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Historical note on glaciology and nanotechnology, by John F. Nye

Wednesday, June 17th, 2015

This personal note, addressed to the glaciological community, is an addition to my lecture "Glaciology Sixty Five Years Ago" which is available to watch here.  I am a former glaciologist who is now working on optics and waves.  The transition happened in a quite logical and natural way.  I was interested in the radio-echo sounding of the Antarctic ice sheet when it first started.  At first, of course, it was not done from an aircraft or spacecraft but from a sledge down on the snow.  As you well know, a pulse of waves is sent out and you time how long it takes for the echo from the bed to come back.

Blog_Pic1

But the pulse that comes back is not exactly the same as the one that was sent out;  it is drawn out, with a long tail.  If the outgoing pulse contains 4 crests the echo might have 12 or many more crests in it.  The reason is simply that the rock bed is not just a mirror;  it is rough.  The front of the echo comes from just underneath the sledge but signals also echo back – are scattered back – by the more distant parts of the bed.  That gives the tail of the echo.

We knew from published photographs that the tail had a fine structure which changed rapidly as the sledge was moved, but to understand the details of how it would change Michael Walford, with two undergraduates, Robert Kyte and David Threlford, built a laboratory-sized analogue model [1] with a transmitter and receiver that used ultrasonic waves instead of radio waves.  Sound in air travels about 10,000 times slower than radio waves in ice, and the Antarctic ice sheet is also about 10,000 times thicker than a laboratory model, 1 m or so in depth.  So it scales down into the laboratory very nicely.  We simulated the rough rock bed by using crumpled cooking foil.  When we displayed the outgoing ultrasonic pulse and its echo on an oscilloscope they looked very like the radio versions.  But there was a feature of the received pulse that drew my attention;  it is shown in simplified form in the following animation.

The received pulse is very short in this example and contains just 4 crests.  But if the transmitter/receiver (carried by the sledge on the ice surface) is moved sideways by less than a wavelength the number of crests increases by 1.

At first there are 4 crests but then there are 5.  The following figure shows what the crests in the returning wave must look like in space.   

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The pulse must contain a dislocation very like an edge dislocation in a crystal, where a plane of atoms comes to an end.  The crests of the waves are the crystal planes.  4 crests sweep past point A, but an observer who moves to point B sees 5, as in the animation.  In the real problem the amplitude of the returning echo is a function of the three space variables x, y, z and the time t.  This example simplifies it to a function of x, z, t.  One can deduce that trains of waves in nature can contain dislocations (of mixed edge-screw type, in general).  That is a very broad conclusion, but it is justified because the cooking foil bed is sufficiently representative of a general scattering object.  Similar observations had been made before, most notably in the discovery as long ago as 1833-1840 by Whewell [2] of amphidromic points in the tides.   The new point was that dislocations are features to be expected in all waves.

This idea, developed with Michael Berry [3], has proved to be extraordinarily fruitful.  We and many others continue to pursue the topic of wave dislocations in optics and in monochromatic waves, where they are called optical vortices.  This is an academic study undertaken for its own inherent interest.  But the historical thread I want to follow here branched into an application of some importance when, following theoretical work by L. Allen, it was demonstrated [4] that a laser beam containing an optical vortex can turn a tiny object by transferring angular momentum to it.  The helical shape of the wavefronts was the operative feature.  It was already known that light can move an object by transferring linear momentum but here was the possibility, using a single laser beam, of a microscopic device, operating on a subwavelength scale, that could both move and turn an object even as small as a single atom – true optical tweezers.  As is well known, laser beams are now used for nano-assembly.  Nano-technology has begun, with all the exciting things it implies for the creation of new materials, and the new vistas it opens in the life sciences (manipulating single cells) and for quantum communication.  Thus the lineage can be traced back, from the manipulation of single atoms, molecules or cells and the nano-assembly of new materials, via optical vortices and dislocations in ultrasonic and other waves, to a study of radio-echo sounding in Antarctica – a scale change by a factor of 109.  Glaciologists may be pleased to know of this historical connection between their subject and nano-technology.

Correspondence

45 Canynge Road, Bristol BS8 3LH, UK.

Email: john.nye@bristol.ac.uk

References

[1] J. F. Nye, R. G. Kyte and D. C. Threlfall 1972, Proposal for measuring the movement of a large ice sheet by observing radio echoes, J. Glaciol. 11, No. 63, 319-325.

[2] S. Ducheyne 2010, Whewell's researches; scientific practice and philosophical methodology, Studies in History and Philosophy of Science A 41, (1), March, pp. 26-40.

[3] J. F. Nye and M. V. Berry 1974, Dislocations in wave trains, Proc. Roy. Soc. Lond A336, 165-190.

[4] R. Allen, M. W. Beijersbergen, R. C. J. Spreeuw and J. P. Woerdman 1992, Orbital angular momentum of light and the transformation of Laguere-Gaussian Modes, Phys. Rev. A45, 8185-8189.

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The Truth is not an option – part 5

Thursday, May 31st, 2012

Part 5. No awards and no allowance for willful ignorance

 “A hallucination is a fact, not an error; what is erroneous is a judgment based upon it.”

—Bertrand Russell, On the Nature of Acquaintance: Neutral Monism (1914)

Bertrand Russell was a philosopher, a pacifist, anti-Stalinist, anti-Nazi, anti-religious, and anti-Vietnam War.  He was also a prisoner of conscience.

In these key cases, more damaging than the misinformation about one scientific issue can be the grave damage to public understanding of the legitimate, constructive roles of science in society.  Science and its applied engineering offshoots (along with the labor and entrepreneurial spirit of millions of citizens and the vision of many leaders) are what built America and other modern developed nations.  Science and engineering have propelled America and the world into the age of planetary exploration, where planetary climate change (all natural, of course) is one of the driving scientific foci of exploration.  The same methods of analysis, but with a far deeper database of observations, are utilized to understand climate change on Earth, where causes in recent decades are clearly both natural and man-made.  The man-made component of climate change on our planet is rising above magnitude of the natural oscillations in many parts of the world.  If there is one thing that planetary science has taught us, it is that planets have many climate tripwires, many nonlinear forcing-and-response functions, many climate change mechanisms that are a challenge to understand from the past and even more difficult to predict.  The ones we know about and understand, when put into physics-based global climate models, are doing a pretty good job replicating the actual recent climate trends; the same models project more rapid and severe climate changes in the future.  Empirical models based on climate and CO2 levels of the geologic past suggest that, compared to baseline models presented by the IPCC, far more severe climate change could be in store for us.  The heat storage capacity and heat conveyance of the oceans, for example, are loaded with both slow and rapid climate change mechanisms, and the slow ones are apt to cause climate to keep warming for centuries even if we could stop greenhouse gas emissions.

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I’ve attended my 5th IGS sponsored (or co-sponsored) meeting of my first year as president…

Monday, February 20th, 2012

Just back from New Zealand, where I attended the 2012 Annual Workshop of the Snow and Ice Research Group (NZ) held in Twizel, New Zealand. What a trip! (See the forthcoming report in ICE for more details.) This means that during the first year of my term as IGS president, I've attended meetings in La Jolla, California, Cambridge, UK, Oslo, Norway, Grenoble, France and now Twizel, New Zealand. The things I've learned at these meetings would have made my head spin, if it were not already spinning from the jet lag.

On my way to and from Twizel (located in the shadow of Aoraki/Mt. Cook), I passed through Christchurch. It was very disturbing to see the damage and suffering that the residents of Christchurch went through as a result of the recent earthquakes. However, I was delighted to have a "flat white" and a bit of shopping at the "container mall" on Cashel Street.

A free day at the end of the trip allowed me to see the blue penguins at the Antarctic Centre in the outskirts of Christchurch. Photos and a video are attached.

Doug MacAyeal

 

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Informal glaciologists meetings

Sunday, February 5th, 2012

Just attended the 16th Alpine Glaciology Meeting in Zurich. These meetings are so productive and informative. The AGM is based on the same concept as the Northwest Glaciologists meetings in the US and Canada. As a young student at University of Washington in Seattle, under Charlie Raymond, we used to travel to Vancouver and Tacoma for those meetings every year, different venue every year. 
It was a great opportunity to meet up with fellow students and the famous names in glaciology like Barclay Kamb, Garry Clarke and Mark Meier and many others. It was a great opportunity for students to practise their presentation skills and learn how to answer probing questions from such greats as Barclay. Once you leaned how to respond to a question from Barclay Kamb you could handle anything. 30 years later these meetings are just as vibrant and stimulating and the students are still taking their first steps in the demanding world of public speaking.

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Submissions to the Journal of Glaciology in 2012

Thursday, January 5th, 2012

2011 is all finished. We got 198 submissions in total, 3 more than we did in 2010. The first count indicated we had only broken even but a recount produced a couple of more submitted papers. So we did break the record as we have done every year since 2006. On top of that, 2010 we had issue 200 for which we 'commissioned' 22 papers.

In all you have been fantastic, we had more papers submitted in December than we have ever had for that month. So obviously glaciologists are not too swept up in the pre Christmas rush, science comes first. Have a super year, I look forward to us breaking the record again in 2012.

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Reaching the South Pole

Sunday, December 18th, 2011

These days it is 100 years since Amundsen and Scott and their teammates reached the South Pole. Amundsen on 14 December 1911 and Scott on 17 January 1912. Both were remarkable expeditions albeit very different. Amundsen, the seasoned polar explorer set out with the sole purpose of being the first to reach the South Pole. Scott wanted indeed to be the first to reach the pole as well, but his expedition also included an extensive scientific explorations program. Amundsen reached the pole first and based on their respective experience in polar travel, Scott never really had a chance. But in discussion with British people about the 'Race for the Pole' I find that their tendency is to make excuses for Scott, 'yes his team was second to reach the South Pole but they did so much scientific work'. Why not give priority to the scientific work done by Scott's team, not the fact that he was 'second to reach the South Pole'? Amundsen was better equipped for the 'race to the South Pole' but Scott's scientific work has enough merit to stand on its own in the world of science. Let that be the legacy of Scott's expedition.

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Welcome to the IGS blog

Thursday, September 15th, 2011

This is the first entry on the IGS blog. The purpose of this blog is to provide a venue for anyone to express their opinions or initiate discussions on anything to do with glaciology.

If you are in the field, the lab, the library or even in your office we'd like to read about your experiences, ideas, your trials and tribulations and even your bugbears.

If you would like to be a contributor to our blog please drop us an e-mail and we will set you up as a blogger.

Magnús

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