So You Want To/Write a Hard Science Fiction Story With Space Travel: Difference between revisions

However, to work as a piece of hard SF with space travel, the writer must go one big step farther: The technology, the mechanics of space travel, the planets, the aliens (if there ''are'' aliens), all the details of that futuristic setting ''must be [[Realism|realistic]].'' The author must take pains to follow the known laws of physics, chemistry, biology, astronomy, and planetology, and how they apply to any areas of engineering that will appear in the story. This means the author must ''know'' the laws of physics, chemistry, etc., or have good access to someone who does. While the laws of the author's fictional universe are allowed to deviate from the laws of [[Real Life]] ''on occasion'', the author must be consciously aware of each of those deviations, must have an excuse for them (even if he never tells the reader this excuse, he must have it in his own head), and above all must take pains to ''limit the damage'' that such departures from reality can potentially do to the story.

Since space travel is involved, it's important to remember that human beings have travelled in space for over five decades now. We ''know'' what is involved in getting from the Earth's surface to low Earth orbit. We know what's involved in landing on a rocky world 400,000 kilometers away. We know what effect microgravity has on human bones and muscles. A realistic story involving space travel must take all this accumulated human knowledge into account. The cartoonish world of 1950s B-movie astronauts having a "navigational error" that sends them to an "uncharted planet" with an Earthlike ecosystem inhabited by alien women who speak English is, and should be, a [[Discredited Trope]] -- but so should portraying space travel like anything other than space travel just because it [[Rule of Cool|looks neater that way]] in your head. You'll just have to dispense with the story making artistic sense so the story can make logical, scientific sense, so be ready for a lot of artistic disappointments.

== Heartbreak Hotel ==

Sadly, the rules of writing about realistic future space travel -- like the rules of writing about anything realistic -- are primarily a set of rules about what you ''can't'' do. The more ideas you have about what you'd ''like'' to have your characters do, the more ways reality will step in and say "No."

First off, here is a list of tropes that are frowned upon in a realistic universe:

* [[All Planets Are Earthlike]] (at least without [[Terraforming]])

* [[Inertial Dampening]]

* [[Reactionless Drive]] (at least until the [[w:RF resonant cavity thruster|RF resonant cavity thruster]], aka the EmDrive, is confirmed)

* [[Space Pirates]]

* [[Stealth in Space]] (and most of the other subtropes of [[Space Does Not Work That Way]])

* [[Teleporters and Transporters]]

There are sound reasons why all of the above tropes will probably not work in [[Real Life]]. It's not ''impossible'' to have them work in a way that doesn't violate the laws of science or good sense, but it will requite extra painstaking labor from the author to make that happen.

==== Doing the Research ====

This should go without saying, but:

If you're going to set your story someplace we already know something about -- like [[Mars (useful notes)|Mars]] or [[Local Stars|Alpha Centauri]] -- for goodness' sake, read up on what we know about the place before you start writing! We've sent space probes to [[Pluto Is Expendable|every planet]] in the solar system, we've accrued reams of data on just about every star that has a name, we've even mapped out the interstellar medium in our neck of the galaxy. The data are out there, and thanks to the Internet they're not even hard to acquire any more.

You wouldn't set a story in the Sahara Desert and have your hero go swimming in one of the "numerous lakes" there. You wouldn't set a car chase in downtown Florence, Italy and then make up the street names and city layout. Similarly, don't set your story on Mars and have your hero swelter in the unbearable heat<ref>I'm looking at ''you'', [[Babylon 5]] novel #1 "Voices" by John Vornholt!</ref>, or put an Earthlike planet in orbit around Alpha Centauri without at least mentioning the bright "B" star that should be visible from time to time in the sky. Making up details about places we ''don't'' have strong data about is one thing, but making up details about places where our existing data would make those details flat-out impossible is quite another.

In other words: Don't give 'em the opportunity to make a [[Did Not Do the Research]] entry for your story's All The Tropes page.

== Doing the math ==

With the exception of ion engines, all of these are mere drawing-board designs at present, and all of them have practical problems. NERVA engines don't have that much better an exhaust velocity than chemical engines, and require shielding to protect the crew (and the ship's more delicate electronics) from their radioactivity. Ion engines have extremely low thrust levels (the engines on the ''Dawn'' spacecraft can, at max throttle, produce about 1/3 of an ounce of thrust). Orion's nuclear putt-putt motor requires an enormous pusher plate that dramatically increases the dead weight the spacecraft has to carry. Controlled nuclear fusion has never been accomplished, at least not in a way that produces more energy than it consumes. Ramscoops rely not only on the controlled nuclear fusion of light hydrogen (which is even trickier than the controlled nuclear fusion of heavy hydrogen), but also on the ability to collect the extremely rarefied interstellar gas without inducing significant drag, which might not even be possible.

But even if controlled nuclear fusion ''does'' become a reality (allowing what [[Robert Heinlein]] called a [http://www.projectrho.com/rocket/torchships.php torch]), that still won't eliminate the need for big rockets if you want to get anywhere in a reasonable amount of time. Sure, your exhaust velocity might now be on the order of (say) 2% of light speed, but the rocket equation still applies. If you want to accelerate at 1''g'' to the half-way point between Earth and Saturn, then decelerate at 1''g'' for the rest of the trip, your total delta-v budget will be about 2% of light speed -- the same as your own exhaust velocity. You'd ''still'' need a mass ratio of ''e'', meaning your spacecraft's fuelled weight will be 2.718 times its empty weight. You're still stuck with a big rocket. And when you get to Saturn, you'll be out of fuel. You'll need to completely refill your fuel tanks if you want to make the return trip to Earth. Forget about the notion of a [[Space Sailing|"ship"]] patrolling the "seas of interplanetary space" for months on end, hopping from planet to planet without refuelling.

== Realistic [[World Building]] ==

We humans evolved on, and (so far) all grew up on, Earth. We instinctively expect the air to be breatheable, the temperature to be liveable, the gravity to be 9.8 m/s², the days to last 24 hours, trees and grass, animals and plants and fungi, et cetera, et cetera.

The sad fact is, though, that no other planet we've detected thus far is even remotely habitable by human standards. The bigger ones are Jupiter-like balls of gas, while the smaller ones are almost universally airless. The few worlds we've found that ''do'' have both an atmosphere and a solid surface have been blanketed in gases that no human can breathe, at pressures anywhere from [[Mars (useful notes)|near-vacuum]] to [[Venus|90 times Earth's sea level]]. While it's theoretically ''possible'' that a planet out there might harbor life as we know it, it would have to fit a long, narrow list of parameters, and even then, the kind of life that might have actually evolved there will most likely be very different from the multicellular-eukaryote-rich biome inhabiting Mother Terra.

In order for a planet to be able to support life as we know it on its surface ''at all'', it will have to lie in a very narrow range of distances from its parent star. Too close, and any water would evaporate. Too far, and any water would freeze. Liquid water -- and life as we know it requires liquid water -- can only exist if the planet lies within that narrow zone where it's receiving just the right amount of energy from its star for the surface temperature to allow it. This is called the star's "comfort zone," or "Goldilocks Zone" (as in: not too close, not too far, but juuuuuuuust right). The exact width of a star's Golilocks zone is a matter of some debate, due to the fact that some atmospheres can trap heat ([[Cough-Snark-Cough|*cough* Venus *cough*]]) and some can't, and a number of other factors that astrogeologists can make whole careers out of. All we can say for sure is that, for a star as bright and hot as the sun, Venus is too close, Earth is clearly within the Goldilocks zone, and Mars is ''probably'' close to the tail end of it.

You'll note that the Martian atmosphere is extremely thin, less than 1% of the surface pressure of Earth's atmosphere. One factor that contibutes to Mars's thin atmosphere is this low surface gravity. Despite being ''farther'' from the sun than the Earth, and thus receiving ''less'' heat that could potentially boil its atmosphere away into space, Mars still has less of an atmosphere than the Earth does. A resonably strong surface gravity may be ''required'' for a planet to retain a thick atmosphere. There are exceptions in our own solar system, of course: Saturn's moon Titan has less than a sixth of Earth's surface gravity yet its surface atmospheric pressure is higher than Earth's, and while Venus is both closer to the sun ''and'' has only 90% of Earth's surface gravity its surface pressure is ''ninety times'' that of Earth's atmosphere. But you need at least ''some'' gravity, and possibly quite a lot of gravity, to retain an atmosphere within the Goldilocks Zone.

== Believable aliens ==

Let's say the idea of a spaceship carying 10 times its empty weight in fuel sickens you. You want the space aboard your space ship to house your colorful characters, dazzling weapons, holodecks, shopping malls, and other fun and excitement -- not deck after deck full of boring old propellant. And you want to allow for long patrols without having to refuel at every destination. So, you elect to go the route of the ''[[Honor Harrington|Honorverse]]'', and equip your spaceship with a gravity-manipulation [[Reactionless Drive]] that allows her to accelerate without throwing material out of her tailpipe. Problem solved, right? And now that you've given your civilization gravity-manipulation technology, that also eliminates your problem of having your characters float around in zero gee; they can now spill liquids without spraying them all over the walls and play ping-pong to their heart's content while riding between the planets.

Second, are you also [[No Conservation of Energy|violating the conservation of energy]]? A 1000 tonne spaceship traveling 1/10 of the speed of light has a kinetic energy of 450 quintillion Joules, equal to 100,000 megatons of TNT. That energy had to come from somewhere. Did it come from burning some sort of fuel on board your space ship, to power the generators? If you used the thermonuclear fusion of hydrogen into helium as your fuel source, and you managed to [[Hand Wave]] a fusion reactor technology that's nearly 100% efficient, you'd have to burn at least 350 tonnes of hydrogen to obtain that much energy, which is a third of your spaceship's own mass. (This isn't as bad a mass-ratio situation as if you'd used a plain-old momentum-conserving fusion rocket, but it's still pretty significant.) And you'll have to burn just as much again to slow your space ship back down at the end of your trip. If this is too much for you, and you decide your reactionless gravity drive simply works by tapping into the magical gravity waves of the universe and surfing along them with only minimal power requirements, then your space ship's kinetic energy is being created ''ex nihilo''. You've got yourself a free energy machine! Just strap your space ship to one end of a long lever, strap the other end to a huge electric generator, and fly in circles. You can generate enough energy to power your entire civilization this way, with no cost in natural resources. This will play absolute ''havoc'' with your fictional economy. You'll have to throw away that whole book you were going to write about your space empires' war over [[Space X|Space Oil]].

The potential damage done to your story by a [[Reactionless Drive]] is just one example of the broader principle. ''Any'' technological marvel that sidesteps the [[Real Life]] roadblocks facing space travel has the potential for unintended consequences. Thermonuclear [http://www.projectrho.com/rocket/torchships.php torchships]? They've got the same "spaceship = weapon of mass destruction" problem that reactionless drives do, albeit on a more manageable scale.

So what do you do when you ''need'' your characters to be able to move between the stars faster-than-light, or [[Transporters and Teleporters|teleport]], or have a [[Tractor Beam]], or do any of the other myriad things that our current best guesses at the law of nature say are impossible? You set the technology up in such a way as to '''limit the damage''' to your story and your setting. Maybe your [[Deflector Shields]] are magnetic, and can only affect charged particles and ferromagnetic metals -- and your spaceship needs to open up holes in its shields to shoot iron slugs or particle beams at an enemy. Maybe the high speeds needed to traverse interplanetary distances in days or hours are imparted not by your space freighter's own engines, but by planetside pushers that will only push it onto a predictable course, thereby eliminating the threat of rogue spaceship commanders turning their vehicles into WMDs. Maybe your transporters only let you beam between one transporter pad and another (unlike the transporters in a certain [[Star Trek|softer SF franchise]]). Maybe the violations of the Laws of Thermodynamics needed to make [[Stealth in Space]] work are curtailed in some way that prevents you from getting useful energy out of any warm object (which, like some types of [[Reactionless Drive]], would have driven your Space Oil companies out of business).

The very worst problem with FTL travel (or even just [[FTL Radio]]) is a certain extremely strange consequence of [[Time Dilation]]. When travelling at any speed, even a brisk walk, relative to somebody else, you'll see his clock move slower than yours -- but he'll see ''your'' clock move slower than ''his''. This extremely counter-intuitive state of affairs means that some distant events in the universe which are in your future are in the other guy's past, and vice-versa. Without FTL travel, though, this isn't a problem. Einstein and Minkowsky established that for any event that's in Oberver A's future and Observer B's past, no matter how far in Observer A's future the event is, it will always be far enough away that any ''light-speed signals'' from this event would not reach Observer A until the event was also in Observer A's past. When plotted on a space-time graph, the signals from the event would stretch out in spacetime in a "light cone," which guarantees that the signal will not reach any observer in the universe until the event is in that observer's past. To put it another way, let's say that in Observer A's reference frame, Event 1 occurs before Event 2, but in Observer B's reference frame, Event 2 occurs before Event 1. Light cones maintains ''causality'' by ensuring that, if Observer A would find out about Event 1 before Event 2, Observer B ''cannot'' find out about Event 2 before information about Event 1 is theoretically available to him.

Here's an example: Suppose Observer A is standing on Earth, and Observer B is in a space ship, coasting in a straight line at 86.60254% of the speed of light. This gives him a gamma (γ) factor of exactly 2. When the space ship passes by Earth, both Oberver A and Observer B synchronize their clocks at 5:00 PM. In Observer A's frame of reference, when his clock reads 7:00 PM, Observer B's clock will read 6:00 PM. However, in Observer B's frame of reference, when his clock reads 7:00 PM, Observer A's clock will read 6:00 PM. At 6:20 PM on Observer A's clock, an event happens on Earth -- the winning State Lottery numbers are announced. At 7 PM on Observer A's clock, this event is 40 minutes in Observer A's past; but at 7 PM on Observer B's clock, this event is still 20 minutes in Observer B's future. It's not just that Observer B ''perceives'' it to be in the future, it really ''is'' in the future, it really hasn't happened yet. What prevents Observer B from knowing about the event before it happens in his reference frame is that it takes ''time'' for any information about the event to reach him. At 6:20 PM in Observer A's reference frame, Observer B's clock would only read 5:40 PM, but Observer B would be 69.282 light-minutes away from Earth; if Observer A radioed the winning lottery numbers to Observer B at this moment, they'd take 521 minutes in Observer A's reference frame to reach Observer B's space ship, at which point Observer B's clock would read 9:40 PM and the event would be 4 hours in Observer B's past. Even if Observer B magically reversed his velocity at 5:40 PM on his clock, so that he was headed ''toward'' Earth at 0.866''c'' instead of away from it from that moment onward, the radio signal would still take 37.128 minutes in Observer A's frame of reference to reach the space ship.

But by going faster than light, even just [[FTL Radio]], you ''can'' receive information about events that are in your own future. You can perceive Event 2, which was caused by Event 1, before Event 1 actually occurs in your reference frame. In our lottery-winning scenario above, suppose Observer A and Observer B had a [[Subspace Ansible]] that allowed instant communication no matter how far apart they were. Observer A could send the winning lottery numbers to Observer B's space ship at 6:20 PM on Oberver A's clock. With instantaneous communication, the numbers would arrive on board the space ship at 5:40 PM on Observer B's clock. If Observer B then sent the same numbers ''back to Observer A'' over the same subspace ansible, they'd arrive on Earth at 5:20 PM on Observer A's clock. Observer A would have the winning lottery numbers ''an hour before they were announced.''

[[Translation: "Yes"|In other words]], '''[[Time Travel]]'''.