That’s a lot.
That means, when you look at a star or other object in the sky that is very, very far away, you’re seeing it as it was, in years ago, as far away as it is, in light years. If it takes eight years for the light to reach you, you’re seeing it as it was eight years ago, etc.
In Junji Ito’s Deathstar Remina, (don’t worry, you don’t have to have read it to continue this article,) the author very specifically points out that the monster planet Remina is 16 light years away at the start of the story, as it’s plot-related that that means the light he is seeing from it was illuminated the same time his 16-year-old daughter Remina (after whom the planet is named) was born.
Shortly, the planet begins moving towards our solar system, (with [un]predictably disastrous results,) and is mentioned at traveling at speeds below and above the speed of light. Now, if it were traveling straight at us, at less than the speed of light, we would see it slowly moving towards us, exactly as you’d expect from any object moving towards you at a reasonable speed.
Why? Well, precisely because the light reflecting off of it, that we see, moves faster than the object, so the object never overtakes its own light.
That’s the important part.
It quickly becomes known that Remina will enter the solar system in under a month. 16 light years in under a month. That’s very, very quick.
If you’ve ever seen an episode of Star Trek where one of the ships travels towards the viewer at warp speed and then stopped, you’ve seen faster than light travel depicted in an unfortunately incorrect way. Ignoring (as we will, for the rest of the article as well) length contraction, time dilation, doppler shift, and other irritating aspects of super high-speed travel, there is one thing fundamentally wrong with this depiction
First of all, the speed should be so high, that no more than a single frame of film would capture the craft on its final approach. More importantly, though, is that what you would actually see is the ship appearing in front of you, and then (while it is still there, because, well, it’s still there) an afterimage of it zooming away from you.
Remember that part earlier where I said, “the object never overtakes its own light”? Time for some charts.
Let’s set up some conceits. For the purpose of this, Earth is stationary and the object at right is moving towards it at two light years per year; that is, double the speed of light. Light from it propagates towards Earth at one light year per year; the speed of light.
Since I can’t really show continuous light, we’ll just show the light at intervals of one year. It’s color-coded per year, and when it is emitted, the distance the object is is color-coded the same. For example, here, the object is eight light years away and emits RED light. When the RED light reaches Earth, the people there see the object as if it was at the RED 8, eight light years away.
One year later, the object is only six light years away and emits ORANGE light. The RED light has only propagated one light year; none of the object’s light has reached Earth yet, so we can’t see it.
One year later, it’s four light years away and emits YELLOW light. None of the light has reached us yet.
It’s now only two light years away, and emits GREEN light. Again, none of its light has reached us.
Suddenly, out of nowhere, the object appears right in our neighborhood! We didn’t see it coming! Why? This is because, moving towards us faster than its own light, it overtook its own light. Since it’s stopped right next to us, we see it now, and a year later…
…we see where it was a year ago, at 2, and a year after that…
…we see where it was a year before that, at 4, and a year later…
…we see where it was a year before that, at 6, and a year later…
…we finally see where it began its journey, eight years ago, at 8. The object literally appears out of nowhere, and then we can watch it appear to travel backwards to where it started (while the object itself is also, well, right next to us.) It seems to split into two after appearing from thin air.
Four years after it started its journey, it’s here with no warning, and four years after that, we see where it started from. (You can save these charts sequentially in a folder, then open one with Windows Photo Viewer. Arrow through them to see it move!)
Now, another question: what happens if it keeps moving? Again, it would appear out of nowhere, and then seem to split into two, one image going back to where it started, and the other going off to where it’s actually going. Wikipedia has a good image of this on its page about tachyons which would have saved you a lot of time if I’d told you about it first!
But wait, isn’t actual travel faster than light prohibited by relativity? Actually, no. Travel AT the speed of light is prohibited, because mass becomes infinite at light speed and thus requires infinite energy to actually attain light speed. Anything that travels faster than the speed of light can do so, provided it always has and always will. Tachyons are theoretical particles that do just that, and have been used in fiction for everything from time travel to space travel.
There’s plenty of artistic freedom taken with depictions of faster than light travel, and that’s quite all right.
Unless, of course, the thing moving faster than light is Deathstar Remina, in which case, we’re all screwed.