How long is a light-year?

Article Category: Units  |   

Exactly how long is a light-year? No, this isn't a trick question, but many people might think so. Unlike what the phrase may suggest at first glance, lightyear isn't a measure of time - it's a measure of distance. The proper definition of a lightyear would be "the distance light travels in a year".

Phenomenon of light - photo

You may wonder, though... just "why we use the light year to measure distance? Why not use snail-years or rocket ship-years instead?" Good point, there! But there's actually a very good reason why light is the best possible thing we can use to measure distance. Two reasons actually.

First, the universe is extremely, immeasurably big. It's much bigger than we can wrap our minds around from our earthly perspective, really. Therefore, when you're trying to measure distances on a cosmic level, you want to rely on the largest measuring unit you can get. So, you need to really look at the fastest thing in existence and see how far it can travel in one year. That gives you a decent baseline that's workable for measuring distances on a cosmic scale. What is the fastest thing in the universe? Come on, you must have figured it out by now...

Light is the fastest thing in the universe

The speed of light is sometimes referred to as the "cosmic speed limit". It was Einstein who first demonstrated it theoretically: that light is the fastest thing in the universe, travelling in the vacuum of space at a whopping 300,000km per second (or 186,000 miles per second, depending on which side of the Atlantic you stand). Nothing can travel faster than that. And if it tried, it would likely blow up and disintegrate… more on that below.

Distances from the earth in light seconds, light minutes and light years
Average distances from the earth, measured in light seconds, light minutes and light years.

E=mc2 shows why nothing moves faster than light

If you look closely at Einstein's famous equation above (where e=energy, m=mass and c=speed of light), you should be able to instantly tell why nothing can possibly move faster than light… because all things asides from light have mass.

To make anything that has *any* mass move as fast as light, it would take a tremendous amount of energy - and this energy would increase in proportion to the mass of the object you're trying to move. In simple terms, you've had to blow something up to make it move faster than light… and even then you'd just fall short and get some bursts and flashes that move just as fast as light does because, well, they're light.

There is however, a kind of exception to this rule, as you'll learn below.

Faster than light: Spooky action at a distance

In recent years, physicists have proposed a possible exception to the nothing-is-faster-than-light rule: it's called "quantum entanglement", and it's a phenomenon that Einstein himself was aware of: he called it "spooky action at a distance". This is what happens when a photon (which is essentially a piece of light) is split in two. When this happens, it appears the two parts retain instant communication no matter how far they are separated in space. And we all know that "instant" is faster than fast… but it could also be reasoned that teleportation is not the same as transportation.

The actual number of miles/km light travels in a year

If you've read this article, you now know why nothing is faster than light. You also know why that makes light convenient as a unit to measure distances: because it's the fastest thing in the universe, which makes it practical to use it as a baseline to track distance measured over a year in space.

So how far does light go in a year? It moves a whopping 9,460,528,000,000 km, equivalent to 5,878,499,817 miles. Those are really large numbers!

Written by George Roberts

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Last update: 01 September 2016

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