Archive for the 'Invisibility' category

Invisibility physics: Kerker's "invisible bodies"

Jul 02 2010 Published by under Invisibility, Optics

(This is a continuation of my “history of invisibility physics” series of posts.  The earlier posts are: Part I, Part II, Part III, Part IV, Part V, Part VI)

The history of invisibility physics truly began with the concept of radiationless motions of charged particles, as described by Ehrenfest in 1910 and Schott in 1933.  There are many more discoveries associated with these and related phenomena, which would eventually be referred to as nonradiating sources.

I would like to jump ahead in the history a little bit, however, and discuss a paper published in the Journal of the Optical Society of America in 1975 by Milton Kerker, entitled, "Invisible bodies".  The article, relatively unknown today, is the first article to describe an object which is invisible in the true sense of the word -- although the object itself is microscopic!

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Lord Rayleigh's comments on invisibility (1884)

Oct 07 2009 Published by under History of science, Invisibility

Found it! I pointed out in my previous invisibility post that R.W. Wood attributes an early discussion of invisibility to Lord Rayleigh in his Encyclopædia Britannica article on optics; however, I couldn't find the quote after browsing Rayleigh's articles and wondered if Wood had miscited Rayleigh's work.

A bit of closer inspection, however, shows that I overlooked Rayleigh's comment, which was buried in a footnote in his article on geometrical optics (Encyclopædia Britannica, vol. 17 (1884, 9th ed.), 798-807), in what I would have considered an unlikely place, namely his discussion of achromatic object-glasses (p. 805).  The footnote is as follows:

Even when the optical differences are not small it is well to remember that transparent bodies are only visible in virtue of a variable illumination.   If the light falls equally in all directions, as it might approximately do for an observer on a high monument during a thick fog, the edge of (for example) a perfectly transparent prism would be absolutely invisible.  If a spherical cloud, composed of absolutely transparent material, surround symmetrically a source of light, the illumination at a distance would not be diminished by its presence.

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The first paper on invisibility? (1902)

Oct 02 2009 Published by under Invisibility, Optics

When discussing the history of invisibility physics, I typically cite Ehrenfest's 1910 paper on radiationless motions as the first publication dedicated to the subject.  Ehrenfest's paper, which attempts to explain how electrons could oscillate in a classical atom without radiating, is a direct precursor to the long history of nonradiating sources and nonscattering scatterers that I've been chronicling on this blog.

However, it turns out that Ehrenfest was not the first author to discuss some form of invisibility!  I recently stumbled across an article in an early issue of the Physical Review: "The invisibility of transparent objects," by R.W. Wood, 1902.  It is not an earth-shattering paper, but it presents some intriguing ideas and suggests that visions of invisibility may go even further back in the sciences... Continue Reading »

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Invisibility physics: "Reflectionless" objects make an appearance

Aug 10 2009 Published by under Invisibility

(This is a continuation of my "history of invisibility physics" series of posts.  The earlier posts are: Part I, Part II, Part III.)

Up through the late 1940s, it seems that the only type of invisibility that authors were considering were "radiationless orbits": motions of charged particles of extended size which in principle could accelerate without emitting radiation.  These are not invisible objects per se, but rather objects that should produce radiation according to conventional wisdom but in fact do not.

A truly invisible object would be one which does not scatter any radiation incident upon it; that is, light which shines on the object is not reflected or absorbed, but instead is transmitted in such a way that it appears to the outside observer as if there were no object present.  But are such invisible objects even possible?

In 1956, a paper appeared in the Journal of Applied Physics which provided at least a partial answer to this question.  In their article, "Reflectionless transmission through dielectrics and scattering potentials," Irvin Kay and Harry Moses demonstrated theoretically that one could construct stratified media that perfectly transmit waves of a given frequency, regardless of the direction of incidence of the illuminating wave.  Light shining on their theoretical media would be completely transmitted, with no reflected light!

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Invisibility physics: Hiding and seeking, all at once!

Jun 19 2009 Published by under Invisibility, Optics

When the first papers on the idea of a "cloaking" device came out in 2006, lots of people were immediately worried that the CIA would soon be peering right over their shoulder from the shelter of invisibility cloaks.  Many scientists, including myself, pointed out the flaw in that reasoning: a "perfect" cloak would direct all light around the outside of the cloak.  This meant that, although the spy couldn't be seen in the cloak, he couldn't see anything from inside!


An illustration of one of the original cloaking concepts from J. B. Pendry, D. Schurig, and D. R. Smith, Science 312, 1780 (2006): rays of light are guided around the interior region, which sees no light.

A recent paper in Physical Review Letters, however, suggests that this "mutual invisibility" can be overcome.  The research described suggests that a different type of cloaking device could be used to enclose a sensing device, and that the sensor would not only be (almost) invisible, but it would be able to detect radiation just as well as when outside the cloak!  The research is intriguing (though it still won't help the CIA quite yet), and it illustrates a different, earlier, technique for making something "not be seen".

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Invisibility physics: can charged particles self-oscillate?

May 31 2009 Published by under Invisibility, Physics

Time to return to my long-delayed series of posts on the history of invisibility physics!  The first two posts were:

  • Acceleration without radiation (1910), describing Ehrenfest's arguments suggesting acceleration without radiation could be possible,
  • Schott's radiationless orbits (1933), describing G.A. Schott's analytical demonstration that a charged spherical shell could move in a periodic orbit without producing radiation.

Our next stop in the study of invisibility physics is a pair of results, one by G.A. Schott in 1937 and another by D. Bohm and M. Weinstein in 1948, in both of which it is suggested that under the right circumstances, not only can an extended charged particle oscillate without radiating, but that it can also oscillate under the influence of its own electromagnetic field, without the application of an external force!

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Optics basics: Inverse problems

Oct 31 2008 Published by under Invisibility, Optics, Optics basics

In previous posts, I've talked at some length about computed tomography (CT) and optical coherence tomography (OCT).  Each of these is a technique for determining information about the internal structure of an object, such as the human body, from exterior measurements of the scattering of electromagnetic waves from the object.  In the case of CT, x-rays are used to measure and image a cross-sectional 'slice' of the human body, while in OCT, broadband visible light is used to probe a few millimeters into the skin or an internal organ of the human body.

Plenty of other techniques exist for measuring the internal structure of objects, using a variety of different types of waves.  Magnetic resonance imaging (MRI) subjects a patient to an intense magnetic field, and makes an image by measuring the radio waves emitted when the field is suddenly switched.  Ultrasound imaging uses ultrasonic waves to probe the soft tissues of the human body, and is used in mammography.

Each of these techniques is quite different in its range of application, but all require nontrivial mathematical techniques to reconstruct an image from the raw scattered wave data.  These mathematical techniques are broadly grouped into a class of problems known as inverse problems, and I thought it would be worth an optics basics post to discuss inverse problems, their common features, and the challenges in solving them.

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New 'cloaking' results? Not really, but interesting anyway

Aug 20 2008 Published by under Invisibility, Optics, Physics

ResearchBlogging.orgAbout a week ago, I reported on another 'teaser' in the media about 'optical cloaks', hypothetical devices which would in principle make objects contained in their core completely invisible. Such devices have gotten a lot of attention, both scientifically and in the press, since the publication of two fascinating theoretical papers in 2006. I recently wrote a post, which can be found here, summarizing those original two papers.

The press reports a week ago suggested another major breakthrough in cloaking research, with headlines such as "Science close to unveiling invisible man" and "Invisibility cloak closer than you think." They were a little confused about what exactly had been accomplished, however: had the researchers made a three-dimensional invisibility cloak, a cloak that works at visible frequencies, or both?

Well, it turns out that they've done neither! This is another example of the press hunting for the best 'hook' for the story, no matter how tangentially related to the actual research. What has been accomplished, however, is the development of low-loss, three-dimensional negative refractive index materials which work for visible wavelengths, which is an important and interesting accomplishment in and of itself. I give a brief answer to the question, "What is a metamaterial?" below the fold, followed by an explanation of the actual results of the recent Berkeley papers and an analysis of how the press got themselves confused again.

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The cloaking craze: A look at the original papers

Aug 13 2008 Published by under Invisibility, Optics, Physics
As I noted a couple of days ago, apparently there has been another significant experimental breakthrough in the development of dielectric cloaking devices. Researchers at UC Berkeley were responsible, though it is a little unclear what exactly the breakthrough is. The results will appear this week in Science and Nature. In the meantime, it seemed like a good time to review the two articles that started the whole cloaking craze.

As I've noted in a pair of previous posts (here and here), the search for objects which can be considered in some sense invisible goes back nearly a hundred years. For the most part, however, the idea that one could make a truly invisible object was considered impossible -- and theory backed up that view.

This changed with the publication of two back-to-back theoretical papers in Science in 2006. The first, by U. Leonhardt, was titled "Optical conformal mapping," and the second, by J.B. Pendry, D. Schurig and D.R. Smith, was titled "Controlling electromagnetic fields". Both papers mapped out strategies -- in a nearly literal sense -- for creating what could be called a dielectric invisibility device. How would such a device work? Let's recall a little basic optics that will help us understand the process...

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Another 'invisibility cloak' teaser!

Aug 10 2008 Published by under Invisibility, Science news

Via several sources (Times Online, via HuffPost and AP, via Pharyngula), I see that there's another pending high-profile release concerning 'invisibility cloaks'. Though the physics behind them is accurate, the media is of course pushing rather hyperbolic headlines again: "Science close to unveiling invisible man."

There isn't enough information to determine for certain what has been done, as these were only 'teaser' releases and the technical reports won't be out until later in the week. Times Online suggests that the important result is the experimental production of a cloak at visible light frequencies, while the AP mentions little of visible light and instead refers to the three-dimensional nature of the experimentally-produced cloak. Whether one or both advances were achieved is unclear, but either would be a major step and would have been achieved surprisingly fast. It is important to note, however, that in any case we're not really that close to an invisible man! There's a lot of practical and theoretical issues still to be solved, including the fact that it isn't clear yet if or how it is possible to make a cloak which is invisible for all frequencies of visible light.

The work was done at UC Berkeley, and I'll say more about it when the papers come out. In the meantime, later this week I'll do a post on the original cloaking papers from 2006, which is the work that started the current 'cloaking craze'.

P.S. Thanks to Mom & Dad, who I now see both sent me links to the story!

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