What is the speed of light in a vacuum?

The story behind the answer

299,792,458 m/s is the exact speed of light in a vacuum, symbolized as c. Since 1983, the meter has been defined by fixing the value of c, so this number isn’t a measurement with uncertainty—it’s a defined constant. Light travels about 300,000 kilometers per second, meaning it circles Earth roughly 7.5 times in one second. The other options are either far too slow or unrealistically high.

Why it matters: in Einstein’s relativity, c is the ultimate speed limit for energy, matter, and information. It also links space and time in E = mc². Memory tip: think “three times ten to the eighth” m/s, or “300,000 km/s.” Another anchor: a light-second is about 300,000 km, and a light-year is the distance light travels in a year at this speed.