Tuesday, June 12, 2007

Light Fantastic: Flirting With Invisibility

The enormity of exploding complexity stalling progress when dealing with disconnected points of view devoid of a viewing point in Science (downsize complexity advance science boundaries and Understanding with StarSteps) : HOW A STEEL DOOR MIGHT BECOME INVISIBLE, OR BETTER YET, A SEE THROUGH WINDOW - We are accustomed to thinking of metals as being completely opaque. However, ordinary glass is just a dense a many metals and harder than most, yet transmits light quite readily. Most matter is opaque to light because the photons of light are captured and absorbed in the electron orbits of the atoms through which they pass. This capture will occur whenever the frequency of the photon matches one of the frequencies of the atom. The energy thus stored is soon re-emitted, but usually in the infra-red portion of the spectrum, which is below the range of visibility, and so cannot be seen as light. There are several ways in which almost any matter can be made transparent, or at least translucent. One method is to create a field matrix between the atoms which will tend to prevent the photon from being absorbed. Such a matrix develops in many substances during crystallization. Another is to raise the frequency of the photon above the highest absorption frequency of the atoms. A beam of energy through a frequency multiplier penetrates the metal on one side and acts upon any light that reaches it in such a way that the resulting frequency is raised to the X-ray and cosmic ray spectrum. At these frequencies, the waves pass through the metal readily. As the waves leave the metal on the inner side, they again interact with the viewing beam, producing beat frequencies which are identical with the original frequencies of the light (a rough analogy this system could be compared to the carrier waves of a radio broadcasting station, except that the modulation is applied 'upstream', instead of at the source of the carrier.

June 12, 2007
Light Fantastic: Flirting With Invisibility

Increasingly, physicists are constructing materials that bend light the “wrong” way, an optical trick that could lead to sharper-than-ever lenses or maybe even make objects disappear.
Last October, scientists at Duke demonstrated a working cloaking device, hiding whatever was placed inside, although it worked only for microwaves.
In the experiment, a beam of microwave light split in two as it flowed around a specially designed cylinder and then almost seamlessly merged back together on the other side. That meant that an object placed inside the cylinder was effectively invisible. No light waves bounced off the object, and someone looking at it would have seen only what was behind it.
The cloak was not perfect. An alien with microwave vision would not have seen the object, but might have noticed something odd. “You’d see a darkened spot,” said David R. Smith, a professor of electrical and computer engineering at Duke. “You’d see some distortion, and you’d see some shadowing, and you would see some reflection.”
A much greater limitation was that this particular cloak worked for just one particular “color,” or wavelength, of microwave light, limiting its usefulness as a hiding place. Making a cloak that works at the much shorter wavelengths of visible light or one that works over a wide range of colors is an even harder, perhaps impossible, task.
Nonetheless, the demonstration showed the newfound ability of scientists to manipulate light through structures they call “metamaterials.”
Obviously the military would be interested in any material that could be used to hide vehicles or other equipment. But such materials could also be useful in new types of microscopes and antennae. So far, scientists have written down the underlying equations, performed computer simulations and conducted some proof-of-principle experiments like the one at Duke. They still need to determine the practical limitations of how far they can bend light to their will.
The method is not magic, nor are the materials novel. Physicists are taking ordinary substances like fiberglass and copper to build metamaterials that look like mosaics of repeating tiles. The metamaterials interact with the electric and magnetic fields in light waves, manipulating a quantity known as the index of refraction to bend the light in a way that no natural material does.
“There are some things that chemistry can’t do on its own,” said John B. Pendry, a physicist at Imperial College London. “The additional design flexibility with introducing structure as well as chemistry into the equation enables you to reach properties that just haven’t been accessible before.”
When a ray of light crosses a boundary from air to water, glass or other transparent material, it bends, and the degree of bending is determined by the index of refraction.
Air has an index of 1. Water’s index of refraction is about 1.3. That is why rippling water waves distort the view of a pond bottom, for instance. It is refraction that makes a straw in a glass of water look as if it is bending toward the surface, and fish swimming in a pond look closer to the surface than they really are.
Diamonds have a refractive index of 2.4, giving them their sparkling beauty.
For visible light, transparent materials like glass, water and diamonds all have an index of 1 or higher, meaning that when the light enters, its path bends inward, closer to the perpendicular. Because the index is uniform throughout a material, the bending occurs only as the light crosses a boundary.
But with metamaterials, scientists can now also create indexes of refraction from 0 to 1. In the Duke cloaking device, the index actually varies smoothly from 0, at the inside surface of the cylinder, to 1, at the outside surface. That causes the path of light to curve not just at the boundaries, but also as it passes through the metamaterial.
Metamaterials first took center stage in a scientific spat a few years ago over a startling claim that the index of refraction could be not just less than 1, but also negative, less than 0. Light entering such a material would take a sharp turn, almost as if it had bounced off an invisible mirror as it crossed the boundary.
The refractive index depends on the response of a material to electric and magnetic fields. Typically within a material, electrons flow in a way to minimize the effects of an external electric field, producing an internal electrical field in the opposite direction. But that is not universally true. For some metals like silver, an oscillating electric field induces a field in the same, not opposite, direction.
Victor G. Veselago, a Russian physicist, realized in the 1960s that if it were possible to find a material that responded in a contrarian way not just to electric fields and but also magnetic fields, a result would be a negative index of refraction.
Dr. Pendry was among the first to start making metamaterials in the late ’90s, building a structure of thin wires that responded to electrical fields in a way opposite most materials. He also designed one that reacted similarly to magnetic fields.
Dr. Smith, then at the University of California, San Diego, attended a talk by Dr. Pendry at a conference in 1999. He and his colleagues built the first metamaterial to combine electric and magnetic behavior.
The journal Physical Review Letters rejected his scientific paper describing the experiment, considering it simplistic and uninteresting. Only then did Dr. Smith come upon Dr. Veselago’s work on negative refraction and the larger implications of the experiment. “We had it, but we didn’t realize it,” said Dr. Smith, who is now at Duke. “Then I rewrote the abstract, and it was accepted.”
That set off a contentious back and forth that lasted several years between researchers who made and measured negative-refraction metamaterials and those who said that the experiments showed nothing of the sort, that negative refraction was at best an illusion and violated the laws of physics.
Part of the difficulty in resolving the controversy was that the negative refraction experiments were at microwave wavelengths. Designing metamaterials for shorter wavelengths and higher frequencies like visible light is more difficult, because fewer materials are transparent at the higher frequencies.
“Just look around the room,” Dr. Pendry said. “How many things can you see through? Not many. You’re running out of road.”
This year, researchers at the Ames Laboratory in Iowa and Karlsruhe University in Germany reported making a metamaterial that had a negative index of refraction for a visible wavelength.
Some critics remain unmollified. Nicolás García of the Spanish National Research Council still calls Dr. Pendry’s statements on negative refraction “propaganda.” But today, most physicists accept the negative refraction interpretation.
The debate did highlight limits of metamaterials. They are dispersive, meaning the angle of refraction depends very sensitively on the frequency of light, and they are lossy, meaning that they absorb energy from the light as it passes through.
Nonetheless, Dr. Pendry has proposed that negative refraction materials can be used to make a “superlens” because they sidestep a process called diffraction that blurs images taken via conventional optics.
Researchers led by Xiang Zhang, a professor at the University of California, Berkeley, have demonstrated that a thin, flat piece of silver can indeed produce such images, able to resolve two thin lines separated by 70 billionths of a meter.
“You put your object on one side and your image will be projected on the other side,” Dr. Zhang said.
The superlens can also preserve detail lost in conventional optics. Light is usually thought of as having undulating waves. But much closer up, light is a much more jumbled mess, with the waves mixed in with more complicated “evanescent waves.”
The evanescent waves quickly dissipate as they travel, and thus are usually not seen. A negative refraction lens actually amplified the evanescent waves, Dr. Pendry calculated, and that effect was demonstrated by Dr. Zhang’s experiment. A negative refraction could someday lead to an optical microscope that could make out tiny biological structures like individual viruses.
The main limit now is that an object has to be placed very close to the lens, within a fraction of a wavelength of light.
Another possible use would be for a DVD-type recorder. The finer focus could allow more data like high-definition movies to be packed in the same space, perhaps the entire Library of Congress on a platter the size of today’s DVD, Dr. Zhang said.
The metamaterials researchers also look for new problems to solve. “Now it’s sort of fired up our imaginations to do this cloaking thing,” Dr. Pendry said, “because we realized we could actually make one using these materials.”
In May 2006, Dr. Pendry and Dr. Smith proposed a design that would cloak a single microwave frequency. By October, Dr. Smith’s group at Duke demonstrated a working version, although simplified and imperfect. Dr. Smith’s microwave design cannot be adapted to visible light, because the energy absorption problem becomes too great.
This year, Vladimir M. Shalaev of Purdue displayed a different design, avoiding the absorption problem. He said it would cloak visible light, albeit just a single wavelength at a time. “We can make our cloak for any of these colors but not for all of them simultaneously,” Dr. Shalaev said. “At least, it starts looking like it’s doable.”
He said he hoped to build the design, which requires tiny rods arrayed around a cylinder, in a few years. Metamaterials could also be used for other novel devices. Dr. Shalaev suggested an “anticloak” that would trap light of a certain wavelength. “That could be used as a sensing device,” he said.
Whether the cloak could be made big enough to cover a teenage wizard or an alien spaceship is another question. “I’m fairly pessimistic knowing what I know now,” Dr. Smith said.
Dr. Shalaev said it would be a challenge. “I don’t know,” he said. “We hope it is possible.”

Tuesday, June 05, 2007

The Universe, Expanding Beyond All Understanding

(StarSteps) The Curvature of All Natural Law: If it is space that is expanding, it is difficult to understand why we have never detected the increasing distance between the earth and the moon or the sun. No attempt is made to explain why the space which exists between the individual atoms, and between the component parts of those atoms, should not expand also.
None of these difficulties, of course, invalidate any of the mathematical laws from which the concepts have been derived, but they do emphasize the great need for explanations which are more compatible with reason and understanding. For instance, in the above case would it not be simpler to assume that the degree of separation which exists between the galaxies, when considered as individual bodies, is apparently increasing because they occupy opposite portions of the sine curve of natural law?
If we exchange our postulate of linear laws and a "curved space" for a concept which incorporates the curvature of natural law, we find that we have not thereby destroyed or invalidated any of our present mathematics, but we have achieved a position from which the operation of the natural laws can be pictured by the mind and can be charted upon paper. Thus we have taken a great stride in the direction of understanding.

NYT June 5, 2007
The Universe, Expanding Beyond All Understanding
When Albert Einstein was starting out on his cosmological quest 100 years ago, the universe was apparently a pretty simple and static place. Common wisdom had it that all creation consisted of an island of stars and nebulae known as the Milky Way surrounded by infinite darkness.
We like to think we’re smarter than that now. We know space is sprinkled from now to forever with galaxies rushing away from one another under the impetus of the Big Bang.
Bask in your knowledge while you can. Our successors, whoever and wherever they are, may have no way of finding out about the Big Bang and the expanding universe, according to one of the more depressing scientific papers I have ever read.
If things keep going the way they are, Lawrence Krauss of Case Western Reserve University and Robert J. Scherrer of Vanderbilt University calculate, in 100 billion years the only galaxies left visible in the sky will be the half-dozen or so bound together gravitationally into what is known as the Local Group, which is not expanding and in fact will probably merge into one starry ball.
Unable to see any galaxies flying away, those astronomers will not know the universe is expanding and will think instead that they are back in the static island universe of Einstein. As the authors, who are physicists, write in a paper to be published in The Journal of Relativity and Gravitation, “observers in our ‘island universe’ will be fundamentally incapable of determining the true nature of the universe.”
It is hard to count all the ways in which this is sad. Forget the implied mortality of our species and everything it has or has not accomplished. If you are of a certain science fiction age, like me, you might have grown up with a vague notion of the evolution of the universe as a form of growing self-awareness: the universe coming to know itself, getting smarter and smarter, culminating in some grand understanding, commanding the power to engineer galaxies and redesign local spacetime.
Instead, we have the prospect of a million separate Sisyphean efforts with one species after another pushing the rock up the hill only to have it roll back down and be forgotten.
Worse, it makes you wonder just how smug we should feel about our own knowledge.
“There may be fundamentally important things that determine the universe that we can’t see,” Dr. Krauss said in an interview. “You can have right physics, but the evidence at hand could lead to the wrong conclusion. The same thing could be happening today.”
The proximate culprit here is dark energy, which has been responsible for much of the bad news in physics over the last 10 years. This is the mysterious force, discovered in 1998, that is accelerating the cosmic expansion that is causing the galaxies to rush away faster and faster. The leading candidate to explain that acceleration is a repulsion embedded in space itself, known as the cosmological constant. Einstein postulated the existence of such a force back in 1917 to explain why the universe didn’t collapse into a black hole, and then dropped it when Edwin Hubble discovered that distant galaxies were flying away — the universe was expanding.
If this is Einstein’s constant at work — and some astronomers despair of ever being able to say definitively whether it is or is not — the future is clear and dark. In their paper, Dr. Krauss and Dr. Scherrer extrapolated forward in time what has become a sort of standard model of the universe, 14 billion years old, and composed of a trace of ordinary matter, a lot of dark matter and Einstein’s cosmological constant.
As this universe expands and there is more space, there is more force pushing the galaxies outward faster and faster. As they approach the speed of light, the galaxies will approach a sort of horizon and simply vanish from view, as if they were falling into a black hole, their light shifted to infinitely long wavelengths and dimmed by their great speed. The most distant galaxies disappear first as the horizon slowly shrinks around us like a noose.
A similar cloak of invisibility will befall the afterglow of the Big Bang, an already faint bath of cosmic microwaves, whose wavelengths will be shifted so that they are buried by radio noise in our own galaxy. Another vital clue, the abundance of deuterium, a heavy form of hydrogen manufactured in the Big Bang, in deep space, will become unobservable because to be seen it needs to be backlit from distant quasars, and those quasars, of course, will have disappeared.
Eventually, in the far far future, this runaway dark energy will suck all the energy and life out of the universe. A few years ago, Edward Witten, a prominent theorist at the Institute for Advanced Study, called a universe that is accelerating forever “not very appealing.” Dr. Krauss has called it simply “the worst possible universe.”
But our future cosmologists will be spared this vision, according to the calculations. Instead they will puzzle about why the visible universe seems to consist of six galaxies, Dr. Krauss said. “What is the significance of six? Hundreds of papers will be written on that,” he said.
Those cosmologists may worry instead that their galaxy cloud will collapse into a black hole one day and, like Einstein, propose a cosmic repulsion to prevent it. But they will have no way of knowing if they were right.
Although by then the universe will be mostly dark energy, Dr. Krauss said, it will be undetectable unless astronomers want to follow the course of the occasional star that gets thrown out of the galaxy and is caught up in the dark cosmic current. But it would have to be followed for 10 billion years, he said — an experiment the National Science Foundation would be unlikely to finance.
“This is even weirder,” Dr. Krauss said. “Five billion years ago dark energy was unobservable; 100 billion years from now it will become invisible again.”
It turns out that you don’t actually need dark energy to be this pessimistic about the future, as Dr. Krauss and Dr. Scherrer point out. In 1987, George Ellis, a mathematician and astronomer at the University of Cape Town, in South Africa, and Tony Rothman, currently lecturing at Princeton, wrote a paper showing how even ordinary expansion would gradually carry most galaxies too far away to be seen, setting the stage for cosmic ignorance.
Dark energy speeds up the picture, Dr. Ellis said in an e-mail message, adding that he was glad to see the new paper, which adds many astrophysical details. “It’s an interesting gloss on the far future,” he said.
James Peebles, a Princeton cosmologist, said there were more pressing worries. We might be headed toward a universe that is “asymptotically empty,” he said, “But I have the uneasy feeling that the U.S.A. is headed into asymptotic futility well before that.”
You might object that the inhabitants of the far future will be far more advanced than we are. Maybe they will be able to detect dark energy — or the extra dimensions of string theory, for that matter — in the laboratory. Maybe they will even be us, in some form or other, if the human race manages to get out of the solar system before the Sun blows up in five billion years. But if relativity is right, they won’t be able to build telescopes that can see past the edge of the universe.
It’s not too late to start thinking about sending out the robot probes that could drift down through alien skies eons from now with, if not us or our DNA, at least a few nuggets of wisdom — that the world is made of atoms and that it started with a bang.
The lesson in the meantime is that we don’t know what we don’t know, and we never will — a lesson that extends beyond astronomy.
Einstein once said, “The Lord God is subtle but malicious he is not.”
I wondered in light of this new report whether it might be time to revise that quotation. Max Tegmark, a cosmologist at the Massachusetts Institute of Technology, told me the problem was not malice but human arrogance — a necessary but unfortunate condition for scientific progress.
“We have a tendency to put ourselves at the center of the universe,” he said. “We assume all we see is all there is.”
But, as Dr. Tegmark noted, Big Bang theorists already suppose that basic aspects of the universe are out of sight.
The reason we believe we live in a smooth, orderly universe instead of the chaotic one that is more likely, they say, is that the chaos has been hidden. According to the dominant theory of the Big Bang, known as inflation, an extremely violent version of dark energy blew it up a fraction of a second after time began, stretching and smoothing space and pushing all the wildness and chaos and even perhaps other universes out of the sky, where they will never be seen.
“Inflation tells us we live in a messy universe,” Dr. Tegmark said. Luckily we never have to confront it.
Ignorance is us, or is it bliss?