You want to get there, and you want to get there fast. How do you do it?
Well if you're Dale Earnhart Jr., you stomp down on the accelerator. If you're Usain Bolt, you push balls-to-the-wall and run like the wind. If you're Lance Armstrong, you dope up and hope no one notices.
But what if you're a starship captain? Fortunately, you are not real, and so have an advantage over the rest of us. You can engage the warp drive, or the quantum jump engines, or fly through a wormhole and explore the Delta quadrant.
For argument's sake, let's assume you're a real starship captain. That is, you need to obey as many laws of physics as humanly possibly. Basically, this is Hard Science Fiction–think Mark Watney on Mars: even though that dust storm (that extremely rarified 1 percent of Earth's atmosphere dust storm) could have never in a million years blown you off your feet, you still need to obey physics to get back to Earth.
Let's go out on a limb and say that in 500 years we have fusion down pat–as in, it's no longer perpetually 30 years in the future but in actually installed down in engineering on deck 10. Let's inch even farther along that limb and say that we have anti-matter production and containment solved, and know how to derive lots of useful energy from its reaction with normal matter. Deal? Ok, so we've more than inched–we're hanging by a thread off that limb, but these are realistic technical goals within the next few centuries, and they at least let us begin to address the ugly truth: Interstellar space travel is very hard and will require unthinkably enormous amounts of energy.
Let's jump tracks for a moment. What is our goal as a starship captain? Let's keep it simple and easy (and dire): our colony on the third planet of Alpha Centauri just sent out a distress signal. Aliens are attacking, and they desperately need our help. Of course, they actually sent out the distress signal 5 years ago, but the transmission didn't reach us until now because, as we all know, radio waves are a form of light, which travels at the speed of, uh, light. What do we do?
Full speed to Alpha Centauri! Helm, lay in a course! Engage the engines! Full thrust! We will avenge them!
You see, Alpha Centauri, the closest star system to us, is nearly 5 lightyears away. Let's assume for the moment that our starship can sustain a substantial thrust for an extended period of time. Just for convenience, and because it's nice to have Earth-like gravity on our ship, let's say the thrust is 1 g. Furthermore, assume our ship weighs about 1000 tons, and that we're able to extract ALL of the energy out of our anti-matter reaction and dump it into propulsion. Easy-peasy, right? At this thrust, we'll max out at about 96% of the speed of light halfway through our journey before we flip the ship around and start slowing down in time to not collide with the third planet of Alpha Centauri at relativistic and therefore splat-inducing speeds.
We have a problem.
In this 1000-ton-ship scenario, it takes about 5000 tons of anti-matter to get us there. Did I mention that the 1000 tons of ship has to include fuel? Oops. It does.
Basically, going 1 g at 100% anti-matter fuel conversion, it still takes about 5 times the amount anti-matter than you can store on your ship. Remember–you have to accelerate the fuel too. No free rides!
But let's look on the bright side. Even though our fictional starship didn't actually make it to Alpha Centauri, our non-trip took us only 3.77 years! From our perspective. From the point of view of everyone back on Earth, and the poor besieged souls on the third planet of Alpha Centauri, our non-trip took us 6.66 years. That's because as we approached the speed of light, from our perspective, the distance between Earth and Alpha Centauri shrunk. By as much as 72% at our highest speed. From their perspective, our clocks slowed down. If they could have watched us through our windows with an impossibly powerful telescope, they would have seen us moving in slo-mo aboard our ships.
Ok, so we can't sustain 1 g of acceleration for 3.77 years. How many g's can we sustain, and still be able to store all the fuel required for the trip?
About 15%-18% of a g. That's right, you'll be running around on your ship with the same amount of gravity as on the moon. And you'll have a lot of time to do it–10.5 years. 12 years according to observers.
Let's inch back towards reality. Just an inch. Suppose that our fuel conversion rate is not 100% like we assumed. Let's make it something more within the realm of physics. Like, say, 10% (still generous, if you ask me). Now we're talking about a 34 year trip. 34.5 years for observers because we only (only!) ever got up to about 28% of the speed of light.
You see why riveting, fast-paced interstellar hard science fiction is tricky? And we never even addressed the problem of propellant–you know, the stuff you have to heat up and shoot out the back of your thrusters.
And then there's the single particles of dust that will destroy your ship while traveling at a fraction of the speed of light. And if you're fortunate to not run into a dust particle, then the 1 atom of hydrogen per cubic meter of space will slam into your forward hull so fast that it will soon overheat and melt. And if you can somehow cool off your hull from all the interstellar hydrogen running into it, there's always the background radiation of space itself–the leftover glow of the big bang.
Huh, you say? Isn't that stuff harmless?
Yes, for us it's harmless, traveling very slowly. But speed up in any direction, and those harmless microwaves can blueshift into sunburn inducing ultraviolet waves, or worse, x-rays. But nothing a little lead shielding on your hull can't handle, right?
Uh, better slap on an extra 1000 tons of lead to your 1000 ton spaceship. Which means you'll need more fuel. Which means … crap …you died from old age trying to save the Alpha Centarians, Captain. Sorry.
Just stick to the solar system for your hard SciFi, ok? Think Pluto. Pluto's nice.
For fun, you can try out your own interstellar travel calculations here: http://nathangeffen.webfactional.com/spacetravel/spacetravel.php
Use the calculator to plan your space travel at your own risk. I am not responsible for any accidental time dilation, space contraction, shrinkage (the spaceship kind), old age from excessive space travel, bone density loss, impotence, baldness, and/or death from any misuse of the calculator. Proceed at your own risk.