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The speed of light is 1.8026 × 10 12 furlongs per fortnight (1.8026 terafurlongs per fortnight). By mass–energy equivalence , 1 firkin is equal to 3.249 366 76 × 10 24 firkin· furlong 2 / fortnight 2 (≈ 3.478 × 10 15 BTU , or 3.669 × 10 18 joules ).
To help compare different orders of magnitude, the following list describes various speed levels between approximately 2.2 × 10 −18 m/s and 3.0 × 10 8 m/s (the speed of light). Values in bold are exact.
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 ≈ 200 000 km/s (124 000 mi/s); the refractive index of air 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.
The paradoxical aspect of each of the described thought experiments arises from Einstein’s theory of special relativity, which proclaims the speed of light (approx. 300,000 km/s) is the upper limit of speed in our universe. [1] [4] [5] The uniformity of the speed of light is so absolute that regardless of the speed of the observer as well as ...
The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in vacuum c = 299 792 458 metres per second (approximately 1 079 000 000 km/h or 671 000 000 mph).
Spatial variation of the speed of light in a gravitational potential as measured against a distant observer's time reference is implicitly present in general relativity. [3] The apparent speed of light will change in a gravity field and, in particular, go to zero at an event horizon as viewed by a distant observer. [4]
The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. Because the light starts and finishes in the same place, only one clock is needed to measure the total time; thus, this speed can be experimentally determined independently of any clock synchronization scheme.
By timing the eclipses of Jupiter's moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun. [1] Using modern orbits, this would imply a speed of light of 226,663 kilometres per second, [2] 24.4% lower than the true value of 299,792 km/s. [3]