light - year is the distance that light travels in one year.
light - year is the distance that light travels in one year.
what is the distance that light travels in one year called?
light - year
according to the equivalence principle of general relativity, the rules of special relativity are "locally" valid in small regions of spacetime that are approximately flat. in particular, light always travels locally at the speed "c"; in our diagram, this means, according to the convention of constructing spacetime diagrams, that light beams always make an angle of 45° with the local grid lines. it does not follow, however, that light travels a distance "ct" in a time "t", as the red worldline illustrates. while it always moves locally at "c", its time in transit (about 13 billion years) is not related to the distance traveled in any simple way since the universe expands as the light beam traverses space and time. in fact the distance traveled is inherently ambiguous because of the changing scale of the universe. nevertheless, we can single out two distances which appear to be physically meaningful: the distance between the earth and the quasar when the light was emitted, and the distance between them in the present era (taking a slice of the cone along the dimension that we've declared to be the spatial dimension). the former distance is about 4 billion light years, much smaller than "ct" because the universe expanded as the light traveled the distance, the light had to "run against the treadmill" and therefore went farther than the initial separation between the earth and the quasar. the latter distance (shown by the orange line) is about 28 billion light years, much larger than "ct". if expansion could be instantaneously stopped today, it would take 28 billion years for light to travel between the earth and the quasar while if the expansion had stopped at the earlier time, it would have taken only 4 billion years.
expansion of the universe
the index of refraction (or refractive index) is a way of measuring the speed of light in a material. light travels fastest in a vacuum, such as in outer space. the speed of light in a vacuum is about 300,000 kilometers (186,000 miles) per second. the refractive index of a medium is calculated by dividing the speed of light in a vacuum by the speed of light in that medium. the refractive index of a vacuum is therefore 1, by definition. a typical singlemode fiber used for telecommunications has a cladding made of pure silica, with an index of 1.444 at 1500 nm, and a core of doped silica with an index around 1.4475. the larger the index of refraction, the slower light travels in that medium. from this information, a simple rule of thumb is that a signal using optical fiber for communication will travel at around 200,000 kilometers per second. to put it another way, the signal will take 5 milliseconds to travel 1,000 kilometers in fiber. thus a phone call carried by fiber between sydney and new york, a 16,000-kilometer distance, means that there is a minimum delay of 80 milliseconds (about formula_1 of a second) between when one caller speaks and the other hears. (the fiber in this case will probably travel a longer route, and there will be additional delays due to communication equipment switching and the process of encoding and decoding the voice onto the fiber).
optical fiber