Radiance is a far-future science fiction setting that I used for a linked tabletop scenario and freeform at Sydcon last year, and will revisit in RADIANCE: RELICS at Eye-Con over Easter.
For the most part, Radiance is Plausibly Hard SF, with futuristic science and technology that includes "things that may or may not be possible, but can still be considered plausible or reasonable, at least until proved wrong by future discoveries". Some of the technology in Radiance is considered improbable or even impossible by conservative physicists, but certainly not all physicists. Some is well within the range of possibility.
I am not a scientist. Some of the people reading this may have a better scientific understanding of these issues than I do, and disagree with me about the plausibility of some of this science. The important thing is that "plausibly hard" science fiction is an aesthetic choice – I enjoy science fiction in which the technology presented is interesting and yet plausible. It's not intended to be a valid prediction of future history (as if that could be possible), but a fictional universe which feels like it could exist.
The scientific and technological assumptions of Radiance include:
- Navigable Wormholes are still controversial among physicists, with about as much evidence in favour as there is against. Warpgates are basically navigable wormholes. These are an engineering challenge for very advanced civilisations (requiring exotic matter to work), and were constructed in the deep past of the Radiance universe.
- Faster-than-light travel with time-travel effects because any method of travelling FTL creates time-travel paradoxes (according to General Relativity and the propagation of information through the universe). Wormholes are no different – they avoid the problem that acceleration to the speed of light requires an infinite amount of energy, but closed timelike curves (i.e., time travel) and causality violations. If it is possible to travel faster than light, then it is possible to time travel. Either the universe is acausal, or causality doesn't work the way we think it does, or faster-than-light travel is only possible in a way that does not allow causality violations.
The option that I've chosen means that travelling through a wormhole is a form of time travel, but wormholes cannot be created in a way that allows causality violations or paradoxes. One theory states that stable wormholes are possible, but that any attempt to use a network of wormholes to violate causality would result in their decay; this is how warpgates work in Radiance. This is a bit complicated – I'll describe it in detail later.
- Reactionless Drives are controversial but considering that Radiance already has exotic matter (involved in warpgates), they could be possible. This is definitely an aesthetic issue, allowing spacecraft to accelerate to high speeds without subjecting characters to enormous g-forces or flooding living compartments with cushioning foam.
- Primitive non-terran life has evolved in extreme environments (non-carbon-based life, cold-chemistry, and so on). There's not a lot of "xenogen" life in the setting, but the organisms that are known have some very exotic biologies.
- The galaxy has few or no current alien civilizations as an explanation for the Fermi paradox – the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilisations and the lack of evidence for, or contact with, such civilisations. In the history of life on the Earth only one species has developed space flight capability and radio technology; this lends credence to the idea that technologically advanced civilisations are a rare commodity in the universe.
- Advanced mechanical nanotechnology exists in some form, although not every possible use for nanotech proved to be possible or has been pursued.
- Very rapid terraforming of uninhabitable planets into habitable worlds is possible given very advanced nanotech. Some of the worlds of the Radiance universe were terraformed over centuries (rather than millennia or tens of millenna) by previous civilisations. The current civilisation of the Radiance setting do not have this capability.
- Digital uploads of human minds is not impossible, given a greatly advanced understanding of human neuroscience. The primary cultures of Radiance are now hostile to the idea of uploading human minds, but it happened in the distant past, and if advanced nanotech was developed that can record human minds and brains, these methods could be replicated.
- Ubiquitous information technology is a basic assumption. Computers are practically ubiquitous right now. In my pocket I have a multifunction device that can connect me to a global information network, record pictures and audio, not to mention make telephone calls, multimedia messages, and email. And I hardly have the most advanced mobile phone on the market.
Most people in Radiance have immediate personal access to local versions of the internet, communications, and information databases. These computers are integrated almost invisibly into clothing or personal accessories. Interfaces are sophisticated and intuitive. Only in unusual circumstances does anybody have to rely on just their personal knowledge or abilities – most of the time, you have computer assistance at your fingertips.
So what doesn't exist in Radiance? Alien civilisations that are very humanlike in appearance and culture. A galactic monoculture or uniform future society, whether human or alien. Faster-than-light travel allowed due to handwavium physics. Psionics and fantastic abilities such as telepathy or "the Force". Force fields, disintegrator weapons, and energy beam weapons that violate basic physics and engineering. Spacecraft that act like WWII fighter planes. There's nothing inherently bad about stories that include this kind of thing (many people have gained a lot of enjoyment from Star Wars), but I'm aiming for a plausible and consistent universe, so I have tried to avoid fantastical elements like these.
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