An old stand-by in the realm of science fiction is that of the force field. It keeps things in - or out - by exerting a pure restraining force in space, and yet is invisible to the eye. It might seem strange that this futuristic technology is fact, not fantasy; stranger still that its realisation is no breakthrough of our nuclear age, but has been available to civilisations for millennia: it is called glass.
Glass is a daily miracle. The transparency that allows us to take it for granted defies the laws of physics. We know that gases and liquids may be translucent, while solids, as their name suggests, generally have a visual inscrutability as well as a structural obduracy. But not glass. It is indeed a force field, able to hold objects in a fixed and demarcated space despite its invisibility. Equally miraculously, it is chemically transparent too: the test-tube and the whole modern investigative apparatus of physical and biological sciences is only possible because of glass's near-inertness to almost every reagent.
We have lost our sense of wonder in the presence of this anomaly. That wonder must have been immense when glass was first discovered; here, at last, people may have thought, was true alchemy - the transmutation of worthless, common sand into an awesome substance far more precious than mere vulgar gold. We can still glimpse some of that sense of bafflement when we see animals confronting glass. For them it is there and not there, incomprehensible and running counter to all their intuitions.
We have not entirely squandered our reverence for glass, although it manifests itself in a curious way. If the fact of the substance is no longer cause for amazement, its destruction still carries a heavy and atavistic charge. The sound of breaking glass is one of the most frightening: in its sudden, shattered chime there is a suggestion of some music of the spheres being lost, of a disorder entering the world.
In part this arises from the immediate disappearance of glass's restraining function: its magic is necessarily holistic. Slivvers of glass are also a betrayal by their lethal sharpness of the implicit vitreous promise of control and smoothness. And broken glass suggests intrusion: most glass that we encounter is in the form of windows. Smashed panes are synonymous with the rupture of a building's protective shell, a penetration of our inner sanctum.
Perhaps, too, glass derives an associative power from its use in the special 'looking glasses' - mirrors. The broken mirror has always been a potent symbol of a wrecked world. Yet clearly to break a mirror means in fact to break its glass: earlier mirrors were made from polished metal, and so would never have broken, only faded or become scratched. The tabooed breaking of a mirror, with its seven-year curse of ill-fortune, must refer to the smashing of the glass through which we see ourselves. It is as if the glass were a boundary separating us from that reversed world. Cracking the glass is to crack open the barrier which kept them asunder, unleashing the counterfactuality we see there. In the mirror we retain our deepest if occulted sense of the power of glass.