Archive for the ‘Satellites’ Category

The James Webb Space Telescope

May 10, 2018

Last week I was on holiday in Southern California. Lucky me.

Lucky me indeed. During my visit I had – by extreme good fortune – the opportunity to meet with Jon Arenberg – former engineering director of the James Webb Space Telescope (JWST).

And by even more extreme good fortune I had the opportunity to speak with him while overlooking the JWST itself – held upright in a clean room at the Northrop Grumman campus in Redondo Beach, California.

[Sadly, photography was not allowed, so I will have to paint you a picture in words and use some stock images.]


In case you don’t know, the JWST will be the successor to the Hubble Space Telescope (HST), and has been designed to exceed the operational performance of the HST in two key areas.

  • Firstly, it is designed to gather more light than the HST. This will allow the JWST to see very faint objects.
  • Secondly, it is designed to work better with infrared light than the HST. This will allow the JWST to see objects whose light has been extremely red-shifted from the visible.

A full-size model of the JWST is shown below and it is clear that the design is extraordinary, and at first sight, rather odd-looking. But the structure – and much else besides – is driven by these two requirements.

JWST and people

Requirement#1: Gather more light.

To gather more light, the main light-gathering mirror in the JWST is 6.5 metres across rather than just 2.5 metres in the HST. That means it gathers around 7 times more light than the HST and so can see fainter objects and produce sharper images.


Image courtesy of Wikipedia

But in order to launch a mirror this size from Earth on a rocket, it is necessary to use a  mirror which can be folded for launch. This is why the mirror is made in hexagonal segments.

To cope with the alignment requirements of a folding mirror, the mirror segments have actuators to enable fine-tuning of the shape of the mirror.

To reduce the weight of such a large mirror it had to be made of beryllium – a highly toxic metal which is difficult to machine. It is however 30% less dense than aluminium and also has a much lower coefficient of thermal expansion.

The ‘deployment’ or ‘unfolding’ sequence of the JWST is shown below.

Requirement#2: Improved imaging of infrared light.

The wavelength of visible light varies from roughly 0.000 4 mm for light which elicits the sensation we call violet, to 0.000 7 mm for light which elicits the sensation we call red.

Light with a wavelength longer than 0.000 7 mm does not elicit any visible sensation in humans and is called ‘infrared’ light.

Imaging so-called ‘near’ infrared light (with wavelengths from 0.000 7 mm to 0.005 mm) is relatively easy.

Hubble can ‘see’ at wavelengths as long as 0.002 5 mm. To achieve this, the detector in HST was cooled. But to work at longer wavelengths the entire telescope needs to be cold.

This is because every object emits infrared light and the amount of infrared light it emits is related to its temperature. So a warm telescope ‘glows’ and offers no chance to image dim infrared light from the edge of the universe!

The JWST is designed to ‘see’ at wavelengths as long as 0.029 mm – 10 times longer wavelengths than the HST – and that means that typically the telescope needs to be on the order of 10 times colder.

To cool the entire telescope requires a breathtaking – but logical – design. There were two parts to the solution.

  • The first part involved the design of the satellite itself.
  • The second part involved the positioning the satellite.

Cooling the telescope part#1: design

The telescope and detectors were separated from the rest of the satellite that contains elements such as the thrusters, cryo-coolers, data transmission equipment and solar cells. These parts need to be warm to operate correctly.

The telescope is separated from the ‘operational’ part of the satellite with a sun-shield roughly the size of tennis court. When shielded from the Sun, the telescope is exposed to the chilly universe, and cooled gas from the cryo-coolers cools some of the detectors to just a few degrees above absolute zero.

Cooling the telescope part#2: location

The HST is only 300 miles or so from Earth, and orbits every 97 minutes. It travels in-to and out-of full sunshine on each orbit. This type of orbit is not compatible with keeping a gigantic telescope cold.

So the second part of the cooling strategy is to position the JWST approximately 1 million miles from Earth at a location known as the second Lagrange point L2.

At L2 the gravitational attraction of the Sun is approximately 30 times greater than the gravitational attraction of the Earth and Moon.

At L2 the satellite orbits the Sun in a period of one year – and so stays in the same position relative to the Earth.

  • The advantage of orbiting at L2 is that the satellite can maintain the same orientation with respect to the Sun for long periods. And so the sun-shade can shield the telescope very effectively, allowing it to stay cool.
  • The disadvantage of orbiting at L2 is that it is beyond the orbit of the moon and no manned space-craft has ever travelled so far from Earth. So once launched, there is absolutely no possibility of a rescue mission.

The most expensive object on Earth?

I love the concept of the JWST. At an estimated cost of $8 billion, if this is not the most expensive single object on Earth, then I would be interested to know what is.

But it has not been created to make money or as an act of aggression.

Instead, it has been created to answer the simple question

I wonder what we would see if we looked into deep space at infrared wavelengths.”. 

Ultimately, we just don’t know until we look.

In a year or two, engineers will place the JWST on top of an Ariane rocket and fire it into space. And the most expensive object on Earth will then – hopefully – become the most expensive object in space.

Personally I find the mere existence of such an enterprise a bastion of hope in a world full of worry.


Many thanks to Jon Arenberg  and Stephanie Sandor-Leahy for the opportunity to see this apogee of science and engineering.


Breathtaking photographs are available in galleries linked to from this page


SI Superheroes

January 12, 2016

Somehow this episode of SI Superheroes came out last May (2015) and I didn’t notice!

If anything, this is even better than the first episode – perhaps because it’s more focussed on a single theme without the need to introduce all the characters.

In case you are unfamiliar with the work of NIST, the US National Institute for Standards and Technology, they are basically the US version of NPL and are a very serious organisation. In my recollection, this is only the second output from NIST that has featured laugh-out-loud moments (which I will not reveal!).

I can foresee great things for these characters.

Remember that Superman, Batman and their friends and foes inhabited a (DC) universe of paper comics for decades.

Then they became TV cartoon characters.

And only relatively recently have they become the stars of the current genre of all action, computer-graphic laden movies.

I wonder if they will be recruiting for a male with slightly older looks to play Dr. Kelvin…


Incidentally, the number 9,192,631,770 displayed on the side of the cartoon satellite is the number of oscillations a Caesium atom that defines what we mean by the passage of one second.

At places like NPL and NIST we can make clocks based on Caesium atoms that very perfectly realise this definition.

The atoms in these super-clocks vibrate at  9,192,631,770.000 000 ± 0.000 001 oscillations per second and form the basis of Universal Coordinated Time (UTC)  that is used throughout the world.

One of the difficulties which Major Uncertainty may have tried to exploit is that the number of oscillations per second changes very slightly with changes in the physical environment of the atom.

Some of the environmental parameters that matter for clocks mounted in space are:

  • the strength of the gravitational field,
  • any accelerations that the atom experiences,
  • the  speed of the clock with respect to the person (often on the ground)  counting the oscillations,
  • the temperature of the walls surrounding the atoms.

Anyway – all is well now that the League of SI Superheroes has done their job again.

How Apollo Flew to the Moon

September 6, 2015
The Moon photographed above some beach grass in Northumberland

The Moon photographed above some beach grass in Northumberland

On my recent holiday in Northumberland, I both photographed the moon, and read about how almost 50 years ago, human beings landed on its surface.

This article is a review of the book I read: ‘How Apollo Flew to the Moon‘ by W. David Woods.

Staring at the moon and considering what we now know about its distance from Earth, its size, and its inhospitable surface, is an exercise in bridging emotional and intellectual understanding.

I have long-considered that the Apollo programme of manned spaceflights to the Moon to have been an exemplar of the power of human intellect, and overall one of humanity’s exceptional achievements.

The enormous cost of the programme (4% of the US Federal budget in 1967) was – in my opinion – well justified by the cultural shift it engendered.

We went to the Moon and discovered the Earth‘ is a truth expressed by many, including several of the early astronauts.

However this book is not about the cultural impact of the programme, but about how the journey was made. For anyone with a technical disposition the book will fascinate.

I took all 500 pages of the book on holiday with me and self-indulgently read it slowly from cover to cover: it was enormously enjoyable.

After an overview, the book follows the Apollo 11 mission through all its stages, sprinkling in astronaut comments and explaining the differences between earlier and later missions.

There are many fascinating details, but what came through to me above everything was NASA’s pervasive mindset of constantly, painstakingly, meticulously and expensively planning for failure.

The philosophy of not just being aware that an operation may fail, but making detailed plans for what you will do when it does is a lesson for anyone who wants a complex plan to succeed.

And not only were there back-up plans for failure, there were plans for failure of the back-up plans! Only at one or two key points in the entire mission were there operations which simply had to work.

So, for example, when their spacecraft fired a rocket engine to leave Earth’s orbit and head towards the Moon –  or rather where the Moon was going to be in three days time – the rocket burn placed them into a so-called ‘free-return trajectory‘.

Thus if something went wrong on the voyage, or the rocket engine failed to fire – the spacecraft would sail around the Moon and head straight back to Earth.

When launched towards the Moon, the Apollo spacecraft was placed in a “Circumlunar-free-return-trajectory” . This meant that unless they did something positive to enter the Moon’s orbit, they would return to the Earth. Picture by NickFr Licensed under Public Domain via Wikipedia

Overall, the book is a great read for the technically minded. And in addition to the narrative there are occasional superlatives – like ‘vista-points’ on a highway – where you can stop and simply wonder.

  • The total mechanical output power of five first stage rockets was 60 GW. This is equivalent to peak electrical supply of the entire United Kingdom.
  • On its return from the moon, its speed just before entry into the Earth’s atmosphere was more than 11 kilometres per second.
  • Since Apollo 17 returned in 1972. no human being has been more than 500 miles from Earth’s surface.

Sun Spotted in Teddington

January 9, 2014
A picture of the Sun taken on 9th January 2014 from Teddington UK

A picture of the Sun taken on 9th January 2014 from Teddington UK. Click for more detail. Image is courtesy of Peter Woolliams 2014.

The Sun – as you know well – shines night and day, but direct visibility of the Sun has been in short supply in Teddington these last two weeks.

But at break this morning I caught two of NPL’s astronomical gurus sipping lemon tea and enjoying the sunshine streaming through the windows. I sat with them and it felt so good.

We chatted about telescopes and things, and I must have said something amazing because later in the day Peter Woolliams sent me an e-mail with the picture above.

“Michael, inspired by you (and the unusual presence of the sun) I dashed back home at lunchtime to grab the first bit of solar image data for a few months, just caught the sun before the clouds rolled in… and the netbook battery expired…Rapid processing, see attached….”

I was astonished. First of all at the beauty of the image and second at the rapidity with which Peter had got to work. Another colleague joined in a discussion and she sent a me a link to the astonishing Helioviewer site.

At Helioviewer you can look at satellite images of the Sun for any particular moment in the last few years and create your own movies such as the one below. The movie below shows 6 hours of images (not much happens) but it is astounding nonetheless – especially when you notice the scale image of the Earth in the lower left corner.

And so I found myself wondering which image to be more moved by: the breathtaking Helioviewer with its movies and whizzy interface, or Peter’s astonishing image – surely the most astounding image produced in Teddington today.

Between the two I would vote for Peter’s, but the winning image of the day is one I can’t share with you: it exists only in my mind.

Inspired by Peter and Andrea Sella I looked out the skylight at Jupiter. Using first binoculars, then a small telescope, and then finally the telescope I bought for Maxwell I saw Jupiter’s disc and its 4 Galilean moons perfectly arranged in a line.

Seeing the image of Jupiter and its satellites which had astonished Galileo, and provided crucial evidence for Newton’s theory of Universal Gravitation, I felt in touch not only with vastness of the universe, not only with my family who I dragged upstairs to see it, but with history too.


How did Peter create the image above?

“Images taken during my lunchbreak (just before the sun vanished behind cloud where it has been for the past few months!). 80mm William Optics refractor with Lunt B600 CaK filter onto a DMK41 mono camera. 600frame stacks processed in Autostakkert, post processed in Registax6 (wavelets and gamma stretch), Microsoft ICE (to stitch the disk together from 2 images) and GIMP to colorize. AR1944 is very complex and generated an X class flare yesterday. The sun is very low in the sky so higher magnification images were impossible, it’s nice to see the sun putting on a good show given all the warnings of it being past Solar Maximum!”

Cosmic rays surprise us again

April 7, 2013
The Alpha Magnetic Spectrometer being tested at CERN by being exposed to a beam of positrons.

The Alpha Magnetic Spectrometer (AMS-02)being tested at CERN by being exposed to a beam of positrons. (Picture from Wikipedia)

[Text and figures updated on April 9th 2013 due to insight from Ryan Nichol: Thanks]

The team running the Alpha Magnetic Spectrometer (AMS-02) have produced their first set of results. And as expected, they are full of surprises.

AMS-02 is an awesomely complex device – too power-hungry, heavy and complex to be placed on its own space platform, it was attached to the International Space Station 18 months ago on the last space shuttle mission. I wrote about this here.

It has with 650 separate microprocessors, 1118 temperature sensors and 298 active thermostatically-controlled heaters. It is basically a general-purpose particle detector like those found at CERN, and represents the culmination of nearly one hundred years of ‘fishing for particles’ in the high atmosphere.

  • First we flew balloons and found that ‘radiation levels’ increased as we went higher.
  • Then we discovered a ‘zoo’ of particles not yet observed on Earth – positrons, muons, pions, and anti-protons.
  • Then we discovered that ‘cosmic rays’ were not ‘rays’ but particles. And we realised that at the Earth’s surface we only observed the debris of collisions of ‘cosmic ray’ particles with the atoms in the upper atmosphere.

Where did these primary cosmic ray particle from?  What physical process accelerated them? Why did they have the range of energies that we observed? What were they? Protons? Electrons? Positrons? We just didn’t know. The AMS-02 was sent up to answer these questions.

I have found much of the comment on the results incomprehensible (BBC Example) with the discussion being exclusively focussed on ‘dark matter’.  So I thought I would try to summarise the results as I see them based on reading the original paper.

Over the last 18 months (roughly 50 million seconds) AMS-02 has observed 25 billion ‘events’  (roughly 600 per second). However, the results they report concern only a tiny fraction of these events – around 6.8 million observations of positrons or electrons believed to be ‘primary’ – coming straight from outer space.

  • They found that – as is usual for cosmic rays – there were fewer and fewer particles with high energies (Figure 1 below)
  • Looking at just the electrons and positrons (i.e. ignoring the protons and other particles they observed) there were only about 10% the number of positrons compared with electrons, but that the exact fraction changed with energy (See Figure 2 below)
  • They found that there were no ‘special’ energies – the spectrum was smooth.
  • They observed that the particles came uniformly from all directions  – the distribution was uniform with variations of greater 4% very unlikely.
  • The electron and positron fluxes followed nearly the same ‘power law’ i.e. the number of particles observed with a given energy changes in nearly the same way – indicating that they probably have the same source.

They conclude very modestly that the detailed observation of this positron ‘spectrum’ demonstrates…

“…the existence of new physical phenomena, whether from a particle physics of astrophysical origin.”

I like this experiment because it represents a new way to observe the Universe – and our observations of the Universe have always surprised us. Observations have the power to puncture the vast bubbles of speculation and fantasy that constitute much of cosmology. I am sure that over the 20 year lifetime of this experiment, AMS-02 will surprise us again and again.


Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

AMS Figure 6

Figure 2: Graph of the fraction of positrons compared with electrons as a function of energy in billions of electron volts (GeV). The ‘error’ bars show the uncertainty in the fraction due to the small number of events detected.

A message from Mars

August 12, 2012
Curiosity on Mars looking at the rim of Gale Crater

Curiosity on Mars looking at the rim of Gale Crater. Picture from NASA.

The pictures from Mars take my breath away. I am lost for words to describe the bravado and brilliance of the engineers and scientists who placed this robotic laboratory so gently onto the surface of Mars.

NASA should be very proud. Americans should be proud too: they paid for it -around $10 per head. And we should all reflect on just what human beings can achieve when we put our minds to something.

The brilliantly-conceived mission has revived talk about a possible manned mission to Mars. And I have even heard people talk again about the possibility of colonising the Moon or Mars. While such extrapolations are understandable I think it is important  to understand that human beings cannot live anywhere except planet Earth.

This is not fundamentally true. It is conceivable that we could create ‘bubbles’ of survivability far distant from the Earth. Overlooking the many challenges (e.g. the increased radiation doses; the atrophy of muscles in reduced gravity; and the creation of a stable microbial population etc.), I will concede that it  is possible that we could create ‘bio domes’ in which relatively large groups could live for extended periods away from the surface of Earth. But there is one problem that any colonists will not easily overcome: energy.

Curiosity is nuclear-powered which will allow it to operate through the Martian winter and at night when solar power is weak. Our putative colony might be nuclear-powered too, but for how long? Let’s say (optimistically) that the initial colonists brought with them enough nuclear power to last a century. In that century the colonists would undoubtedly achieve great things. But after the nuclear power station was shut down, what would they do? It is unlikely that a colony could develop the capability to build and fuel a new nuclear power station in just a century. Even if they struck oil on Mars – and refined hydrocarbons flowed easily from the rock – they would be of little use because there is no oxygen in the atmosphere with which to burn the fuel. Ultimately, the colonist’s engineers would find that the only sustainable method of generating energy was solar power.

Back on planet Earth we are in a similar position to our putative Mars colonists. Using fossil fuels we have achieved great things. But our use of fossil fuels is now affecting the flow of energy on and off the planet. Worryingly, the evidence that this is happening is becoming irrefutable. We could use nuclear power for a century or two. But if we want to replace all current energy use with a sustainable source then we have no choice: we need to capture 0.01% of the solar energy which reaches Earth’s surface. Yes. We need just one ten-thousandth part of the 123 000 000 000 000 000 watts of solar energy that constantly warms the Earth’s surface.

Earth is a ‘bio-dome’ driven by solar power. The flow of energy on and off our planet allows plants to thrive – and they provide us with the food and resources we need to live from day-to-day. It is the only place in the Universe where humans can live sustainably. If we want to avoid disturbing the climate that creates this home to which we are uniquely adapted, then we need a truly sustainable energy source. And there is only one

If our planet’s engineers and scientists can put the Curiosity rover onto Mars, and if taxpayers can fund this noble mission, then surely we can collectively decide to live sustainably on Earth and ask our scientists and engineers to make it possible. Can’t we?


Earth. The only place in the Universe in which human beings can live. But can we live here sustainably? Picture from Apollo 17 courtesy of NASA.

Is this a picture of Earth?

March 21, 2012
One of these images is a photograph of the Earth. The other isn't. So which is which? And what is the other one? (Images courtesy of NASA)

One of these images is a photograph of the Earth. The other isn't. So which is which? And what is the other one? (Images courtesy of NASA)

Friends. Fellow humans. We live on an amazing planet. And I feel priveliged to belong to the first generation in all of Earth’s history who have seen our planet as viewed from space. As we beat ourselves up for our collective failure to safeguard our planet, I feel it is worthwhile to pause and realise just how recently we acquired a truly global perspective.

The image on the left is a photograph of the Earth taken on a Hasslebad camera by an astronaut on Apollo 17 who, 28,000 miles out from Earth, looked out the window and happened to find the Earth illuminated fully. Since you can see Antarctica in daylight you can tell this must be in the southern hemisphere summer. When the camera was returned to Earth, the film was developed and the image revealed – there were no digital previews in 1972!

The image on the right is a fabrication. It uses ‘data’ acquired by a low Earth orbit satellite (Suomi) which is cleverly pasted together as described here.

Illustration of the way in which the right hand image was fabricated

Illustration of the way in which the right-hand image was fabricated. Despite being acquired by a satellite at a height of around 300 miles, it simulates the view from much further away. Image courtesy of NASA.

So what do we conclude? The view is no less amazing for having been simulated. And the whole Earth perspective it represents is as much a philosophical perspective as a physical one. But despite the ubiquity of a similar image as the default iPhone desktop, I find the original more emotionally powerful. The fact that an individual human being took the picture on the boldest adventure of a generation somehow resonates with me.

The six and a half minute video below describes in more detail how the images are made – it is shockingly complicated. Enjoy 🙂

If I ruled the world…

March 9, 2012

The video above is a time-lapse movie of the view from the International Space Station (ISS) as it flies through the night. And I share it with you for the simple reason I love the video. In itself it seems to me a justification of the installation of a ‘picture window’ on the ISS, and possibly of the entire project. As I mentioned before, if I ruled the world, I would chill out weekends on the ISS, put on some nice music and just float in the cupola and watch the Earth drift by.

I didn’t recognise many of the land masses on view, but at 1 minute and 9 seconds the space station is looking from over the Alps in Northern Italy down along the length of Italy. It then flies across Northern Greece, Turkey – Cyprus is visible – Israel and then Iraq at 1 minute 20 seconds.

Have a nice weekend.


The 'cupola' on the International Space Station. This is where I would hang out if I were an astronaut.

My big problem with astronomy

February 22, 2012
Pretty Galaxy

A Pretty Galaxy - the subject of erudite speculation by astronomers and mindless reporting by hacks. The circle marks the apparent location of a black hole called HLX-1. Don't believe the colours - the picture is 'data' and not a photograph. The galaxy - which is inferred to be spiral in shape even though we see it edge on - is called ESO 243-49

Recent stories in Wired and The Register illustrate perfectly everything I hate about popular astronomy. First of all, you can see these are both routine hacks by comparing them with the press release.

Don’t get me wrong: I am filled with admiration for astronomers: their instruments are astounding; the maths and physics of observing is inspiring; and of course the Universe is just breathtakingly beautiful. What irritates the pants off me is the ridiculous desire to ‘explain’ what they observe. What we end up with is a pretty picture and a fantastical, unverifiable ‘sciency’ tale. Frankly we would be better of with just the pretty picture and good old fashioned ‘fairy’ tale.

To explain what I mean I have reproduced extracts from the ‘Wired’ article below in blue with what the article should (IMHO) have said.

Wired: The Hubble space telescope has spotted a supermassive black hole floating on the outskirts of a large galaxy.
Actual: Scientists looking at data from the Hubble Space Telescope have inferred the existence of a black hole near a large galaxy.(How?)

Wired: The location is odd because black holes of this size generally form in the centers of galaxies, not at their edges. This suggests the black hole is the lone survivor of a now-disintegrated dwarf galaxy.
Actual: The location is odd because evidence indicates black holes of this size are generally  found near the centers of galaxies, not at their edges. Scientists don’t understand this.

Wired:The black hole — named HLX-1 — is 20,000 times more massive than the sun, and is situated 290 million light-years away at the edge of the spiral galaxy ESO 243-49.
Actual: The black hole — named HLX-1 — is estimated to be 20,000 times more massive than the Sun (how?), and is estimated to be 290 million light-years away at the edge of the spiral galaxy ESO 243-49

Wired: Hubble detected a great deal of energetic blue light coming from the black hole’s accretion disk — a massive collection of gas and dust that spirals into the black hole’s maw, generating x-rays. But scientists studying Hubble’s data also noticed the presence of cooler, red light, which shouldn’t have been there.
Actual: Hubble detected blue light  and red light. They inferred that the blue light came from an accretion disk. But they couldn’t understand the red light.

Wired: Astronomers suspect the red light indicates the existence of a cluster of young stars, roughly 200 million years old, orbiting around the black hole. These stars, in turn, are the key to explaining the chaotic history of the supermassive black hole.
Actual: Astronomers could explain the red light if there were young stars orbiting around the black hole. They even thought up a story about these unobserved stars that might actually exist.

Wired: HLX-1 was likely formed at the center of a dwarf galaxy that once orbited ESO 243-49. But in this dog-eat-dog universe of ours, large galaxies often swallow up their smaller brethren. When the dwarf galaxy came too close to ESO 243-49, the larger galaxy plucked away most of its stars, leaving behind the exposed central black hole. The force of the galaxies’ collision would have also triggered the formation of new stars, explaining the presence of a young stellar cluster around the black hole. The cluster’s age, 200 million years, gives a good estimate of when the merger occurred. HLX-1 may now be following the same fate as its parent galaxy, slowly getting sucked into ESO 243-49. But researchers don’t know the details of the black hole’s orbit, so it could also possibly form a stable orbit around the larger galaxy, circling as the isolated reminder of a vanished dwarf.
Actual: HLX-1 was likely formed when a space dragon called PTMD-X1 laid an egg, which grew into a blue headed X-ray dragon. Astronomers speculate that the dragon’s mother died when it was just 200 million years old  causing the youngster to cry tears which then turned into stars through a process astronomers call tear-star -formification. The blue colour of the stars shows the dragon was sad and astronomers hope that it is happier now and has made friends.

Does the Grand Canyon Exist?

November 2, 2011
An image of the Grand Canyon showing clearly that what we see is merely the absence of the rock which was previously there.

An image of the Grand Canyon showing clearly that what we see is merely the absence of the rock which was previously there. Picture from NASA Earth Observatory.

The wonder-ful NASA Earth Observatory this week sent me an image of the Grand Canyon created by overlaying satellite imagery with a detailed topographic map. And as I stared in wonder I could see clearly that the Grand Canyon did not in fact exist!

What I saw was that the Grand Canyon was a phenomenon defined by absence – the absence of the rock which has been removed. The rock which has been removed was – if you like – the ‘positive’ – the actual ‘stuff’ which exists. And what remains is the ‘negative’ a phenomenon made entirely out of… absence! And this made me think about electrons…

As you may know, Dirac’s conception of particles and anti-particles imagines a vacuum state, geographically analogous to the elevated high plains. He imagined that things could be added to this vacuum state to create a phenomenon we call a ‘particle’ , for which the geographic analogy would be a mountain. Nobody could doubt that a mountain exists!

But in Dirac’s conception, something can also be subtracted from this vacuum state to make an anti-particle, for which the geographic equivalent would be a valley. Or a canyon. Anti-particles would be defined by absence.

Staring at the undeniable existence of the Grand Canyon and realising that it didn’t really exist, I felt something click. I felt that the wonders and mysteries of the  microscopic world were somehow being played out at another scale – a scale so large that this time I almost failed to recognise them for a different reason!

It’s a funny old world.


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