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
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
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
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
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
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.
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
Gain Assisted Superluminal Light
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
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
"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.
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
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
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."
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
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
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
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:
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.