The universe is very big.
It’s so big that our terrestrial-evolved brains have trouble really understanding how far distances between stars really are. The closest star to us (other than our sun) is Proxima Centauri at 25 trillion miles away. That’s so far, it would take light 4.2 years to reach it.
This presents a challenging problem for gallant space cadets exploring the universe and battling aliens. In order to cross the distances even to the closest stars would take a very very long time. If you were to travel at the same speed as you drive on the highway, it would take about 44 million years to get to a neighboring star! (Without bathroom breaks.)
New intelligent species could evolve back on Earth before you got back!
The solution we humans typically employ to cross vast distances is to travel faster. But the problem is that there’s a cosmic speed limit that can’t be broken. You can’t go faster than the speed of light – typically denoted mathematically as c, about 300 million meters per second, in case you were wondering.
Why You Can’t Travel Faster Than Light
Anything with mass behaves differently when it gets accelerated to speeds approaching c. You can thank Albert Einstein for this little nugget from special relativity… the Lorentz factor (Hendrik Lorentz had a roll in it too).
The m0 term is the “rest mass” of the space ship, person or electron you’re trying to accelerate. On the other side, m, is the “relativistic mass.” I won’t go into the details, but this derives from the total energy of a system (in the E=mc2 sense.) What this equation says, basically, is that as an object’s velocity (v) increases, it will behave as if the mass has increased. For the most part, in our experiences evolving here on earth, things simply don’t move all that fast. The top speed of a cheetah for example is about 36 meters per second.
The speed 36 m/s is very small compared to 300 million m/s. The ration is even smaller when squared. So for all the fancy terms on the right hand side of the equation above are practically no different than 1. In other words, unless you’re travelling an appreciable percentage of light speed, your mass doesn’t behave any differently than if it’s at rest.
But what happens when v gets close to c? All of a sudden, the term on the bottom gets smaller. In fact if v=c, you run into the “divide by zero” problem. The relativistic mass becomes infinite. And it takes an infinite amount of energy to move it.
The problem then facing the science fiction writer who wants a heroine who can gallivant around the universe with her K205 railgun at the ready in a timeframe that doesn’t involve the evolution of new species, is how to overcome the distance problem and sound at least semi-believable in doing so.
Breaking the Lightspeed Barrier
In theory there are ways around this.
There are physicists who spend their carriers working through general relativity equations, searching for potential solutions to the problems of interstellar travel. Enter the notions of wormholes and warp drives.
The basic “cheat” here is figuring out ways to shorten the distance between two points.
When general relativity came around shortly after the end of World War One, it changed how physicists thought about space and time. Einstein showed that gravity could be explained mathematically as a curvature in space around an object with mass. Space itself could change!
It only took a few months following the general relativity publication for Karl Schwarzchild and Hendrik Lorentz to publish solutions to Einstein’s field equations for a point mass, effectively describing a black hole (though the general concept in the classical sense had been proposed over a century earlier). This extreme warping of spacetime eventually gave rise to another idea where by two points on a plane could be brought together through a shorter tube. In 1957 John Wheeler coined the term “wormhole.” In principle, this solved the distance dilemma.
If the two star systems are too far away, one could simply warp space and time to bring them closer together. You don’t have to go faster than light and you can still get there.
It’s important to point out that this is one of those notions where the math comes first. There’s nothing in the mathematics that says this can’t be done, however that doesn’t mean it’s easy. For one thing, the kind of warping we’re talking about typically takes huge amounts of energy–like all the energy contained within a star. And there are other practical considerations-like not getting turned into a human spaghetti noodle by gravitational tides. However, as we used to say in physics undergraduate classes… those are engineering problems.
The other big idea along these lines is a little more recent and was inspired by the Star Trek franchise. In 1994, Miguel Alcubierre proposed a solution to Einstein’s field equations whereby a spacecraft could contract spacetime in front of it and expand spacetime behind it, effectively travelling faster than light without actually breaking any laws.
Unfortunately for would be Captain Kirks, accomplishing this requires matter with an energy density less than than of a vacuum–in other words a “negative mass” (and a lot of it at that)–which we’ve never observed and many people say is not actually possible.
Recent work from Alexey Bobrick and Gianni Martire as well as work from Erik Lentz that has come out just this year suggests that perhaps you can get by without the requirement of negative energy, although the catch seems to be that while you can in principle have an isolated bubble that moves practically faster than light, you still have an acceleration problem. And generating the bubble may not be possible from a spacecraft within. But that work is a hopeful step that there might be “some” way of actually travelling faster than light.
Incorporation into Science Fiction
As a genre, science fiction has a spectrum of scientific realism interpreted on a scale of hardness. One one hand you have hard where the author makes a serious attempt at keeping the technology realistic. A good example of this might be Andy Weir’s “The Martian” where an astronaut on a Mars mission is left for dead and has to survive on his own until he can be rescued. There’s no faster-than-light travel and the science is at least reasonably believable. On the soft end you have something like the Star Wars franchise–where the science takes a back seat to the action. You just get in your Millennium Falcon and go make the Kessel Run in less than 12 parsecs, regardless of what a parsec is.
When building their fictional universes, authors have to make a decision about how true to the science they want their stories to be. And a lot of the time that depends on the type of story that they want to tell.
When I was writing First Command, my goal wasn’t to write hard science fiction. The story at its core is about leadership. It’s action first, science second. That’s not to say that I didn’t think at all about the science while writing it though.
What is important, even in pew pew kerplewy stories, is a set of consistent rules. Readers of speculative fiction will often accept whatever rules you establish up front as a writer. What’s critical, is loyalty to those rules as the story evolves.
In Cassi’s universe, spacecraft achieve faster than light travel via a transit process, similar to the Alcubierre drive. The completely fictional part is that they need to line up precisely between two stars to generate their warp bubble. This forces spacecraft to travel along specific lines, like train tracks when travelling between solar systems, and looking ahead in the series, this presents a set of interesting possibilities and restrictions for interstellar travel.