Astrophysics (Index)About

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, 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.

Further reading:

Referenced by pages:
astronomical quantities
black-body radiation
broad-line region (BLR)
Brackett series
Cherenkov radiation
chirp mass (Mc)
Compton wavelength
cyclotron radiation
Doppler shift
escape velocity (Ve)
focal length
general relativity (GR)
Great Debate
gravitational-wave detector
gravitational wave spectrum
innermost stable circular orbit (ISCO)
Jeans escape
kinetic energy (KE)
kelvin (K)
light cone
Lorentz transformation
Lyman series (L)
Maxwell's equations
Michelson interferometer
observable universe
OH/IR source
particle horizon
Paschen series
Planck function
Planck units
pulsar timing array (PTA)
radiation pressure
Rayleigh-Jeans law
redshift (z)
relativistic energy
relativistic invariance
relativistic speed
rotation period
Schwarzschild radius
supernova (SN)
spacetime diagram
special relativity (SR)
Stefan-Boltzmann constant (σ)
superluminal motion
synchrotron radiation
Sunyaev-Zel'dovich effect (SZ effect)
time standard
terrestrial time (TT)
wavenumber (ν)
Wien approximation