Dr. James Van Allen 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 Speculative Fiction 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 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.
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 lightyear 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 lightyear is roughly where you'd end up if you spent the entire national debt of the US on petroleum fuel.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 SI prefixes rarely appear in fiction, and that's before we get anywhere near the scales of galaxies and subatomic particles. This is because most writers aren't that good at or are too lazy to implement mathematics. If it sounds like a number made up by a child (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 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 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 that 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. Even in a billion years (assuming a sentient Earth species still exists by then - given that the sun will have grown bright and hot enough to make living on Earth a real pain, it's likely we'll have already jumped planets or found a way to move the Earth to a safer distance) when Andromeda begins its final approach to the Milky Way, it would still theoretically take an Andromedan civilisation a considerable amount of time to reach the arm of the Milky Way where the Solar System resides.
Some would consider this one of the Acceptable Breaks from Reality. If the characters didn't travel through space at 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 Time Dilation and shorten the time spent from the characters' perspective, put the characters into some kind of suspended animation (and just fast forward through their journeys), or even have entire generations of characters that would live and die on the ship before they even reached a known extrasolar planet (meaning the audience would say They Wasted a Perfectly Good Character or treat the new generations as a Replacement Scrappy), and so on... If Star Trek, 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 high-Kardashev Higher-Tech Species; 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 Motion—regardless of what the exhaust is or how it imparts the energy to expel it).
Two related tropes are Medieval Stasis and Modern Stasis, where society stays the same for thousands of years. It is a subtrope of Artistic License – Space, which features a list of tropes that try to treat space in ways that wouldn't realistically work. Compare with What Other Galaxies? and Everything in Space Is a Galaxy. See also: MST3K Mantra, Bellisario's Maxim, Watsonian versus Doylist.
Examples go into subpages:<!—index—>
- No Sense of Distance
- No Sense of Time or entropy
- No Sense of Mass and size
- No Sense of Energy
- No Sense of Velocity
- No Sense of Units or demographics.