Explaining Planck by analogy

Explaining physics to the public is hard. Most physicists do a lousy job of conveying a summary of what their research really means and why it is important, without the use of jargon and in terms that can be readily understood. So it is not particularly surprising that occasionally non-experts trying to translate these statements for the benefit of other non-experts come up with misleading headlines such as this, or this.

Just to be clear: Planck has not mapped the universe as it was in the first tiny fraction of a second. (To be fair, most other reports correctly make this distinction, though they differ widely on when inflation is supposed to have occurred.) I think this is an important thing to get right, and I'm going to try to explain why, and what the CMB actually is.

However, I'm going to try to do so with the help of an analogy. This analogy is not my original invention – I heard Simon White use it during the Planck science briefing – but I think it is brilliant, simple to understand and not vastly misleading. So, despite the health warning about analogies above, I'm going to run with it and see how far we get.

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.

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.

LftW: 25th June

Sorry about the relative lack of posting, this will be rectified soon. Here are some of the things I noticed last week.

Physics links:

• As you will no doubt already have heard from elsewhere, there is a press conference scheduled at CERN for the 4th of July, in which updates on the status of the searches for the Higgs boson at the ATLAS and CMS detectors will be provided. You've probably also heard the rumours that the results will feature higher statistical significance than the previous results from last December, and possibly even an official announcement of "discovery".
  
  I believe these rumours originated from Peter Woit's blog (see here and here), and they have caused a bit of a kerfuffle in the blogosphere. Matt Strassler rather bad-temperedly complained about "non-particle-physicist bloggers" spreading rumours, and Jon Butterworth also seemed a little upset, claiming spoilers would be "bad for science". Personally, I don't see how that argument works. Sure, I understand any scientist only wants to make an official statement in a professional capacity when they're sure they've got the analysis correct. And a large collaboration has to have rules to govern behaviour of individuals, so they're well within their rights to want to avoid leaks. But I don't see how, if the leak is made, publishing a blog post clearly marked as "rumours" is bad for science. If anything, it simply ensures that more people tune in to the press conference to check whether the rumours are actually true. (I suppose this makes it slightly more likely that the servers providing live streams from CERN will fail.)

• Still on the topic of the Higgs, Matt Strassler also got rather annoyed with a New York Times article on the topic: here and followed up here. Again, it's not something that bothered me too much, but at least in this case I do recognise the feeling of seeing some arcane aspect of your field slightly misrepresented in a summary in the popular press and getting worked up about it. (If the rumours above turn out to be true then this principled objection will be even less relevant!)

• On a completely different note, here is an amusing little game that incorporates special relativity (hat tip to Shaun). 

Other links:

• In the UK, the Education Secretary Michael Gove apparently wants to do away with GCSE qualifications for 14-16 year-old students and go back to the old O-level exams. I don't have any sensible comments to make about the advisability or otherwise of such a plan, but I did see Peter Coles had posted a link to a GCSE Science paper from 2006. It is absolutely astonishing. Question 3 is a particular classic.

• Two weeks ago Jonathan Portes caught George Osborne implicitly admitting his economic policy had been wrong for the last two years. Last week he caught the G20 admitting they'd been wrong too.

• I recently learned about the confluence of the Rio Negro and the Solimões river (which later becomes the Amazon) in Brazil, where the waters of the two rivers, which are very different colours, flow side-by-side for nearly 6 kilometres without mixing (because they are at different temperatures, and flow at different speeds). This makes for nice dramatic photographs, and I couldn't resist including one here:

Confluence of Rio Negro and the Solimoes.

Talent vs Hard Work or Nature vs Nurture

Listening to back episodes of the Four Thought series I highlighted last week, I found a talk given by Matthew Syed on the 9th of May this year which I thought was well worth listening to. Syed is a former international table tennis player and now a sports journalist at The Times. His thesis was that "talent" and "natural ability" are overrated and provide no guide to future success in most endeavours. Instead it is repeated and focussed practice of complex skills that actually enables people to master them, so everything is really down to hard work rather than some innate ability. According to him, even child geniuses will, on inspection, actually turn out to have simply compressed their hundreds or thousands of hours of practice into a relatively short period of time.

The reason this point is important, he argues, is that we too often take the opposite view, which inevitably leads people to not practise enough and consequently underachieve. A relevant example he cites is of students (particularly at state schools in the UK) just accepting as a fait accompli that they "just don't have the brains to go to university" or that they "just don't have the ability to do maths" or "don't have a talent for languages". Equally dangerous is the fact that we glamourise "effortless" achievement. He cites some studies which appear to show that young students praised for their natural ability or talent in some field will tend to avoid stretching themselves in future assignments in order to maintain that impression, whereas those who are praised for their effort are more willing to take on tougher challenges. (During the few years that I tutored undergraduates beginning their physics degrees at Oxford, I encountered all of these examples!)

To some extent obviously nature must play a role, as Syed admits. Sprinting and basketball are two examples he gives of sports where a genetic advantage is very important. But the more complex the activity that is to be performed, the more time must be spent to train muscles and brain to achieve success, and the greater the relevance of hard work over innate "talent". So activities such as playing the piano, hitting a cricket ball, or doing physics are all sufficiently complex to reward more dedicated practice.

While I found the talk itself interesting, I'm not sure I am completely convinced. Perhaps this is simply years of received wisdom speaking, but I find it hard to believe that enough training of muscles and neural networks can render all genetic pre-dispositions irrelevant. Especially at the elite level in any activity, where surely all participants have put in similar levels of training. I am sure that a person's natural mental state — encompassing such things as boldness in unfamiliar situations — must also be very important.

Having said that, however, almost none of us ever operate at an elite level in any sphere. Below this rarefied level, efficient training is almost certainly more important than anything else. Mental states can also — to an extent — be learned. I certainly agree that there is no justification for school pupils to ever consider themselves incapable of learning any particular skill, or for teachers to ever encourage that belief. That just sells young people short, and is a disservice to those regarded as talented as well.

I should add that I am also convinced this is one of the main reasons for the disproportionate under-representation of women in science, and in physics in particular.

The further this message is disseminated, the better.

Collective Marvelling

Another, very quick, post to say that as some of you might have noticed, this blog is now part of a fledgling blog network called Collective Marvelling. I've put a little tab at the top of this page describing the network and linking to it. The individual blogs in the network are also among those linked in the panel on the right. If you haven't already checked them out, you should!

Steven Weinberg: The Crisis of Big Science

A few weeks ago, via Peter Woit's excellent blog, I was alerted to a recent article in The New York Review of Books by Steven Weinberg, titled The Crisis of Big Science. Woit added his own thoughts on the article, and there are many interesting opinions expressed in the comments section there. However, I felt that a very important question was not satisfactorily answered, as I will try to discuss here.

Weinberg provides a brief and personal overview of the development of particle physics experiments, starting from Rutherford's pioneering gold foil experiment in 1911 (the "experimental team consisted of one postdoc and one undergraduate [...] supported by a grant of just £70 from the Royal Society of London"), through the invention of cyclotrons, the postwar development of accelerators and the construction of dedicated accelerator facilities at Fermilab and CERN, to the current multi-billion dollar Large Hadron Collider (LHC) which (we hope) will confirm a detection of the Higgs boson later this year, and might yet — if we are lucky — provide exciting clues about the correct particle physics theory at higher energy scales than we have yet probed.

One of the obvious themes in this short history is the enormous and growing cost of building the experimental facilities required to increase our knowledge about particle physics. Obtaining the money for these facilities is always difficult, and sometimes impossible. Weinberg reflects in particular on the cancellation in the 1990s of the Superconducting Super Collider (SSC):

Then in 1992 the House of Representatives canceled funding for the SSC. Funding was restored by a House–Senate conference committee, but the next year the same happened again, and this time the House would not go along with the recommendation of the conference committee. After the expenditure of almost two billion dollars and thousands of man-years, the SSC was dead.  

One thing that killed the SSC was an undeserved reputation for over-spending. There was even nonsense in the press about spending on potted plants for the corridors of the administration building. Projected costs did increase, but the main reason was that, year by year, Congress never supplied sufficient funds to keep to the planned rate of spending. This stretched out the time and hence the cost to complete the project. Even so, the SSC met all technical challenges, and could have been completed for about what has been spent on the LHC, and completed a decade earlier.

(The claim that projected costs only increased because of insufficient spending every year is interesting, and something I hadn't heard before. I don't know whether it is true, but it sounds plausible.)

This is not simply a problem for particle physics. The same pattern holds — though at a somewhat lower cost level — for my own field of astrophysics and cosmology. Recently, the US Congress decided to scrap funding for NASA's James Webb Space Telescope and the US has also decided to terminate the IXO (X-ray) and LISA (gravitational waves) projects, which caused the European Space Agency to choose to fund a mission to explore Jupiter's moons (known as JUICE) over projects that would have been more relevant to cosmology.

In the end, the point is that the political support for a such massive science experiments is hard to obtain. Weinberg says

During the debate over the SSC, I was on the Larry King radio show with a congressman who opposed  it. He said that he wasn't against spending on science, but that we had to set priorities. I explained that the SSC  was going to help us learn the laws of nature, and I asked if that didn't deserve a high priority. I remember every word of his answer. It was "No."

Now this is the question I'd like to focus on. Let's leave aside for now the question of whether a convincing science case can in fact be made for an SSC-like accelerator or JWST-like space mission (Peter Woit's post discusses this for a new particle accelerator); I will assume that the proposed experiment, whatever it may be, will indeed help us "learn the laws of Nature." Is this justification enough for vast government funding?