Speed of light can be calculated, "often with less than 5% error". Interferometry is another method to find the wavelength of electromagnetic radiation for determining the speed of light. A coherent beam of light (e.g. from a laser), with a known frequency ("f"), is split to follow two paths and then recombined. By adjusting the path length while observing the interference pattern and carefully measuring the change in path length, the wavelength of the light ("λ") can be determined. The speed of light is then calculated using the equation "c" = "λf". Before the advent of laser technology, coherent radio sources were used
Variable speed of light While the measured value was later provided by Eddington in 1919, Albert Einstein went through several versions of light speed theory between 1905 and 1915, eventually concluding that light speed is constant when gravity does not have to be considered but that the speed of light cannot be constant in a gravitational field with variable strength. In the same book Einstein explained that he intended light speed to be a vector when it was described by coordinates in a reference frame. Einstein was not explicit about whether the speed of light would change in a gravity field, or whether just