Conventional wisdom, even to this day, dictates that accelerating charges necessarily give off electromagnetic radiation. This is seen, for instance, in large-scale particle accelerators (synchrotrons), such as the Tevatron at Fermilab and the soon-to-be-operational LHC at CERN: the charged particles moving around the ring are constantly shedding radiation over a range of frequencies, including X-rays.
In the first post in my series on the physics of invisibility, however, we discussed a little-known 1910 paper by Paul Ehrenfest, in which he demonstrates theoretically that one can have accelerating extended distributions of charge which produce no radiation fields. Ehrenfest was attempting to explain one of the most vexing problems of physics at the time: the presence of electrons in the atom. The atom was known to have electrons moving about within it, and these electrons should have been radiating constantly, according to the known physics of the time, but were not seen to do so.
Soon after Ehrenfest's paper, Bohr produced his model of the atom, which eliminated the need for radiationless orbits and ended most speculation on atomic structure. Ehrenfest's work was mostly forgotten, but other researchers independently discovered other radiationless motions of charges, and this research would lead eventually to more detailed studies of invisibility. One of the most important researchers on radiationless motions was G.A. Schott, who in 1933 produced a beautiful and amazing theoretical result* which we discuss in this post.
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A couple of years ago, a number of physicists made international news (some descriptions here and here) by proposing that "cloaking devices" were theoretically possible to construct. Two papers appeared consecutively in Science Magazine in May 2006, one by U. Leonhardt of the University of St Andrews, Scotland (Science 23 June 2006: Vol. 312. no. 5781, pp. 1777 - 1780), and the other by J.B. Pendry of Imperial College, London and D. Schurig and D.R. Smith of Duke University (Science 23 June 2006: Vol. 312. no. 5781, pp. 1780 - 1782). Both papers describe how, with the proper materials, one could create devices which 'guide' light around a central core region without distortion, effectively making the cloak, and whatever sits in the core, invisible. This idea is illustrated by the figure below, from the Pendry paper, which shows how light rays could be guided around the core:
These papers have generated so much interest that it is fair to say that they have created their own subfield of optical science, what one might call 'invisibility physics', and numerous research groups are busy concocting their own invisibility schemes or attempting to construct a Leonhardt/Pendry-style device.
It is interesting to note, however, that the study of objects which are in some sense 'invisible' is not really new, and in fact there is a century-long history of scientists studying objects which may be considered, one way or another, undetectable.
I happen to know a lot about the history of such objects, so I thought I'd start yet another long-running series of posts, this one on invisibility physics. We start today with a discussion of what may be the first paper of this type, written by none other than the remarkable physicist Paul Ehrenfest.
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