History SoYouWantTo / WriteAHardScienceFictionStoryWithSpaceTravel

You could go the easy route, and look up how long it took for the Voyager 1 space probe to fly from Earth to Saturn, and just assume that you space ship flies about as fast as the Voyager probes did. But when you do look it up, you balk -- it took nearly ''three years'' for Voyager 1 to make this trip! You can't have your intrepid [[SpaceCadet space cadets]] waiting around for three years just to get to Saturn, they've got important space adventures to have, space wars to fight, and space women to woo. You want them to get to Saturn a lot quicker. So, you give them a space ship that never runs out of fuel -- or uses fuel that's so efficient that it won't run out even if it runs its engines continuously between Earth and Saturn.

Let's say you decide to limit your space ship to a cruising acceleration of 1''g'', so that the crew will experience thrust equal to Earth's surface gravity. After all, this is a ''hard'' science fiction story, right? You can't just go slinging InertialDampening around. Any acceleration your ship undergoes will be experienced by your crew as G forces. So, if they're going to be cruising under engine power for anything longer than a few minutes, you'll probably want to keep your acceleration down to 1''g''. 1''g'' works out to an acceleration of 9.8 meters per second per second -- at the end of one second, you'll be going 9.8 m/s, at the end of two seconds, you'll be going 19.6 m/s, and so forth.

Now, how long will the trip take? Let's plug our numbers into the second equation above. We know d, and we know a. We can assume v[[subscript:0]] = 0, if we're taking off from Earth at a standing stop. (We won't actually be; our space ship will be taking off from Earth orbit, which means it'll be moving at about 7800 m/s, and our zero-velocity reference point here is our destination -- Saturn -- which is orbiting at a different speed as the Earth. But our overall speed is going to be so great that these little speed differences shouldn't matter much.) This is what we get:

Hmmm! It takes 360,000 seconds to decelerate from 3500 km/sec to 0 -- exactly the same amount of time it took to accelerate from 0 to 3500 km/sec! And if we ran the numbers, we'd see that on this second leg, we covered exactly the same 640,000,000 km we covered in the first half of the trip. The two legs of the journey are ''mirror images'' of one another. Once we've figured out how long it takes to get to the half way point, we know that it'll take exactly as long to go the rest of the way (assuming our deceleration during the second leg is equal in magnitude to our acceleration during the first leg).

So ... using our constant-acceleration trajectory, it takes 360,000 seconds to reach the half way point, and another 360,000 seconds to go the rest of the way, for a total of 720,000 seconds to get from Earth to Saturn. That's 8.3 days, a little over a week. It's still not something your space cadets can do on their day off, but it's a heck of a lot quicker than the 3 years Voyager 1 took to make the trip -- and unlike Voyager 1, your space ship ends up at rest relative to Saturn so it can stay there as long as you need it to.

Now ... what if our heroes can go faster than this? Let's say we've decided we need shorter flight times, so ''screw it'', we're introducing InertialDampening technology into our story, like in the ''Literature/HonorHarrington'' or ''Franchise/StarTrek'' universe. Now our space ships can accelerate at 1,000 ''g'', or 9,800 m/sec[[superscript:2]], and we should be able to get to Saturn much faster. In fact, using the first equation above, it looks like we should be able to accelerate to 30,000,000 m/sec in less than an hour -- that's a tenth of the speed of light.

And there's where we run into problems. Because above about a tenth of the speed of light, acceleration doesn't affect velocity in a straightforward manner any more. Your clock runs [[TimeDilation a little slower]] to a fixed observer than it does to you. Your momentum is slightly higher to a fixed observer than your acceleration history says it should be. The universe shrinks slightly in the direction you're moving. In short, you run smack-dab into ''special UsefulNotes/{{relativity}}'', and now the math gets a '''lot''' more complicated. For one, the change in your velocity per second, given a constant acceleration from your space ship's reference frame, now depends on your current velocity -- which turns the relationship between the two into a first-order differential equation.

(As you can see, a massive object such as a space ship can never achieve 100% of the speed of light, because its momentum would be infinite -- it would take an infinite amount of energy to accelerate to ''c''.)

... where T is "proper time" (the number of seconds elapsed in your space ship's frame of reference), t is the number of seconds elapsed from a fixed observer's frame of reference, d is distance travelled from a fixed observer's frame of reference, a is acceleration in the space ship's frame of reference (which is assumed to be held constant), Tanh is hyperbolic tangent (there's a button for this on most scientific calculators), and [=ArcCosh=] is inverse hyperbolic cosine.

So, let's say you want your inertial-dampener-equipped space ship to accelerate at 1,000''g'', or 9800 m/sec[[superscript:2]], and you want to follow the same course you did with your piddling little 1''g'' space ship -- accelerate to the half-way point between Earth and Saturn, 640 million km away, and then decelerate for the same distance to arrive at Saturn with v = 0. How much time will that take, as far as the folks back on Earth are concerned? For that, we can use equation 2 above:

The folks back on Earth measure 11,600 seconds, or about 3.2 hours, for the space ship to reach the half way point. Meanwhile, how much time has elapsed for the folks on board the space ship? For that, we need equation 1 above:

... or a tad over 1/3 of the speed of light. This explains why our proper time T and our Earth time t are so close together: At 1/3 of light speed, the gamma factor is only about 1.07, and our space ship was only going this fast near the end of this leg anyway. If we'd taken a longer trip -- say, from Earth to Sedna in the Kuiper belt, or from Earth to [[UsefulNotes/LocalStars Alpha Centauri]] -- we'd have more distance in which to accelerate, which would let us get closer to the speed of light, and the relativistic effects would have been more pronounced.

Unfortunately, these equations only address the situation when your space ship starts out at 0 velocity. If you want to apply these equations to a situation where you start out already moving at (say) 1/5 of light speed, they get even more complicated, and often times will not have already been derived for you. For example, the equation for the amount of time a fixed observer measures that it takes your accelerating space ship to cross a given distance, assuming you started out with a velocity that gave you an initial gamma factor of γ[[subscript:0]], is this hairy beast:

If your space ship is close to a large gravity source like a planet or a star, and is moving at a low enough speed, it isn't going to travel in a straight line. The gravity of the big object will cause your spacecraft to follow an ''orbital'' trajectory.

All orbits are shaped like ''conic sections.'' If your space ship is moving slower than the "escape velocity" -- or more precisely, if its total kinetic energy and gravitational potential energy is less than the gravitational potential energy it would have at an infinite altitude -- its orbit will be shaped like an ellipse. If it's moving ''precisely at'' the escape velocity, its orbit will be shaped like a parabola. If it's moving faster than the escape velocity, its orbit will be shaped like a hyperbola. This is assuming, however, that your space ship spends all its time from this moment forward in free-fall, without firing its engines.

Parabolic and hyperbolic orbits are basically escape trajectories. The spacecraft leaves the big object in question and never comes back. An elliptical orbit, on the other hand, is stable, and allows your space ship to go around and around the big object over and over again. It's what most people think of when they hear the word "orbit." An [[http://en.wikipedia.org/wiki/Ellipse ellipse]] looks an oval-ish shape, with two points inside it called the "foci" (plural of focus). In an elliptical orbit, the ''center'' of the big object you're orbiting is going to be at one focus, while the other focus won't contain anything at all.

Suppose you want your space ship to orbit the Earth in a perfect circle 200 kilometers above the surface, like the Space Shuttle does. What will the period (P) of that orbit be? If we want to use the equation above -- P[[superscript:2]]M = A[[superscript:3]] -- we'll first have to find the combined ''mass'' of the Earth and your space ship in solar masses. The Sun's mass is about 2 x 10[[superscript:30]] kg, while the Earth's mass is about 6 x 10[[superscript:24]] kg. Let's say your space ship weighs in at 1000 tonnes, i.e. a million kg. Here's the mass of the Earth in solar masses:

... and here's the combined mass of Earth and your space ship in solar masses:

Now, we need the semi-major axis, that is, the radius of the orbit. We're at 200 kilometers altitude, so that means the orbital radius is 200 kilometers, right? ''Wrong.'' The radius is the distance from our space ship to the ''center'' of the Earth. The Earth is 6371 kilometers in radius, on average, so the orbit's radius is 6571 km. For our formula, we need this expressed in Astronomical Units. One A.U. is 149,598,000 km, so the value we need for A is:

How impractical is it? Well, let's take the example of the trip to Saturn discussed above, where our space cadets undergo a continuous 1''g'' acceleration to the half way point, and a continuous 1''g'' deceleration for the rest of the trip. We established that their velocity is 3,500,000 m/s at the half way point, so we need 3,500,000 m/s of delta-V to get that far, and another 3,500,000 m/s of delta-V to brake to a halt, for a total delta-V requirement of 7,000,000 m/s. If their space ship's engines had an exhaust velocity of 4,500 m/s, the same as the Space Shuttle's main engines, what mass ratio (M/M[[subscript:e]]) would be required to attain 7,000,000 m/s of delta-V?

e[[superscript:1555]] works out to an absolutely gargantuan 3.7 x 10[[superscript:675]]. In other words, your space ship must carry 3.7 x 10[[superscript:675]] times as much fuel as its own empty mass! To put that into perspective, the estimated mass of the ''entire observable universe'' (excluding exotic forms of mass such as dark matter) is only some 10[[superscript:53]] kilograms. A space ship with a very modest 1000 kg empty mass would have to carry 3 x 10[[superscript:625]] observable universes' worth of fuel.

But even if controlled nuclear fusion ''does'' become a reality (allowing what Creator/RobertAHeinlein 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. Let's try the trip to Saturn under a continuous 1''g'' acceleration again, only this time let's give the space ship "torchship" engines with an exhaust velocity of 2% of light speed, or 6,000,000 m/s. The trip still requires 7,000,000 m/s of delta-V, so:

e[[superscript:1.17]] works out to 3.22. So the space ship's fuelled weight will need to be 3.22 times its empty weight. This means it's ''still'' going to have to carry 2.22 times as much fuel as its empty mass.

Another factor that can mean no life-bearing planets are possible in a given star system is the lack of heavy elements. The Milky Way galaxy is over ten billion years old. When it first formed, it consisted almost entirely of hydrogen and helium; almost no heavier elements (like carbon and the other elements necessary for organic life) existed. Several generations of stars have been born and died since then, and some of the more spectacular star deaths have peppered the interstellar medium with heavy elements synthesized by those stars' death throes. The sun, for instance, is a third-generation star -- the cloud of gas and dust out of which it formed contained material expelled by a supernova which, in turn, had formed out of an earlier cloud that contained material from an even earlier supernova. This is why there was enough carbon, oxygen, silicon, iron, etc. to form solid, rocky planets and organic molecules. Astrophysicists refer to all elements heavier than helium as "metals" (even if the element in question is oxygen or neon), and sometimes call a star system's heavy element abundance its "metallicity."

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 ''Literature/HonorHarrington'' series, and equip your spaceship with a gravity-manipulation ReactionlessDrive 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.

First, if you allow them to accelerate without pushing anything, they are now violating one of the most basic laws known to physics: the ''conservation of momentum''. In the real world, you can't apply a force to an object in one direction without causing an equal-and-opposite force on some other object. Rockets fly up because their exhaust flies down. Jumping up pushes the Earth ever-so-slightly downward; falling back to the ground afterward pulls the Earth ever-so-slightly up. By letting your space ship violate this basic law, you're saying that momentum ''is not always conserved.'' What other circumstances in your universe will cause momentum not to be conserved? Do the laws of Newton simply get held in abeyance every time someone switches on a gravity generator? Are there natural phenomena that accomplish the same thing?

Second, are you also [[NoConservationOfEnergy 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 HandWave 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 [[SpaceX Space Oil]].

Third, if any 1000 tonne space ship can easily accelerate to a tenth of light speed, then every two-bit spaceship owner has in his possession a weapon of mass destruction. Those 100,000 megatons of TNT-equivalent kinetic energy will act like 100,000 megatons of ''actual'' TNT if they strike a planet. Want a future populated by plucky tramp space-freighters and sneaky space pirates? It ain't gonna happen if every ship is a Hiroshima-on-steroids waiting to happen. Every spacecraft captain will be on too short a leash. Any spacecraft that even ''looks'' suspicious will be killed before it can become a threat. (And, yes, ''all'' fast-moving spacecraft, and even stationary spacecraft, will eventually be detected -- there ain't no StealthInSpace.) Any civilization that didn't take these precautions wouldn't ''be'' a civilization for very long. This might work as a setting for your future totalitarian dystopia, but is hardly the right world for romantic swashbuckling adventures.

FTLTravel is one of the bigger thorns in the side of the Hard SF genre. Special Relativity makes it absolutely clear: it is physically impossible to accelerate an object with any kind of mass so that it's moving faster than the speed of light. Even accelerating an object ''to'' the speed of light would require an infinite amount of energy. However, we've also pretty much established that there are no other technological species on any planet in the Solar system other than Earth. If we want to have space adventures involving high-tech aliens, we'll have to travel to other star systems, and the distances involved are so enormous that it would take years to get from one star to another if you were limited to sub-light speeds. Science Fiction writers have had to compromise, [[note]] (the alternative being to assume that their characters have a lot of patience, long lives, or can reach [[TimeDilation relativistic speeds)]][[/note]] and allow ''some'' means of travelling faster-than-light which didn't turn their universe into something totally unrecognizable to a modern reader. Therefore, the ability to move faster-than-light has received more attention in SF than any other fantastic concept as to ways to Limit The Damage of having it around.

The very worst problem with FTL travel (or even just FTLRadio) is a certain niggling consequence of TimeDilation. 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 way-counterintuitive 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 FTLRadio, 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 SubspaceAnsible 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.''

Maybe faster-than-light travel only works between certain [[PortalNetwork rare points in space]], and your ships must maneuver in normal space to get to and from them. Maybe FTL movement is impossible within some large distance from a gravity source, requiring your space ships to leave the solar system -- or at least leave Earth orbit -- before they can go FTL. Maybe your space pirates ''can'' jump to hyperspace at the first sign of trouble, but so can your space cops, and they have FTL weapons they can shoot at each other while in hyperspace.

Carlin's observation evokes one of the core appeals of [[MohsScaleOfScienceFictionHardness hard science fiction]] to some. A sufficiently hard science fiction story, even if it's set in another star system, could ''really happen'' -- or at least, it should be very difficult for the readers to come up with ways that it ''couldn't'' happen.

* StealthInSpace (and most of the other subtropes of SpaceDoesNotWorkThatWay)

->''"Did you ever notice when you're jacking off, that it's more of a turn-on to fantasize about the girl next door than it is to fantasize about a supermodel? Because with the girl next door, you're thinking, hey, ''this could really happen!''"''
-->--'''Creator/GeorgeCarlin'''

If you're going to set your story someplace we already know something about -- like UsefulNotes/{{Mars}} or [[UsefulNotes/LocalStars Alpha Centauri]] -- for goodness' sake, read up on what we know about the place before you start writing! We've sent space probes to [[PlutoIsExpendable 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'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.

Even if a planet happens to lie within the Goldilocks Zone, that's no guarantee that it can harbor surface life -- let alone that life will actually arise there on its own, or that said life will have had sufficient time to evolve to the point where space-faring beings can emerge. The atmospheric pressure must both be high enough for liquid water to exist, and that can't happen unless the planet has sufficiently strong surface gravity to keep its atmosphere from escaping into space. The formula for determining the surface gravity of a planet is as follows:

The v[[subscript:e]] for, say, the Space Shuttle's main engines is about 4500 meters per second. In order to orbit the Earth, the Space Shuttle must travel at 7800 meters/sec, and it must be about 300 kilometers above the Earth's surface (it requires another 1000-1500 m/s of delta-v to lift it that high and overcome atmospheric drag along the way). This means it needs a total delta-v of around 9000 m/s, which is twice its own exhaust velocity. From the rocket equation above, this means its M/M[[subscript:e]] ratio must be ''e''[[superscript:2]], or 7.39. The shuttle's weight with fuel must be over ''seven times as high'' as its weight without fuel! Discarding its spent solid rocket boosters in mid-flight (a trick similar to [[http://www.projectrho.com/rocket/multistage.php staging]]) can help a little, but not much.

So, what value should we plug in for d? The distance from Earth to Saturn, right? Well, unfortunately, Earth and Saturn are both in orbit around the sun, which means they ''move''. Their movements are more complicated than the simple equations I've written above, but for a first approximation, let's say you plan your trip to happen when Earth and Saturn are closest together, i.e. both on the same straight line from the sun (what astronomers call "opposition"). Looking up their orbital parameters on Wikipedia, we see that the average Earth-sun distance is 1.0 A.U., and the average Saturn-sun distance is 9.582 A.U., so the two will be about 8.5 A.U. apart at closest approach. (They ''could'' actually be closer. Saturn's orbit is rather eccentric, varying between 9 AU and 10 AU from the sun; but 9.5 A.U. is close enough for a first approximation. And waiting for Earth and Saturn to be at opposition ''when'' Saturn happens to be at perihelion will mean your space cadets will be sitting around tapping their toes for years.) 1 A.U. is about 150 million kilometers, so 8.5 A.U. works out to around 1,280,000,000 kilometers.

e[[superscript:1555]] works out to an absolutely gargantuan 3.7 x 10[[superscript:675]]. In other words, your space ship must carry 3.7 x 10[[superscript:675]] times as much fuel as its own empty mass! To put that into perspective, the estimated mass of the ''entire universe'' (excluding exotic forms of mass such as dark matter) is only some 10[[superscript:53]] kilograms. A space ship with a very modest 1000 kg empty mass would have to carry 3 x 10[[superscript:625]] universes' worth of fuel.

This last requirement is a real buzzkill, as it eliminates damn near every bright star you can see in Earth's night sky. Big, bright stars like Sirius A only live for a few hundred million years before they run out of gas. (The [[Film/BladeRunner candle that burns twice as bright lasts half as long]], after all.) Red giant stars like Arcturus ''had'' a long, stable lifetime as a dimmer star in the past, but will only last for a couple of million years at the red giant stage -- so if they did harbor life bearing planets in the past, those ecosystems were snuffed out when the star expanded to its current red giant state, and any planets in the star's ''new'' Goldilocks Zone won't have long enough for UsefulNotes/{{evolution}} to run its course.

* HumanoidAliens

* TeleportersAndTransporters

So what do you do when you ''need'' your characters to be able to move between the stars faster-than-light, or [[TransportersAndTeleporters teleport]], or have a TractorBeam, 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 DeflectorShields 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 [[Franchise/StarTrek softer SF franchise]]). Maybe the violations of the Laws of Thermodynamics needed to make StealthInSpace work are curtailed in some way that prevents you from getting useful energy out of any warm object (which, like some types of ReactionlessDrive, would have driven your Space Oil companies out of business).

The third problem with FTL travel is more practical: ''we don't know how to do it in RealLife''. Every attempt to come up with a way to do so has run into intractable problems. Quantum entanglement can occur instantaneously across vast distances, but it can't convey any actual information faster than ''c''. The Alcubierre space warp requires the energy output of an entire sun just to create, and there's no guarantee that you could actually make the space warp ''move'' -- and even if you could, there's even ''less'' of a chance that it could move faster than ''c''. Wormholes, if they even exist, will spontaneously collapse faster than it's possible to traverse them. You, as the writer, will have to ''invent'' a way to travel faster than light, and then cover all the repercussions of the method you come up with.

||At 50% of ''c''||&#gamma; = 1.15||
||At 86.6% of ''c''||&#gamma; = 2||
||At 90% of ''c''||&#gamma; = 2.29||
||At 99% of ''c''||&#gamma; = 7.09||
||At 99.9% of ''c''||&#gamma; = 22.37||
||At 100% of ''c''||&#gamma; = ∞||

* γ = 1 / sqrt (1 - v[[superscript:2]]/c[[superscript:2]])

||At rest||γ = 1||
||At 10% of ''c''||γ = 1.005||
||At 50% of ''c''||γ = 1.15||
||At 86.6% of ''c''||γ = 2||
||At 90% of ''c''||γ = 2.29||
||At 99% of ''c''||γ = 7.09||
||At 99.9% of ''c''||γ = 22.37||
||At 100% of ''c''||γ = ∞||

(As you can see, a massive object such as a space ship can never achieve 100% of the speed of light, because its mass would be infinite -- it would take an infinite amount of energy to accelerate to ''c''.)

At speeds much less than the speed of light, which is the pseed we're dealing with in the Earth-to-Saturn example, the formulas relating speed, acceleration, time, and distance travelled while undergoing constant acceleration are pretty straightforward. Ignoring the sun's gravity (which can indeed be neglected for a ship that accelerates at one g) we have:

-->--'''GeorgeCarlin'''

But even if controlled nuclear fusion ''does'' become a reality (allowing what Creator/RobertAHeinlein 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 [[SpaceSailing "ship"]] patrolling the "seas of interplanetary space" for months on end, hopping from planet to planet without refuelling.