Answer 1:
It is difficult to answer the "what?" or
"why?" questions about nature, and science
can most of the time answer the "how?"
questions. Light moving is simply a consequence
of nature, of natural processes. Our largest
source of visible light on Earth is the sun. The
sun is a massive fusion reaction, where extreme
temperatures and pressures result in elemental
hydrogen being converted to helium. This comes
with an immense amount of energy, and some of that
energy is released as light. It is difficult to
say if anything is "making" light move.
Light moves through space and air as a wave
and a particle, traveling and interacting with the
environment. To stop light is an interesting
concept, as it implies dissipating light energy of
a given source at one instant. However, it is
important to note the first law of
thermodynamics, that there is always a
conservation of energy. Energy is neither
created or destroyed. To this end, if an
object absorbs all the light from a source, such
as a black asphalt road, that light energy is
converted to heat because energy is conserved. To
understand more of how light interacts with
objects, search for physics lessons on
absorption, reflection, and transmission.
Light is very interesting in that it behaves
like both a wave and a particle, and this is
known as the wave-particle duality. This is
a fairly complex topic in quantum
mechanics, as light can be mathematically
explained in some cases by treating it as a
particle, and other cases by treating it as a
wave, but the actual behavior is difficult to
comprehend. With this in mind, light travels
through air at different speeds depending on what
medium it is in, and anything that can
interact with light, such as water vapor in the
sky in clouds, will alter or attenuate
propagation, creating dim skies on a cloudy day or
pretty colors during a sunset. In this way,
natural things such as clouds or man made
things such as walls can stop light. Again, to
understand more of how light interacts with
objects, search for physics lessons on absorption,
reflection, and transmission.
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Answer 2:
According to the second postulate of special
relativity, light is the fastest thing in the
universe, and it travels at constant speed in
vacuum. This is the nature of light. As long as
light is produced, it will move.
We can always “stop” light by turning off its
source. Consider stars. Stars produce light by
nuclear fusion. When stars die, they no
longer emit light. Of course, the light that
already got emitted before the star's death will
continue on moving until something else stops
it---like encountering matter. When light hits an
opaque material (like your body), light gets
absorbed. This is why you make shadows---your body
absorbs most of the light and stops it from
reaching the ground. We can also stop light by
trapping it by reflection. There is a
phenomenon called“total internal
reflection.” Put a water-proofed light source
under a bath of water. If you point light directly
up the surface of water, light will pass through
the water. However, as you change the angle of the
light source (making it more horizontal little by
little) you should reach the critical angle
where no light escapes the water. This is the same
strategy they use when cutting diamonds to make
them sparkle. They also use this strategy for
fiber optic cables to make fast internet
possible.
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Answer 3:
Good question - this one had physicists puzzled
for some time. Light has a duality as both
particles and waves - it is the particle nature
that allows light to move, since waves normally
only pass through a medium. You see this as
alternating electrical and magnetic fields, which
is the wavelike nature of light (electric
fields create magnetic fields when changing and
vice-versa, so this is how the wavelike
explanation works).
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Answer 4:
It is actually almost impossible to stop
light. Light is "made of" what we call
"photons". Photon has no mass, and
Einstein's special relativity says that any
particle with zero mass must travel with the
"speed of light", at least in vacuum.
Light usually slows down a little bit when it
travels through a material, but it is extremely
difficult to completely stop it. Recently this has
been achieved to some extent by using very special
techniques.
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