Book Review: Slow Light (2011) by Sidney Perkowitz

by Ley Hood Hong

16.1_Book_CoverLight moves incredibly fast and we have utilised this property well, making high-speed internet ubiquitous nowadays. Despite current photonics technologies which exploit the manipulation of light, the true nature of light has yet to be fully understood, especially its bizarre behaviour to act as both particle and wave. And how about entangled lights? Two entangled particles of light, regardless of their distance apart, any consequences made on one of the particles will immediately have the same effect on the other. These peculiarities of light have delighted (pun intended) scientists for ages.

The concepts of invisibility, teleportation, faster-than-light travel, and the book title itself are the topics highlighted in Slow Light by Sidney Perkowitz, a professor emeritus of physics at Emory University. We are always amazed when we see these phenomena in action as depicted in sci-fi or fantasy movies, especially the famed Harry Potter’s invisibility cloak or Star Trek’s warp drive. Ever wonder how close science and technology are to achieve these “magic”? If you have, then this is the book to whet your curiosity palate. In his book, Perkowitz discusses the capability of our current technologies in achieving such fictional ideas and how these ideas influence researches in science.

If slow glass is placed at a location in the past such that the light will take, say, 50
years to travel through slow glass, the ‘past’ of that location 50 years ago might
be seen and the effects may be similar with this figure. Photo:

Perkowitz first tackles faster-than-light travels (FTL). Our current technology can only move a spacecraft to 0.005% of the speed of light, the record from NASA’s New Horizons probe directed for Pluto. However, theory of relativity sheds light on making FTL plausible via warp drives, hyperspace or wormholes. Albeit being clichés in sci-fi, all these terms actually have their roots in the theory of relativity

These methods utilise space-time distortion to change the speed of light and the spacecraft. Imagine saving time by walking through a tunnel piercing through a mountain than climbing over it – an analogy by the author in explaining wormhole travels. As Perkowitz noted, wormholes are proposed to be formed by two linked black holes, but to date, there have been no hints of naturally formed wormholes. Perhaps until humanity has found a way to effectively manipulate the space-time fabric, the possibility of FTL will remain far from grasp.

‘Invisibility’ achieved by Susumu Tachi and his group. Photo:

Another lesser known idea pointed out by Perkowitz refers to a science fiction story by Bob Shaw, “Light of Other Days”, where light is slowed using a medium with huge refractive index so that light would take a thousand years to travel across it. Such material is coined as ‘slow glass’. Scientifically speaking, to fabricate slow glass would require medium with huge refractive index (in the order of trillions), and to date there is no material that could provide such value yet. Not to mention a thin slow glass could not possibly hold a decade’s worth of light energy. Also, due to its high refractive index, no light would enter slow glass as all incoming light will be reflected.

The properties of entangled light, mentioned earlier, can serve as the working principle for quantum teleportation. Neat. But does this mean teleportation is within reach? Yes, but not quite the sort that we’d expect, as instead of transferring an object or a person, quantum teleportation is used to transfer information. Although researchers have eleported atoms, hindered by quantum mechanical principles, this is not possible for macroscopic object. However, quantum teleportation has found its use in cryptography and has been developed for commercial use especially in the field of communications, where security is of paramount importance in data transmission through the internet (such as online funds transfer).

The author described a few types of invisibility that is technologically available – radar invisibility for stealth aircraft, virtual display invisibility (which is actually an illusion), and the invisibility of Harry Potter’s cloak. Radar invisibility has shown splendid achievements: a modern stealth aircraft can appear as a size of a bird in the radar! The experiment on virtual transparency was first attempted by Susumu Tachi and his group in 2003. They achieved invisibility simply by projecting the image of B onto A (where B is behind A) and make A appear to be “invisible”. As for ‘real’ invisibility, in principle this is achievable through the manipulation of light ray using a “cloak” such that light curves around an object and continue in its original direction. In 2010, researchers have successfully cloaked objects up to centimetres in size in the visible light region. This field of research is still in its infant stage, but current result shows a bright future towards achieving the dream of personal invisibility.

I thoroughly enjoyed Slow Light, and Perkowitz also provided a list of further reading on specific topics, divided into fictional and technical reading. In my opinion, I would say this book is rather easy to digest as the author did a good job in explaining complicated concept in a simple manner.


Ley Hood Hong is a PhD student at the Faculty of Science, Universiti Teknologi Malaysia. He has a Master’s degree in Physics from the same university. He can be contacted at [email protected]. Find out more about Hood Hong by visiting his Scientific Malaysian profile at

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