The speed of light is the same for all observers. When you talk about how fast something’s moving, you usually have to pay attention to where you’re standing as you watch that motion. Suppose you’re on a train traveling 100 kilometers per hour. You decide to get out of your seat and walk toward the dining car at the front of the train, so you amble up the aisle at a speed of 2 kilometers per hour. How fast are you moving? Relative to your seat on the train, you’re moving 2 kilometers per hour. But relative to the track under the train, you’re moving 102 kilometers per hour—the speed of the train plus your speed on the train. That’s the way it works for most things—but not for light. Back in 1887, physicists A. A. Michelson and E. W. Morley did an experiment that showed the speed of light does not depend on the movement of the observer. That means no matter how fast or slow you move, light always moves at the same speed. Imagine, for instance, that Marco is on a spaceship traveling toward the sun at 99.99 percent of the speed of light, and Sophia is on a spaceship heading away from the sun at 88.6 percent of the speed of light. Even though they’re heading in different directions, sunlight is rushing past them both. •  How fast is that light traveling relative to Marco? It’s traveling at the speed of light—about 300 000 kilometers per second. •  How fast is that light traveling relative to Sophia? It’s traveling at the speed of light—about 300 000 kilometers per second. Marco’s speed and Sophia’s speed don’t matter. The speed of light is always the same. Nothing can travel faster than 299 792 kilometers per second, the speed of light in a vacuum. We chose Epsilon Eridandi 3 for our party because, by galactic standards, it’s right next door, just 10 light years from Earth. (A light year is the distance light can travel in a year.) At the speed of light, it would take 10 years for a spaceship from Earth to get to this nearby star. By contrast, a journey at light speed from Earth to the center of the Milky Way galaxy would take a whopping 26 000 years. Back to Thinking Like Einstein         © Exploratorium | Credits