Speed of light

The speed of light in <a href="/facts/Vacuum/BTk3goyV">vacuum</a>, commonly denoted c, is a universal <a href="/facts/Physical_constant/uI2QvAHw">physical constant</a> that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour). It is exact because, by a 1983 international agreement, a <a href="/facts/Metre/hPhzc1CV">metre</a> is defined as the length of the path travelled by <a href="/facts/Light/jeofpMn4">light</a> in vacuum during a time interval of 1⁄299792458 <a href="/facts/Second/jtFxnJXx">second</a>. According to the <a href="/facts/Special_relativity/vsFFc63E">special theory of relativity</a>, c is the upper limit for the speed at which conventional <a href="/facts/Matter/osyc04UN">matter</a> or <a href="/facts/Energy/wd8PrQM7">energy</a> (and thus any <a href="/facts/Signal/41YqUJ8c">signal</a> carrying <a href="/facts/Information/3LdrzXtJ">information</a>) can travel through <a href="/facts/Space/tq6LUM3C">space</a>.
All forms of <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a>, including <a href="/facts/Visible_light/jeofpMn4">visible light</a>, travel at the speed of light. For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. Much <a href="/facts/Starlight/nXTYlHSz">starlight</a> viewed on <a href="/facts/Earth/HLLmDfj0">Earth</a> is from the distant past, allowing humans to study the history of the universe by viewing distant objects. When <a href="/facts/Data_communication/1yAhaANd">communicating</a> with distant <a href="/facts/Space_probe/AeWRBITP">space probes</a>, it can take minutes to hours for signals to travel. In <a href="/facts/Computing/Kghtm9fX">computing</a>, the speed of light fixes the ultimate minimum <a href="/facts/Latency_(engineering)/6Bjhn30H">communication delay</a>. The speed of light can be used in <a href="/facts/Time_of_flight/DSINDYyn">time of flight</a> measurements to measure large distances to extremely high precision.
<a href="/facts/Ole_R%C3%B8mer/1lqXzA7F">Ole Rømer</a> first <a href="/facts/R%C3%B8mer%27s_determination_of_the_speed_of_light/44UvclHw">demonstrated in 1676</a> that light does not travel instantaneously by studying the apparent motion of <a href="/facts/Jupiter/95m7XjMT">Jupiter</a>'s moon <a href="/facts/Io_(moon)/p3Uc8FmK">Io</a>. Progressively more accurate measurements of its speed came over the following centuries. In a <a href="/facts/A_Dynamical_Theory_of_the_Electromagnetic_Field/zbe0J5rW">paper</a> published in 1865, <a href="/facts/James_Clerk_Maxwell/jtYyze61">James Clerk Maxwell</a> proposed that light was an <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic wave</a> and, therefore, travelled at speed c. In 1905, <a href="/facts/Albert_Einstein/CNTet8qu">Albert Einstein</a> postulated that the speed of light c with respect to any <a href="/facts/Inertial_frame_of_reference/1U6TrdG3">inertial frame of reference</a> is a constant and is independent of the motion of the light source. He explored the consequences of that postulate by deriving the <a href="/facts/Theory_of_relativity/qGHj3eTC">theory of relativity</a> and, in doing so, showed that the parameter c had relevance outside of the context of light and electromagnetism.
<a href="/facts/Massless_particle/65Xng6Vx">Massless particles</a> and <a href="/facts/Field_(physics)/FJdAKeRA">field</a> perturbations, such as <a href="/facts/Gravitational_wave/4XWN3NtG">gravitational waves</a>, also travel at speed c in vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the <a href="/facts/Observer_(special_relativity)/t3yB2Bt9">observer</a>. Particles with nonzero <a href="/facts/Rest_mass/aRloR0nW">rest mass</a> can be accelerated to approach c but can never reach it, regardless of the frame of reference in which their speed is measured. In the <a href="/facts/Theory_of_relativity/qGHj3eTC">theory of relativity</a>, c interrelates <a href="/facts/Spacetime/uRIN1x4b">space and time</a> and appears in the famous <a href="/facts/Mass%E2%80%93energy_equivalence/ExXawGcc">mass–energy equivalence</a>, E = mc2.
In some cases, objects or waves may appear to travel faster than light (e.g., <a href="/facts/Phase_velocities/m43q3X33">phase velocities</a> of waves, <a href="/facts/Superluminal_motion/hAPdgkbr">the appearance of certain high-speed astronomical objects</a>, and particular <a href="/facts/Quantum_effects/BRc3Mzgr">quantum effects</a>). The <a href="/facts/Expansion_of_the_universe/f6Uvt79B">expansion of the universe</a> is understood to exceed the speed of light beyond <a href="/facts/Hubble_volume/aHRo0zN8">a certain boundary</a>.
The speed at which light propagates through <a href="/facts/Transparency_and_translucency/PRryNNW6">transparent materials</a>, such as glass or air, is less than c; similarly, the speed of <a href="/facts/Electromagnetic_waves/Ynx7ujCW">electromagnetic waves</a> in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the <a href="/facts/Refractive_index/yjkzvgeE">refractive index</a> n of the material (n = ⁠c/v⁠). For example, for visible light, the refractive index of glass is typically around 1.5, meaning that light in glass travels at ⁠c/1.5⁠ ≈ 200000 km/s (124000 mi/s); the <a href="/facts/Refractive_index_of_air/FV97Am25">refractive index of air</a> for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c.

Speed of light open-in-new

Speed of light