[IP] The Economist: How to weave a cloak that makes you invisible
Begin forwarded message:
From: Bob Rosenberg <bob@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Date: May 25, 2006 10:06:58 PM EDT
To: dave@xxxxxxxxxx
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@xxxxxxxxxxxxxxxxxxxxxxxxxx
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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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