The Tau of Light Speed: How Propulsion Changes Everything

There are a huge number of sub-genres of science fiction. They all have their own tropes, archetypes, and ways of doing things. The characteristics of any given one vary wildly, because sci fi is such a broadly-defined category and because it lends itself to wild flights of imagination into universes as disparate as their creators’ minds.

A number of these categories, often grouped under the heading of ‘hard’ sci fi (we’ll cover how problematic the hard/soft distinction is in another post), tend to include space ships and interstellar travel. This is the classic vision of high-tech sci fi and brings me onto the main thrust of this article: there are two basic ways those ships reach the stars, and each technology fundamentally influences the character of the universes they help drive.

The challenge, both for fictional and real-life space travel, is that space is big. Astronomically so, if you will. Take the light year, the basic unit of measurement we use for the distances between stars: The mean distance from our planet to the Sun (known as one Astronomical Unit, or AU) is 149,597,871 km. One light year, or the distance light travels in one Earth solar year, measures just over 63,239 AU or 9.4605284 × 1012 km. Our nearest star, Proxima Centauri, is just over 4.2 light years away.

We simply can’t cover those kind of distances using conventional chemical rockets like those we currently use to get into orbit and which sent astronauts to the moon. Even our fastest, which launched the Voyager probes out of the gravity well of our star and into interstellar space, only travel at 16.3 kilometers per second. The speed of light is 299,792.5 km/s.

This is also where that wonderful genius, Einstein, comes back to kick us in the teeth. According to the General and Special Theories of Relativity he devised, which – mostly – have been proven correct by experimental data as well as further theoretical study, the speed of light acts as the effective speed limit for the universe. Nothing can go faster than light. Ever. Sir Isaac Newton may be the deadliest son-of-a-bitch in space, but Einstein’s the biggest bastard.

You see the problem. If we can’t go faster than light then, even assuming we can approach light speed in any meaningful way, it is going to be a long time before any of humanity’s ships reach the stars. When they do, the children of mankind will be scattered across the vastness of space, unable to communicate effectively or create any kind of interconnected civilisation. Because subjective time moves ever more slowly the closer you get to light speed, a few years of ship-time on any of these interstellar voyages will equate to decades or even centuries back home. Any travellers or explorers heading to other solar systems will return to entirely different worlds from which they left; they will be orphaned in time.

If, on the other hand, we can somehow negate the light speed barrier then it becomes a different ballgame altogether. With space travel taking weeks or months of normal time rather than decades or centuries, we will be able to communicate and spread much further than we would travelling at sub-light speeds. We can go and explore the cosmos, see its wonders and terrors, and generally fly around the galaxy like we own the place (assuming someone else doesn’t already). This future is brighter, and holds the prospect of an interconnected human diaspora, a civilisation such as the the ones imagined in Star Trek or Dune. It is worth noting that it is now at least theoretically possible to travel faster than light without negating Einsteinian Relativity, so the real future may actually be a form of warp drive. NASA is apparently working on it, but unfortunately for us all the technology is still at its earliest stages of development.

As I mention above, the distinction between sub-light and FTL travel fundamentally changes the feel of the universes created by works of science fiction, and the stories told within them. Universes in which it is extremely difficult or impossible to break the light speed barrier tend to be darker, grimmer, places with humanity scattered across the stars. The stories told in them, such as Alastair Reynolds’ Revelation Space trilogy, are almost without exception ‘hard’ sci fi, and Reynolds’ work is a perfect example.

The Revelation Space universe is a very dark, dangerous place filled with inhuman monsters and nanotechnology gone awry. Humans have spread across multiple worlds, but are often impoverished and eking out a meagre existence in the fallen civilisations built by their forefathers. The kilometer-long, heavily armed ships which travel between the stars – ‘lighthuggers’ – are crewed by ‘Ultras’, post-humans modified with nanotechnology and physical augmentations to be more effective in combat, think faster, work in zero-gravity, and for other more esoteric purposes. Travel between systems takes decades, and the appearance of a lighthugger above a planet heralds either trouble or once-in-a-generation opportunity for acquiring new technologies. There is also the positing of an answer to the Fermi Paradox, which clearly influenced Mass Effect’s Cosmicist, genocidal ‘Reapers’.

The inability to easily travel faster than light in the Revelation Space universe (for, as it turns out, you can but it’s an incredibly bad idea) is a core factor in how the narrative plays out and how the central concepts it addresses are developed throughout the novels set there.

Other novels, such as Tau Zero, by Poul Anderson, take the concept of relativity and time/mass-dilation at relativistic speeds to extremes in different ways. Instead of Reynolds’ sprawling space-opera, Anderson’s novel limits itself to a single crew of 50, confined within the hull of a ship sent to explore a nearby star system. Halfway through the journey, they encounter a micro-nebula which damages their deceleration system, and because of the nature of their engine (the Bussard Drive, a real-life proposal for interstellar travel), they must keep accelerating in order to survive.

Anderson’s crew ends up a very long way from home and, due to the time-dilation effects of adding velocity at near-light speeds, tens of billions of years into the future. I shan’t spoil the ending, but the novel deals adeptly with the psychological and social effects of being isolated from your civilisation, confronting the inevitable death of your entire species, and the basic insignificance of not only our individual human lives but our whole universe. Like Revelation Space and many other excellent ‘hard’ sci fi works, it is able to perform these quite remarkable feats of psychological and social imagination in part due to the basic constraints of the universe it posits.

Just as the constraints of a sonnet help poets explore new possibilities, so the bonds of Relativity help push science fiction to new and often terrifying places. While there is much excellent sci fi featuring faster than light travel, the gloom seemingly inherent in the acknowledgement of the limit of light speed forces the fictional envelope to be pushed in unexpected ways. Placing limits on our ability to overcome the laws of nature does rankle somewhat, especially in fictional genres often so freely untethered from them, but the ways in which those limits change our responses can be absolutely fascinating, even inspiring. Much as I hope we can circumvent the laws of physics and travel to the stars quickly, the inverse proposition produces some excellent fiction.

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