Sharp Blue: The Subjective Horizon


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The objective distance to the stars is measured in light years. The speed of light imposes an objective upper limit for the speed of travel across our galaxy. However, when considering extrasolar colonisation, this isn’t the measure of the size of the universe that is most relevant. Instead we must be concerned with how big the universe seems. Even our very best designs for starships will probably only be able to manage speeds of about a tenth that of light, which means that voyages even to the nearest stars will take many decades or even centuries. Space seems big. I call the surface marking the furthest distance to which humanity will choose to explore the “subjective horizon”. At the moment, the subjective horizon is lurking somewhere out around Mars. In the next few decades, I imagine that we’ll push it out to Jupiter and then on to the Kuiper Belt. The Solar System belongs to us, but not yet the stars.

If we were to invent technologies that greatly expanded our lives then the subjective horizon would be pushed out into the galaxy. If I were probably going to live for a thousand years then a hundred year trip to Alpha Centauri and back would still seem like a major commitment but it would no longer take my entire life. If I knew I would probably live to be ten thousand years old then it might seem like a trip subjectively comparable to taking a year out to cruise around the world today. If I thought that I might live forever then the subjective horizon might be pushed out into our galactic arm.

It might seem like effective immortality would effectively give us the universe. However, I don’t think this would be the case. There will be more factors at work. I think it’s likely that the amount of technological progress that could occur over a period comparable to that of an interstellar voyage will be vast. Before our putative immortal humans have a chance to enjoy their great longevity, technology will have advanced to allow much more impressive things. I fully expect that within a century of the achievement of effective immortality, we will have migrated entirely to non-biological substrates. Humanity will have been replaced by post-humanity. Our new non-biological minds will almost certainly enable us to think millions of times faster than mere humans. Suddenly, the stars would seem millions of times further away. Even worse than that, the degree of connectivity of posthuman civilisation would be staggering by today’s standards. Posthuman entities would be reluctant to travel even light seconds away from the bright centre of their civilisation because they’d feel terribly, terribly lonely. It would take subjective weeks for messages encoded in pulses of light to crawl between the Earth and the Moon. The subjective horizon would then rapidly close in about the posthuman civilisation as that civilisation falls into the Singularity. First the outer planets, then the asteroids, then Mars, then the Moon and finally even other continents would seem too far away to be of any consequence.

The future will contain wonders and terrors for beyond our ability to comprehend, but the stars might remain forever beyond our reach.

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