Despite common misperceptions, a light year is a measure of distance, not time. It is the only feasible way to measure distances in the universe because the universe is so vast that measurements of distance used on earth would quickly get too large to be comprehensible. One light-year is the equivalent of six trillion miles, and scientists often discuss and study stars that are hundreds of thousands of light-years away.
The other reason light-years are a necessary unit of measurement when discussing outer space is that much of what we see in space (stars, galaxies, etc.) are so far away that we are actually looking back in time when we see them from earth. Light-years are needed as the unit of measurement because it reminds us of the speed of light – that light can only travel so fast, and so what we observe is limited by the amount of time it takes the light to get to us.
Our universe is likely much larger than we know, as we can only see 14 billion light years away because the universe has only existed for that amount of time. Therefore, the farthest we can see is 14 billion light-years away, but this also means that what we see at that distance is as it existed 14 billion years ago. This severely limits our ability to know the universe surrounding us, as what we see will always be in the past. It doesn’t effect us substantially on earth or even when viewing our solar system, but it is undeniable an impediment to our knowledge of the rest of the universe. Thinking about our inability to know the extent of the universe really puts earth and our lives here into perspective.
The Observable Universe
