->''"Space is ''big''. '''''Really''''' big. I mean, you might think it's a long way down to the road to the chemist, but that's ''peanuts'' to space. ''Listen''..."''
-->-- '''Reported opening lines of the eponymous ''Franchise/TheHitchhikersGuideToTheGalaxy'''''
[[http://en.wikipedia.org/wiki/James_Van_Allen Dr. James Van Allen]] [[note]] for whom the [[http://en.wikipedia.org/wiki/Van_Allen_radiation_belt Van Allen Radiation Belt]] is named [[/note]] was once asked by a reporter to 'define space'. He replied, "Space is the hole that we are in."
Most people can't get their minds around just how big the universe is. So it should come as little surprise that most SpeculativeFiction writers can't either.
This is chiefly true of creators of TV, film, and video game SF. Creators of ''written'' science fiction can be positively obsessive about accuracy[[note]] but on the other hand, [[SturgeonsLaw sometimes they're not]][[/note]]. If your qualitative yardstick is based around an author's ability to describe distances, this may be a useful way to distinguish good print science fiction from bad print science fiction. And it's why a lot of science fiction fans don't like the movie and TV adaptations of their favorite books and stories. The usual blend of AdaptationDecay and bad research is a surefire way to leave the adaptation with no sense of ''scale''.
On the other hand, "Space is so ridiculously huge that there is absolutely no realistic way that anyone could ever travel to anywhere even remotely interesting in the lifespan of most major civilizations", while not a total deal-breaker, does rule out an awfully broad range of plots.
For example, consider that a light year is on the order of 10 ''quadrillion'' metres or nearly ''six trillion'' miles. Let's assume your family car uses about 2 and a half gallons (11.37 litres) of fuel per 100km - about 25 mpg - and a gallon (2.55 litres) costs about $4 USD (i.e. 1.6 USD/1 Euro per litre) to traverse it. This means that one light year is roughly where you'd end up if you spent the entire national debt of the US on petroleum fuel [[note]]and at 60 miles per hour, it would take 11 million years to drive there[[/note]]. At the opposite end, an atomic nucleus is on the order of a ''quadrillionth'' of a meter. That's ten-to-the-power-of-negative-fifteen of a meter, or a femtometer. Such outrageous [[http://en.wikipedia.org/wiki/SI_prefix SI prefixes]] rarely appear in fiction, and that's before we get anywhere near the scales of galaxies and subatomic particles. [[WritersCannotDoMath This is because most writers aren't that good at or are too lazy to implement mathematics]]. If it sounds like [[EleventyZillion a number made up by a child]] ([[VideoGame/TheWorldEndsWithYou Attention all yoctograms!]], septillion seconds), the writer might have actually taken it seriously.
A way of explaining the scale of the universe is to use [[http://wikipedia.org/wiki/Fermi_estimation fermi style estimation]] to the nearest powers of ten. The solar system is about a million times the width of the Earth while the Milky Way galaxy is a 100 million times the width of the solar system, and the observable universe is a million times the width of the Milky Way. The size of the universe beyond that is speculation, though the observable universe may be but a speck in the larger universe, assuming it's not infinite.
Another example which often comes up is the idea of beings coming to our galaxy from another galaxy. While there's no reason why a writer ''can't'' introduce beings from the nearest galaxy intent on contacting/conquering the Milky Way, there would have to be a ''pretty dang good reason'' to travel the incredibly vast distances separating galaxies -- distances which make traveling between stars seem like a little hop.
Some would consider this one of the AcceptableBreaksFromReality. If the characters didn't travel through space at [[FasterThanLightTravel thousands of times the speed of light]], it wouldn't be very interesting, unless the focus was just the spaceship itself. Either you'd have to make the ship incredibly powerful to max out TimeDilation and shorten the time spent from the characters' perspective, put the characters into some kind of [[HumanPopsicle suspended animation]] (and just fast forward through their journeys), or even have entire [[GenerationShip generations of characters that would live and die on the ship]] before they even reached a known extrasolar planet (meaning the audience would say TheyWastedAPerfectlyGoodCharacter or treat the new generations as a ReplacementScrappy), and so on... If Franchise/StarTrek, for example, was realistically scaled, it'd be a ''lot'' less interesting.
When adding examples, it may be wise to consider the capabilities of the faction in question. What is "unrealistic" for a low-tech harder-SF group may not be so for a [[JustForFun/AbusingTheKardashevScaleForFunAndProfit high-Kardashev]] HigherTechSpecies; after all, what we can do now would be outlandish to our medieval ancestors, so who's to say a society centuries if not millennia more advanced than us can't invent a "unrealistically" light yet superstrong material? On the other hand, some things are ''laws of physics'', not limits of technology, and the difference is an important one (any ship that expels an exhaust to propel itself, for example, functions by the Tsiolkovsky rocket equation, which is basically a special case of the Second Law of Motionóregardless of what the exhaust is or how it imparts the energy to expel it).
Two related tropes are MedievalStasis and ModernStasis, where society stays the same for thousands of years. It is a subtrope of SpaceDoesNotWorkThatWay, which features a list of tropes that try to treat space in a way that it wouldn't realistically work. See also: MST3KMantra, BellisariosMaxim, WatsonianVersusDoylist.
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!!Examples go into subpages:
* SciFiWritersHave/NoSenseOfTime or entropy
* SciFiWritersHave/NoSenseOfMass and size