Light is the fastest object in the cosmos, but its speed depends on what it’s going through.
Light’s speed limits the cosmos. Our finest spacecraft cannot fly faster than light, according to physics.
How fast is light?
Light travels at 186,000 mph (300,000 km/s), or almost 700 million mph (more than 1 billion km/h). That’s fast enough to round the world 7.5 times in one second, while a standard passenger flight would take nearly two days (without fuel breaks or layovers).
For much of human history, we assumed light moved instantly. According to Britannica, physicist Ole Roemer measured the speed of light (c) using Jupiter’s moons in the late 1600s.
James Clerk Maxwell developed electromagnetic ideas in the early 19th century. Light is formed of electric and magnetic fields, hence electromagnetism may describe its behavior and theoretical speed. With a 30-kilometer inaccuracy, that figure was 299,788 kilometers per second.
Physicists utilized lasers to measure the speed of light with a 0.001 inaccuracy in the 1970s. The speed of light is used to establish length units, thus humans have settled on 299,792.458 kilometers per second.
However, light doesn’t always go swiftly. It slows in air, water, diamonds, etc. Light’s speed is measured in a vacuum. In a prism, light energy bend differently, generating rainbows.
Space, a vacuum, is faster than light. Light from Proxima Centauri takes two years to reach Earth, while light from the sun takes eight minutes. Astronomers measure immense distances in space using light-years, the amount of light that travels in a year.
This universal speed restriction makes telescopes time machines. Astronomers see 500-year-old light from a 500-light-year star. The cosmic microwave background is radiation from the Big Bang’s infancy, 13 billion light-years distant. Modern astronomy depends on the speed of light, which actually molds our worldview.