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What is Light?

Light, form of energy visible to the human eye that is radiated by moving charged particles. Light from the sun provides the energy needed for plant growth and plants convert the energy in sunlight into storable chemical form through a process called photosynthesis. Petroleum, coal, and natural gas are the remains of plants that lived millions of years ago, and the energy these fuels release when they burn is the chemical energy converted from sunlight. When animals digest the plants and animals they eat, they also release energy stored by photosynthesis.

Scientists have learned through experimentation that light behaves like a particle at times, and like a wave at other times. The particlelike features are called photons. Photons are different from particles of matter in that they have no mass and always move at the constant speed of 300,000 km/sec (186,000 mi/sec). When light diffracts, or bends slightly as it passes around a corner, it shows wavelike behavior. The waves associated with light are called electromagnetic waves because they consist of changing electric and magnetic fields.

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Nature of Light

To understand the nature of light and how it is normally created, it is necessary to study matter at its atomic level. Atoms are the building blocks of matter, and the motion of one of their constituents, the electron, leads to the emission of light in most sources.

German-born American physicist Albert Einsteinís elegant equation E=mc2 predicted that energy could be converted to matter. Using a linear accelerator and high-energy laser light, physicists have done just that. This 1997 Encarta Yearbook article describes their success.
 


Scientists Create Matter Out of Light

Physicists at the Stanford Linear Accelerator Center (SLAC) in California have succeeded in producing particles of matter from very energetic collisions of light. The team, which included researchers from Stanford University, the University of Rochester in New York, the University of Tennessee in Knoxville, and Princeton University in New Jersey, published an account of their work in the September 1, 1997, issue of the journal Physical Review Letters.

Scientists have long known that matter can be converted to energy and, conversely, energy can be converted to matter. In 1905 physicist Albert Einstein quantified the relationship between matter and energy in his famous equation E=mc2, in which E is energy, m is mass, and c is the speed of light (300,000 km/sec [186,000 mi/sec]). In an atomic bomb blast, a very small amount of matter is converted to its equivalent in energy, creating an immense explosion.

Scientists have also created matter from energy by bombarding heavy atoms (atoms made up of many protons and neutrons) with high-energy radiation in the form of X rays. Collisions between the X-ray beam and the atoms created matter in the form of sets of electron and positron particles, a phenomenon known as pair production. Positrons are particles that have the same weight and amount of charge as electrons, but positrons are positively charged, while electrons are negatively charged.

In the recent experiments at SLAC, physicists accelerated a beam of electrons to nearly the speed of light. They then aimed a split-second pulse of high-energy laser light directly at the electron beam. Occasionally a photon (a tiny, discrete unit of light energy) collided with an electron. The photon then recoiled from the collision and rebounded into oncoming photons from the laser beam with such violence that the resulting energy was converted into an electron-positron pair. Over several months of such experiments, the physicists were able to produce more than 100 electron-positron pairs.

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Light Theories

The earliest speculations about light were hindered by the lack of knowledge about how the eye works. The Greek philosophers from as early as Pythagoras, who lived during the 5th century BC, believed light issued forth from visible things, but most also thought vision, as distinct from light, proceeded outward from the eye. Plato gave a version of this theory in his dialogue Timaeus, written in the 3rd century BC, which greatly influenced later thought.

Some early ideas of the Greeks, however, were correct. The philosopher and statesman Empedocles believed that light travels with finite speed, and the philosopher and scientist Aristotle accurately explained the rainbow as a kind of reflection from raindrops. The Greek mathematician Euclid understood the law of reflection and the properties of mirrors. Early thinkers also observed and recorded the phenomenon of refraction, but they did not know its mathematical law. The mathematician and astronomer Ptolemy was the first person on record to collect experimental data on optics, but he too believed vision issued from the eye. His work was further developed by the Egyptian scientist Ibn al Haythen, who worked in Iraq and Egypt and was known to Europeans as Alhazen. Through logic and experimentation, Alhazen finally discounted Platoís theory that vision issued forth from the eye. In Europe, Alhazen was the most well known among a group of Islamic scholars who preserved and built upon the classical Greek tradition. His work influenced all later investigations on light.

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Modern Theories

Planckís theory remained mystifying until Einstein showed how it could be used to explain the photoelectric effect, in which the speed of ejected electrons was related not to the intensity of light, but to its frequency. This was consistent with Planckís theory, which suggested that a photonís energy was related to its frequency. During the next two decades scientists recast all of physics to be consistent with Planckís theory. The result was a picture of the physical world that was different from anything ever before imagined. Its essential feature is that all matter appears in physical measurements to be made of quantum bits, which are something like particles. Unlike the particles of Newtonian physics, however, a quantum particle cannot be viewed as having a definite path of movement that can be predicted through laws of motion. Quantum physics only permits the prediction of the probability of where particles may be found.

 

The probability is the squared amplitude of a wave field, sometimes called the wave function associated with the particle. For photons the underlying probability field is what we know as the electromagnetic field. The current world view that scientists use, called the Standard Model, divides particles into two categories: fermions (building blocks of atoms, such as electrons, protons, and neutrons), which cannot exist in the same place at the same time, and bosons, such as photons, which can (see Elementary Particles). Bosons are the quantum particles associated with the force fields that act on the fermions. Just as the electromagnetic field is a combination of electric and magnetic force fields, there is an even more general field called the electroweak field. This field combines electromagnetic forces and the weak nuclear force. The photon is one of four bosons associated with this field. The other three bosons have large masses and decay, or break apart, quickly to lighter components outside the nucleus of the atom.

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Gain Assisted Superluminal Light Propagation

by Dr. Lijun Wang

 

The Speed of Light Is Exceeded in Lab

Scientists have apparently broken the universeís speed limit. For generations, physicists believed there is nothing faster than light moving through a vacuum - a speed of 186,000 miles per second. But in an experiment in Princeton, N.J., physicists sent a pulse of laser light through cesium vapor so quickly that it left the chamber before it had even finished entering. The pulse traveled 310 times the distance it would have covered if the chamber had contained a vacuum.

This seems to contradict not only common sense, but also a bedrock principle of Albert Einsteinís theory of relativity, which sets the speed of light in a vacuum, about 186,000 miles per second, as the fastest that anything can go.

But the findings--the long-awaited first clear evidence of faster-than-light motion--are "not at odds with Einstein," said Lijun Wang, who with colleagues at the NEC Research Institute in Princeton, N.J., report their results in todayís issue of the journal Nature.

"However," Wang said, "our experiment does show that the generally held misconception that ínothing can move faster than the speed of lightí is wrong." Nothing with mass can exceed the light-speed limit. But physicists now believe that a pulse of light--which is a group of massless individual waves--can.

To demonstrate that, the researchers created a carefully doctored vapor of laser-irradiated atoms that twist, squeeze and ultimately boost the speed of light waves in such abnormal ways that a pulse shoots through the vapor in about 1/300th the time it would take the pulse to go the same distance in a vacuum.

As a general rule, light travels more slowly in any medium more dense than a vacuum (which, by definition, has no density at all). For example, in water, light travels at about three-fourths its vacuum speed; in glass, itís around two-thirds.

The ratio between the speed of light in a vacuum and its speed in a material is called the refractive index. The index can be changed slightly by altering the chemical or physical structure of the medium. Ordinary glass has a refractive index around 1.5. But by adding a bit of lead, it rises to 1.6. The slower speed, and greater bending, of light waves accounts for the more sprightly sparkle of lead crystal glass.

The NEC researchers achieved the opposite effect, creating a gaseous medium that, when manipulated with lasers, exhibits a sudden and precipitous drop in refractive index, Wang said, speeding up the passage of a pulse of light. The team used a 2.5-inch-long chamber filled with a vapor of cesium, a metallic element with a goldish color. They then trained several laser beams on the atoms, putting them in a stable but highly unnatural state.

In that condition, a pulse of light or "wave packet" (a cluster made up of many separate interconnected waves of different frequencies) is drastically reconfigured as it passes through the vapor. Some of the component waves are stretched out, others compressed. Yet at the end of the chamber, they recombine and reinforce one another to form exactly the same shape as the original pulse, Wang said. "Itís called re-phasing."

The key finding is that the reconstituted pulse re-forms before the original intact pulse could have gotten there by simply traveling though empty space. That is, the peak of the pulse is, in effect, extended forward in time. As a result, detectors attached to the beginning and end of the vapor chamber show that the peak of the exiting pulse leaves the chamber about 62 billionths of a second before the peak of the initial pulse finishes going in.

That is not the way things usually work. Ordinarily, when sunlight--which, like the pulse in the experiment, is a combination of many different frequencies--passes through a glass prism, the prism disperses the white lightís components.

This happens because each frequency moves at a different speed in glass, smearing out the original light beam. Blue is slowed the most, and thus deflected the farthest; red travels fastest and is bent the least. That phenomenon produces the familiar rainbow spectrum.

But the NEC teamís laser-zapped cesium vapor produces the opposite outcome. It bends red more than blue in a process called "anomalous dispersion," causing an unusual reshuffling of the relationships among the various component light waves. Thatís what causes the accelerated re-formation of the pulse, and hence the speed-up

In theory, the work might eventually lead to dramatic improvements in optical transmission rates. "Thereís a lot of excitement in the field now," said Steinberg. "People didnít get into this area for the applications, but we all certainly hope that some applications can come out of it. Itís a gamble, and we just wait and see."

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SCIENTISTS claim they have broken the ultimate speed barrier: the speed of light

by Jonathan Leake

In research carried out in the United States, particle physicists have shown that light pulses can be accelerated to up to 300 times their normal velocity of 186,000 miles per second.

The implications, like the speed, are mind-boggling. On one interpretation it means that light will arrive at its destination almost before it has started its journey. In effect, it is leaping forward in time.

Exact details of the findings remain confidential because they have been submitted to Nature, the international scientific journal, for review prior to possible publication.

The work was carried out by Dr Lijun Wang, of the NEC research institute in Princeton, who transmitted a pulse of light towards a chamber filled with specially treated caesium gas.

Before the pulse had fully entered the chamber it had gone right through it and travelled a further 60ft across the laboratory. In effect it existed in two places at once, a phenomenon that Wang explains by saying it travelled 300 times faster than light.

The research is already causing controversy among physicists. What bothers them is that if light could travel forward in time it could carry information. This would breach one of the basic principles in physics - causality, which says that a cause must come before an effect. It would also shatter Einsteinís theory of relativity since it depends in part on the speed of light being unbreakable.

This weekend Wang said he could not give details but confirmed: "Our light pulses did indeed travel faster than the accepted speed of light. I hope it will give us a much better understanding of the nature of light and how it behaves."

Dr Raymond Chiao, professor of physics at the University of California at Berkeley, who is familiar with Wangís work, said he was impressed by the findings. "This is a fascinating experiment," he said.

In Italy, another group of physicists has also succeeded in breaking the light speed barrier. In a newly published paper, physicists at the Italian National Research Council described how they propagated microwaves at 25% above normal light speed. The group speculates that it could be possible to transmit information faster than light.

Dr Guenter Nimtz, of Cologne University, an expert in the field, agrees. He believes that information can be sent faster than light and last week gave a paper describing how it could be done to a conference in Edinburgh. He believes, however, that this will not breach the principle of causality because the time taken to interpret the signal would fritter away all the savings.

"The most likely application for this is not in time travel but in speeding up the way signals move through computer circuits," he said.

Wangís experiment is the latest and possibly the most important evidence that the physical world may not operate according to any of the accepted conventions.

In the new world that modern science is beginning to perceive, sub-atomic particles can apparently exist in two places at the same time - making no distinction between space and time.

Separate experiments carried out by Chiao illustrate this. He showed that in certain circumstances photons - the particles of which light is made - could apparently jump between two points separated by a barrier in what appears to be zero time. The process, known as tunnelling, has been used to make some of the most sensitive electron microscopes.

The implications of Wangís experiments will arouse fierce debate. Many will question whether his work can be interpreted as proving that light can exceed its normal speed - suggesting that another mechanism may be at work.

Neil Turok, professor of mathematical physics at Cambridge University, said he awaited the details with interest, but added:

"I doubt this will change our view of the fundamental laws of physics."

Wang emphasizes that his experiments are relevant only to light and may not apply to other physical entities. But scientists are beginning to accept that man may eventually exploit some of these characteristics for inter-stellar space travel.

 

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