This post is a review of the book Invisibility: The History and Science of How Not to be Seen by Gregory G. Gbur. (Full disclosure: Greg is a colleague and a friend; we overlapped in graduate school).
There is a lot of public curiosity about this topic (invisibility) and the book is well poised to address it. Here are some pressing questions the book answers:
Can an object be perfectly hidden, according to the principles of science?
Short answer: it's not clear.
Long answer: it seems to be almost impossible if the object is made of natural materials (this is what the theorems say). On the other hand, it seems to be possible if the object is a substance specially designed by human beings (metamaterials). This is what the experiments show.
In any case, all concrete demonstrations so far have involved some kind of conditional invisibility - the object disappears only when viewed from a certain angle, or using only certain colors of light, or only when it is made of a certain kind of material.
Are there different types of invisibility?
The book broadly categorizes these into two types.
1. Active invisibility: This indicates invisibility caused by some device which measures as well as generates light to create the illusion.
Examples include those by Susumu Tachi (he made an invisibility cloak which recorded the scene behind the wearer using a camera and then projected it from the front) and Alaina Gassler's prize-winning use of similar ideas to remove the blind spots due to the A-pillars in her family car. (I think all cars should have this).
The book does not deal much with this kind of invisibility.
2. Passive invisibility: This indicates invisibility caused by some device which only guides the light illuminating the object. Examples are (refractive index matching) such as when pyrex glass is made to vanish in mineral oil, beads disappear in water, and refraction through calcite is used to cloak the background.
This is mainly the kind of invisibility that the book describes.
What is the scientific basis of invisibility?
Light can be thought of as consisting of waves or rays.
Invisibility can be thought of in terms of waves. Things become visible by scattering light (like when we shine a torch on them). So the way to stop visibility is to avoid light scattering. This can be done using the fact that light is made of waves and can show destructive interference.
Greg's book traces the history of this line of scientific research starting with Ehrenfest's paper on accelerating but nonradiating electric charges (for the experts - remember quantum physics had to be invented because point charges, i.e. electrons in atoms, could not keep to stable orbits around the nuclei - they would radiate away their energy; Ehrenfest found extended charge distributions that accelerated but did not radiate).
Among others, the book describes the relevant contributions of Emil Wolf (Greg's PhD advisor, who also taught me Complex Analysis) on light scattering (the theorems mentioned above).
Finally the book arrives at the work of Veselago, Pendry and Leonhardt, who basically started the field of artificial (human-made) substances that allowed for cloaking better than what was available in nature. These substances are called metamaterials.
Invisibility, in these arrangements, can be explained by thinking of light as consisting of rays. In these designs, light rays flow like water around an obstacle so that downstream it is not possible to infer of the existence of these obstacles.
As we all know, rays are deflected when light goes from one medium to another (this is why straws look bent as light from it exits from water into air); what changes at the interface is the refractive index. So refractive index engineering, aimed at bending light appropriately, can be used to create invisibility.
The general field is now called Transformation Optics: it involves warping space to govern light flow (if this reminds you of general relativity, you are bang on - the mathematics is the same).
Outlook
According to the book, research on invisibility cloaks has been extended to interesting applications like thermal cloaks (e.g. to prevent heat loss), sea cloaks (to protect oil rigs, etc. from rogue waves in the ocean), acoustic cloaks (for noise cancellation purposes, etc.), and seismic cloaks (to protect buildings from earthquake waves). All of these involve the manipulation of waves and creation of their destructive interference.
I thought it was interesting (though in hindsight perhaps inevitable) that Greg also discusses the 'anti-cloak' - a device that cancels a cloak, and makes the object visible. There is also now a field of 'illusion optics' which replaces the real object by a desired illusion.
Summary
Overall a great read and handy reference. The prose is direct and the scientific technicalities are handled very well. The book covers the history of the subject nicely, including ancient myths, references in the popular literature and film. Interestingly, no magicians are mentioned.
Bonus
As a special treat the books ends with two appendices, one for building your own invisibility device, and the other, an 'invisibibliography' (Greg's word) - a list of invisibility stories from the literature.
Comments