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> why do many science fiction writers and science fiction movies want to introduce faster than light travel?

Because this way you keep the human timescales for all characters. It allows you to tell stories more familiar to your audience while introducing elements that we wouldn’t be able to introduce without this plot device.

If you embark on a relativistic trip to Kepler-186f, when you return you will find a completely changed planet. The only way this would work is if every character spends most of the time at relativistic speeds reuniting from time to time as the whole universe ages around them.

That’s a nice premise, BTW.

Most writers did not take science classes in college; they got a liberal arts degree and not a physics degree. On the other hand there are a few writers who do a better job of things.

Vernor Vinge does a great job in A Deepness In The Sky of creating a human space–fairing society that has to deal with some degree of relativity. Their ships only manage a 0.3× the speed of light, but they also use suspended animation to hibernate for most of their voyages. A trip where you age a year might mean that hundreds of years pass on the planet you’re going to. One character founded a vast trading coalition that was enormously successful in spite of this and other handicaps.

Greg Egan wrote a book called The Clockwork Rocket that is set in a universe where the space–time metric is different. This has huge effects on all aspects of physics from relativity to quantum mechanics, and he worked out all of those implications before writing the story. You can learn a lot about our universe just by contemplating all of the ways that this fictional Riemannian universe is different from our own Lorentzian universe. For example, in the Riemannian universe there is no fixed upper bound on speeds, light travels at different speeds based on its color, and thermodynamics is backwards so plants gain energy not by absorbing light but by emitting it.

> Since you could get to anywhere in the universe almost instantly by travelling sufficiently close to the speed of light

But the universe is billions of light years across - if someone is travelling at just under the speed of light, wouldn't that mean it would take them billions of years (as experienced on the ship) to traverse the universe?

> why do many science fiction writers and science fiction movies want to introduce faster than light travel?

My thought on this, from a purely sci-fi perspective, is that accelerating close to light speed would use a phenomenonal amount of energy, and take a long time to do. Furthermore, time is experienced differently for those on the ship and those outside it. So with these points in mind, some kind of "hack" that allows you to fold spacetime and jump several lightyears instantaneously (as experienced by everyone) makes sense.

> wouldn't that mean it would take them billions of years (as experienced on the ship) to traverse the universe?

No, because time passes more slowly in the ship frame. This means it’s billions of years for your friends back home, but it could be hours for you.

I get that time passes slower for those outwith the ship, but I'm struggling to wrap my head around how long it would take those on the ship to travel large distances at close to light speed - whenever I think on this topic, my head almost bursts with awe and wonder!

Let's consider a ship, travelling from Earth, and say it somehow accelerates almost instantly to just below light speed, and travels for a distance of 1 light year. How much time would pass for those still on Earth, and for those on the ship?

1y for Earth and 0 time for the ship.

If it's zero time for the ship, what I'd they wanted to travel a different distance?

It's not really zero, but the closer you get to c, the closer to zero the duration experienced by the astronauts will be.

Ursula Le Guin's Hainish Cycle explores how an interstellar society would function if people could travel this way: an instant for the traveler, but decades back home. Despite glossing over all the technobabble that other scifi finds so fascinating, these books, more than any others I've read, show just how great the distances are between stars.

Funny thing is that this difference of passed time that remains after the spaceship slows down has more to do with acceletation and GR than speed and SR.

The fact that you can describe this effect from the point of SR (by swuinting a bit) means that you can get GR time dilatation from SR time dilatation with no additional input.

The moment that we knew that the acceleration is the same as the gravity and that the speed of light in vacuum is constant we only had to think long and hard to see that gravity causes time to flow differently in different places.

From an outside point of view the ship takes 53K years to cross our galaxy, from inside it looks like an instant. Time flows at different speeds.

Star Wars would have been a very different story if the Rebels had arrived at the Death Star 10k years after everyone aboard had died.

OTOH, by the time the Death Star arrives at Yavin, the Rebel civilisation will have collapsed a couple times and won’t even remember why they even got there in the first place.