How to Destroy the Universe (10 page)

• Natural camouflage

• Stealth tech

• Invisibility cloak

• Hiding spacecraft

• Metamaterials

• Not being seen

Harry Potter's invisibility cloak gets him out of many a scrape, but could you ever build such a camouflage garment for real? A working invisibility cloak wouldn't just be a boon for young wizards, but for the military, in medicine and for anyone hoping to pull a sickie without getting spotted in town by their boss. The good news is physicists now think they can do it.

Camouflage is found throughout nature. Moths, tigers, even some fish that look virtually transparent underwater, are all making use of techniques that make them less visible, in turn helping them to evade predators or creep up on prey. Humans are no exception. We've
become adept at hiding ourselves. Soldiers, for example, have perfected the art of blending in with their surroundings. But simple camouflage isn't quite what we mean by invisibility—the ability to completely disappear from view. Stage magicians have long made use of cunning arrangements of mirrors to create the illusion of invisibility. In practice, though, no one wants to be rattling around with glass plates bolted to their body. It would hardly be very stealthy.

Invisibility of a sort has been in use by the military since as far back as World War II, when aircraft designers began building military planes that were invisible to radar. The German Horton Ho 2-29 was a “flying wing” style aircraft coated with a radar-absorbing mixture of wood glue and charcoal. Luckily for the Allies, the war ended before the Germans could put the plane into service. In the 1960s, American engineers rolled out their first stealth aircraft, the Lockheed SR-71 Blackbird. This was a spy plane that used basic stealth technologies to surreptitiously photograph Soviet bases.

Today, the most famous stealth aircraft is the B-2 Spirit stealth bomber. Despite its wingspan of 50 m (160 ft), to a radar installation it looks no bigger than an aluminum marble. It achieves this first of all by its
shape. The B-2's angles are designed so that a radar beam coming in from any direction will not be reflected directly back to the source—which would mean detection for the plane. Instead, it's rather like a disco glitter ball, scattering the radar beam off in all directions. Designers do this by carefully shaping the surfaces and by avoiding internal right angles—a radar beam striking a right-angled corner from any direction will be bounced right back to its source (just like a squash ball hit into the corner of the court will come right back at you). For this reason weapons and engines are all mounted internally. On some stealth fighter aircraft, even the pilot's head can cause an unwanted reflection, which is usually solved by coating the canopy glass with a thin layer of reflective gold. The exact shape that minimizes radar signature is hard to calculate, and so must be done using computers. The fact that the first mainstream stealth aircraft—the F117-A Nighthawk stealth fighter—had an angular polyhedral shape is largely because computers in the 1970s were only powerful enough to model the radar return from flat surfaces, with none of the sleek lines that would later grace the B-2. The weird shape of stealth planes makes them inherently unaerodynamic, and therefore quite unstable to fly. They get round this by using computers again, this time on board to constantly adjust the flight surfaces—flaps, rudder and so on—to stop the plane from careering out of control.

Shape isn't the only consideration. Like the early Ho 2-29, modern stealth aircraft are coated with radarabsorbent paint. Rather than wood glue and carbon, though, this contains tiny iron balls that soak up the radar energy, turning it into heat which can then be lost to the air rushing over the plane. Some stealth aircraft also add cool air to the engine exhaust to reduce their heat signature and even chemicals to minimize the formation of water vapor, which itself can have a strong radar signature. With all this technology on board, a single B-2 costs $2.67 billion. It is literally worth more than its weight in gold.

Some early attempts to make aircraft stealthy involved putting lights on them to try and match their brightness to that of the background sky. Military pundits predict that as wars against technologically inferior opponents become more common, modern military aircraft could go the same way—as visual camouflage becomes more important than radar invisibility. This wouldn't be done using simple lights though, but flatpanel display screens to show an image on the aircraft's exterior of what's behind it, so rendering it virtually invisible to any onlookers. Similar technology has already been demonstrated by researchers at the University of Tokyo, who have made Harry Potterstyle invisibility cloaks. Images of what's behind the
wearer are relayed from video cameras to a projector, which displays them on the cloak's silvery fabric. The results are impressive, reducing the wearer to little more than a ghostly outline with objects behind them clearly visible.

The Tokyo team's set-up is crude. But the technology is set to improve drastically as cameras become ever smaller (think of the size of the camera in your mobile phone) and display screens become thinner and more flexible. For example, the Media Lab at MIT in Boston (and, in fact, many other groups around the world) are developing screens so thin and flexible they're known generically as “electronic paper.” This is a kind of invisibility that exists in practice—not just in theory—in the world today. Of course, pedants might assert, however, that this is merely an illusion. Can we do better?

In H.G. Wells's novel
The Invisible Man
, the scientist Griffin uses a cocktail of chemicals to change the refractive index of his body to that of air. Refraction is a property of substances caused when their density slows down the passage of light through them. This makes the light tend to bend toward the “normal” (a line perpendicular to the interface between two media—see opposite page) as it moves into a denser medium, and away from the normal as it moves into a medium that's less dense. In reality, Wells's scheme for invisibility is unworkable—at least for living things—as changing the refractive properties would mean altering the chemical properties of living tissue, which isn't likely to do the tissue's owner all that much good.

A more promising route is to try to deflect the light around the object that you want to make invisible. But how? In his book
The Physics of Star Trek
, US scientist Lawrence Krauss tries to explain the cloaking devices that are used by the Klingon and Romulan alien races in the show to render their interstellar battle cruisers invisible. Krauss's explanation makes use of Einstein's
general theory of relativity, our best theory of gravity, which works by ascribing gravity to curvature of space and time.

Because general relativity dictates the stage on which the whole of physics is played out, it means that it's not just solid objects that feel the force of gravity but beams of light as well. By distorting space in just the right way, it is possible to deflect light beams around an intervening object so that they emerge on the same paths they would be traveling on were the object not there, and space not distorted. It's a great idea. The only trouble is that doing this requires a truly colossal amount of matter.

In 1919, British astronomer Sir Arthur Eddington carried out one of the key tests of general relativity, which involved measuring the degree to which starlight passing close to the Sun is bent by its gravity. Eddington confirmed Einstein's prediction that light grazing the Sun's surface is deflected by a minuscule 1.75 seconds of arc—less than one two-thousandth of a degree. And that's by the entire mass of the Sun. Even “gravitational lensing”—the phenomenon exploited by astronomers to see quasars at the edge of the visible Universe (see
How to see the other side of the Universe
) requires the mass of an entire intervening galaxy cluster to do the bending. That's many hundreds of billions of Suns! Certainly not something the average Romulan spacecraft could fit in its cargo hold.

Perhaps the most promising attempt at proper invisibility lies in the research of British physicist Professor John Pendry. In 2006, Pendry together with colleagues from the United States put forward a theoretical scheme whereby light could be channeled around an object by a special kind of material that acts in much the same way as the gravitational system envisaged by Lawrence Krauss—so that the light emerges on the other side traveling on the same path it would be on were the object and the cloak not there. Pendry likens it to water in a stream flowing around an obstruction.

His hypothetical substance is called a metamaterial—a material that has been carefully engineered on the smallest scales to have a specific set of properties that cannot be found in nature. Pendry's metamaterial would have to be very special, refracting the light away from the normal despite being optically denser than the surrounding air (remember, optically dense media typically refract the light the other way, toward the normal). This strange property is known as negative refractive index. In 2008, a multinational team of researchers actually succeeded in fabricating the first real examples of these weird, light-bending metamaterials. They are made by perforating pieces of silicon with a carefully designed pattern of holes—each just a hundred nanometres in diameter (about one ten-thousandth of a millimeter). These holes guide the
light through the silicon in just the right way to give the desired negative refractive index.

So far, the team have only demonstrated two-dimensional metamaterials, and tiny ones at that—just a few billionths the size of a postage stamp. The next step will be a metamaterial that can create a negative refractive index in all three dimensions. Team member Dr. Jensen Li believes this will be possible in the next few years.

A working invisibility cloak would have obvious military applications, to camouflage planes, tanks, ships and individual soldiers. And whereas existing stealth systems act specifically on waves of a particular frequency such as radar or light, metamaterials work at all frequencies simultaneously. There are peaceful applications aplenty as well. Don't like those plans to build a wind farm overlooking an area of natural beauty? Clad them in some of Professor Pendry's meta-material and you'll never know they're there—until you accidentally blunder into one, that is. Pendry even believes there could be medical applications. A meta-material cloak could be made to deflect not just beams of light, but any kind of electromagnetic wave—including strong magnetic fields. And so, for example, in MRI scanners—where it's not possible to
have any metal objects or instruments present because of the magnetic fields involved—these objects could be wrapped in metamaterial to block their interaction with the magnetism. And, as with any breakthrough area of scientific research, there will be umpteen other applications too, many of which no one will have seen coming.

• Young's experiment

• Why bands?

• Coherent light

• Monochrome vision

• Lasers

• Quantum double slits

• Schrödinger's equation

In the busy 21st-century world, many of us might wish we could be in more than one place at the same time. For subatomic particles this isn't a problem. According to the abstruse laws of quantum mechanics, particles of matter also behave like waves—enabling them to be not just in two places at once, but everywhere. One simple experiment gave physicists the insight they needed to unravel the laws of quantum theory.

The double-slit experiment is probably the most startling demonstration of quantum physics. It was first carried out before quantum theory was even a twinkle
in the eye of Max Planck, Einstein and colleagues. Nobel prize-winning US quantum physicist Richard Feynman would later remark that pretty much every aspect of quantum physics is encapsulated by this astounding experiment. So what is it?

British physicist Thomas Young was the first person to perform this experiment in 1801. Young was trying to figure out whether light is made of particles or waves. To do this he shone a beam of light onto a screen with a pair of narrow slits cut into it. The light passed through the slits to illuminate a second screen, this time with no slits. Young postulated that if light was made of particles then the second screen would be evenly illuminated by the light from the two slits. But if it was made of waves there should be a pattern of bright and dark bands, known as “interference fringes.” And this second possibility is exactly what Young observed.