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The **speed of light** (or **light speed**, or, more precisely,
the **speed of light in a vacuum**, symbolized as **c**)
is a fundamental constant, precisely 299,792,458 meters per second
(because the meter is currently defined as *c/299,792,458*)
which is roughly 186,000 miles per second,
and is the speed at which any EMR is measured to travel through
a vacuum. Due to this less-than-infinite speed, observing at astronomical
distances is also looking "backward in time".

Outside a vacuum, light moves slower than this, e.g., in liquid water, about 25% slower. Air has a much smaller effect, but glass (including fiber optics) has the same order-of-magnitude reduction as water, and according to the science of optics, it this reduction that makes glass lenses function.

The *speed of light* (in a vacuum) always measures to the same
quantity, even though light acts like waves. (This is unlike sound
waves: if the medium in which the sound waves are traveling is also
moving relative to you, sound waves are passing you in the
same direction are moving that much faster.)
Relativity spelled out how the speed of
light can be constant in this respect,
and takes it to be more than merely the speed of EMR:
it is the maximum speed at which any influence occurs,
such as the effects of gravity and it is the speed of gravitational waves.
Quantum mechanics appears to have effects that transcend
the speed of light, but in very limiting contexts.

http://en.wikipedia.org/wiki/Speed_of_light

aberration

astronomical quantities

black-body radiation

broad-line region (BLR)

Brackett series

Cherenkov radiation

chirp mass (M

Compton wavelength

cyclotron radiation

Doppler shift

ergosphere

escape velocity (V

focal length

frequency

general relativity (GR)

Great Debate

gravitational-wave detector

gravitational wave spectrum

IceCube

inflation

innermost stable circular orbit (ISCO)

Jeans escape

jet

kinetic energy (KE)

kelvin (K)

light cone

Lorentz transformation

Lyman series (L)

mass

Maxwell's equations

Michelson interferometer

observable universe

OH/IR source

particle horizon

Paschen series

photon

Planck function

Planck units

pulsar timing array (PTA)

radiation pressure

Rayleigh-Jeans law

redshift (z)

relativistic energy

relativistic invariance

relativistic speed

relativity

rotation period

Schwarzschild radius

supernova (SN)

spacetime diagram

special relativity (SR)

Starshot

Stefan-Boltzmann constant (σ)

superluminal motion

synchrotron radiation

Sunyaev-Zel'dovich effect (SZ effect)

time standard

terrestrial time (TT)

wavenumber (ν)

Wien approximation