).
  • Speaking of the popular press, Time magazine has seen fit to nominate the Higgs boson as one of the candidates for "person of the year" (you can vote for it if you like). As if that weren't ridiculous enough, the accompanying description must surely qualify as one of the worst pieces of science journalism ever: literally every single sentence is wrong. Worth a look — even if you're not a physicist, you might be able to spot the mistakes.
  • I've recently discovered a series of super-slow-motion videos of lightning strikes knocking around on the internet. These are shot at several thousand frames per second, and really show the details of how the charge leaders meander towards the ground before the main secondary stroke follows. Even more interesting are some videos showing lightning travelling upwards, from ground to cloud. Here's a video:

    and here's a popular-level description of the phenomenon.
  • Another particularly cool video I saw some time ago was this one, showing the synchronisation of 32 metronomes placed on a flexible platform:

Other links:
  • This one is part science, part history and part politics. Have you ever wondered what the connection is between prehistoric plankton from the Cretaceous era, and the distribution of votes for Obama? Of course you haven't, but now you can find out anyway. 
  • On a more serious note: one of the striking things about the US election was how completely wrong Republicans and the right-wing were in their predictions of the outcome. Especially so since there were so many people who were able, by simple analysis of the available facts, to arrive at predictions that were much better (Nate Silver, for one). There is lots to be said about this, I suppose. One striking observation, which maybe hasn't been made enough of, is the hope that people realise that if the right-wing media can spout such rubbish (not to say lies) about the polls, perhaps the rest of what they say is rubbish too
  • And following on from that, an interesting article from someone on the right of American politics: The Revenge of the Reality-Based Community.
  • It's not just in the US that facts have a well-known anti-right-wing bias of course. And it's not just US right-wing politicians who dislike them. When economist Jonathan Portes of the National Institute for Economic and Social Research appeared before the UK parliament and explained some fairly elementary economics that happens to contradict the ideology of the Conservative Party, he received thinly veiled threats from MP Jesse Norman (watch; transcript). This drives other economists to express their equally thinly veiled contempt. Norman responds, and Wren-Lewis dismisses him again.
  • And finally, something more cheery: some time ago I posted the story about the dog who climbed a mountain with me in the Himalayas. I thought that dog was pretty awesome, but then I saw this dog, who seems to have done an even harder climb!

    (She does seem to have a protective harness though, which probably gives her an unfair advantage ...)

Thursday, November 22, 2012

The Structure of a Scientific Revolution

A few days ago, Peter Coles posted an interesting comment about skepticism on his blog. In it, he showed the first graphs with data showing evidence for the acceleration of the expansion of the Universe from observations of distant supernovae. His point was that though the plot isn't much to look at, the statistical evidence for cosmic acceleration from supernovae data was quickly established and very soon became very widely believed.

Coles goes on to make some points about global warming skepticism, and most of the long discussion in the comments that follow is concerned with this. Which is unfortunate, as none of that discussion provides an exception to the general rule that arguments about global warming are a complete waste of time, because people have already decided what they want to believe before they start, and are subsequently impervious to any evidence, logic or persuasion. So although you may want to read the discussion there, let's not talk about the climate here.

Instead, I want to stick with the cosmology and pick up on a question that is more interesting. Let's take a quick look at the picture of the supernovae data from the High-Z Supernova Search Team and the Supernova Cosmology Project:
Evidence for dark energy from the Hubble diagram of supernovae. The top panel shows the distance modulus of the supernovae as a function of redshift, with three different theoretical model curves. The bottom panel shows the residual distance modulus relative to that in the model with the dotted curve (a Milne universe).
As Coles points out, most people who are not cosmologists (and some who are) look at these data and find them deeply unconvincing. The error bars are large. The points are widely scattered, and at least both the top two curves seem to give reasonably good fits. On top of that, there appear to be two distinct groups of supernovae: one group rather close to us, and the other much further away at higher redshifts (and therefore observed as they were a long time ago). Any inferences we draw from relative calibrations of these two groups rest on the assumption that deep down they are the same kind of objects, and that the Universe they existed in remained the same kind of Universe.

So why exactly did this cause such a revolution in the field? How did so many physicists become convinced, almost overnight, of the existence of this mysterious dark energy, that constitutes most of the energy of the Universe, but that we didn't understand then and don't understand now?

Saturday, November 10, 2012

Does Nate Silver get his maths wrong?

Although I haven't had much time to write anything new for this blog recently, like most people I know I have of course been paying close attention to the unfolding circus of the US Presidential election. Now that all the action is done and dusted, there is a lot of discussion on the internet about the predictions made by poll aggregators, the best-known of whom is the New York Times blogger and statistician Nate Silver. And although I'm not going to talk about politics, I do want to make a comment about probabilities and the statistics of poll aggregation.

The reason Silver has been in the news so much is because even when a lot of innumerate and possibly partisan journalists were making a huge amount of noise about "Romneymentum" and predicting a landslide Republican victory, Silver calmly analysed the actual polls and stated that the odds continued to favour an Obama victory. For this he attracted an enormous amount of flak in the right-wing press and blogs, and a substantial section of the mainstream media refused to believe him. (See here if you'd like a frequently-updated list of everyone who attacked Silver and ended up looking stupid.) Even on election eve, when Silver gave Obama more than a 90% chance of victory, such impartial websites as the BBC continued to describe the result as being on a "knife-edge".

Of course now that Obama has won, the pendulum has turned the other way, and Silver is being hailed as a mathematical genius, a wizard, a guru and much else besides. This adulation rather ignores the fact that there were several other bloggers and poll aggregators who also predicted an Obama win. Some of these are Sam Wang at the Princeton Election Consortium, Drew Linzer at Votamatic, Pollster at the Huffington Post, Simon Jackman and so on. (To be fair, Silver does have a much more prominent position than the others, being at the New York Times. He did also provide very insightful regular explanations and updates, and some nice interactive graphics to help explore the data, so one can forgive the extra attention given to him.)

But the interesting question is not whether Nate Silver does a better job of predicting elections than, say, Donald Trump. How does he compare with other, equally numerate and mathematically minded poll-aggregating bloggers? And there is really a difference between them: for a large part of October, Silver rated Obama's chances of re-election at only around 65-75%, whereas Sam Wang regularly claimed it was more like a 99% chance. Or have a look at Votamatic, where the predicted median of electoral college votes for Obama has been fairly constant at 332 (the correct number) since July, compared to Silver's prediction of closer to 290 at the same time. You'll also notice, if you play with the graphics at FiveThirtyEight, that the margin of error Silver quoted on his prediction was significantly larger than that the other forecasters gave.

So in summary, Nate Silver made more conservative predictions and was more pessimistic about Obama's re-election chances than his direct competitors. (Why so? As I understand it, Wang and Linzer only include information from published poll data, appropriately weighted for freshness and sample size. Silver included also what he calls "fundamentals" — his estimate of other underlying factors, such as economic data and so on, which might influence the final outcome. Silver's exact model is proprietary, but for an educated guess at how it works, explained in layman's terms, see here.) In the end though, Obama was re-elected, as all the forecasters said he would be. Since the only differences between them lay in exactly how confident they were in their prediction (and also whether they thought he would win Florida ... ), and the election can only be run once, how do you decide whose model is better?

A common argument you may have heard in favour of Silver before Obama's most recent victory is that he correctly predicted 49 out of 50 state results in 2008. Add to that his predictions this year and his tally is now 99 correct out of 100. This is obviously impressive, and clearly you should trust Silver and his maths more than basically every conservative hack who attacked him.

But the problem is that Silver may be too accurate. To put it bluntly, if you say the probability of your prediction being correct is around 70%, you should expect to be wrong 30% of the time, not 1% of the time. So if you predict the right result 99% of the time but quote probabilities of only 70%, you are probably overstating the error bars on your prediction. On the other hand, if someone else says he is 99% sure and is right 99% of the time, his model might be better.

OK, some obvious caveats. It is misleading to count all 50 state predictions as successes for Silver's model. I doubt even Donald Trump would have had trouble predicting Utah would go Republican and New York would go Democrat. In reality, there were only around 10 or 11 states that were at all competitive in each election cycle, so adjusting for that would mean lowering Silver's actual success percentage. Also, his model didn't predict all results with equal confidence — for instance, maybe he gave only a 75% probability for Obama to win Ohio, but a 99% probability for him to win Minnesota.

How should one account for this in quantitatively comparing models? Unsurprisingly, Sam Wang himself suggests a method, which is to assign each prediction a Brier score. This takes the square of the difference between the predicted probability of an event occurring — e.g., 0.75 for a prediction of 75% probability — and the post facto probability (i.e. 1 if it did happen, 0 if it didn't). In the case of multiple predictions (for multiple states), you then average the Brier score for each prediction. Thus the Brier score punishes you for getting a prediction wrong, but it also punishes you for hedging your bets too much, even if you got the end result right. Getting a Brier score of exactly 0 would mean you knew the results in advance, getting a score of 0.25 is equivalent to just guessing randomly (if there are only two possible outcomes).

Have a look at Wang's result tables: it looks as though he did slightly better than Silver in predicting the Presidential race, and much better in predicting the Senate results. As he rather pithily put it,
additional factors used by FiveThirtyEight – “fundamentals” – may have actively hurt the prediction. This suggests that fundamentals are helpful mainly when polls are not available.
Anyway, the point of this post, despite the deliberately provocative title, was not really to attack Nate Silver. Even if he did hedge his bets and overstate his error bars a little, given his very public platform and the consequent stakes, I think that's understandable. The Princeton Election Consortium and Votamatic really don't have the same exposure, so Wang and Linzer would have faced less general opprobrium if they had made a mistake. Also, perhaps more statistics are needed to really make a judgement — though let's not have another election for some time please!

But I think this does illustrate a subtle point about probabilistic predictions that most of the media seem to have missed, so it is worth pointing out. And I feel a little personal guilt for being too pessimistic to believe Sam Wang before last Wednesday!

Tuesday, October 16, 2012

Black Sun

While I'm at it, let me also reblog this beautiful picture taken by Jim Lafferty:

Yes, that's the Sun. The image is recorded at the H$\alpha$ wavelength and then colour-inverted: the other black dots around it are stars.

There's a short description of the features of the photograph here.

Ignoring the evidence: politics as usual

I have not had time to write a new post of my own for some time because real life has intervened, partly in the form of my day job (a project I am working on is nearing completion, and I am also preparing several seminar talks and grading a student's thesis), and partly in the form of other inconvenient distractions (writing job applications for next year, trying to find a new apartment and so on).

I hope to find some time to rectify this in the near future, but in the meantime I thought I would reblog a post by Simon Wren-Lewis, titled When policy ignores evidence: badgers and austerity. As you can guess from the title, it deals with just two examples demonstrating the apparent incompatibility of evidence-based rationality and politics. In fact, these examples seem to show politicians not so much failing to account for the scientific evidence as deliberately ignoring it.

This is a theme I wrote a bit about here. If that piece sounded rather cynical, well, things like this are the reason why.
 When policy ignores evidence: badgers and austerity: [...] Badgers get, and spread, TB. As a result, the UK government is about to begin a large scale cull of badgers in Gloucestershire and Somerset. No one likes the idea of killing badgers. But cattle (or occasionally alpacas) dying from TB is no fun either. So the badger cull is just one of those necessary bad things that have to be done to prevent something even worse happening. Environmentalists are up in arms, but that is just because badgers look cute and cattle do not.

Except that is not what the evidence suggests. Following various small scale randomised badger culling trials, the UK government set up an independent group of scientists (the ISG) to evaluate the evidence. In 2007 the government published the report (pdf). It concluded as follows:
“The ISG’s work – most of which has already been published in peer-reviewed scientific journals – has reached two key conclusions. First, while badgers are clearly a source of cattle TB, careful evaluation of our own and others’ data indicates that badger culling can make no meaningful contribution to cattle TB control in Britain. Indeed, some policies under consideration are likely to make matters worse rather than better. Second, weaknesses in cattle testing regimes mean that cattle themselves contribute significantly to the persistence and spread of disease in all areas where TB occurs, and in some parts of Britain are likely to be the main source of infection. Scientific findings indicate that the rising incidence of disease can be reversed, and geographical spread contained, by the rigid application of cattle-based control measures alone.”
On Sunday 30 eminent UK and US scientists published a letter in the Observer. They write: “As scientists with expertise in managing wildlife and wildlife diseases, we believe the complexities of TB transmission mean that licensed culling risks increasing cattle TB rather than reducing it.” One of the signatories described the government’s policy as crazy, and suggested vaccination and biosecurity was a better solution. The Guardian reports the chair of the ISG as saying “I just don't know anyone who is really informed who thinks this is a good idea." The current government chief scientist said: "I continue to engage with Defra [the relevant government ministry] on the evidence base concerning the development of bovine TB policy. I am content that the evidence base, including uncertainties and evidence gaps, has been communicated effectively to ministers." In other words, ministers know what scientists are saying and have decided to ignore them.

So what is going on? One of the strongest pressure groups in the UK is the National Farmers Union (NFU) [...] The NFU are convinced that culling badgers will reduce the incidence of TB in cattle, and government policy is following that belief, rather than the scientific advice it commissioned. (For more details, see George Monbiot here.) The BBC reports Defra Minister David Heath as saying "No-one wants to kill badgers but the science is clear that we will not get on top of this disease without tackling it in both wildlife and cattle." Dare I say weasel words.

[...] With austerity we did not have randomised trials: we had one almost globalised trial, starting in 2010, and one eighty years earlier. The evidence this time round is becoming clear: the harmful effects are much greater than many had assumed.

[...] perhaps the two cases are not so different. The problem with austerity is that too many people of influence just know that high government debt is always and everywhere a bad thing. Too many think it is just obvious that when a country has difficulties in selling debt that must imply cutting it back as quickly as possible, in the same way that it is obvious that killing badgers must reduce the spread of TB. And perhaps too many people see badger culls as part of a battle between farmers and environmentalists, just as austerity is a weapon in a battle over the size of the state.

Maybe we are just naive in thinking that as the evidence against austerity accumulates, and as those that were once for it change their mind, the policy will change. As Wolfgang Munchau writes (FT): “As hordes of frustrated European economists know only too well, macroeconomic analysis in general does not play a role in eurozone policy making.” So the policy goes on, in both the UK and the Eurozone, and I do not like to think about what might happen in the US if Romney wins. While I would never advocate a totally uncritical acceptance of the views of scientists, we are an awfully long way from that position, as unfortunately many badgers are about to find out.

Wednesday, September 19, 2012

Extortion in Prisoner's Dilemma

This week I stumbled across a fascinating paper by William H. Press and Freeman J. Dyson titled "Iterated Prisoner's Dilemma contains strategies that dominate any evolutionary opponent," published in the Proceedings of the National Academy of Sciences. I think it is fair to say it is the most interesting thing I have read this year. Press and Dyson have somehow discovered something completely new and profound in a field that has already been studied extensively for decades, and have written a beautifully clear and concise paper that you require only some elementary linear algebra to understand. This is the kind of paper we would all love to write, and I've found it hard to stop thinking about it since I first read it a few days ago.

(Incidentally, there is also a physics connection here. Although Press is a computational biologist at the University of Texas he happens to have written several papers in theoretical astrophysics and cosmology, including a famous one with Paul Schechter in 1974, which is one of the most highly cited theoretical papers in all of cosmology. He is also one of the authors of the Numerical Recipes books, which anyone who has ever done any programming has probably used at some point. Dyson is best known to physicists for his work on the theory of quantum electrodynamics, in particular for being the first person — other than Feynman — to recognise the importance of the Feynman diagram. According to Steven Weinberg he was "fleeced" of a Nobel. He's also a remarkable 88 years old. You can read a beautiful character portrait of this complicated man here.)

Tuesday, September 18, 2012

The dog who climbed a mountain

Three years ago today I took this photograph of a dog on the summit of a Himalayan peak, nearly 6000 metres above sea level.

Dog on the summit of Hanuman Tibba (5932 m) in the Indian Himalayas. 

It wasn't the easiest peak in the world, he was just a pretty amazing dog. Below the fold you can read the story of how he made it there — and as you will see, there was another powerful reason, apart from the dog, that keeps the day quite fresh in my memory.

This piece was first written in 2009 as a report for trustees of the A.C. Irvine Travel Fund, who helped finance my trip, and was later published in the Oxford Mountaineering Journal. It does contain some strong language, for which I apologise.

Saturday, September 15, 2012

Calling on Germany

In the New York Review of Books, George Soros has written an excellent long piece on the future of the European Union in light of the Eurozone economic crisis. If what happens in the EU in any way affects your life (and it almost certainly does), you should read it. There is a lot to ponder over in there, including a brief history of European cooperation, an astute diagnosis of the economic problems, and a bold call for action that is unlike anything that has been said anywhere else. It is also written from the perspective of a heartfelt supporter of the idea of a united Europe.

It is that call for action that gathered the most attention in the press, making several newspaper headlines. Soros is quite blunt: he says straight out that at every stage in the crisis, Germany's intention has been to do the absolute minimum required to avert disaster — with the end result that no resolution has been reached, and the can is just a few metres further down the road.
The policies pursued under German leadership will likely hold the euro together for an indefinite period, but not forever. [...] If and when the euro eventually breaks up it will destroy the common market and the European Union. Europe will be worse off than it was when the effort to unite it began, because the breakup will leave a legacy of mutual mistrust and hostility. The later it happens, the worse the ultimate outcome. That is such a dismal prospect that it is time to consider alternatives that would have been inconceivable until recently.

In my judgment the best course of action is to persuade Germany to choose between becoming a more benevolent hegemon, or leading nation, or leaving the euro. In other words, Germany must lead or leave.
Strong words indeed, and not an angle I have seen advocated (explicitly) by anyone else yet. He then goes on to explain a bit more.
Since all the accumulated debt is denominated in euros it makes all the difference who remains in charge of the euro. If Germany left, the euro would depreciate. The debt burden would remain the same in nominal terms but diminish in real terms. The debtor countries would regain their competitiveness because their exports would become cheaper and their imports more expensive. The value of their real estate would also appreciate in nominal terms, i.e., it would be worth more in depreciated euros.

The creditor countries, by contrast, would incur losses on their investments in the euro area and also on their accumulated claims within the euro clearing system. The extent of these losses would depend on the extent of the depreciation; therefore creditor countries would have an interest in keeping the depreciation within bounds.

The eventual outcome would fulfill John Maynard Keynes’s dream of an international currency system in which both creditors and debtors share responsibility for maintaining stability. And Europe would escape from the looming depression. The same result would be achieved, with less cost to Germany, if Germany chose to behave as a benevolent hegemon. That would mean (1) establishing a more or less level playing field between debtor and creditor countries and (2) aiming at nominal growth of up to 5 percent, in other words allowing Europe to grow its way out of excessive indebtedness. This would entail a greater degree of inflation than the Bundesbank is likely to approve.

Whether Germany decides to lead or leave, either alternative would be better than to persist on the current course. The difficulty is in convincing Germany that its current policies are leading to a prolonged depression, political and social conflicts, and an eventual breakup not only of the euro but also of the European Union. How to persuade Germany to choose between either accepting the responsibilities and liabilities that a benevolent hegemon should be willing to incur or leaving the euro in the hands of debtor countries that would be much better off on their own? That is the question I shall try to answer.
As I said, go and read the full argument.

One thing that is generally accepted by everyone is that the ultimate resolution of the Eurozone crisis can only come about through a relative decrease in the wages and prices in debtor countries (Spain, Italy, Portugal et al.) compared to Germany, in order to restore their competitiveness. This can happen in one of two ways: slightly higher inflation in Germany than in other countries, or deflation in other countries and stable prices in Germany. Most people also agree that achieving this entirely through deflation in debtor countries is not only highly unlikely to work, but also exacts a terrible and unnecessary human cost.

This is why Soros argues for Germany to make the sacrifice and accept moderate inflation. He accepts that this is a very hard pill for the Germans to swallow:
[T]he Bundesbank remains committed to an outmoded monetary doctrine that is deeply rooted in German history. Following World War I, Germany had a traumatic experience with inflation; consequently it recognizes only inflation as a threat to stability and ignores deflation, which is the real threat today.
The genuineness of this fear of any inflation is not in doubt. Nevertheless, if you think about it, it is a little bit strange. It is often argued that the Weimar inflation was one of the driving factors behind the rise of the Nazis, and that this contributes to the national psychological scarring. But surely this is the wrong historical lesson. As a few people have argued before, the period of the most dramatic growth in the Nazi share of the vote — from 2.6% in 1928 to 37.8% in 1932 — was long after the awful inflation, and coincided almost exactly with the Great Depression, during which the deflationary policies of Chancellor Brüning dramatically increased unemployment and lowered national income. If we must bring the Nazis into it, we should at least draw the right conclusions.

Thankfully, since Soros wrote his piece, events have moved quickly in Europe and it looks as though a combination of the ECB, the German courts, Spanish firmness and Angela Merkel's surprising new attitude have gained the upper hand over the president of the Bundesbank. Which is just as well, because as Kevin O'Rourke pointed out two years ago, looking at the massive unemployment, civic unrest and success of extremist parties in Europe today would not otherwise leave much room for optimism.

Saturday, September 8, 2012

Why are plants green?

Yes, I know, because chlorophyll is green. But why is that? This is a genuine question, so if any readers of this blog can answer it for me, I'd be grateful.

Let me explain a bit more. Yesterday I stumbled across an old post by Peter Coles asking why the Sun isn't green. If you haven't wondered this before, you should have a read. The reason for asking is that the intensity of light emitted by the Sun — which is very similar to a blackbody spectrum at a temperature of close to 6,000 Kelvin — peaks at the wavelengths associated with green.
Some example blackbody spectra. The Sun's surface temperature is about 5,800 K.
Naively therefore one might expect the Sun to appear green, rather than the colour it does, which is ... white (depending a little on where it is in the sky).

I won't explain here why the Sun doesn't appear green, or indeed why we don't see any green stars at all, because that's been done very well elsewhere on the internet. In particular you might want to check out this video explanation. As perhaps you might have suspected, the answer is less to do with physics than with biology of the human eye.

In fact, it's quite easy to come up with a simple ex post facto rationalisation of why we should have evolved so as to perceive sunlight as neutral white colour. 'White', after all, is a somewhat vague and flexible concept, as anyone who has operated a digital camera would know.

But in light of this, what I don't have such a simple explanation for is the fact that plants are green. Chlorophyll appears green because it uses light from the red and blue parts of the spectrum to power photosynthesis, and reflects green wavelengths. But why has it evolved in such a way as to reject precisely that part of the solar spectrum where the intensity of light is highest? Would it not have conferred an evolutionary advantage to make better use of this incident energy?

As I said, I don't know the answer to this little puzzle. Perhaps the explanation comes from chemistry — maybe there is simply some restriction on the possible chemical pathways by which CO2 and H2O can be converted into organic molecules, which all evolutionary variants would have to respect, and which renders light of certain frequencies unsuitable. Or perhaps it comes from some quirk of evolutionary history — one of those little inefficiencies that occur due to historical accident, and which are themselves evidence for the action of evolution, like the panda's thumb.

Either way, if you know the answer or can hazard a guess at it, please do tell me.

Things to Read, 8th September

I have been neglecting this series over the summer, so there are rather a lot of things to highlight today.

Science items:
  • The most interesting piece of physics news over the last few months has been that the wealthy Russian investor and physics fan, Yuri Milner, decided to award a 'Fundamental Physics Prize' to 9 theorists he chose himself. The amount of the award is a staggering $3 million each, wired overnight into the physicist's bank accounts — as many people have pointed out, this makes the prize worth far far more than the Nobel Prize. Suddenly being a physicist almost appears to be a lucrative career choice. The lucky physicists were Nima Arkani-Hamed, Alan Guth, Alexei Kitaev, Maxim Kontsevich, Andrei Linde, Juan Maldacena, Nathan Seiberg, Ashoke Sen and Ed Witten. All big names in theoretical physics.
  • Unsurprisingly, the award of this prize generated a lot of debate. For one thing, it is a huge amount of money. For another, the prizes were awarded basically on the whim of one man, who is not a physicist himself. But perhaps the most controversial thing was reported in the New York Times:
    Unlike the Nobel in physics, the Fundamental Physics Prize can be awarded to scientists whose ideas have not yet been verified by experiments, which often occurs decades later.
    This was picked up on by Peter Woit, who is of course well-known for his dislike of some of the work on string theory that the said physicists are famous for, and was highly critical of the prize, including in an article for an Italian newspaper (English transcript from his blog). I guess most physicists would share his unease with the prize, if perhaps not with string theory itself. It was quite amusing to read Matt Strassler finding himself in a position of agreement with Woit but managing to attack him nevertheless (I get the feeling he really dislikes Woit).
  • Motivated by this discussion, Strassler did put up a very interesting and nuanced personal defence of string theory. Whether you have an opinion on 'The String Wars' or not, you will probably have heard strong criticisms of string theory. This piece will provide some balance.
  • Among the other worthwhile physics discussions were two guest posts at Cosmic Variance: by Terry Rudolph and Douglas Finkbeiner. Both discuss recent research work (in quantum mechanics and searches for dark matter, respectively) and both include interesting little insights into the world of academic publishing.
  • Outside physics, I saw a piece by Dr. Dave Hone in the Guardian science section that includes the lines:
    [...] we have dinosaurs everywhere around us. We have dinosaurs nesting in trees in our gardens, tiny dinosaurs that can hover and fly backwards and feed on nectar, aquatic dinosaurs that live in Antarctica. There are dinosaurs that we eat, we have dinosaurs that can circle the globe without landing, intelligent and puzzle-solving dinosaurs, tall and flightless dinosaurs that can sprint at over 40 miles and hour, and brightly coloured and beautiful dinosaurs almost too numerous to mention.
Other items:
  • Of course the big news over the summer has been the Olympics. It's been impossible to miss the outpourings of national pride in British newspapers at Team GB's medal tally and I have heard also of despondency in countries such as Russia and Australia. But what is the benefit to a nation from this arms race for medals? This was discussed, rather provocatively perhaps given the timing, by Ian Johnson in the New York Review of Books
  • There's been a lot of discussion in the US about the Republican presidential ticket, and in the UK about the dire state of the national economy, now in a longer slump than during the Great Depression. Paul Krugman made the obvious connection, and labelled Chancellor of the Exchequer George Osborne "Britain's Paul Ryan." That post includes a graph that should be shown to everyone who still believes austerity in the recession is the answer to anything:

    One could forgive Krugman for feeling rather frustrated (and perhaps a little smug), because this is pretty much exactly what he predicted back in 2010:
    The British government’s plan is bold, say the pundits — and so it is. But it boldly goes in exactly the wrong direction.
    See also here.
  • Of course some people might remember the 20 "leading economists" who wrote an open letter to The Sunday Times supporting George Osborne's fiscal plans back before the 2010 election. Well, some time ago the New Statesman had the bright idea to go back and ask them if they still stuck by their advice: 9 of the 20 had changed their minds, 10 refused to comment, and only one still agreed with himself.
  • You might wonder what a poor politician is to do: if you have only a degree in History and an ideology as substitutes for genuine competence, you naturally rely on the advice of professional macroeconomists. But when the economists disagree with each other, which ones should you listen to? Jonathan Portes had a post laying out some sensible criteria for judging who to take seriously. Unsurprisingly, people like Krugman, Simon Wren-Lewis and Martin Wolf passed the test, but none of the Sunday Times economists did. Portes notes that not all those who pass his test always agree with each other — but he still always takes them seriously.
  • The New York Review of Books carries a review of two books on the current global and American economic mess by Paul Krugman (End This Depression Now!) and Joseph Stiglitz (The Price of Inequality: How Today's Divided Society Endangers Our Future). What particularly interested me here was learning just how radical Stiglitz appears to be.
  • Incidentally, for those who were interested in the Niall Ferguson episode featured here, John Cassidy in the New Yorker has a longer look at the arguments, and comes to the same conclusions: Ferguson is a lightweight who is being repeatedly knocked to the canvas, and this "isn't a fair fight".
  • And finally, since I'd like to end on an uplifting note, here's a video to cheer us all up.

Thursday, August 30, 2012

The largest patterns in the Universe

In addition to the developing series of posts on probability and statistical inference, I also want to write another series discussing the patterns in the distribution of galaxies, clusters of galaxies and dark matter in the Universe: how we reconstruct these patterns from observation, and how we can use this information to learn about the very distant past back near the time of the Big Bang some 14 billion years ago. This is what much of my day-to-day research is about, so I can claim more expertise on this subject than on some of the others I post about.

I'll try to keep the majority of the discussion at a level suitable for readers with an interest in cosmology, but no detailed technical knowledge of it, though I hope to include enough information to interest more expert readers as well. However, rather than constructing a systematic development of ideas from first principles I'm afraid I will flit about like a butterfly, alighting on topics that are of particular interest to me at the moment! Questions and feedback are welcome via the comments box.

Today's post is about the homogeneity of the distribution: the absence of pattern.

Wednesday, August 22, 2012

The disgrace of Niall Ferguson

(The content of this post is somewhat beyond the usual remit of this blog.)

A month or so ago, a friend drew my attention to the 2012 BBC Reith Lectures given by Niall Ferguson, a Harvard professor of economic history. This is a particularly prestigious lecture series on BBC Radio 4. Past lecturers have included many famous names, from Bertrand Russell in 1948 to Aung San Suu Kyi in 2011; I would recommend several previous lectures. But on listening to Ferguson, I was so annoyed by the combination of factual errors, non sequiturs, deliberate misrepresentations and schoolboyish debating tactics I heard that I almost composed a long diatribe for this blog to point them all out. In the end I refrained, partly for the sake of my blood pressure.

Now, however, Ferguson is back in the news, for an execrable cover story in Newsweek attacking Barack Obama's economic policy that contains many of the same tactics he employed in his Reith lectures, and I can no longer pass up the opportunity to comment. Luckily the Newsweek story has stirred up such justifiable disdain everywhere that I will be able to outsource much of the detailed commentary and merely provide you with a round-up of the reasons why you should never believe a word Ferguson says or writes.

Monday, August 13, 2012


I just wanted to make a couple of quick points about the system of leaving comments on this blog. I've been away on holiday for a couple of weeks (I tried to arrange some posts in advance so you wouldn't notice) and on my return I notice that this blog has been flooded with comments that Blogger has picked up in its (rather good) spam filter. These comments are all of a similar style — effusive in their general praise for my blog, but reluctant to specify any details, and always advertising some other website as well. Now it may be that I have a particularly enthusiastic fan who writes in broken English, but more likely the spam filter is doing its job very well. Therefore I won't reinstate those comments.

However, I noticed that the spam filter also picked up a few genuine comments as well (since reproduced here). Generally speaking I don't check the spam filter so such comments could remain lost forever. So if you are a real person trying to leave a comment on a post but you don't see it appear, you should try to (a) make sure your comment addresses the content of the post and not simply some vague generalities, (b) avoid gratuitous plugs for irrelevant websites, and (c) not suspiciously post and delete too many comments in too short a time. In case your insightful contribution is still lost to the æther, explain the problem on here and I will try to reinstate your comment.

Saturday, August 11, 2012

The Shining Mountain

At this sort of time every year I find my thoughts turning to the mountains, and today for no particular reason I wanted to write a little about a particular favourite of mine. There are a few peaks around that are sometimes claimed to be the most beautiful, inspiring or aesthetically pleasing in some way — the Matterhorn and Ama Dablam are ones that are mentioned very often — but for me, nothing quite matches up to Changabang in the Garhwal Himalaya.

Instead of attempting to describe the mountain myself, let me quote from the description in a book called Scottish Himalayan Expedition, written by the mountaineer and author W. H. Murray in 1950. By all accounts, Murray was a wonderful man with a deep love of mountains, and an ability to express that love in the written word that I've never found with any other writer. This little paragraph itself is one of the main reasons that Changabang features at the top of my personal list of favourite mountains:
The nearest of the great peaks, Rishi Kot, turned to us an edge like a cutlass but black as gun-metal, whereas Changabang, its neighbour, by day the most like a vast eye-tooth fang, both in shape and colour — for its rock was a milk-white granite — Changabang in the moonlight shone tenderly as though veiled in bridal lace; at ten miles' distance seemingly as fragile as an icicle; a product of earth and sky rare and fantastic, and of liveliness unparalleled, so that unawares one's pulse leapt and the heart gave thanks — that this mountain should be as it is.
Murray's description of the entire expedition sitting out in the freezing night air, wordlessly eating their dinner while staring transfixed at this glorious summit above them, has stayed with me ever since I first read it as a teenager. In fact the entire book is excellent; if you can find it anywhere, you should read it.

Murray and his companions would have been looking at Changabang from the south-west, its most spectacular profile. Here's a photograph of it from the same angle, taken by Doug Scott in 1974:

Bonington, Haston, Hankinson and Scott admiring Changabang, 1974. Photo by Doug Scott.

The ridge on the right drops down to Shipton's Col, named after the great mountaineer and explorer who first crossed it while mapping the Nanda Devi Sanctuary in 1936. (Shipton's connection with the name of this blog was explained here.) This photo is from the expedition by a team of British and Indian climbers who made the first ascent of Changabang in 1974, by the south-east face. I recently found Doug Scott's original report of the climb in the Alpine Journal online here. It contains several other beautiful black-and-white photographs of Changabang and the surrounding peaks. Here's yet another photograph with a slightly different view:

The most beautiful mountain in the world
Dougal Haston in front of the West Wall. Photo by Doug Scott.

Towering above the climber, Dougal Haston — who, you'll notice, is carefully rehydrating himself in the medically recommended manner — is the famous West Wall of Changabang, with the left skyline dropping down to the Bagini Pass (apparently that's not the real Bagini Pass; see the correction in the comments below). Despite appearances, the West Wall is actually mostly too steep for any great quantities of snow to remain on it; its colour in the photo is due, as Murray noted, to the whiteness of the granite.

The West Wall played an important role in mountaineering history: in 1976, a two-man British team — Peter Boardman and Joe Tasker — made a dramatic and revolutionary ascent of it using 'big wall' techniques that no one had previously thought were possible in the high Himalaya. It took them 25 days. Part of their training for the route in England involved spending nights hanging in hammocks inside industrial meat freezers. Boardman's book about the ascent, The Shining Mountain, is one of the all-time classics of mountaineering literature — again, if you get your hands on it, read it. In some ways Boardman was not a dissimilar writer to Murray, his passion and the simple poetry of his language shining through (please excuse the pun) on every page. After their disappearance on Everest in 1982, the Boardman-Tasker prize has been awarded every year for outstanding mountain literature.

Viewed from the north, Changabang is in some ways less impressive, as at 6864 metres high it is somewhat lower than its immediate neighbour Kalanka (6931 m). This is what it looks like from the Bagini glacier:

Changabang and Kalanka north faces
Kalanka (L) and Changabang (R) from the north, the Bagini Pass on the right (not quite, see the comments below!). Photo: unknown.

Less impressive in some ways maybe, but from a mountaineer's perspective the grandeur of that sweeping vertical north face is awe-inspiring. Frank Smythe described it as "a peak that falls from crest to glacier in a wall that might have been sliced in a single cut of a knife." That north face was first climbed — Alpine-style, no less! — by Andy Cave and Brendan Murphy in 1997, though Murphy sadly died in an avalanche on the descent. In a neat symmetry, Andy Cave won the Boardman-Tasker award in 2005.

For the time being, I believe the northern perspective of Changabang is the only one available to mountaineers, since the Nanda Devi Sanctuary remains closed to expeditions (Though I may be wrong on this: it is possible that partial access to the Sanctuary has recently been allowed again, and it is also possible that Changabang can be seen from the southwest from some location outside the Sanctuary).

At some point in the future, I have promised myself a visit to Changabang. Not to climb it, which would be beyond my capabilities, but merely to see it for myself, from the Bagini glacier if the Sanctuary remains closed. Until then, I must content myself with other people's photographs, and Murray's words.

Saturday, August 4, 2012

Why don't more scientists enter politics?

This is a question posed by Shaun over at The Trenches of Discovery, in a piece I would urge you to read. He argues that scientists are not shy of expressing opinions on matters of political interest, and scientists are well-qualified (some would say better qualified than most) to offer opinions on, and to frame, policies on matters of great importance to society. So why are they under-represented in politics?

I found this a bit of a thought-provoking question. As readers will have noticed, this particular science blog is not backward in expressing political opinions, so some sort of considered response appears to be in order. If you click through to Shaun's post you can see some of my immediate reactions in the comments; however, while I am happy to shoot from the hip over at his blog, I will try to express myself more coherently and thoughtfully on my own.

Saturday, July 28, 2012

Things to Read, 28th July

Physics items:
  • New results are in from the dark matter search at the XENON100 detector in Gran Sasso in Italy. See here for technical slides, or read the summaries at Quantum Diaries or Cosmic Variance. Basically, they see no dark matter events above the background expectation, so the exclusion limits on WIMP mass and cross-section are extended. As Sean Carroll points out, the results appear to be in some tension with some other experiments, but these things depend on theoretical assumptions about the nature of dark matter.
  • In the Guardian, Jon Butterworth muses about the 'impact' of the discovery of the Higgs, and on the usefulness of 'impact' in general as a guide for deciding what scientific research to fund. There are some worthwhile points there, especially about who should get to decide which science has an 'impact' and which not. But I felt on some issues the piece was rather confused — he first seems to suggest that "a sense of progress" and the satisfaction of simply "adding to the body of human knowledge" are sufficient justification for funding fundamental research that may have no other forseeable benefit, but later says he disagrees with the statement that "the acquisition of knowledge is somehow morally or intellectually superior to the application or dissemination of knowledge". So which is it?
    I wrote about my own views on the justification for funding science research that has no obvious economic, medical or social payoff here. I do think it is justifiable, even if one can safely predict that there will never be any direct benefits from such research (serendipitous spin-offs, like the common examples of the internet or MRI, are a separate issue, but cannot be guaranteed). So it annoys me to see people constantly trying to justify studying the Higgs boson in terms of what hypothetical technological advances it might bring. Face it, there won't be any — if you don't believe me, here is Steven Weinberg saying much the same (among other interesting things!), or click here to see just how hard it is to come up with any remotely plausible technological speculations consistent with known physics.
    The point is that there already exist good enough reasons for funding fundamental research, so let's leave out all the rubbish about Higgs-powered spaceflight and so on, shall we?
  • Readers may have already seen elsewhere the story of the physicist Paul Frampton, who has been in an Argentine jail for several months, facing charges of drug smuggling. It's a sad and frankly incredible story; I see (via Peter Woit) that Frampton expects he might be released soon (though he's been wrong about this before).
Other items:
  • There is a fantastic super-slow-motion video of a lightning strike, shot at 7207 frames per second, available here, with a nice little explanation of the mechanism of the strike. I'd highly recommend it!
  • A Canadian wonders whether the Euro is worth saving at all.
  • Is there a magic button for the UK government to press? Do they want to press it?
  • On a more cheerful note, some time ago Peter Coles posted Patrick Barrington's lovely comic poem I Had A Hippopotamus. It begins like this:
    I had a hippopotamus; I kept him in a shed
    And fed him upon vitamins and vegetable bread.
    I made him my companion on many cheery walks,
    And had his portrait done by a celebrity in chalks.

    His charming eccentricities were known on every side.
    The creature’s popularity was wonderfully wide.
    He frolicked with the Rector in a dozen friendly tussles,
    Who could not but remark on his hippopotamuscles.

    Barrington's best known poem The Diplomatic Platypus is also great fun (and has a strangely similar theme).

Wednesday, July 25, 2012

Be very, very afraid

Paul Krugman cannot believe the lack of terror among German politicians about the spectre of a Greek exit from the Euro. Do they really think that if Greece is forced out, Spain will not be forced to follow?

In fact the disturbing news from Germany has got most economists spooked — or at least most economists who aren't willfully blinkered. Some time ago, I noted here that Simon Wren-Lewis was holding out some hope for the survival of of the Euro, because he felt sure Eurozone officials would eventually see that hard-line moral posturing was detrimental to their own interests (apart from being wrong). Sadly, it appears not even he is so optimistic any more:
Sometimes it seems as if Germany and its supporters are like a poker player with a very weak hand, who has managed to convince all the other players that their hand is much stronger than it is. But there is a danger that you may get so good at playing this bluff, that you may stop looking at your cards and actually believe you have a strong hand. Or worse still, that although your hand is weak, you deserve to have the better cards, and therefore you do have the better cards.
The situation with German public opinion is now so dire that although he is careful not to explicitly say so, I think Wren-Lewis now agrees with Greek economist Yanis Varoufkis' assessment that Germany actually doesn't want to solve the Eurozone crisis.

Which brings us back to Krugman's take on the foolish lack of terror.

Tim Duy agrees, asking whether a panic button even exists in Europe:
I doubt we will need to wait much longer to learn the outcome of Grexit. But the devastating train that is the debt crisis keeps rolling right along, currently crashing through Spain's economy.

And make no mistake, European policymakers have learned nothing from the Greek experience. One gets the sense that policymakers think the prescription was correct, but that the patient was simply unwilling to take the medicine. Where Greece failed, Spain will succeed, or at least so it is hoped [...] Spain is doing the right thing, apparently. It's just the markets that have it all wrong [...] And in return for this bailout, Spain will be pushed further down the same path of never ending recession as Greece. Because if once you don't succeed, try, try again. European policymakers will pursue the same path because they know of no other [...]

In my view, the lack of panic is downright scary. Is Europe completely devoid of new ideas? Or is everyone simply on vacation?
Surely, you feel, we will wake up soon and learn this was all a dream? Surely somebody will remember which country's banks it was that lent the money to Greece, Spain, Ireland, Italy, Portugal et al. in the first place, and which country's banks therefore stand to lose most if they collapse? Surely someone will remember who was "the sick man of Europe" before the introduction of the Euro helped turn their economy around?

But no. The words 'face', 'spite', 'chop' and 'nose' come to mind.

Saturday, July 21, 2012

How to update your beliefs in light of new evidence

This post is the second in a series on probability and statistical inference, and follows this one, in which I asked readers a question that I had once put to applicants to Oxford in physics interviews. If you have not yet answered the question in the poll, you should click on the link above and try it first! The poll will remain open for anyone who would like to try answering the question before reading the answer below.

Monday, July 16, 2012

Things To Read, 16th July

When I first decided to start collecting together links to interesting things on the internet and putting them into one blog post per week, I was envisaging something like a less frequent version of this. Instead these posts have become something slightly different: longer, because I like to add short comments on the articles I link to; and perhaps more selective. This meant that these posts were becoming a little too much work! I also don't always find enough physics links to highlight each week.

So I have decided to embrace the change: these posts will remain a regular feature, but no longer necessarily a weekly feature. Instead I shall put them out as and when I have collected enough items I'd like to point out and briefly comment on. The title of the series will also be amended to reflect this.

Anyway, since I have already started writing something for today, I will include a few items:
  1. I was particularly interested in this little post by Julianne Dalcanton at Cosmic Variance, about the employment chances for physicists who choose to, or for whatever reason have to, leave the world of academia. The statistics — at least in the US — appear to be relatively encouraging, and somewhat better than for "lab-based" biologists or chemists. Julianne has her speculations as to why this should be so; I suppose they sound quite plausible.
    I think the other point she makes is really important too — students enrolling for physics PhD programmes really ought to be aware that the odds against them ever getting a permanent academic position equivalent to that of their advisor are very small. Say the average professor sees one student through to a PhD every two years. Over a career of 40 years, that's 20 completed PhDs, and yet when that professor retires, only one permanent faculty position becomes available. Odds like that mean that, unlike the UK government, every sensible PhD student should be aware of the need to have a Plan B. This applies also to those of us who have been lucky enough to get a foot on the ladder in the form of a post-doc job (though I don't seem to have taken my own advice yet!).
  2. Tucked away in the Guardian, I saw this report on the treatment in Pakistan of Nobel-Prize-winning particle theorist Abdus Salam, both before and after his death in 1996.
  3. Anyone who has been following the US Presidential elections will know that there has been some amount of kerfuffle recently about who was in charge of which company when. You could see this for a little perspective, or doubtless there are countless other places you could read about it. Anyway, that's not what interests me: I don't have a vote in this election, and if I did, I wouldn't need stories about Bain Capital to know that Romney is a ridiculous candidate.
    Instead, I think the significant occurrence from the last week was this quote from Barack Obama:
    [I]f you’re a head of a large private equity firm or hedge fund, your job is to make money. It’s not to create jobs. It’s not even to create a successful business – it’s to make sure that you’re maximizing returns for your investor. Now that’s appropriate. That’s part of the American way. That’s part of the system. But that doesn’t necessarily make you qualified to think about the economy as a whole ...
    Phew. At last some politician has come out and said something sensible. I would have thought it was fairly obvious that the analogy between running a country and running a business was completely wrong — after all, I doubt there is a company anywhere in the world that sells its product primarily to its own employees — but still most politicians continue to treat people like idiots. This is not restricted to the US: in Britain the analogy used by the Tories is different — that government budgets are like family budgets, complete with credit cards and tightening belts — but the underlying fallacy is basically the same (which family buys goods and services primarily from itself?). I don't know exactly what rhetorical devices politicians use in Germany, Spain, Greece and the like, but from the utter mess they have made of the Euro, one can surmise that they must be equally stupid. So one-and-a-half cheers for Obama for attempting to right that trend.

Sunday, July 15, 2012

A question of probability

In my post today, I'd like to ask readers of this blog a little question on probability that we* used a few years ago as an interview question for Oxford physics admissions.

The Oxford interview process is quite famous — or notorious, depending on your point of view — in the UK, and every year, come admissions time, newspapers run stories containing collections of "impossible" or eccentric questions that have supposedly been asked of candidates by malicious interviewers. In some newspapers the slant then given to the story is that the entire interview process is, by design or by coincidence, weighted against applicants from state schools, who are less well prepared for such shocks and are reduced to nervous wrecks.

I don't wish to enter into such a political discussion here, but in case any prospective interview candidates were to read this blog post I'd like to assure them that the interviews — at least in physics and at least in my experience — are certainly not like this. Interviewers realise that nervous applicants do not reflect their true potential, so they do not deliberately add to the tension and will in most cases attempt to create as relaxed an atmosphere as possible. They also realise that interviews, because of their highly subjective nature, are statistically not very good indicators of the true physics ability of the student. It is well-known that performance in the physics aptitude test applicants are required to take is a better predictor of their subsequent performance when at Oxford, so test results are substantially weighted up relative to interview scores when making decisions on who to admit. (You're also probably better off reading Oxford's own guide to sample interview questions than anything in the press!)

Anyway, I digress. The point of this post was the question of probability, not a discussion of admissions procedures. I thought the question was quite a nice one, but as it turned out no interviewees answered it correctly when it was first put to them, and after being given a gentle hint or two, almost all of them got to the right answer. So it was pretty useless at distinguishing between applicants and as a result not a very successful interview question. On top of that, I've also recently discovered that it is explained in great detail in a best-selling popular book on probability, so it probably won't be used again — which is why I feel it is safe to disclose it here! So here it is, slightly reworded:
You learn about the existence of some rare disease, X, which is known to affect 1 in every 10,000 people. Being a bit of a hypochondriac, you are afraid that you may have disease X, so you go to your doctor for a blood test. The doctor tells you that the test for this disease is accurate 99.9% of the time. To your horror, the test result comes back positive. What is the probability you have disease X?
I won't give the answer immediately (update: the answer is now explained here), because I'd first really like to know how readers of this blog would answer it. So I'd like to encourage everyone who reads this post to vote in the following poll -

What is the likelihood you have disease X?

Please don't be shy about voting! If you get it wrong, well, you've done no worse than some of the brightest A-level students in the UK. On the other hand, you could have the satisfaction of getting it right and displaying your knowledge to the world (the poll's completely anonymous, but still). Feel free also to comment on this post, but please don't give the answer away if you know it.

The reason that I've posted this question and the reason I'd like to know people's answers is that although it wasn't a good interview question, I think it is still a good way to get people to think about probability, and the question of how to deduce information from limited evidence. These are topics which I would like to discuss in a series of further posts over the next few weeks, so I'll use this as a starting point.

* I took part in admissions interviews between 2007 and 2010 as part of a tutoring job while finishing my doctorate. This particular question was one suggested by my (senior) colleague for use in one of those years.

Monday, July 9, 2012

LftW: 9th July

Physics links:
  • Update: Another link worth highlighting this week is a background piece on the Higgs by Steven Weinberg from the New York Review of Books, taken from the introduction to a book due to be published next month. As ever, Weinberg is clear, precise and interesting.
  • Perhaps inevitably given the momentous nature of the results announced at CERN last week, there has been much discussion about who should be awarded the Nobel Prize for (a) the discovery of the Higgs and (b) the theoretical prediction of the Higgs around 50 years ago. Of course, the existence and future discovery of the Higgs was regarded as so certain that (b) has already been a topic of much debate for several years. For those who have not already read it, Frank Close's book The Infinity Puzzle provides an excellent summary, and (somewhat surprisingly) Mark Thoma's economics blog quotes some of the relevant sections. Summarised in a sentence, the problem is that six theorists including Peter Higgs can claim to have predicted the mechanism by which the Higgs boson gives mass to elementary particles, but the Nobel can only be awarded to three at most, leaving a complex decision for the committee to make. Close has recently been reiterating his recommendations and urging haste, since Nobels cannot be awarded posthumously. Already only five of the six survive.
  • The problem of who should get the prize for (a) is perhaps even more difficult. Although it is hard to imagine anything more Nobel-worthy than the discovery of a new elementary particle, Nobel Prizes can only be awarded to individuals, not collaborations, and most commentators seem to agree that it would be hard to single out any especially deserving individuals from the excellent team efforts of ATLAS or CMS. (This has been done before, for instance when John Mather and George Smoot got the 2006 Prize for their roles in COBE, but the choice now is less clear-cut.) Peter Woit wants the rules to be changed so the prize can be awarded to both complete groups and also CERN engineers, which I suspect is unlikely to happen. He would also like the award to be made this October, which is probably also unlikely, given that the detailed papers on the discovery will not even be submitted to a journal until the end of this month. (And I would guess it is even more unlikely for a theory prize to be awarded in October as Frank Close wants, since strictly speaking it isn't yet proven that the discovered particle is the Standard Model Higgs boson. But of course mine is a relatively uninformed opinion!) 
Other links:

Wednesday, July 4, 2012

Found the Higgs, not the mermaids

Once again not a long post — in fact I was in the middle of typing a simple congratulatory message when I noticed that Rhys got there before me. The facts to take home from today's presentations can of course be found on many physics blogs: basically, a particle has been found, at a mass of around 125 GeV, it is a boson, so far it behaves as we would expect a Standard Model Higgs to behave (there are some small discrepancies which may or may not be indications of the deviations that all theorists would love to see). So, as Rhys says: congratulations to everyone, and we hope to learn more soon.

In the meantime though, I notice (h/t Rob) that the most popular science articles on the BBC website at the moment are the following:
Higgs discovery at CERN and mermaid non-discovery in the US
We've found the Higgs, but not the mermaids.
Which suggests that although science marches on, science education still has some way to go.

Tuesday, July 3, 2012

What is the Higgs, and why is tomorrow's announcement important?

Although the title suggests this will be a long, detailed post full of explanations of particle physics, it actually won't. This is partly because I haven't the time or inspiration to write one, but mostly because it has been done much better elsewhere. Instead I'd like to draw your attention to a few excellent background pieces so that you can be well-prepared for what promises to be a very important press conference tomorrow (indeed Peter Woit now suggests that ATLAS and CMS will each present nearly-5-sigma evidence).

The first of these is a new post by Shaun at partner blog The Trenches of Discovery, which provides a nice layman-level overview of why discovering the Higgs would be cool, but theorists hope it isn't just the Higgs. A similar theme was discussed by Jester (Adam Falkowski) at Résonaances. He has also got a series of countdown posts, each explaining some aspect of Higgs physics, but these posts are all at a more technical level than Shaun's. At Cosmic Variance, John Conway has reprised an old Higgs overview post (from 2007) which is again aimed at the non-expert. There are other pieces I won't link to now, because these cover most of the bases for anyone interested.