Hey! You Got Science in my Science Fiction (Essays)

The Speed of Light

The bane of any science fiction writer's existence is the speed of light. Why is it such a problem? Is it something we'll someday figure out how to overcome? Many shows like Star Trek seem to blithely assume it will. While I agree that you shouldn't let science get in the way of a good story, it can make your story a touch more believable if you put a little effort into answering the question, "how do my people get around the galaxy?"

This is a complex subject. Please make comments if I confuse you and I'll try to make it better.

The fundamental problem is called "special relativity." Special relativity says that physics has to work the same in every "reference frame," and no single "reference frame" is better than any other. What's a reference frame? It's a viewpoint that moves through space at a constant speed (smaller than the speed of light). So if Alice is on earth and Bob is flying by in a spacecraft, they are in different reference frames. Alice is in a reference frame moving at speed zero, and Bob is moving at rocket speeds.

The other thing special relativity says is that it's equally valid to say that Bob and his spaceship are sitting still and that Alice and the earth are moving.

Nothing mind bending so far.

So here's the first weird thing. No matter what reference frame you're in, light always moves the same speed. So if Bob fires a laser from the nose cone of his ship, it looks to him like that light ray is moving at 300,000,000 meters per second (I'll just use the letter "c" to stand for this number from now on). Alice also sees the light moving at speed c. She sees Bob and his spaceship chasing the light rays at some other speed (let's call it "v") which is less than c. To her, it looks like the light rays from Bob's laser are only moving at a speed c minus v faster than Bob.

 How does that work? How is it that Bob sees the light ray speeding ahead of him at speed c, while Alice sees the light ray getting ahead of Bob at speed c minus v?

The answer is, To Alice, Bob's clocks run slower and both he and his ship get shorter in the direction they're moving. These changes compensate so that something that looks like it's going at speed c in Bob's reference frame based on his wonky clocks and yardsticks, is actually going at speed c minus v.

Oh, you knew that time slows down as you get near the speed of light already? And what does that have to do with putting a speed limit on a starship?

Hang on, I'm getting there. This is a little tricky.

Here's what you might not have known. Because reference frames are all the same, in Bob's reference frame, it's Alice's clocks that run slower and she is the one getting shorter in the direction she's moving. And the best part? Science says they're both right.

Okay, I realize that doesn't quite make sense if you're hearing it for the first time. How can both of them look shorter? They can't both be right about that, can they?

They can.

Here's an example which kind of conveys the idea. Suppose Alice and Bob are both on earth, are both the same height, and are standing very far away (a few hundred feet or so). Each of them thinks the other one looks smaller, even though both know that's how perspective works. Far away things look small. Also, if each of them holds up a yardstick, each will think the other's yardstick looks smaller. So when Bob watches Alice measure her tiny self using her small yardstick, she appears to measure herself as bigger than she appears to be--and vice versa.

Let's think about another experiment. Alice is in a moving train car with a light bulb in the exact center. When the light is turned on, the light rays reach the front and back of the car at the same time.

On the other hand, Bob, who is standing at the station, sees the light hit the back of the car first--because the back wall of the train car is moving to meet the advancing light rays, while the front of the car is moving away so the light rays have to catch up to it.

What this tells us is the words "at the same time" are not meaningful. They apply only to a single reference frame.

Indeed, in the viewpoint of Pat, who's riding a faster train than Alice's, the light hits the front of the train before the back.

There is, however, something Alice, Bob, and Pat all agree on.

First, another piece of terminology. An exact time and place, the combination of when and where, is called an "event." My house at 4pm, or McDonald's at 5pm are examples of events. In the above example, the time and place where the light rays hit the front of the car is an event. Let's call it F. Likewise, the time and place the light rays hit the back of the car is also an event. Let's call it B.

For Alice, to get from F to B you'd have to travel at infinite speed (after all, F and B happen at the same time). For Bob, to get from B to F (which is the direction you'd have to go in because B happens first), you'd have to travel faster than light. For Pat, to get from F to B (which is the direction you'd have to go in because F happens first), you'd have to travel faster than light. All agree that to move from one of those events to the other, you'd have to travel faster than light.

And this, it turns out, is a generic problem for anyone traveling faster than light. If you have a starship that travels from event A to event B at a speed faster than light, for some observer it traveled infinitely fast, and for another, the ship went in the reverse direction. If you fire a gun with a bullet that goes faster than light (event A) and it triggers the death of an alien some distance away (event B), then it means for some reference frame, the alien died before you pulled the trigger.

Causality breaks. And unless you plan to start writing books in which you can read the pages in any random order, causality matters.

So what can you do? Take a shortcut.

It may take an hour's driving to get from your house to McDonald's using the roads and traveling 60mph because the nearest bridge across the river is 30 miles away. However, if you take the rowboat, you can be there in 5 minutes.

What if you need to get from earth to Alpha Centauri in an hour? You need a shortcut. One kind of shortcut is called a "wormhole." Basically, it's a sort of portal that connects one part of space to another.  Hyperspace is another sort of shortcut. You first travel to another place where distances are shorter, you travel in that place for a while at a speed slower than light, then you come back to normal space. If you've heard of the Alcubierre Warp Drive, this is also a kind of shortcut--although it's a little more complex to describe.

Did that make sense? I hope it did. At least a little.