The trouble was the Rim.

The Rim as such was not a strict boundary. Although the Oort cloud, the great “bubble” of leftover material from the formation of the Solar system, was imagined as the Rim by the media and the public, the Rim was just the distance from Human settlements where space travel was no longer practical, and it was far closer than the Oort cloud. Not that it really mattered – space is really big, and the human mind is not evolved to imagine the huge distances in space. The human mind simplifies and makes up its own lines in order to comprehend the world. In the human mind, the Rim is as intangible as the Frontier of the Old West. You can’t point at it on any map, but suddenly you find yourself there.

Expansion within the Rim was still possible to a certain extent. There was still the Asteroid belt and Kuiper belt. It was possible to terraform Venus. If someone found a way of shielding a colony on Mercury from the rays of the sun, there would be a near-endless supply of heavy ores. All these solutions were incredibly costly, and in the end, they wouldn’t solve the problem, only delay it. Mankind had to go past the Rim. And to do that, it was necessary to invent a way to go faster than light.

There is a slight problem: the light barrier is not just an idea. It’s the law. Einstein’s general relativity says that unless you are a massless particle, like for instance a photon, you can’t reach the speed of light.

However, there are ways of circumventing the light barrier. Most of these are hypothetical and depends on more energy than there is mass in the entire universe. Some are possible, and through quantum technology, a few of them were even made practical.