If I had to pick one word that was synonymous with astronomy, light-year would definitely make the short list. It is a word that is sprinkled liberally through almost any news report on space and it embodies the feelings of immense size that arise when we ponder the Universe. And yet, it’s also frequently misunderstood.
Let’s start with clearing up one popular misconception: it has nothing (or very little) to do with time. Ignore the word year. It’s a distraction. When we’re talking about light-years, we’re actually talking about a way to measure distance. A light-year is just the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong. In short, it’s simply the distance a beam of light will travel in one year.
What we’re doing is using time as a proxy for distance. It’s not such a crazy idea. Have you ever been on your way to meet friends at a restaurant or movie theater and, while en route, called to tell them you’re running late? What do you usually say? If you’re like me, you might tell them “I’m about fifteen minutes away”. You’re using how long it will take you to get there as a substitute for how far away you are. Astronomers do the same, but use a beam of light as their reference. If we can figure out how long it will take light from a star to get here, we know something about how far away that star is.
Why do things this way? Partly because the distances we deal with in space are immense. They are mind-boggling, ginormous, colossal, stupendous…..astronomical. You really can’t go overboard with the hyperbole on this one. If we were to stick to miles or kilometers we quickly run into very unwieldy numbers just measuring the distance to the closest star to our Sun (a distance of roughly 24,000,000,000,000 miles!). So using a longer yard stick, so to speak, helps keep the numbers at least manageable.
Light is also convenient because, throughout the Universe, all light travels at exactly the same speed: about 670 million miles per hour. We don’t usually think about light “traveling” anywhere because when we turn on a light switch…there it is! We don’t have to wait for the room to light up, it just happens instantaneously. Except that it’s not instantaneous, just insanely fast. In fact, let’s pause for a bit and ponder how ludicrously quick the speed of light is.
Traveling at that speed, you would encircle the globe almost eight times in one second!
Eight times! Around the world! In one second! That’s quick. So if you were to travel off the Earth in a straight line at that speed, you’d get pretty far in the same amount of time, right? Actually, you wouldn’t have even made it to the Moon yet. For that, you’d have to wait half a second more. To get to the Sun at that speed would take you about eight minutes. Can you even imagine it? Traveling at a speed where you cover 200,000 miles every second for eight full minutes would only get you to the center of our Solar System.
If your brain hasn’t imploded yet, here’s another way to think about it: the light from the Sun takes eight minutes to get here. A side effect of that is, when you look at the Sun (which you really should never do!), you’re seeing the Sun as it was eight minutes ago. You’re looking into the past! While this sounds a little crazy, it’s actually something with which you’re already familiar. If you’ve ever seen fireworks, for example, you know that you see the explosion and then a few seconds later you hear it. If you close your eyes during the fireworks show, you’d only have your hearing to know when things were happening. Since it takes some time for the sound to get to you, you’d always be hearing things a few seconds after they happened. You’d be hearing from the past. The same happens with light: we only see something once the light from that event actually gets to our eyes and it takes time to do that. When we’re looking across a room, the time delay is only a few billionths of a second so in practical terms it doesn’t really matter. But when you start looking across large enough distances, the light from those objects is delayed like the sounds are from exploding fireworks.
So, going back to how far we can travel on a beam of light, where would we be after one year traveling at that speed? Surely we would have made some progress across the Galaxy by now! Not even close. We’re not even a quarter of the way to the nearest star! To get there, a dim red star called Proxima Centauri, we would have to travel at 670 million miles per hour for four years! That’s not counting bathroom breaks. So, because of that, we say that Proxima Centauri is four light-years away. Once again, when we look at that star, since it took the light four years to get here, we’re seeing the star as it was in 2008!
So exactly how big is one light year? It’s about six trillion miles. That’s a six followed by twelve zeros. And that’s just one light-year! The Milky Way galaxy in which our Sun and all the stars we see at night reside is 100,000 light-years from one end to the other. To put that into perspective, the span of recorded human history is roughly 5000 years. So light from a star at one end of the Galaxy takes twenty times longer than all of recorded history to get to the other end. If we start moving beyond our Galaxy, it’s just over two million light-years to our nearest galactic neighbor, the Andromeda Galaxy. The light we currently see from that galaxy left there about the same time the ancestors of modern humans were first discovering stone tools.
Our last stop takes us to the edge of the visible Universe. It’s also here where the trickiness of measuring distance in an ever-expanding Universe becomes apparent. The light we see coming from the farthest depths of the Universe has been traveling across the cosmos for almost three times longer than our planet has existed: nearly 14 billion years! But here’s a catch: we can not say that the edge of the visible Universe is 14 billion light-years away. Why? Because the Universe has grown larger in that time! A galaxy whose light took 14 billion years to cross the cosmos to our little planet has, in the intervening aeons, moved even further away. The current physical distance to that remote beacon, if we stopped the Universe from expanding and stretched out a really long tape measure, is just over 46 billion light years! Even in light years, measuring distances across the Universe becomes unwieldy. But measuring in something familiar, like miles, is truly humbling. From here to the edge of our vision spans a distance of approximately 276,000,000,000,000,000,000,000 miles.
And it’s getting bigger every day.