The Economist: How to weave a cloak that makes you invisible

[David Farber <dave () farber net>]

Begin forwarded message:

From: Bob Rosenberg <bob () bobrosenberg phoenix az us>
Date: May 25, 2006 10:06:58 PM EDT
To: dave () farber net
Subject: The Economist: How to weave a cloak that makes you invisible

Dave

Fascinating, and perhaps for IP.

Cordially,

Bob Rosenberg
P.O. Box 33023
Phoenix, AZ  85067-3023
LandLine:  (602)274-3012
Mobile:  (602)206-2856
bob () bobrosenberg phoenix az us

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“Education's purpose is to replace an empty mind with an open one.”
                               Malcolm Forbes

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Optical materials

Cast no shadows
May 25th 2006
From The Economist print edition
http://www.economist.com/science/displaystory.cfm?story_id=6971134

How to weave a cloak that makes you invisible

IN NORSE mythology, a magic cloak granted invisibility to Sigurd, a  
demi-god and
skilled warrior with superhuman strength. Millennia later, a similar  
garment
bestowed invisibility on Harry Potter, a schoolboy wizard. In the  
mortal (or
Muggle) realm, engineers have for years tried with varying degrees of  
success
to build such a device. This week a team of physicists and materials  
scientists
announced it had devised a pattern for a potentially perfect  
invisibility cloak.

Light is an electromagnetic wave, with a longer wavelength than X- 
rays and
ultraviolet, and a shorter wavelength than infra-red, microwaves and  
radio
waves. All these electromagnetic waves are governed by four mathematical
expressions established almost 150 years ago by James Clerk Maxwell.  
These
equations represent one of the most elegant and concise ways to state  
the
behaviour of electric and magnetic fields and how they interact with  
matter.
However, because they are so concise, they also embody a high level of
mathematical sophistication.

The team—Sir John Pendry of Imperial College London with David  
Schurig and David
Smith of Duke University in North Carolina—used the equations to  
devise a way to
cloak an object with a material that would deflect the rays that  
would have
struck it, guide them around it and return them to their original  
trajectory.
Maxwell's equations conserve certain properties—the magnetic field  
intensity,
the electric displacement field and the Poynting vector that  
describes the
electric flux of an electromagnetic field. These properties remain  
the same
when others are altered. The team showed how these fields could be  
manipulated
to flow around objects like a fluid, returning undisturbed to their  
original
paths. The findings were published online this week by Science.

The trick is to use metamaterials: materials that owe their  
characteristics to
features of their structure that are smaller than the wavelength of the
electromagnetic radiation. For light, this is on the scale of tens of
thousandths of a millimetre. Metamaterials can and have been designed  
and made
to possess certain properties, even counter-intuitive ones. Using  
metamaterials
means the scheme should work for all objects, regardless of their shape.
Moreover, unlike other proposed invisibility cloaks, it does not require
knowledge of what is behind the wearer, nor are crude projections  
involved.

So far, so good: the theory is in place. Sadly, the implementation  
lags behind.
Moreover, there are several other difficulties that may prevent a device
conferring total invisibility from being built. The first is that the  
plan
described by Sir John and his colleagues works only for a small range of
wavelengths. A surgeon wearing metamaterial gloves tuned to make his  
hands
invisible might benefit from being able to see exactly where the  
scalpel was
cutting. However, an invisibility cloak designed to hide something  
from people
who were looking for it would not work. An aeroplane shrouded in such  
kit might
be invisible to the human eye but it would be picked up readily by  
radar, which
operates at radio wavelengths.

Even if it ultimately proved possible to make an aeroplane completely  
invisible
at all wavelengths, there would be a further problem. According to  
the laws of
physics, an invisible person would necessarily be blind. In order to  
see light,
the eye must absorb it, but in order for a person to be invisible,  
the body must
not absorb any light. Thus, a spy plane could not be completely  
invisible if it
were to be used for espionage or, indeed, flown at all, since its  
pilots would
need to know its position relative to the ground.


Copyright © 2006 The Economist Newspaper and The Economist Group. All  
rights
reserved.




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