In a discussion on File 770 I was rude about the physics of faster-than-light travel while defending some of the madness of Doctor Who’s Kill the Moon episode and commenter “ccm” replied:
FTL spaceships? Hah! What about a freakin Time Machine that can grow and jettison rooms as needed, produce pretty much anything you need, can travel anywhere and anywhen with no concerns about fuel, weight, etc….and seems to be some kind of living creature as well.
Well fair point.
But I still think that FTL drives are as bad and in someways the madness of whovian physics makes more sense. The absurdity of the Who reality is a kind of realistic realism.
Now that will require some justification.
How to travel through space
Space opera and similar styles of space travel fiction face a problem: the distance between solar systems is vast and there is a fundamental limit to how quickly a spaceship can move. Specifically the speed at which light travels in a vacuum is actually a fundamental constrains of the universe. As far as we know that is just how the rules of our universe are.
Getting round this constraint is a problem for science-fiction and there are numerous solutions.
- Set your story on a single planet or within a single solar system or within some kind of ‘generation ship’ that will take hundreds of years to reach its destination. All fine approaches as far as they go.
- Ignore the problem and just zip around space regardless in your story.
- A warp drive (e.g. Star Trek) that allows a spaceship to exceed light speed by warping the geometry of space around it.
- Wormholes – distort part of the geometry of space so as to create a tunnel-like short cut between the place you are the place you want to be (e.g. Stargate).
- Hyperspace – move out of the usual dimensions into a different space which is connected to our space in someway but with different relative distances. At the right spot come back out of hyperspace (e.g. too numerous to mention).
- Some kind of quantum or probabilistic event – for example the Heart of Gold infinite improbability drive used in the Hitchhiker’s Guide to the Galaxy (used in the story as an alternative to hyperspace)
Before I continue you can read various websites of people who have spent more time and thought on this than I have.
The above approaches 3 to 6 aren’t bad physics – a theoretical warp drive is conceivable for example https://en.wikipedia.org/wiki/Alcubierre_drive . The problem is that each of them solve only one aspect of the light speed problem.
The upper limit of the speed of light arises as a fundamental property of space and time together. For narrative purposes faster-than-light travel is being used to had wave one aspect of that – large distances. However General Relativity throws up more problems than just a universal speed limit. It also results in time being relative to your frame of references and it also means notions of causality become complex when viewed over large distances.
Now for narrative purposes a space-opera wants sensible causality and a single universal time – at least for the events of the story. If it is a complex TV/Movie series universe it is rather like the desire for continuity. Plot should make sense and events should have an order to them (even if presented out of sequence) and be connected by a sensible causality when needed.
Now consider options 3 to 5 above. What do they all have in common? Essentially each one requires people to have mastered technology that allows them manipulate the geometry of space time and/or access other dimensions of the universe. However for plot reasons the people in this universe of warp drive etc must only use it for one thing: spaceship propulsion.
We need to imagine a technologically sophisticated society that have discovered how to manipulate a fundamental aspect of reality AND only apply that technology in one way. This is inherently implausible. A society with the power to build one a FTL spaceship would appear to have everything they need for a time-machine of some kind or another. Further the way the plots work mean that in effect every FTL ship is a kind of time machine as to arrive at your destination at a time EARLIER than you would by traveling at the speed-of-light is the physical equivalent of traveling into the past.
In Ursula Le Guin’s novel The Dispossessed the hero Shevek is developing a new fundamental theory of physics that will allow faster than light communication. The principle of simultaneity allows for Le Guin’s ansible devices (which have appear in other notable works) to make sense within her Hainish novels. In effect every model of FTL used in fiction needs both things – a way of letting spaceships travel faster than a light and a way of stopping it making the universe crazy.
So let’s keep it simple. Instead we will imagine a society that could manipulate space-time to make a warp drive or access hyperspace etc. What would they be able to do with this technology?
- Time travel – is a given. You can manipulate space time. Past-future are no more of a problem than back-forth
- Size doesn’t matter – you can manipulate physical space. You can make things bigger and smaller.
- Energy – you can go anywhere and anywhen so you can access whatever energy you need
Basically this society would have mastered time and the relative dimensions of space. At the same time there are downsides.
- Causality will now be messed up. The connection between events may no longer make general sense. Locally causality will be sensible but overall there will be issues.
- Overall the universe of this society would be plagued with historical inconsistency.
In short the most realistic portrayal of a universe where warpdrive etc is possible would be centered on a time traveler with the technology to manipulate the relative size of some things in a set of story in which causality and overall consistency would be very mutable.
i.e. Doctor Who is realistically an insane universe.