Archive for the ‘Uncategorized’ Category

Error Bar: Update

April 17, 2018

Error Bar

Friends – I have found the author of the above image that I mentioned in my previous post

And I knew them all along! What are the chances of that!

The author is John Kennedy from the Meteorological Office who blogs under the pseudonym ‘diagram monkey’.

The story of the image’s creation can be found here.

It is based on a real similarly-named barthe Aero Bar in San Diego.

The Aero Club Bar

The Aero Club Bar, San Diego

The World of Diagram Monkey

John’s blog contains some wonderful resources.

Do check out his post to find out how he came close to a humiliating early death at the hands of an orange.




April 7, 2018


Back in December 2010 I wrote about Hans Rosling with a post titled:

Hans Rosling: You’re my hero

Sadly, Hans Rosling died in February 2017.

But aware of his imminent death, he worked with his son and daughter-in-law to write a book which captures some key elements of his world view, which he summarises as…


The book has two strands that run through all the chapters.

  • The first strand is that we are tremendously ignorant about the world. Repeatedly he expresses his surprise at our levels of ignorance. And given our access to facts, he asks why we are not just randomly ignorant, but have views which are  systematically wrong. He suggests it is a kind of cognitive bias.
  • The second strand is that we can stop being ignorant if we want to. And with this aim, he and his son invented delightful ways to view developmental data.

I won’t try to precis the book, but as someone who experiences intense anxiety in everyday situations, I found the section “Statistics as Therapy” particularly affecting.

As one specific example I went to the download page of Gapminder and downloaded a Powerpoint file with a graph showing how extreme poverty in the world has changed over time.

Extreme Poverty

It is clear that collectively humanity has made truly astonishing progress in reducing the awfulness of crushing poverty. [The Powerpoint file and the web site includes links so you can chase the data sources and check them.]

Why do we not celebrate this fantastic achievement?

This graph tells a good news story compared with which any news story we have experienced in the last 10 years is irrelevant.

This is a story of an epochal and positive change in the world of which most people – including myself – are largely ignorant.

The idea that the world – while acknowledging all its faults and injustices – is dramatically better than it is has ever been, feels like a balm against the ‘news-ification’ of reality that we experience.

Hans Rosling’s Death

I leave you with a video which I find intensely moving.

It shows Hans, his son,and his daughter-in-law explaining why they wrote the book.

Somehow knowing that he is no longer with us feels like a very intense – and despite the fact that I never knew him, personal – loss.



The fact that I feel Hans Rosling’s death personally is probably a cognitive bias caused by his many engaging video appearances.

This page on the Gapminder website contains links to many of his best videos. I cannot recommend them strongly enough.

SI at the RI

February 23, 2018


MdeP at the RI

On Monday 16th October 2017 I gave a talk at about the International System of Units (the SI) at the Royal Institution (the RI) in London.

It wasn’t a great talk, but it was at the RI. And I stood where Michael Faraday stood!

The RI have now processed the video and produced an edited version: enjoy 🙂

The RI have tended to retain the video of me talking rather than showing the animated PowerPoint slides. If you would like the full multimedia experience, you can download the presentation using the link below.

On the day

I was nervous and arrived ridiculously early with a couple of glass Dewars containing triple point of water cells.

I waited outside the lecture theatre for Martin Davies from the RI to arrive.

When he arrived, he noticed the Dewars and without hesitation he turned to the wall and pointed out the painting above where I was standing, and said:

“What a coincidence: your standing by a picture of Sir James Dewar lecturing in this theatre!”

Henry Dewar at the RI

It’s hard to convey the historical significance of Royal Institution without sounding trite. So I won’t try.

But it is a special place for chemists and physicists alike, and I feel honoured to have even had the chance to stand on that spot.


Thanks to Chris Brookes and Martin Davies for a memorable day.

Why is it so hard to lose weight?

February 21, 2018

After several years of looking, I think I have finally found the answer.

So if people follow a calorie-controlled diet based on government guidelines, almost everyone will put on weight.

Let me explain…

Mifflin St Jeor

The minimal basal metabolic requirements (BMR) of a human being have been the subject of scientific study for more than a century.

The best estimate of our requirements are the Mifflin St Jeor (MSJ) Equations which state that the calorific requirements for men and women are given by:

Men:     BMR = 10 × [Weight in kg] + 6.25 × [Height in cm] – 5 × [Age in years] – 5

Women:     BMR = 10 × [Weight in kg] + 6.25 × [Height in cm] – 5 × [Age in years] – 161

This is the amount of food (expressed as kiloCalories (kCal) per day) required to maintain a given weight and do nothing else: no exercise at all.

A sedentary male lifestyle

The MSJ equations are generally multiplied by a factor to reflect the amount of physical activity one undertakes during the day. And there is considerable uncertainty about which factor applies to any particular individual!

The factor 1.2 is commonly chosen to represent a “sedentary lifestyle”. In a moment I’ll come back to whether this factor is justified or not.

But based on this factor, the blue line on the graph below shows how the actual calorific requirements of a man of my weight and height vary with age. The equivalent graph for women is shown in the next section.

Calories versus Age

The most striking thing about this graph is that the actual amount of calories I need to maintain my weight (1860 kCal/day) is 25% less than the government recommend (2500 kCal/day).

The difference is not a rounding error – it amounts to 640 kCal/day which is a reasonably-sized meal!

A man of my age living a sedentary lifestyle and following government guidelines would put on weight at a rate of several kilograms per year.

The second striking feature of the graph is reduction in calorific requirements with age. The slope of the graphs is 50 kCal/day per decade.

This means that if I was maintaining my weight in my forties, then unless I changed either my eating habits or my exercise habits, I would slowly begin to put on weight.

Eating 50 kCal/day too much amounts to putting on weight at around 2 kg per year.

Is the sedentary lifestyle factor 1.2 appropriate?

One way to assess whether the factor 1.2 applied to the MSJ equations is appropriate is to consider the calorific equivalent of some exercise.

For a man of my weight and height, running 1 kilometre uses up about 74 kCal.

So if I were to run 25 km per week, then this would allow me to eat about another 260 kCal/day and still maintain my weight. This is shown as the red line on the graph above.

Most people would consider running 25 km per week to be quite serious exercise. Comparing this amount of exercise to the work done in a sedentary day makes me think that the factor 1.2 is probably about right.


The equivalent graph for women is shown below

Calories versus Age Women

It shows a similar disparity between government recommendations and actual metabolic requirements, but not quite as dramatically wrong as for men.

Government Guidelines

The reason I searched out the MSJ equations was because I know from experience that if I eat anything close to 2500 kCal per day I put on weight.

Calorific intake is notoriously difficult to estimate with an uncertainty better than about 10%,  but the MSJ figure of about 1860 kCal/day for a man of my age weight and height seems about right.

The UK Government guidelines are – frankly – nonsense, and given that the UK has something of a problem with obesity – not least with people of my age – it would seem a sensible first step to just get this simple factual message about right.

One important step would be to emphasise the reducing calorie requirements with age.

Government guides in the US such as this one are closer to reality, but if you want real information I recommend this helpful calculator.

Gravity: one more thing

January 28, 2018

I am a great admirer of James Clerk Maxwell.

And amongst his greatest achievements was the prediction that waves in electric and magnetic fields should travel at the speed of light.

He arrived at his prediction by considering the observed strength of static electric magnetic fields.

  • For example, studies had established the strength of the force from a given amount of electric charge at a given distance.
  • This electrical force was characterised by a constant called (for historical reasons) the permittivity of free space. It was given the symbol ε0 – the greek letter ‘epsilon’ with a subscript of zero. It was considered to represent in some way how ‘disturbed’ the space was around an electric charge.
  • Similarly, studies had established the strength of the magnetic force from a given electric current at a given distance.
  • This magnetic force was characterised by a constant called (for historical reasons) the permeability of free space. It was given the symbol μ0 – the greek letter ‘mu’ with a subscript of zero. It was considered to represent in some way how ‘disturbed’ the space was around an electric current.

Maxwell analysed these static experiments and predicted that there should be coupled waves in the electric and magnetic fields and that they would travel with a speed of:


And when Maxwell calculated this number he arrived at a number very close to the previously measured speed of light.

He observed that this was unlikely to be a coincidence and concluded that light was a wave in the electromagnetic field.

I can still remember how I felt when – aged 19 – I followed Maxwell’s footsteps and ‘discovered’ this connection: I was gob-struck!

Other waves

This type of formula is typical of expressions for the speed of waves. For example, the speed of a wave on a stretched wire or string is given by:


where T is the tension in the string and m is the mass per unit length of the string.  So a wave will travel quickly when the string is taut and low mass.

And in general we expect the speed of waves to reflect how the medium in which the waves travel responds to a disturbance.

Gravity waves

And that is why last years’ announcement (LIGO, Popular Report) that gravity waves travel at the speed of light is so profoundly important.

This discovery implies that there is a connection between:

  • electricity and magnetism – responsible for just about all the phenomena we experience around us – and…
  • gravity – which is associated with space and time and mass.

Alternatively, it could indicate a connection between them both and something else we don’t know about.

But the experimental fact of this connection astounds me as much if not more than the connection that Maxwell made.

And it makes me wonder just what he would have to say about the discovery.

Now I know this connection is not ‘new’: I can remember being told that gravity waves would travel at the speed of light many years ago.

But the discovery of the experimental fact of the speeds of light and gravity being equal seems to me to be more profound than the mere expectation that it should be so.

You can see more about the discovery in the LIGO video below


Perspectives on Gravity

January 9, 2018

Gravity is such a familiar force that its utterly mysterious nature can sometimes go unnoticed.

Looking at the picture of Earth and Moon bound together in the solitude of the Universe, it is strange to think that all that holds them together is this apparently weak force.

In this article I will do a couple of calculations using Newton’s law of Universal Gravitation. If you know the maths, please check my calculations, and if you don’t, please trust me.

Not so weak

Many people are familiar with the fact that the average gravitational field strength at the surface of the Earth is approximately 9.8 newtons of force for every kilogram of mass. This is sometimes called one ‘g‘.

(This is sometime expressed as 9.8 metres per second per second, but I don’t think that formulation is as clear in this context.)

But what is the gravitational field strength due to the Earth at the Moon? A simple calculation shows it to be just 0.0027 newtons per kilogram – about 0.02% of g.

And yet this weak field is sufficient to bind the Moon to the Earth with a force of 2 × 1020 newtons.

If gravity disappeared (!) and we applied that force to the Moon with a tensile steel cable, it would need to be 1000 km in diameter and would require about half the mass of the Earth to manufacture!

So weak

Many people are familiar with the fact that the tides on Earth are affected by the Moon.

We can work out the gravitational field strength on the side of the Earth nearest the Moon – where the Moon’s gravity opposes the Earth’s gravity: 9.8134727 newtons per kilogram.

Compare this with the gravitational field strength on the side of the Earth farthest from the Moon – where the Moon’s gravity acts with the Earth’s gravity: 9.8134749 newtons per kilogram.

The gravitational field strengths differ by just 0.2 parts in a million. And yet this difference is sufficient to affect the tides!

So very weak

Many people are familiar with the fact that the Earth is bound to the Sun by gravity. And that the Sun is bound to the Centre of the Milky Way Galaxy by gravity.

We can work out the gravitational field strength at the Earth due to the Sun. It is just 0.0059 newtons per kilogram or about 0.06% of g.

And the gravitational field strength at the Sun due to the Galaxy is a breathtakingly small 0.000000002135 newtons per kilogram or just 0.2 parts per billion of the gravitational field strength at the Earth’s surface.

And the lesson is?

There is no lesson here – it is just surprising to me how weak gravitational fields – billions of times weaker than the fields we are familiar with on Earth – can bind stars into galaxies. That’s all.

Good night.


January 8, 2018

Image courtesy fo NASA

The image above shows the Earth on the left and the Moon on the right.

It was acquired by a spacecraftOsiris Rex – a couple of months ago, 10 days after it had just been ‘slingshot’ into an orbit where it will eventually meet up with an asteroid.

The mission is fascinating: it will rendezvous with an asteroid, take a sample from it – and in September 2023, return it to Earth for analysis.

But for me, this picture is worth the project in itself. I find it haunting and surprising.

Most significant is the tiny fraction of the image taken up by the Earth and the Moon. I find it chilling to see this against the blackness of space.

Next is the sense of perspective. The figure below shows where the spacecraft was when it took the image.

Image courtesy of NASA

From my perspective on Earth, the Moon looms large, and its true distance is unimaginable.

In the image the Moon seems less significant than I would have expected, and yet it still drives our tides.


If I place in one hand my anxiety about work, an anxiety which poisons so much of my life.

And in my other hand I place this image of my home in the cosmos.

Then I feel sure that if I could just gain the right perspective, and balance these two realities, then my anxiety would seem smaller and less significant.

And if I could manage that, then the view from a spacecraft deep in space would have meaningfully changed life back here on Earth. Mmmmm.



It’s a shame…

August 2, 2017


Pictured above is the humble grave of James Clerk Maxwell.

By all accounts, he was a kind and humble man, and so in many ways it is an entirely appropriate memorial.

But simple as it is, surely we could show our respect and admiration by as simple an act as mowing the grass? It seems not.

My attention was drawn to the unkempt state of his grave by this article in the Scottish Daily Record.

In death we are all equal.

And I have no doubt that Maxwell himself would have wanted no fuss.

But some people – very few – have led such exceptional lives that it is appropriate for us to collectively mark their mortal remains in a way which shows how much we honour their achievements in life.

This is not an indicator of our belief in any kind of saintliness on their part.

It is rather a statement about us.

It is a statement about what we currently admire and treasure and celebrate.

I have been told that Ren Zhengfei, the founder and President of Huawei Technology visited the grave and was embarrassed and shocked.

To neglect the grave of such a monumental figure says something about us.

It is actually a matter of national shame. And while acknowledging that Maxwell was decidedly Scottish, I draw the boundaries of ‘nation-hood’ more widely.

So how great was James Clerk Maxwell?

Maxwell’s many contributions to our modern view of the world are difficult to summarise without being trite, and they span an enormous range. But here are two of his achievements concerning light.

The first colour photograph taken using Maxwell's prescription. (Credit: Wikipedia)

The first colour photograph taken using Maxwell’s prescription. (Credit: Wikipedia)

Having made a breakthrough understanding of the nature of human colour vision, he used that understanding to describe how to take the first colour photograph.


A picture from Wikipedia showing a young James Clerk-Maxwell at Trinity College, Cambridge. He is holding one of his colour wheels that he used to study colour vision.

Later he became the first person to appreciate that light was an electrical phenomenon.

And the equations he wrote down to describe the nature of light are still those we use today to describe just about all electrical and magnetic phenomena*.

Richard Feynman, the person who made the next step in our understanding of the light said:

“From a long view of the history of mankind — seen from, say, ten thousand years from now — there can be little doubt that the most significant event of the 19th century will be judged as Maxwell’s discovery of the laws of electrodynamics. The American Civil War will pale into provincial insignificance in comparison with this important scientific event of the same decade.”

And Michael de Podesta, the person writing this blog said:

“I named my son after him”

That a true hero should not be honoured in his own land, is a shame on us all.

Surely we could collectively manage to keep the grass on his grave tidy?


*Note for pedants: In fact the equations we use are a simplified form of Maxwell’s Equations devised by Oliver Heaviside after Maxwell’s tragic early death.

Work Experience

August 2, 2017

Film Crew


I had a work experience student with me last week. Let’s call him ‘William’.

On reflection, I am rather concerned about the impression that the “work” he witnessed might have on him.


Firstly, everything was very ‘bitty’: it was hard to concentrate on a single task for any period as long as a half day.

And in between explicit tasks, I spent a fair amount of time composing e-mails. That’s right, I said composing, not writing. Because e-mails are generally not simply ‘written’.

For despite the immediacy of the transmission, words in e-mails have to be chosen as carefully as words in a missive that might travel more slowly.

So even though I may appear to be sitting in front of a computer for an hour, I am in fact ‘composing’: plucking words from the vacuum of possibility, and then distilling the raw words to create clear and unambiguous text.

Anyway, I think that bit may have been a bit boring for him.


Secondly, although primarily temperature-related, it was extremely diverse.

One activity involved measuring the temperature of the air using our non-contact thermometer and hygrometer (NCTAH).

NCTAH in lab with notes

We set up the experiment in one of NPL’s ultra-stable temperature labs which we normally use for dimensional measurements.

The idea was to compare the temperature indicated by NCTAH with four conventional thermometers. However while NCTAH operated beautifully, it was the readings of the conventional sensors I couldn’t understand.

They indicated that objects in the room were hotter than the air in the room by as much as 0.3 °C. Unfortunately I was in a bit of a rush and I was bamboozled by this result. And I am still working on an answer. However I would have liked him to see something simple ‘just work’. Hey, ho.

And finally…

A film crew visited to interview me about the re-definition of the kelvin. They were charming and professional and genuinely interested in the subject.

They shot a long interview one afternoon, and then the next day they must have spent a good two hours filming me walking.

It wasn’t just walking. We spent a fair amount of time opening doors and then walking. Also walking and then opening doors.

Then it was time for a solid 30 minutes of emerging from corridors, and turning into corridors.

I am not sure what I made of the experience, and I am curious to see what the director Ed Watkins will make of the footage. But he and his colleagues seemed happy as they headed off to film at the PTB in Braunschweig, Germany.

And as for what ‘William’ made of it all, I haven’t a clue. It involved quite a lot of just ‘sitting’ and ‘keeping out of shot’.

But I guess he got to see how documentaries are constructed which might have been the most valuable experience of all.

Exactitude and Inexactitude

July 19, 2017

Exactitude and Inexactitude

After being a professional physicist for more than 30 years, I realised the other day that I write for a living.

Yes, I am a physicist, and I still carry out experiments, do calculations and write computer programs.

But at the end of all these activities, I usually end up writing something: a scientific paper; a report; some notes for myself; or a blog article like this.

But although the final ‘output’ of most of what I do is a written communication of some description, nobody ever taught me to write.

I learned to write by reading what I had written. And being appalled.

Appalled by missed words and typographic errors, and by mangled ideas and inappropriate assumptions of familiarity with the subject matter.

Learning to write is a difficult, painful and never-ending process.

And over and over again I am torn between exactitude – which I seek – and inexactitude, which I have learned to tolerate for two reasons.

  • Firstly, a perfect article which is never completed communicates nothing. Lesson one for writing is that finishing is essential.
  • Secondly, an article which has all the appropriate details will be too long and may never be read by the people with whom I seek to communicate.

So in order communicate optimally, I need to find the appropriate tension between the competing forces of exactitude and inexactitude.

This blog 

When I write for this blog, I try to write articles that are about 500 words long. I rarely succeed.

Typically, I write something. Read it. And then add explanatory text either at the start or at the end?

But with each extra word I type, I realise that fewer and fewer people will read the article and appreciate the clarity of my writing.

And I have to acknowledge that if I had written fewer words I might have communicated something to more people.

Or even communicated more by omitting detail people might find obfuscatory

Indeed I have to acknowledge – and this is hard – that I could have even written something erroneous and communicated something to more people.

For example

For example, in the previous article on the GEO600 Gravity Wave detector, I said that “moving a mirror by half a wavelength of light caused the interferometer to change from constructive to destructive interference.”

Now I know what you are thinking: and yes, it only has to move by a quarter of a wavelength of light.

I realised this before I finished the article but it had already taken hours, and I had already recorded the narrative to the movie.

Similarly, my animation showed one of the reflections coming from the wrong side of a piece of glass (!), and it omitted the normal ‘compensator’ plate in the interferometer.

And how many people noticed or complained? None so far.

So the article was published and presumably communicated something, inexactly and slightly incorrectly. And it was not wholly erroneous.

Exactitude and Inexactitude.

Exactitude and Inexactitude are like two mis-matched protagonists in a ‘buddy movie’.

At the start they hate each other, but over the course of ‘a journey’ in which they are compelled to accompany one another, they learn to love each other for what they are, and to accept each other for what they are not.

Inexactitude: You drive me crazy, but I love you.

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