Hello, everyone. This week I’m exploring the idea of travelling to other stars, a staple of science fiction. There are many possible ways of doing this, and I’m going to try to take a quick look at each one.
FTL
FTL refers to faster-than-light, the idea that the speed of light is not the fastest anything can travel. Most authors are very dismissive of this concept, mostly because they don’t understand Einstein as well as they think they do. Einstein’s theories (not laws) suggest that the speed of light is absolute, through a given medium. We have been able to slow light down to a crawl in laboratory experiments. We observe distant galaxies moving faster than light, which we attribute to space expanding, which doesn’t violate any theories or laws. We have also seen things break the lightspeed barrier.
Cherenkov radiation. This is the radiation emitted when a charged particle, usually an electron, passes through a dialectic medium at a speed faster than the phase velocity of light through that medium. Basically the maximum speed of light is determined by the medium it is traveling through. Physics tells us light speed is the fastest anything can travel through a given medium. Cherenkov radiation occurs when something breaks that rule. This isn’t science fiction, folks; this is real. The blue glow in the picture above is caused by electrons moving faster than light through water.
Now, I’m not saying things can move faster than the absolute limit of light we’ve observed, but it should be pointed out that we have only ever observed light from inside a gravity well. We don’t know for certain that the speed of light through the interstellar medium is the same as it is here. We need to get outside the solar system and observe. Maybe then we’ll begin to understand our universe a little bit better.
I don’t know of any modern science fiction stories which ignore Einstein, although it was more common when he was still alive. The only ones I can think of offhand are the stories of E.E. “Doc” Smith, which were being written around the time Einstein was gaining acceptance.
The Lensman series, on the surface, ignores Einstein and gets a bit of ridicule because of that. But the stories don’t, not really. The Berganholm drive of Lensman is a drive that negates the inertia of the craft so that the engines can push it faster and faster. Einstein’s theory tell us that an object can never achieve the speed of light because mass and energy are the same thing, and as an object travels faster, it has more potential energy, so it has more mass, so it requires more energy to propel it faster, and so on. However, if it doesn’t gain energy from traveling faster (inertia-less) then it could travel as fast as the engines could push it. Right? Well, maybe not, but it does a better job of hand-waving (handwavium) than most science fiction. Not to mention that the stories are just damn fun.
Warp Drive
This was a convenient piece of handwavium in Star Trek until someone came along and said: “Why not?” Star Trek uses the warp drive to get from star system to star system. The drive bends space around the ship, technically sending it into warp space, which is kind of like hyperspace (I’ll get to that) but not really. Roddenberry wasn’t a scientist; he was just a really big fan who made it big. However, the idea of warp travel inspired a real scientist, Mexican theoretical physicist Miguel Alcubierre, to try and work out the physics behind a drive of this type. What he came up with is being called the Alcubierre drive. Basically it warps space by compressing the fabric of spacetime in front of the ship and pushing it away behind the ship. This means that the ship moves over a shorter distance through space and doesn’t violate any rules along the way. For example, if the drive works at 1:4 compression, then four light years could be compressed and traveled over like one light year. I’m sure you can see how this would be useful. The only problem is the math required the total energy annihilation of a Jupiter-sized planet to make it work. Not something very practical.
NASA’s Alcubierre
The scientists over at JPL (Jet 
Propulsion Laboratory) liked the idea but not the execution, so they spent a couple of decades trying to make it better. They did. They came up with a new version of the drive in simulations that only requires the mass energy of a Volkswagen Beetle. Much more practical. They are currently working on getting this thing into operation, and also to refine the engine to use less power. They came this far in two decades. What will we have in one or two more?
Spacefold, Fold, and Jump Drives
This idea is taken from our knowledge of space being wrinkled up and not the smooth sheet of classical physics. I’m not sure who first came up with the idea, but it is best described in the 1958 novel Have Space Suit, Will Travel by Robert A. Heinlein. I’m sure you’ve all heard his explanation, either in Interstellar, Stanger Things, or elsewhere, but I’ll go ahead and mention it for those who aren’t sure. Take a piece of paper and draw two points on it, the star where you are and the star where you want to go. The paper is spacetime. Draw a line across it. It takes lightyears to travel between the two stars. Now fold the paper so the two points line up. Jab the pencil through the paper. The hole is the fastest way between the two points, not the line across spacetime.
There is nothing is modern physics that says this can’t be done, and a few things that suggest it can. A subset of this would be wormhole travel, which basically does the same thing. Some other books which make use of this are Foundation by Isaac Asimov, Falcon by Emma Bull, and my own stories, where I use several different drives. I have seven methods of interstellar travel in my stories, because I figure that if it works, someone will be using it.
Hyperdrives
The idea behind hyperdrives is that 
there is a layer (or layers) of reality that are beyond and yet congruent with our spacetime. Imagine another spacetime that is smaller, or where the speed of light is different. You travel at normal speed in hyperspace for a light-month, pop out into realspace, and you have traveled ten lightyears. Cool concept, and not beyond current theoretical physics. Possibly the least likely drive, but real enough that it can be backed up with some physics from hyperspace theory (a real thing) and brane or m-theory (membrane theory) that uses real observation and a lot of math to explain what science fiction authors like to throw into stories because it is cool and convenient. Some notable examples are the Babylon 5 TV series, the Honor Harrington novels by David Weber, This Alien Shore (amazing story) by C.S. Friedman, and my own stories, where hyperdrive is the most common type of drive.
There are many other types of drives in fiction. Asimov had a cool story where the AI pilot of the starship goes insane because, just for moment during the jump, the human crew and passengers are neither alive nor dead, causing some serious confliction with the Laws of Robotics.
For non-FTL solutions, there are generation ships, solar sails, cold sleep, stasis pods, Von Neumann devices… The list is endless. Ask me in the comments if you want to know more about any of these.