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.

Perspective

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.

So…

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.

 

 

My Weight: Good News or Bad News?

January 5, 2018

I spent most of 2017 feeling bad about my weight.

But as I review the data now I realise I can say positive things about my weight which – if I could believe my own words – would leaving me feeling good about my weight.

Alternatively, I could say negative things which would leave me feeling bad about my weight.

In either case the data would be the same. So which view should I take?

The data

The graph below shows my weight determined first thing in the morning for almost every day throughout the last two years.

Weight 2016-2017

2016 was a good year. I lost about 14 kg and felt enormously better. And in 2017 I initially managed to lose another couple of kilos,

Weight 2017

But then, as work became a nightmare, my weight drifted back up at just 10 g per day – a weight gain equivalent to a biscuit or a glass of wine per day.

By the end of the year I managed to catch my breath enough to slowly lose some weight. And that is pretty much where I am at now.

Postive or Negative?

So how should I feel about this data? What story should I tell myself?

Should I say:

“Well done! You managed to keep control of your weight through a difficult year. And your weight is only 1 kg more than it was at the start of 2017. And still 13 kg lower than it was at the start of 2016”?

Or should I say:

“What a mess! You put on 3 kg through the year!

In either case, the data are the same. So actually I think I will just keep the graphs and refrain from telling myself any story at all. Indeed, the real storyline won’t become clear until I find out what happens next.

I will keep you informed.

2018: Here we come!

January 4, 2018

Top Cat Guide to Life

2017 was a difficult year for me, by far the worst of my 17 years at NPL.

But after a break in which I have managed to breathe, become 58 years old, spend time with my family, and read some books, I have resolved to try to focus on the positive.

At first I thought I would follow advice from the  ‘indisputable leader of the gang’, Mr. Top Cat, from New York City, who has published his ‘Guide to Life‘.

But in fact after reading Mr. Cat’s advice, I can inform you that it is not so much feline, as asinine.

However, his positivity and self-assuredness are enviable, if somewhat (respectively) mysterious and misplaced.

My plan is to reflect on the positive – of which there is plenty – while not ignoring the negative. It’s a tough balance, but nobody said life would be easy.

In any case, if you are reading this, I wish you all the best for the remaining 98.9% of 2018.

 

Candles at Christmas Revisited

December 21, 2017

Last week I gave a presentation at NPL on the physics of candles.

Above is a video of the slightly chaotic 32 minute presentation and if you are so inclined, you can download the PowerPoint file here.

I have spoken before about the wonderful physics of candles. But in revisiting the subject I learned that wax is basically not flammable and I felt obliged to mention this inconvenient truth!

The highlight of the talk is using a candle to power a thermo-electric generator, which in turn powers a USB port, which in turn powers a torch, which is brighter than the candle.

nikita-pic.jpg

Sadly, in the rush at the end of the talk I forgot to actually measure the brightness of the torch!

Next time!

And by the way, here is the slow-motion candle-relighting movie that is embedded in the PowerPoint but which doesn’t show up well in the lecture theatre view.

Thanks to Brian Madzima for the videography and editing, and Nikita Mezhnyakov for the photograph.

Impact

December 3, 2017

I can collect my state retirement pension in just 97 months.

The closeness of this date – and its week-by-week countability – is a great comfort to me when I feel under pressure at work.

And one abiding pressure is the requirement that I personally create ‘impact’ from my work.

I had cause to reflect on this when I visited the Royal Society in London last week.

The Royal Society

The Royal Society has a lot of marble and in one portion of that marble is carved its admirable motto.

Nullius in verba

or

Take nobody’s word for it

This is a reflection of the belief of the founding fellows in the primacy of experimental results over beliefs.

In short, it is a declaration that science has to deal with the world as it is, and not as we would wish it to be.

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The Royal Society also has the best door handles I ever seen.

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Evoking the double-strands of DNA, I was told the door-handles had to be re-made because the first batch had the wrong chirality!

And just in case you were in doubt about its prestige, its walls are lined with portraits of its past presidents: Newton, Boyle, Darwin, Kelvin, Rutherford, and…

Hans Sloane

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Hans Sloane was the president of the Royal Society who followed Sir Isaac Newton: a tough act to follow.

But in terms of ‘impact’ I think he may even have exceeded Newton. History records three great achievements.

  • He founded the Chelsea Physic Garden for the study of plants from all over the world. Mmmm. Not bad.
  • He donated his collection of antiquities to found the British Museum. Mmmm. Impressive.
  • But finally, as recorded on a small label underneath his portrait, is by far his greatest achievement: he invented Milk Chocolate!

Citing Nullius in verba, I am disinclined to believe that he really invented Milk Chocolate. I suspect he re-invented it or modified a previous recipe.

But as I imagine him filling out his annual appraisal form, I feel sure that in the ‘impact’ section, he would have mentioned the popularisation of chocolate as his most significant achievement.

And since his death in 1753, literally billions of human beings have experienced momentary pleasure, or relief from anxiety, by simply eating a small amount of chocolate.

In all of history, has humanity ever had a greater benefactor? 

 

 

How do we know anything?

November 18, 2017

How do we know Anything MdeP-NPL

This is an edited video of a talk I gave recently to the NPL Postgraduate Institute about the forthcoming changes to the International System of Units, the SI.

It’s 36 minutes long and you can download the PowerPoint slides here.

It features the first ever public display of the Standard Michael – the artefact defining length in the SM, le systeme de moi, or My System of Units. (It’s shown at about 6 minutes 40 seconds into the video).

The central thesis of the talk is summarised in the slide below:

Measurement

In the talk I explain how the forthcoming changes to the SI will improve future measurements.

I hope you enjoy it.

 

Before understanding comes familiarity

November 14, 2017
IMG_6178.jpg

Averil Horton

It is tough being an adult. Hey. We all know that.

But is especially tough if you realise as an adult that science fascinates you. There are relatively few places where you can go and learn about science  without being condescended to, or treated like a child.

I tried to create such an environment when I ran Protons for Breakfast and my friend Averil Horton is now trialling her ‘Science Club’ with adults.

I am attending as a helper – which is delightfully low stress compared to running a class!

And the key insight of which I have been reminded repeatedly is that experience has to come to before understanding. And for adults, just gaining exposure to the experiential pleasure of hands-on experimentation is so difficult!

I won’t describe the classes in detail, but below I will just post a couple of pictures showing the kinds of things people do. And with the exception of a couple of potentially dangerous things – everyone does everything!

Cutting Potassium…

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Chromatography…

Colours

Growing Silver…

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Burning Magnesium…

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Experiments with density…

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Burning hydrocarbons…

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And we are not even half way through!

Measuring Temperature with Sound

November 12, 2017

Measurement

I have just given the first of a series of five talks for The Training Partnership, a company that provide ‘enrichment’ days for A level students.

Since one of my key messages is the importance of measurement in science, I feel obliged to perform some measurements during the presentation.

I find this worrisome, but I think it works well. When it works!

Anyway, with four more presentations to go I thought I would create a page with links to all the the resources I use in the talk.

PowerPoint

The PowerPoint presentation can be found here. Please feel free to steal animations if you think they will be helpful, but please give credit to NPL.

Software

During the presentation I use:

  • Audacity for capturing acoustic wave-forms and analysing them: it is astonishing software, and completely free.
  • Sound Card Oscilloscope for detecting the resonance within the spherical resonator: it is excellent and free for educational users. It also comes with a built in oscillator, but for the demo it is much clearer if I use a separate device. So I use…
  • Signal Generator, an app for IOS devices. There many others for both IOS and Android but this one is fine and costs £0.99.

And this is the spreadsheet I use to interpret the results from the experiment.

Hardware

In my talk I use the same microphone for all the demonstrations, a commercial lapel microphone from RS Components (RS Stock No.242-8911which costs about £20. Similar devices are available from other suppliers.

I chose this particular model because it more robust than home-made contraptions and has a small head – so it fits inside tubes. Larger microphones will work but they tend to damp acoustic waves more strongly.

I hold it in place with a blob of Blutac.

The miniature loudspeaker I use for the resonator demo is quite specialised. It is from a range of products used in headphones, mobile phones and hearing aids produced by the Knowles corporation.

think the model I used is  from this ‘BK’ series. It requires wires to be soldered onto very tiny terminals, and then wired to a 4 mm jack plug that can connect to a mobile phone.

One alternative would be to dismantle a pair of in-ear headphones and just use the loudspeaker from one earphone.

Tube and Resonator

The metal tube I use in the talk is 1.1 metre long stainless steel tube approximately 9.5 mm diameter. You can also use many other types of tubing such as copper or steel plumbing tube.

In general, longer is better for more accurate measurements at room temperature, but it is obviously more difficult to heat it uniformly.

The resonator is a 3-D printed version of the copper resonator we used to measure the Boltzmann constant and make the most accurate temperature measurements in history.

I have placed the 3-D printing files in a zipped folder hereThere are files for the Northern Hemisphere, the Southern Hemisphere, and the plugs. Creating the resonator is quite complicated and I will write a separate blog post on that later.

Good luck!

 

The Past, Present and Future of Measurement

October 22, 2017

Measurement, the simple process of comparing an unknown quantity with a standard quantity, is the essential component of all scientific endeavours. We are currently about to enter a new epoch of metrology, one which will permit the breath-taking progress of the last hundred years to continue unimpeded into the next century and beyond.

The dawn of this new age has been heralded this week by the publication of an apparently innocuous paper in the journal Metrologia. The paper is entitled:

Data and Analysis for the CODATA 2017 Special Fundamental Constants Adjustment

and its authors, Peter Mohr, David Newell, Barry Taylor and Eite Tiesinga constitute the Committee on Data for Science and Technology, commonly referred to as CODATA. In this article I will try to explain the relevance of CODATA’s paper to developments in the science of metrology.

The Past

The way human beings began to make sense of their environment was by measuring it. We can imagine that our agrarian ancestors might have wondered whether crops were taller or heavier this year than last. Or whether plants grew better in one field rather than another. And they would have answered these questions by creating standard weights and measuring rods.

But to effectively communicate their findings, the standard units of measurement would need to be shared. First between villages, and then towns, and then counties and kingdoms. Eventually entire empires would share a system of measurement.

First units of weight and length were shared. Then, as time became more critical for scientific and technical endeavours, units of time were added to systems of the measurement. And these three quantities: mass, length and time, are shared by all systems of units.

These quantities formed the so-called ‘base units’ of a system of measurement. Many other quantities could be described in terms of these ‘base units’. For example, speeds would be described in multiples of [the base unit of length] divided by [the base unit of time]. They might be [feet] per [second] in one system, or [metres] per [second] in another.

Over the last few hundred years, the consistent improvement in measurement techniques has enabled measurements with reduced uncertainty. And since no measurement can ever have a lower uncertainty that the standard quantity in that system of units, there has been a persistent drive to have the most stable, most accurately-known standards, so that they do not form a barrier to improved measurements.

The Present

Presently, all scientific and technical measurements on Earth are made using the International System of Units, the SI. The naming of this system – as an explicitly international system – represented a profound change in conception. It is not an ‘imperial’ system or an ‘English’ system, but a shared enterprise administered by the International Bureau of Weights and Measures (BIPM), a laboratory located in diplomatically-protected land in Sèvres, near Paris, France. Its operation is internationally funded by the dozens of nations who have signed the international treaty known as the Convention of the Metre.

In essence, the SI is humanity’s standard way of giving quantitative descriptions of the world around us. It is really an annex to all human languages, allowing all nationalities and cultures to communicate unambiguously in the realms of science and engineering.

Founded in 1960, the SI was based upon the system of measurement using the metre as the unit of length, the kilogram as the unit of mass, and the second as the unit of time. It also included three more base units.

The kelvin and degree Celsius were adopted as units of temperature, and the ampere was adopted as the unit of electric current. The candela was defined as the unit of luminous efficacy – or how bright lights of different colours appear to human beings. And then in 1971 the often qualitative science of chemistry was included in the fold with the introduction of the mole as a unit of amount of substance, a recognition of the increasing importance of analytical measurements.

SI Circle - no constants

The SI is administered by committees of international experts that seek to make sure that the system evolves to meet humanity’s changing needs. Typically these changes are minor and technical, but in 1984 an important conceptual change was made.

Since the foundation of the SI, the ability to measure time had improved more rapidly than the ability to measure length. It was realised that if the metre was defined differently, then length measurements could be improved.

The change proposed was to define what we mean by ‘one metre’ in terms of the distance travelled by light, in a vacuum, in a fixed time. Based on Einstein’s insights, the speed of light in a vacuum, c, is thought to be a universal constant, but at the time it had to be measured in terms metres and seconds i.e. human-scale measurement standards. This proposal defined a metre in terms of a natural constant – something we believe is truly constant.

The re-definition went well, and set metrologists thinking about whether the change could be adopted more widely.

The Future

Typically every four years, CODATA examine the latest measurements of natural constants, and propose the latest best estimate of the values of a range of natural constants.

Measurement Graphic

This is a strange. We believe that the natural constants are really constant, not having changed measurably since the first few seconds of our universe’s existence. Whereas our human standards are at most a few decades old, and (as with all human standards) are subject to slow changes. Surely, it would make more sense, to base our measurement standards on these fundamental constants of the natural world? This insight is at the heart of the changes which are about to take place. The CODATA publication this week is the latest in a series of planned steps that will bring about this change on 20th May 2019.

Constants Graphic

After years of work by hundreds of scientists, the values of the natural constants recommended by the CODATA committee will be fixed – and will form the basis for the new definitions of the seven SI base units.

What will happen on 20th May 2019?

On the 20th May 2019, revised definitions of four of the base units of the SI will come into force. More than 10 years of careful measurements by scientists world-wide will ensure that the new definitions are, as closely as possible, equivalent to the old definitions.

The change is equivalent to removing the foundations underneath a structure and then inserting new foundations which should leave the structure supported in exactly the same way. However the new foundations – being based on natural constants rather than human artefacts – should be much more stable than the old foundations.

If the past is any guide to the future, then in the coming decades and centuries, we can anticipate that measurement technology will improve dramatically. However we cannot anticipate exactly how and where these improvements will take place. By building the SI on foundations based on the natural constants, we are ensuring that the definitions of the unit quantities of the SI will place no restriction whatever on these future possible improvements.

The kilogram

The kilogram is the SI unit of mass. It is currently defined as the mass of the International Prototype of the Kilogram (IPK), a cylinder of platinum-iridium alloy held in a safe at the BIPM. Almost every weighing in the world is, indirectly, a comparison against the mass of this artefact.

On 20th May 2019, this will change. Instead, the kilogram will be defined in terms of a combination of fundamental constants including the Planck constant, h, and the speed of light, c. Although more abstract than the current definition, the new definition is thought to be at least one million times more stable.

The new definition will enable a new kind of weighing technology called a Kibble balance. Instead of balancing the weight of a mass against another object whose mass is known by comparison with the IPK, the weight will be balanced by a force which is calculable in terms of electrical power, and which can be expressed as a multiple of the fundamental constants e, h and c.

The ampere

The ampere is the SI unit of electrical current. It is presently defined in terms of the current which, if it flowed in two infinitely thin, infinitely long, parallel wires would (in vacuum) produce a specified force between the wires. This definition, arcane even by metrologists’ standards, was intended to allow the measurement of the ampere in terms of the force between carefully constructed coils of wire. Sadly, it was out of date shortly after it was implemented.

On 20th May 2019, this will change. Instead, the ampere will be defined in terms of a particular number of electrons per second, each with an exactly specified electrical charge e, flowing past a point on a wire. This definition finally corresponds to the way electric current is described in textbooks.

The new definition will give impetus to techniques which create known electrical currents by using electrical devices which can output an exactly countable number of electrons per second. At the moment these devices are limited to approximately 1 billion (a thousand million) electrons per second, but in future this is likely to increase substantially.

The kelvin

The kelvin is the SI unit of temperature. It is currently defined as the temperature of the ‘triple point of water’. This temperature – at which liquid water, solid water (ice) and water vapour (but no air) co-exist in equilibrium – is defined to be 273.16 kelvin exactly. Glass cells re-creating this conjunction are located in every temperature calibration lab in the world, and every temperature measurement is a comparison of how much hotter a temperature is than the temperature at one position within a ‘triple point of water cell’.

On 20th May 2019, this will change. Instead, the kelvin will be defined in terms of a particular amount of energy per molecule as specified by the Boltzmann constant, kB. This definition finally corresponds to the way thermal energy is described in textbooks.

The requirement to compare every measurement of temperature with the temperature of the triple point of water adds uncertainty to measurements at extremely low temperatures (below about 20 K) and at high temperatures (above about 1300 K). The new definition will immediately allow small improvements in these measurement ranges, and further improvements are expected to follow.

The definition of the degree Celsius (°C) in terms the kelvin will remain unchanged.

The mole

The mole is the SI unit of ‘amount of substance’. It is currently defined as the amount of substance which contains the same number of ‘elementary entities’ as there are atoms in 12 grams of carbon-12. The change in the definition of the kilogram required a re-think of this definition.

On 20th May 2019, it will change. The mole will be defined as the amount of substance which contains a particular, exactly specified, number of elementary entities. This number – known as the Avogadro number, NA – is currently estimated experimentally, but in future it will have fixed value.

The specification of an exact number of entities effectively links the masses of microscopic entities such as atoms and molecules to the new definition of the kilogram.

The ‘New’ SI

On 20th May 2019 four of the seven base units of the SI will be re-defined. But what of the other three?

The second is already defined in terms of the natural frequency of microwaves emitted by atoms of a particular caesium isotope. The metre is defined in terms of the second and the speed of light in vacuum – a natural constant. And the candela is defined in terms of Kcd, the only natural constant in the SI that relates to human beings. So from 20th May 2019 the entire SI will be defined in terms of natural constants.

SI Circle - with constants

The SI is not perfect. And it will not be perfect even after the redefinitions come into force. This is because it is a system devised by human beings, for human beings. But by incorporating natural constants into the definitions of all its base units, the SI has taken a profound step towards being a system of measurement which will enable ever greater precision in metrology.

And who knows what features of the Universe these improved measurements will reveal.


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