What Does That Mean?
One of the most commonly used terms in astronomy is the compound word “light-year”. I posted a tweet about light-years a while back and I got a private message from someone telling me that it scared them. I don’t see how it could be, but then again, I’m so used to it I don’t think twice about using that unit of measure. And that’s all it is — a unit of measure.
Put simply, a light-year is the distance light travels in a year at an average speed of 186,282 miles per second (roughly 300,000 meters per second if you think in metric). The nearest star to us is about 4.3 light-years away. The next nearest spiral galaxy to us — the Andromeda Galaxy — is about 2.5 MILLION light-years away. So, knowing a distance to something tells you how long it takes for light from that object to reach us.
When I was a kid, I used to outside with a flashlight and send little blasts of light up to the sky. All things being equal, in one second, those little beams traveled immensely fast and were gone before I’d even turned off the switch. Of course, as a kid, I didn’t know about our atmosphere absorbing light, and dust bouncing it around, but the concept was still sound. Light travels incredibly fast, and if you send light to the sky, it’s headed out to space never to return.
If you think about this concept of light-speed for a bit, you can come up with all kinds of interesting ideas. Like, the light you see from Andromeda left it before modern humans evolved on our planet. Or, the light you see from the Sun shows you how our star looked just under 10 minutes ago. Or, if you look at Mars in the sky, you’re seeing it as it was as little as 4.3 minutes ago or as much as 21 minutes ago. (This is because Mars’s orbit is elliptical and at certain times it’s farther from us than other times.)
Light-travel time affects communications. For example, signals going out to the Cassini spacecraft travel at the speed of light, and they take about an hour and a half to get to the probe’s antennas. Our earliest radio and TV transmissions are spreading out radially from the planet — at the speed of light. They’ve gone not quite 100 light-years out to space. If there’s anybody within that expanding signal radius, then they’re detecting us as we were back in the early 20th century. Maybe that’s scarier than thinking of light speeding along across the light-years. Our early radio and TV programs really don’t say much about what we were actually like — but they do give insight into what we found funny, scary, and interesting. And, light-years from anywhere, our presence is heralded by that expanding ring of electromagnetic debris. It’s an interesting and sobering thought.