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."

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 ...)

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?

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!

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.

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.)

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.

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.

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.