### speed of light

**(c, light speed, speed of light in a vacuum, light's speed)**
(fundamental constant measured as the speed at which light travels)

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

Outside a vacuum, light moves slower than this, e.g., in liquid
water, about 25% slower. Air has little effect, but glass (and
fiber optics), for example, 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, the waves are traveling past you that much
faster.)
Relativity gave details of the manner in which the speed of
light can be constant in this respect,
and showed it to be more than merely the speed
of EMR: it is the speed at which any influence occurs, especially
including gravity, for example, being the speed of gravitational waves.
Quantum mechanics appears to have effects that transcend it,
but in very limiting contexts.

(*physics,EMR*)
**Further reading:**

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

**Referenced by pages:**

aberration

astronomical quantities

black hole model

black-body radiation

broad-line region (BLR)

Brackett series

Cherenkov radiation

chirp mass (M_{c})

Compton wavelength

cyclotron radiation

Doppler shift

ergosphere

escape velocity (V_{e})

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

Starshot

Stefan-Boltzmann constant (σ)

superluminal motion

synchrotron radiation

Sunyaev-Zel'dovich effect (SZ effect)

time standard

terrestrial time (TT)

wavenumber (ν)

Wien approximation

Index