Archive for July, 2012

Sorry I can’t be with you this week

July 21, 2012
Villa Monastero in Varenna, Italy

The Villa Monastero in Varenna, Italy.

I hate international travel. And I would happily see out my days in sunny rainy Teddington without regret. But travel is part of my work and so next week I am off for a week to teach at the International School of Physics “Enrico Fermi”. And I have to confess I am looking forward to it.

The summer school is about Metrology, and I feel honoured to have been invited to speak. Some of the other speakers have Nobel Prizes to their name, and so it is a prestigious affair. But despite my nervousness, I am eager to go, mainly because of the setting. The Villa Monastero in Varenna is situated on Lake Como, and to my eyes looks like an earthly paradise. As I stare at the images I can almost hear the sound of the waves lapping on the steps…

View from the terrace of the Villa Monastero, Varenna

View from the terrace of the Villa Monastero, Varenna

Sadly, I do have to do some work [Note to self: you really must prepare a talk before you go]. I have an onerous two hours teaching on the penultimate day of the summer school, talking about the Boltzmann constant – a subject which has been on my mind almost every day for the last 5 years. And I also have to write a paper following the outline of my presentation. But at the moment, I can think only of the prospect of sitting on the monastery steps by the lake, breathing the fresh mountain air, and gazing at the mountains.

They already have a statue of me in the gardens of the Villa Monastero, Varenna

They already have a statue of me in the gardens of the Villa Monastero, Varenna. Cheers.

Anyway. The blog posts may be a bit thin on the ground next week. And if there was anything that I said I was going to do for you by next week, well, it’s not going to happen. Sorry. Have a nice week.

The incredible lightness of being wrong

July 18, 2012
BBC GLobal Fat Scale

Where I lie on the BBC Global Fat Scale. Under average for the UK – over average for the world. But the text in the ‘info’ box (shown enlarged in the figure below) has  been changed. Click for larger image.

One of the most interesting features of modern media is that if one makes a mistake, it can be corrected quickly and the original error then disappears. Anyone who accidentally witnessed the transient error is then left with a feeling of bewilderment when they try to show the mistake to their friends. They may even experience a feeling of paranoia. I take advantage of this feature regularly on this blog, and the more august BBC took advantage this week when it erred in an article on Body Mass Index (BMI).

Both myself and one of my international network of informants noticed that the ‘hover over’ information text stated that having an above ‘normal’ BMI caused increased mortality – an increased risk of death. As I mentioned in a previous article, this appears to be not true. In fact being ‘overweight’ (BMI 25 to 30) appears to reduce mortality. Even being ‘obese’ (BMI 30 to 35) appears to give a reduced risk of death. Before I could write to the BBC about this, they removed the offending text, leaving me and my informant scratching our heads.

BBC GLobal Fat Scale

Detail from the image at the head of the article.

This matters. If the BBC, or the government, or a health authority, encourages people to move from the ‘obese’ or ‘overweight’ categories to the ‘normal’ category, then they are encouraging them to die earlier. Generally we consider ‘dying younger’ to be a ‘bad thing’. But that is what the statistical evidence indicates and that presumably is why the BBC chose to remove this reference.

There are two lessons to learn from this episode.

Firstly, reputable media, such as the LA Times, make a note on a page to record the fact that it has been updated since publication. This should be routine in a public organisation such as the BBC. They should not try to re-write history.

Secondly, the article in itself is profoundly flawed. It ranks a large number of countries but fails to note that the bottom half of the list are countries in which the populace is profoundly and tragically undernourished. The mortality data indicate that the ‘normal’ BMI categories may well have been drawn up in a time when our population was malnourished. Being overweight may not the most beautiful condition, but the data tells us that in terms of living longer, its the best condition to be in. And that is something this chubby 52 old (BMI = 27) feels pretty good about.

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Published first at 12:09 a.m. on the 19th July 2012

Not all mass derives from the Higgs!

July 15, 2012
Tevatron-Higgs-results-20120702-mr

An unspectacular representation of ‘the Higgs result’. Slightly more events of a particular kind have been seen when particles collide with a particular energy. Picture from DOE Tevatron.

My last course in particle physics at University represented the high water-mark of my understanding of the subject. But I was already struggling. So in the matter of the Higgs discovery my scientific qualifications, offer me only the slightest of advantages.

Fortunately the man who was responsible for what understanding I once possessed, David Bailin, responded to my plea for help:

As you say, the Higgs mechanism gives masses to all (truly elementary) particles, … but that certainly does not account for most of the mass of ordinary matter like us. Most of the mass of neutrons and protons derives from non-perturbative strong interaction effects in the gluons (and quark-anti-quark pairs) that bind the valence quarks in the nucleon.

The key words here are ‘truly elementary. As I understand this, the Higgs mechanism is responsible for all the mass of electrons, muons, and quarks. But it is not responsible for all – or even most – of the mass of composite particles such mesons, protons and neutrons (glossary here). I think this is an interesting subtlety to the more generally “Higgs gives mass” simple story that has been widely reported. This simple story is true in that the quarkquark interactions that give rise to most of the mass of ordinary matter would not have got started if the Higgs field had not given some initial mass.

The idea is that as the Universe cooled, the Higgs field first gave truly elementary particles their initial mass, and then later on these particles interacted and acquired the bulk of their additional mass as a consequence.

Is that clearer now?

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Full Quote from David Bailin

As you say, the Higgs mechanism gives masses to all (truly elementary) particles, except those like the photon, gluons (and graviton) that are protected by an unbroken gauge symmetry. But that certainly does not account for most of the mass of ordinary matter like us. Most of the mass of neutrons and protons derives from non-perturbative strong interaction effects in the gluons (and quark-anti-quark pairs) that bind the valence quarks in the nucleon. That’s what the lattice QCD people have spent so long and so much money trying to compute. The nucleon masses would hardly change even if there was no Higgs effect. Gravity is coupled to anything with a non-zero energy momentum tensor, and that certainly includes the “condensate” in the nucleons, as well as the Higgs field.

Bubbles

July 12, 2012
Gianluca Memoli views the world through a bubble. But then don't we all?

Gianluca Memoli views the world through a bubble. But then don’t we all?

Do you think of bubbles as objects of childish fascination? Beautiful, ephemeral, but ultimately peripheral to the main branches of science, life, and business? Well if you did think that, then that would be a BIG mistake. Because bubbles of one kind or another are absolutely central to your life.

Last weekend I got to help at the NPL stand at the Royal Society Summer Science Exhibition. It was a busy day and by the end of it my voice was hoarse. My reward was that I reminded myself – with a little help from my colleague Gianluca Memoli – just how ubiquitous and important bubbles are.

  • I remembered that all biological life consist of cells, which are nothing more than bubbles formed spontaneously when fats (lipids) are mixed with water. The miracle of life itself exists only inside a protective bubble!
  • I remembered returning a kettle to John Lewis because it was too noisy – noise created by the bubbles in the water.
  • I remembered that the golden sound of the sea upon the shore – Shhhhhh…….. Shhhhhhh…… is nothing more than the sound of bubbles collapsing.
  • I remembered that when we wait for bread to bake and reach its optimal state of scrumptious edibility – we are simply waiting for the bubbles inside it to reach the right size.
  • I remembered that I love champagne, a wine in which the value is increased ten-fold because of the presence of bubbles.
  • I remembered that I owe the value of my house the existence of a property bubble – wait a minute! That’s not Physics!

On the NPL stand Gianluca was explaining how sound – more generally ultrasound – could be used to create, move and manipulate bubbles. And if his research pans out as planned, he will be able to use the oscillations of micro-bubbles to measure the properties of fluids.

As I wandered back to Waterloo Station across the Hungerford Bridge I gazed across the bustling river at the hubbub of the South Bank. Reflecting on those things which rivers make one reflect upon, I realised that I was lost in nothing less than a …thought bubble. Bubbles really are everywhere: watch out for them!

Another thought on Higgs

July 9, 2012
Higgs boson: Proton-proton collisions as measured by Cern

Another incomprehensible image typically used to illustrate stories about the Higgs Boson. It shows ‘things’ shooting out from the point where two protons have been smashed together. Picture stolen from The Guardian

I had one more thought about the recent discovery of the Higgs particle: if the Higgs is the particle which gives ‘mass’ to all the other particles, then surely the nature of the Higgs must be linked to the nature of gravity?

As you may be aware, the concept of ‘mass’ enters our lives in two quite distinct ways: as inertial mass and as gravitational mass.

  • Inertial mass is the property of an object which makes it harder to speed up or slow down. This is encapsulated in Newton’s Second Law of Motion: that the amount of force required to achieve a given acceleration is related to the inertial mass of the object.
  • Gravitational mass is the property of an object which makes it attract other objects at a distance through space. This is encapsulated in Newton’s Law of Universal Gravitation: that all the matter in the universe attracts all the other matter with a force which is inversely proportional to the square of the distance between the objects, .

Einstein was fascinated by the simple observation that inertial and gravitational mass were – as well as can be measured – always exactly equal. From this insight he was inspired to derive his General Theory of Relativity which is based on the central tenet – the principle of equivalence – that these two types of mass are not in fact two distinct properties, but one single property.

When scientists say the Higgs particle is responsible for giving ‘mass’ to all the other particles, they mean inertial mass. But this will also have a gravitational effect. I have not seen any discussion of this feature in the news, but surely, if the Higgs particle gives rise to both types of mass, it must provide some kind of link between the two different manifestations of mass. Discovering any kind of connection at all between the Electroweak force, the Strong force and Gravity would really be a major step forward in our understanding of the Universe.

Or maybe I have completely missed the point?

Higgs: Smashing News!

July 6, 2012
Higgs Boson Picture

An incomprehensible image typically used to illustrate stories about the Higgs Boson. It shows ‘things’ shooting out from the point where two protons have been smashed together.

You don’t need me to tell you that scientists at CERN announced this week that they had found the elusive Higgs particle. This is the latest in a long line of significant discoveries that we have made by ‘smashing things together’.

  • First we smashed rocks together and made ‘sparks’, which are hot mixtures of dust and gas. The atoms within the gas can be disturbed enough to ‘wobble’ and give out light.
  • Then we created sustained ‘flames’ and exposed different substances to the bombardment from atoms in the flame. By analysing the ‘spectrum’ of light emitted we came to understand that there were many different types of  atoms.
  • Then we discovered the only common stable particle which is still – to the best of our knowledge – truly elementary: the electron.
  • Then we began smashing electrons into things, and discovered X-rays.
  • Then we discovered radioactive emanations and began to smash these ‘alpha, beta and gamma’ emanations into other pieces of matter.
  • Then we discovered the structure of atoms and the existence of the nuclei of atoms, and began to smash more and more things into nuclei, eventually breaking them apart.
  • Then we began to smash one bit of a nucleus into another bit.
  • Then we began to smash electrons into other bits of matter. And looking at the ‘spectrum’ of the debris, deduced that the particles we called protons and neutrons were made out of still smaller particles which (ré Joycing) we called quarks.
  • And now smashing the bundles of quarks that we call protons into each other, we have seen a bright peak in the ‘spectrum’ of debris emitted and inferred the existence of another particle of nature – the long-predicted Higgs particle.
We should all be pleased. Each household in the UK contributed around £3 per year to this activity and so the scientists have  really been acting on all our behalves. This discovery is certainly as much ours as it theirs. It is a jewel of knowledge, and a small particle of good news in a gas of gloomy stories. Smashing 🙂

Slinky Drop

July 4, 2012

David Beckham was not known for excellence at Physics. But his intuition about where a ball would go when he kicked it was astonishing. And though our skills are not quite so exceptional, we all rely on our intuition about the how the world is going to behave in unexpected situations.

But  sometimes our intuition fails us completely.

I recently experienced such a failure. As the video shows, if a slinky spring is suspended from one end, hanging under its own weight, and then the support is removed, the slinky falls in an unexpected (to me) manner. The top of the spring moves down, but initially the bottom of the spring does not move at all until the top collides with it.

Slinky

Illustration of the stages of falling of a ‘slinky spring’. The top falls first and the bottom remains completely motionless until the top hits it. This wasn’t what I expected.

I don’t intend to try to explain this to you. But I did spend some time trying to explain it to myself. I couldn’t think how to model a slinky spring, so instead I modelled a similar situation – a set of masses connected by springs, initially suspended and then dropped.

Slinky 2

A system of weights and springs. Initially the weights (1 kg) were separated by springs at their normal length (30 cm). When the system was suspended, each spring stretches by a different amount. When dropped, this system should display the same effect as the slinky but be easier to model.

I was able to write a short program to solve all the sums for how the springs and masses behaved, using Newton’s laws of motion thousands of times to see how the masses moved. I then worked out the extension of each spring over time and results are shown in two graphs below. The key understanding is that each mass responds only to gravity and the two springs above it and below it in the chain.

The first graph shows how the springs first stretched, with springs near the top extending more until they settle down, and then after 3 seconds the top spring is removed.

Slinky Graph 1

Graph showing the extension of each spring in the model versus time. The spring at the top extends more because it has to support the weight of the entire ‘chain’ of masses. The graph below shows in detail what happened in the red dotted section – after the top spring was ‘cut’.  Click for Larger Version

The graph below shows detail from the first graph.  When released, the top springs reduce their extension quickly, but the bottom springs didn’t change their extension at all until the the upper weights made them move. If one draws a line on the graph its slope corresponds to the speed of a ‘sound wave’ propagating down the chain of masses and springs.

Slinky Graph 2

Graph showing the extension of each spring in the model versus time after the top spring was ‘cut’.  Springs at the bottom are not affected until the disturbance reaches them. Click for Larger Version

So now I feel I understand what is happening. But each time I see the video, I am still amazed. You can see the beautiful video made by the ever enthusiastic Veritasium here. I hope it inspires you as much as it inspired me.
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The model I solved considered 10 x 1 kg masses separated by springs with a regular length of 30 cm and a spring constant of 500 newtons per metre. The expected speed of sound is approximately 6.7 metres per second, and the simulation gives a result close to this.

Overweight?

July 1, 2012
BMI Mortality

The upper curve shows the relationship between body mass index (BMI) and mortality expressed as relative risk of death compared to those with a BMI in the range 22.5 to 25. The data indicates quite robustly that being in the overweight category is ‘protective’. The data point at BMI=18 includes data for all people with lower BMI and the data point at BMI=35 includes data for all people with higher BMI. The lower curve shows the distribution of BMI in the population used for the study.

Friends, I am concerned about my weight. In particular I am concerned about whether being overweight (i.e. with a body mass index or (BMI) in the range 25 to 30) is genuinely bad for me, or whether it just makes me feel bad.

I have looked at this issue before and expressed my puzzlement at how ‘normal’ ever came to be defined as having a BMI in the range 20 to 25, when as far as I could tell, it has never coincided with the central range in the population.

My puzzlement appears to be vindicated by research which shows that the relative risk of death – mortality – is lower for people who are overweight compared with people in the ‘normal range’. These conclusions are backed up by other studies. But even so, it is important to understand how the research was done in order to appreciate what it really tells us.

The research followed 11,834 individuals in Canada from 1994/5 to 2006/7, and saw how mortality was affected by their BMI at the start of the study in 1994/95. Let me stress this. It saw how the BMI statistic in 1994/5 affected the rate at which they died in the subsequent 12 years. This large population included men and women, smokers and people who never smoked, and people in all age groups.

Could the ‘BMI effect’ have been protective in young people, but harmful in older people? This might make sense, since young people are less likely to die in any case. This might have masked the effect that I would have expected to see: that being overweight was harmful. The researchers controlled for that and looked at how the relative risk of death varied with BMI categories for various sub-populations within the group. Surprisingly – to me at least – the effect was seen in all categories.

BMI Mortality versus age

BMI Mortality versus age for different subpopulations within the study. The risk of death is relative to those with BMI in the ‘normal’ range. The lowest and highest BMI points include data for all individuals at lower or higher BMI. I have missed out the confidence indicators (error bars) because they make the graph too confusing.

Mortality and Morbidity. This report recorded the BMI of a population at a point in time, and studied correlations between the BMI and the rate at which people died in the following 12 years: This is called the risk of mortality. It did not record whether the individuals concerned became ill or unwell, and did not study how their BMI affected their chance of becoming unwell – that is called the risk of morbidity. I do not have data to hand but I would be pretty sure that being in the ‘overweight’ category (as defined by a BMI in the range 25 to 30) would be a significant risk factor for diseases such as cardio-vascular disease and type II diabetes.

This data was taken amongst a population which, for possibly the first time in human history, has enjoyed essentially unrestricted access to food for several generations. This is an astonishing cultural achievement. If the data really does fall in this way for other populations – including that in the UK – it will be very interesting to understand why that occurs. It will also be interesting to hear doctors argue that ‘chubby’ people like me should lose weight – something which will increase my risk of death!


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