The speed of light (in a perfect vacuum), is a numerical
constant in physics usually denoted by the letter c. It is
299,792,458 m/s exactly, since the meter is defined to be the
distance light travels through vacuum in 1/299,792,458 of a
second. The speed of light is considered to be an "ultimate
cosmic speed limit". This is because massive ("massive" as in
"having mass", not the colloquial "really fuckin' big")
particles and objects can attain speeds that approach light
speed, but never actually reach it. As the photons which make up
light do not have any rest mass, they not only can travel at
this speed, but must travel at this speed - hence the name
"speed of light" to refer to c. According to special relativity
and the experiments that back it up, the speed of light is the
same for all inertial observers. When light passes through
materials, such as water, air, or glass, the photons (light
quanta) do travel slower, and in certain materials can even be
brought down to a mere walking pace. However, it shoul