Space travel, we've all thought about it. Whether you're an astronomy student or a Star Wars fan, it's difficult to avoid wondering what kinds of worlds might be out there. If there are billions of stars in our own galaxy, and billions of galaxies in the observable universe, that means that there is an unthinkable number of planets out there ready to be explored.
But how are we going to get there?
We've sent humans to the moon and unmanned probes through the solar system, but our current technology takes too long, and these places are just too far. We need new methods of propulsion to get there faster.
First, we need to recall some basic physics: Newton's second law of motion. This law is summarized in the equation F= ma, whereby the force imparted on an object is equal to that object's mass times the acceleration that results. What this means is that if you want an object to accelerate more quickly, you either provide more force or make the object very light. This is why we spend so much money making everything on a spaceship as light as possible because we want all of the force from the engines to get the ship going as fast as we can, and for a given force, a smaller mass will mean a greater acceleration.
But there's an obvious limit to this approach since we can accelerate something like a subatomic particle to almost the speed of light, but no humans can fit inside, so we won't be getting very far. Instead, we must focus our attention on producing a greater force with which to propel a spacecraft. Currently, we use combustion to get a spaceship off the earth and into space.
This utilizes the chemical energy stored in hydrocarbons by burning them in the presence of oxygen. This is an exothermic reaction that releases energy, and this is the energy that provides propulsion for the spaceship. As it happens, combustion is the same type of energy we use in our cars, so by the standards of a spacefaring civilization, it's extremely primitive. Also, all that fuel is very heavy, which is why you always see space shuttles launching with huge fuel tanks because we need so much of it just to escape Earth's gravity. Surely there's a better way to go, right?
Well, we have recently begun to play with nuclear energy. Although it gets a bad rap along the way, these accidents are the byproduct of nuclear fission, which involves the splitting of massive radioactive nuclei like uranium. An even better technique that generates way more energy and involves no radioactive waste is nuclear fusion, whereby small nuclei like hydrogen fuse together, and the tiny bit of mass that is lost in this process is converted to pure energy. This is dictated by Einstein's famous equation, e = mc^2, which says that energy and matter are equivalent and related by the square of the speed of light. This is a huge number, so when squared it's even bigger, and that means tiny amounts of matter can be converted into enormous amounts of energy.
This is fundamentally different than a combustion reaction where atoms rearrange their combinations, this is the conversion of the matter directly into energy, which is the process that makes the Sun and every other star radiate with such incredible intensity. Truly, nuclear fusion is the energy of the Sun, and although we are still trying to figure out how to get a fusion reactor to produce more energy than we put into it, it's not outrageous to think that this technology could eventually be mastered and even placed on a spacecraft. This would allow us to go through space dozens of times faster than we do now, making manned missions to the outer solar system possible.
Other such powerful engines could involve the annihilation of matter and antimatter, which again converts matter to pure energy that could accelerate a spaceship. This also poses some serious challenges, like the containment of antimatter, which annihilates anything it touches. But it remains a mere technological challenge.
Solar sails have also been proposed, which take advantage of radiation pressure exerted by sunlight, allowing us to sail the cosmic seas. So this sounds pretty great, and new kinds of rockets could help us navigate our solar system with ease. But that's just not going to be enough. Even with these futuristic propulsion systems, it would still take many years to get to even the closest stars beyond our Sun.
how do we go faster?
Unfortunately, there is a very firm limit as to how fast anything can go, and that's the speed of light. While it may seem arbitrary, this cosmic speed limit is necessary for the universe to be consistent with logic, and our inability to go faster than this has nothing to do with technology, it's just how the universe works.
Einstein's special relativity describes the bizarre things that happen as we approach the speed of light. Time slows down, distances shrink, and we gain mass according to specific equations. These equations, which have been verified to high degrees of precision countless times, tell us that it would take literally infinite energy to accelerate anything with mass to the speed of light.
Beyond this, surpassing the speed of light would result in absurdities like imaginary time and a violation of causality, whereby effects could precede their causes, so this cosmic speed limit is necessary for the universe to make any sense at all, and we will absolutely never be able to break it. This is a pretty big bummer, because even light, which moves as fast as anything possibly can, still takes a hundred thousand years just to go from one end of our galaxy to the other, let alone to other distant galaxies, and we just don't have that kind of time. Isn't there some loophole to all this?
Well, yes there is, but it's not a loophole, it's a wormhole. A wormhole is a rip in the space-time fabric. We have to bring up Einstein again because he told us that space and time are actually one thing: space-time, which is warped around massive objects. If space-time can wraparound mass, it can also rip due to energy, since mass is energy, and it is hypothesized that one could rip a hole in the space-time fabric such that one could travel through it and pop out in some other part of the universe. This does not violate special relativity because nothing would be traveling faster than the speed of light. Technically, a spaceship wouldn't have to go any faster than an airplane. It is simply that traveling through a wormhole could act as a shortcut to another place in the universe, like the way Mario goes through pipes in the Mushroom Kingdom.
This is usually what is implied by a warp drive in science fiction, as this kind of warping is the only way to quickly get from one place in space to a totally different one, and believe it or not, this idea of shredding through space-time is based on some pretty sound mathematics. The only problem is that our calculations show that the amount of energy that would be required to generate such a wormhole is absolutely astronomical. It is an amount of energy that makes a futuristic fusion engine look like a AAA battery. Nevertheless, it is a finite number, which always leaves room to hope that we could one-day master this technology too, whether in a thousand years or a million. I can think of no better motivation to put our petty differences aside and come together as a species than the promise of exploring every nook and cranny of the universe. If mankind manages to survive long enough, and unlock the mysteries of longevity to boot, maybe I'll see you out there, since I'll definitely be first in line.