The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. That's about 186,282 miles per second — a universal constant known in equations as "c," or light speed.  According to physicist [[Albert Einstein]]s [theory of special relativity](https://www.space.com/36273-theory-special-relativity.html), on which much of modern physics is based, nothing in the universe can travel faster than light. The theory states that as matter approaches the speed of light, the matter's mass becomes infinite. That means the speed of light functions as a speed limit on the whole [universe](https://www.space.com/52-the-expanding-universe-from-the-big-bang-to-today.html). The speed of light is so immutable that, according to the U.S. [National Institute of Standards and Technology](https://www.nist.gov/si-redefinition/meet-constants), it is used to define international standard measurements like the meter (and by extension, the mile, the foot and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit [Kelvin](https://www.livescience.com/temperature.html). But despite the speed of light's reputation as a universal constant, scientists and science fiction writers alike spend time contemplating faster-than-light travel. So far no one's been able to demonstrate a real warp drive, but that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics. A #lightyear is the distance that light can travel in one year — about 6 trillion miles (10 trillion kilometers). It's one way that astronomers and physicists measure immense distances across our universe. Light travels from the [moon](https://www.space.com/55-earths-moon-formation-composition-and-orbit.html) to our eyes in about 1 second, which means [[The moon]] is about 1 light-second away. Sunlight takes about 8 minutes to reach our eyes, so the [sun](https://www.space.com/58-the-sun-formation-facts-and-characteristics.html) is about 8 light minutes away. Light from [Alpha Centauri](https://www.space.com/5094-alpha-centauri.html), which is the nearest star system to our own, requires roughly 4.3 years to get here, so Alpha Centauri is 4.3 light-years away. "To obtain an idea of the size of a light-year, **==take the circumference of the Earth (24,900 miles), lay it out in a straight line, multiply the length of the line by 7.5 (the corresponding distance is one light-second), then place 31.6 million similar lines end to end==**," NASA's Glenn Research Center [says on its website](https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/how_long_is_a_light_year.htm). "The resulting distance is almost 6 trillion (6,000,000,000,000) miles!" Stars and other objects beyond our [solar system](https://www.space.com/16080-solar-system-planets.html) lie anywhere from a few light-years to a few billion light-years away. And ==everything astronomers "see" in the distant universe is literally history==. When astronomers study objects that are far away, they are seeing light that shows the objects as they existed at the time that light left them. [[Time Travel]] This principle allows astronomers to see the universe as it looked after the [[Big Bang]], which took place about 13.8 billion years ago. ==Objects that are 10 billion light-years away from us appear to astronomers as they looked 10 billion years ago — relatively soon after the beginning of the universe — rather than how they appear today.==