Stardrive

The stardrive is the ship system that carries a Pax Abyssi ship from one star system to another. The pilot locks a destination star, charges the drive, turns the ship onto the star and then flies it up to the speed of light by hand, with the drive multiplying the ship's forward thrust. When the ship reaches 1.0c, the speed of light, the jump fires, the ship enters the tunnel, and it emerges in the destination system. The design goal is that the pilot makes the crossing, with a moment of achievement when the speed readout touches 1.0c.

Flying the stardrive

Lock. Select a star, on the galaxy map or in the sky, and press FTL LOCK. The lock takes a random three to ten seconds to acquire. It refuses the star system the ship is already in.

Arm and charge. Pressing the drive runs its checks in order (a target, a lock, fuel, power, a working drive) and names the first one that fails. If all pass, the drive hums and charges for about three seconds. DUMP CHARGE throws the charge away and returns the drive to ready.

Align. The pilot turns the ship until its nose is within 10 degrees of the locked star. The drive will not engage outside that cone.

The run to light speed. On ENGAGE the dampers come off and the drive multiplies the ship's forward thrust twenty-fold. From rest, about fifteen seconds of held throttle brings the ship to the speed of light. A low rumble starts at 0.8c. Engaging the dampers or pressing ABORT RUN stops the run.

The jump. At 1.0c, with the nose still on the star, the jump fires: a boom, a white flash, stars stretching into streaks, and the tunnel. If the nose has drifted off the star, the jump waits until the pilot brings it back inside the cone.

Arrival. Dropping out plays the drive's power-down and the ship emerges in the destination system.

The numbers, put in perspective

The run to light speed is where the game's numbers meet real physics most directly, so it is worth working them through.

Acceleration. Reaching 300,000 kilometres per second in about fifteen seconds means accelerating at roughly 20,000 km/s every second, about two million times Earth's surface gravity. No crew and no structure could survive that for a fraction of a second. The game accepts it as part of the fiction, alongside the drive itself.

Energy. Special relativity sets the energy needed to reach a speed $v$. A body of mass $m$ carries kinetic energy

$$E_\mathrm{k} = (\gamma - 1)\,mc^2, \qquad \gamma = \frac{1}{\sqrt{1 - v^2/c^2}},$$

where $c$ is the speed of light and $\gamma$, the Lorentz factor, grows without limit as $v$ approaches $c$ ^[1]. For a 100-tonne ship at half the speed of light, $\gamma = 1.155$ and the kinetic energy is about $1.4 \times 10^{21}$ joules. That equals the rest-mass energy of about 15.5 tonnes of matter: the complete annihilation of nearly eight tonnes of antimatter with eight tonnes of ordinary matter, with every joule going into motion. At 0.99c, $\gamma = 7.1$ and the bill rises about fortyfold. At exactly $c$ it is infinite. This is why nothing with mass can reach the speed of light, and why the game's jump at 1.0c is fiction from the first step.

Antimatter. In the game's design, the drive pays for its speed in antimatter, and it is the right fuel to imagine: when a gram of antimatter meets a gram of ordinary matter, all two grams become energy, $E = mc^2 = 1.8 \times 10^{14}$ joules, about the yield of a 43-kilotonne explosion. Nothing else stores energy so densely. The catch in reality is supply. Particle accelerators make antimatter an atom at a time, in quantities far too small to weigh.

Time and light. A real ship near light speed would see its own clocks run slow compared with the stars' (at 0.8c, $\gamma = 1.67$, so the crew would age 60% as fast as the people they left). It would also see the sky distort. Starlight arriving from ahead would crowd towards the direction of travel and shift towards the blue, and the view behind would thin and redden.

The boom. Sound needs a medium to travel through, and interplanetary space is close to empty, so there is nothing to carry a boom and nothing to break at 1.0c. A sonic boom happens when an aircraft outruns its own pressure waves in air. The nearest real analogue for light is Cherenkov radiation, the blue glow in a reactor pool, emitted when a charged particle moves through water faster than light travels in water. It cannot happen in a vacuum, where nothing outruns light. The boom is a sound designed to mark the moment.

Why the gaps are so wide

The fiction needs a faster-than-light drive because real distances between stars are enormous. The nearest star, Proxima Centauri, is 4.24 light years away. Voyager 1, humanity's farthest spacecraft, is leaving the Solar System at about 17 kilometres per second ^[2]; at that speed Proxima would be about 75,000 years away.

Even the stardrive's planned top speed of 5,000 times the speed of light would leave the Galaxy large. Proxima would be about 7.4 hours away, the region of human settlement in the game's setting, roughly 800 light years across, about two months, and the Milky Way's disc, about 100,000 light years across, some twenty years.

Real ideas for fast interstellar travel

Physics offers two kinds of answer, neither of them a stardrive.

The first stays below light speed and makes the ship small. Directed-energy proposals would push wafer-scale probes of a few grams on light sails with powerful ground-based lasers, to more than a quarter of the speed of light, reaching the nearest star in about twenty years ^[3]. Nothing with people aboard fits that plan.

The second bends spacetime instead of moving through it quickly. In 1994 Miguel Alcubierre found a solution of Einstein's equations of general relativity in which a region of space contracts ahead of a ship and expands behind it, carrying the ship along at any speed while it never locally outruns light ^[4]. The catch is that the geometry requires regions of negative energy density, which no known form of matter provides in bulk. Applying the quantum limits on negative energy, Pfenning and Ford showed that the total energy needed to hold such a bubble together is physically unattainable ^[5]. Warp metrics remain a useful way to study what general relativity allows; they are not an engineering design.

Faster-than-light travel carries a further problem that no drive design escapes. In relativity, observers moving relative to each other disagree about which distant events happen at the same time ^[1]. A signal or ship that could outrun light could, for some observers, arrive before it left, and such travel could be used to send messages into one's own past. Most physicists read that as a strong sign that nothing travels faster than light.

See also

• Milky Way
• Star catalogue, the real stars you can lock
• Sol
• Orbit
• Science in Pax Abyssi, on how the wiki labels fiction

References

1. 1Einstein, A. (1905). Zur Elektrodynamik bewegter Körper. Annalen der Physik 322, 891-921. doi:10.1002/andp.19053221004
2. 2NASA. Voyager 1. NASA Science. science.nasa.gov/mission/voyager/voyager-1/
3. 3Lubin, P. (2016). A Roadmap to Interstellar Flight. Journal of the British Interplanetary Society 69, 40-72. arxiv.org/abs/1604.01356
4. 4Alcubierre, M. (1994). The warp drive: hyper-fast travel within general relativity. Classical and Quantum Gravity 11, L73-L77. doi:10.1088/0264-9381/11/5/001
5. 5Pfenning, M. J. and Ford, L. H. (1997). The unphysical nature of 'warp drive'. Classical and Quantum Gravity 14, 1743-1751. doi:10.1088/0264-9381/14/7/011