February 26, 2016 Filed in: Review
Richard Taylor, CAP Councillor for Affiliates, Teacher Merivale High School, Ottawa
The University of Ottawa’s Physics Department has initiated a lecture series they call “Eureka Talks”. The first of these talks was given Saturday January 30, 2016 by Sir John Pendry of Imperial College, London, on the topic of metamaterials and invisibility.
Sadly, I must inform you that Harry Potter’s invisibility cloak is still not available. Even worse, if Harry was invisible, he wouldn't be able to see anything — he’d be in total darkness, with all the light rays bent around him and none going into his eyes!
On the other hand, there have been some remarkable developments in our ability to manipulate light and other EM waves, beyond the capabilities of bulk materials like traditional lenses. Sir John gave a quick historical introduction to optics which served as a solid base for understanding what came next. (Note: Dr. Krich of U of Ottawa’s Physics department introduced Sir John Pendry and informed us that he had investigated the correct protocol for referring to a knight. When there are no other knights present with the same first name, ‘Sir John’ is the proper form.) Snell’s Law of refraction is a useful concept, but you have to think outside the traditional box and consider the refractive index as a geometrical transform and vice versa. This means that the geometrical transformations of Einstein’s General Relativity can be translated into an effective index of refraction and then onward to gravitational lenses.
For the invisibility cloak, theorists including Sir John have worked out that it is possible to bend light around a region and make it invisible, but this requires materials with a negative refractive index. This doesn’t make sense with the traditional interpretation of Snell’s Law, but I told you we'd have to think outside the box. With a dose of Maxwell’s Laws, Sir John has come up with a geometrical formula for this kind of bending, and even better, has given experimentalists the design for nano-scale patterns that will create the effect of a negative index of refraction. When etched into copper sheets, these patterns can deflect radar signals around an object: the ultimate cloaking device!
Working with the even shorter wavelengths of light presents a bit more of a challenge, but there has been some progress. The latest experiments are with a fishnet structure of silver and magnesium fluoride that has a negative refractive index. Alternately, it’s not necessary to make something completely invisible. As Sir John pointed out, if you can make something appear infinitely flattened, then it will be invisible on an appropriate flat surface. He demonstrated this effect by showing how a carefully sculpted prism of calcite could make a triangular region appear exactly like a flat mirror. On the opposite extreme of wavelengths, Sir John has also proposed some ideas for deflecting earthquake waves around sensitive buildings like nuclear power plants by drilling a series of holes in a particular pattern around the area to be shielded. This would change the refractive properties of the Earth itself.
The talk went past the originally scheduled time period, but the audience of about 150 was eager for more and had many questions, so the second part of the talk — “What is it like to be a Physicist” — was rather short. I was quite impressed with the nuggets of advice that Sir John tossed to us in that brief time:
- Any memorable mistakes? — So many mistakes they are no longer memorable. A theorist must have MANY ideas, and most must be thrown away.
- Advice for starting Physicists? — Practice! If practicing math problems and logic puzzles is not fun for you, then perhaps Physics is not the right career for you. Communicate! You must be able to explain your ideas to other people. Collaborate! Build good relationships. Modern science is all about teamwork.
After the talk, the audience was invited to tour the University of Ottawa’s Advanced Research Complex, but that will have to be the topic of another article.
If you weren’t able to be there, you can watch a similar lecture
given at the Imperial College of London in 2013.