Pavlos (pavlos) wrote,

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Pointless Gravity Banks


I feel like writing something, but nothing in particular (this trend may continue), so here is a rant about a particular piece of SciFi physics. This rant is of utterly no import to your life and you can safely forego reading it.

Normally, Ian Banks' "Culture" series of science fiction novels is extremely friendly to the sensitivities of people who know anything about real science. In fact it's the most plausible and scientifically restrained series I know of, especially considering it's about a super-advanced civilisation with a galactic extent.

For the most part there are no weird telepathic or otherwise mystical powers. Weapons emit vaguely harmful radiation, or even projectiles, and not cunning sleep-inducing rays. There's no time travel, holographic apparitions, or other messing up of reality. Banks wisely avoids novelty planets, with most of the action taking place on spaceships or terraformed environments. When the characters use a quaint vehicle, such as a train, it's really intended as a quaint vehicle and behaves mundanely as one. There is strong AI, but its relationship with humans is very sound, and indeed its absence would be unrealistic. People don't drive their own spaceships but sensibly let the AI do it. The whole society and the characters are quite rational and plausible.

There are some dramatic concessions, but they're handled quite well. There's faster than light travel, but it does cost a fantastic amount of energy, and it is still quite time-consuming. Banks has cleverly avoided the "wormhole" model of practically every other SF plot and instead we have ships travelling through hyperspace at varying speeds. There's teleportation but it's risky and (apparently) limited in distance. There are some ultra-complex and ultra-hard materials, and some fancy force fields (used for holding things together over vast distances, tearing things apart with unimaginable force, carrying tea trays and displaying colour patterns). The Culture people look like humans, which is dramatic license, but the other major races look plausibly different. All that is fair enough.

But then there's this one cringeworthy moment in Consider Phlebas, which hits one of the pillars of real physics with a sledgehammer, apparently for the value of a small gag. The protagonist and his supporting characters, who are pirates of some sort, are looting an abandoned structure in a huge space-station of the rotating type. The ground is the inside surface of the rotating cylinder, and the spin provides the equivalent of gravity, in perfectly understandable low-tech fashion. The station is huge, so variations over small heights and speeds are minimal. Then one of the characters jumps over a parapet, forgetting that his anti-gravity harness works against gravity but not against inertia and falls to his death.

Ouch! Ouch ouch ouch! My scientific lobe is hurting. It is a basic premise of modern physics, called the Equivalence Principle that no experiment could distinguish the condition of falling in a gravitational field from the condition of being at rest. Another way of stating this is that the gravitational and the inertial masses of an object, in other words the solutions for m in F=ma and F=GmM/r^2, are empirically identical. A series of elaborate experiments were set up in the 20th century to measure any discrepancy between the two physical concepts we call mass and, to my amateur understanding of physics, no such discrepancy was found. There are some intriguing proposals involving chiral experiments, but if I understand correctly these are purely conjectures at this stage.

So the guy's "anti-gravity" harness should have fucking worked, it should have gently and reliably propelled him away from the inner surface of the giant rotating cylinder with an acceleration just under one g, his feet should have connected gently with the ever-so-slightly-less curved surface of the outer (lower) floor, and the heist should have gone smoothly as planned. And Einstein would have approved.


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