Relativity is a fundamental concept in physics and was instrumental in the development of the Relative Worlds Universe. Telling a story like this without diving into a bunch of technical exposition means that some of the details about how the Universe works might not be immediately obvious just from listening to the story. To help alleviate this, I’m going to list the relevant points here. This page will be intended for the layman, meaning you won’t need a physics degree to follow it. If you would like further details, Wikipedia has a nice overview. If you would like a historical perspective I highly recommend the book A Perfect Theory by Pedro G. Ferreira.
What Relativity is NOT:
Relativity isn’t “how things relate to each other” or “all things are connected”, but if I ask people what they think it is these are among the most common replies I get. So, first and foremost, I want to cut this expectation off at the pass. Relativity isn’t magic. It isn’t a feeling.
What Relativity IS:
It is math based on boundary conditions, just like a lot of physics. There are actually two relativity theories, both discovered by Albert Einstein: Special Relativity and General Relativity. Special Relativity (the simpler one) comes from two fundamental ideas that Einstein used as a starting point. Please remember this is a layman version intended to give the overall idea.
The two ideas are:
1) The laws of physics are the same for all observers.
2) The speed of light is the same for all observers.
From these ideas we can derive a number of things, including the following:
1) Time is not a constant.
2) Length is not a constant.
3) Energy and Mass are “two sides of the same coin”.
4) The speed of light is a speed limit. Nothing can be faster.
General Relativity is far more complex. This is where the curvature of space-time was discovered as well as the notion of an expanding universe. Interesting stuff, but that’s all I’ll say about it here since Special Relativity alone is responsible for the main ideas in Relative Worlds.
One thing that happens when you calculate relationships between time, position, and speed using special relativity is that our understanding of classical (Newtonian) physics breaks down at speeds approaching the speed of light. The ‘extra’ mathematical terms that usually have a negligible influence begin to dominate under these extreme conditions. The resulting physics seems strange to us because we never encounter those speeds (at least for anything that has mass) in our day-to-day lives. But this is how the universe ACTUALLY works. And most science fiction ignores this! Once we understand these equations, some new conclusions can be drawn for a science fiction universe based on real science if we somehow had a way for ships to travel near to the speed of light. Here are some of them, all of which play a role in Relative Worlds:
1) Nothing can travel faster than the speed of light. This includes spaceships AND data transmissions.
2) Travelers moving near the speed of light will experience time moving more slowly. For example, a 100 light year journey would take a ship slightly longer than 100 years to travel from the point of view of the place of departure as well as the destination (and everywhere else!) but to the traveling crew it might feel like mere hours or days or weeks have passed (depending on the speed). Yes, this means that those who don’t also travel at high speed will have died of old age. Thus, the common science fiction trope of “freezing” crew to travel long distances without aging is not needed. Instead, a relativistic journey could be like a long plane ride to the passengers, except you arrive at your destination far into the future. It’s like forward time travel with each trip, with reverse time travel being impossible.
3) If the human race were to colonize in this way (say, along one arm of the Milky Way galaxy as is the case in Relative Worlds) then the frontier would be younger and less advanced than Earth and the early colonies. In addition, the older colonies would be old enough for human life to have evolved into something else. This allows for species diversity in a science fiction universe without running into any human-like aliens originating from anywhere else. They could be out there, sure, just not in this expanding section of this particular galaxy.
4) Since information travels at the speed of light (maximally), then a colony 100 light years from the nearest colony would be 200 years away from any “distress call” bringing in help beyond ships that were already in route. There’s a stark isolation for low traffic colonies. This isolation also means that technology will be different everywhere you go as local technological advancements would spread slowly.
There are certainly other considerations. Galaxy-wide laws and how to enforce them. Protecting against the spread of disease to colonies without a built up immunity to them. Economies. Politics. A whole science fiction universe comes out of this understanding of a speed limit and what it means to be able to travel just shy of it.