Let’s talk about speed. Everything has a maximum speed. Your car does, you do, even Usain Bolt does. But even if he’s been hailed as the fastest man alive, Usain Bolt has nothing on the fastest-moving thing in the universe: light. Light travels at approximately 186,000 miles per second. If Usain Bolt can run 100 meters in 9.58 seconds, light is around 30 million times faster – it travels 100 meters in a mere 333 nanoseconds (0.000000333 seconds).
Light doesn’t just travel fast – it travels the fastest. The speed of light is considered the maximum at which anything in the universe can travel, whether it’s matter or energy or anything else. Theoretically, the speed of light is the fastest humanity can hope to travel in any car or spaceship that could ever be built.
For short distances or even distances that seem long on an earth scale, such as the distance to the horizon, this speed makes light travel appear instantaneous to us. After all, the human eye can only perceive down to millisecond changes (which is considerably slower – about 1million times slower – than nanoseconds), meaning that light doesn’t appear to travel from its source to us; it’s just suddenly there where we can see it. For example, if you flip the switch on the light in your kitchen (as long as it’s not one of the LED bulbs that take a second to warm up), the room is immediately brighter, and you don’t notice any lag between flipping the switch and actually seeing light.
But what if the light is traveling HUGE distances? To consider this, let’s increase the scale considerably, and move from thinking about traveling on earth to thinking about the universe. The distance from the sun to the earth is approximately 92 million miles, which means that it takes light around 8 minutes to travel from there to here. So if the sun were to suddenly be extinguished (which is impossible, so don’t worry about that too much) we would have another eight happy minutes in the sunlight before we realized what had happened.
So you’re thinking, eight minutes, that’s not that long of a lag, I couldn’t even run a mile in eight minutes. (Or maybe you could, which is awesome.) In other words, an eight-minute lag doesn’t seem like that big of a deal, since not a whole lot generally happens in such a short period of time. But consider that the sun is the closest star to our planet, which means that the light we see from all the other stars in the sky is substantially older. The closest star to earth, Proxima Centauri, is 4.24 light-years away, meaning that if you went outside tonight and looked at it through a telescope, you would be seeing it the way it looked four years ago – and would have to wait another four years until you could see what it looked like on this date. And that’s the closest star. Some of the starlight you see when you go outside tonight is hundreds of millions of years old, having traveled to us across vast distances of space.
The implications of this time lag between earth and the stars we see are incredibly useful for astronomers. Since the light reaching us from far-off places in the universe has taken so long to get here, it offers astronomers a glimpse into what the universe looked like when it was very young. From this, they can start to form a more complete model for how the first stars were born, how galaxies developed, and how the universe came to be what it is today in the billions of years after the Big Bang.
So tonight when you get out of your car after work or running errands, appreciate that you’re seeing the beginnings of the universe when you look up at the sky – and then revel again in the speed of light as you flip the switch when you walk into your house and can immediately see where you’re going, without having to wait a few million years for the room to light up.