Archive for the 'Physics' category

The other meaning of "dimension" and its use in physics

Aug 27 2009 Published by under Physics

Thanks to the advent of relativity theory, and string theory in recent decades, there's a lot of talk in physics about space having extra, unseen dimensions -- up to 11 spacetime dimensions in one version of string theory!  These days, the word "dimension" in physics immediately evokes Twilight Zone imagery:

There is a fifth dimension, beyond that which is known to man. It is a dimension as vast as space and as timeless as infinity. It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of man's fears and the summit of his knowledge. This is the dimension of imagination. It is an area which we call the Twilight Zone.

(Fun fact: the introduction to the show changed pretty much every year it was on.  See Wikipedia for the text of all the intros!)

The term "dimension", however, has another meaning in physics: a more mundane one, but equally important.  This other type of dimension, used in what is known as dimensional analysis, has been used to gain surprising insight into difficult physical problems.

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Why I left experimental particle physics - a meandering story

Aug 19 2009 Published by under ... the Hell?, Personal, Physics

Some time ago, I promised that I would tell the story of my transition from experimental particle physics to theoretical optics.  With a lot of busy stuff going on at work and my research blogging efforts mired in some rather difficult reading, this seemed like a good time to share the story!

Before I begin, though, let me make a disclaimer: none of this should be interpreted as bashing on particle physics!  I have a soft spot in my heart for the field, and the research is getting very exciting with the gradual startup of the LHC.  This is really a story of how I chose my particular field of research, and how that decision involved balancing the nature of the research with the actual day-to-day work involved.  Most of my choices and rationalizations at the time were from the perspective of an unknowingly clinically depressed graduate student who hadn't quite figured out what he wanted to do with his life.

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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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Scientific cranks: Going strong since at least 1891

Jul 30 2009 Published by under ... the Hell?, History of science, Physics

It is easy to assume that scientific crankery is a relatively new phenomenon, perhaps fueled by the completely non-intuitive, sometimes intimidating nature of many modern scientific theories.   In physics, for instance, most cranks spend their time attacking Einstein's theories of relativity and the theory of quantum mechanics, both of which go against "common sense."

While browsing the older journals, however, I came across an example of crankery from 1891, well before the advent of "modern" physics!  The crankery practically jumped off the page at me as I was skimming the table of contents in the Philosophical Magazine.  An image of the page in question is below; see if you can spot what caught my eye (click to enlarge):


Does anything strike you?

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7 responses so far

Thomas Levenson's Newton and the Counterfeiter

Jul 26 2009 Published by under History of science, Physics

About a month ago, I noted that Thomas Levenson's book Newton and the Counterfeiter (2009) is now available:


The book is the story of how the great scientist Isaac Newton, after making the discoveries which electrified the scientific world, took a job as the Warden of the Royal Mint, an official charged with protecting the nation's currency.  In this role, he came into contact, and conflict, with a criminal mastermind and counterfeiter William Chalconer, and the two would play a game of cat-and-mouse with life literally at stake.

When I've told people about this book, they ask, "For real?"  They naturally assume that the book is historical fiction, but it is in fact a true story!

I bought the book immediately, but didn't read and review it right away: I figured that a book with such subject matter would naturally be an instant hit!  But as Tom Levenson noted on his own blog, the book has not gotten the publicity it needs (I would say deserves), so I thought I'd do my own part to draw people's attention to it.

It deserves your attention, too: if you're a fan of history, a fan of science, a fan of true crime stories, a fan of economics, or just interested in reading a good, true, tale, Newton and the Counterfeiter is well worth your time.

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8 responses so far

Maxwell on Faraday

Jul 25 2009 Published by under History of science, Physics

I'm working on a few longer posts at the moment, but in the meantime I thought I'd share a nice little passage I came across while looking through James Clerk Maxwell's A Treatise on Electricity and Magnetism (1873).  Maxwell, of course, was the scientist who theoretically put together, for the first time, a complete set of equations of electricity and magnetism, the eponymous Maxwell's equations, and also used these equations to postulate that light is an electromagnetic wave.

I've blogged a lot about the accomplishments of Michael Faraday, who did most of the experimental legwork which allowed Maxwell to make his discovery.  This relationship was not lost on Maxwell, who had nothing but unadulterated praise for his predecessor in the introduction to his own text:

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Lord Rayleigh vs. the Aether! (1902)

Jul 09 2009 Published by under History of science, Optics, Relativity

(Note: This is an attempt to get myself rolling on my long-ignored series of posts explaining Einstein's theories of relativity.  It's also a really cool experiment in the history of science.)

One of the most fascinating aspects of 19th century physics is that many remarkable ideas and ingenious experiments were motivated by a physical hypothesis which we now know to be incorrect: namely, the aether.   When light was demonstrated to have wavelike properties in the early 1800s, it was natural to reason that, like other types of waves, light must result from the excitation of some medium:  after all, water waves arise from the oscillations of water, sound waves arise from the oscillation of air, and string vibrations are of course the oscillations of string.  The hypothetical medium which carries light vibrations was dubbed the "aether", due to its unknown, "aetherial" nature.

A lot of scientists speculated on the physical properties of the aether, and sometimes this speculation produced lasting results in other fields; for instance, Earnshaw's theorem was originally conceived to try and describe the forces involved in the aether's oscillation.

By the late 1800s, however, more and more research cast doubt on the very existence of the aether, notably the Michelson-Morley experiment (to be discussed below).  In response, theoreticians produced more and more "patches" to the aether theory, until at last Einstein published his special theory of relativity, which eliminated the need for an aether and in fact suggested that the idea of an aether was incompatible with the experimental evidence.

Before this happened, however, at least one brilliant researcher took up the challenge of testing one of the "patches" to the aether.  Lord Rayleigh (1842-1919), distinguished physicist and eventual Nobel Prize winner, conceived of and carried out a very clever optical experiment to see whether objects shrink in the direction of motion, a phenomenon known as length contraction.

As is often the case, even though the experiment was unsuccessful, we can still learn many useful lessons about the workings of science from it!

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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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Barkla shows that x-rays have polarization (1905)

Jun 06 2009 Published by under History of science, Physics

It is one of the quirks of scientific progress that many great experiments are forgotten as the things they demonstrate become common knowledge in the scientific community.  A good example of this is the 1890 experiment of Otto Wiener, which I blogged about as my very first "official" science history post.  Wiener constructed a beautiful experiment to demonstrate that it is the electric field, not the magnetic field, which is the "active" ingredient in light.  Nowadays, this observation is just taken for granted, and relatively few books discuss the experiment which proved it.  This is not an injustice, though, as much as an expedience:  certain physical phenomena can be understood perfectly well without going into the historical origins of the discovery, and physics students have plenty of much more relevant topics  to worry about.  Nevertheless, there's a lot of interesting work that isn't talked about much anymore.

As research for my in-progress textbook, I've recently been looking into the original X-ray diffraction experiments of the Braggs circa 1912.  While reading through their 1915 book on X-rays and Crystal Structure, I found a passing reference to the first observations of polarization of X-rays.  Not being able to help myself, I tracked down the original source...

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7 responses so far

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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5 responses so far

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