Slow glass is a concept first introduced by Bob Shaw in his 1966 short story "Light of Other Days".

In an ordinary transparent medium, such as air, glass, or water, the speed of light is reduced from what it is in a vacuum. The difference in light speeds between two media is what causes refraction at their boundary: think of a prism. The actual difference in speed, however, is relatively small.

Now imagine a pane of glass in which the speed of light is so slow that a ray can take as long as years to traverse it. This is slow glass. The thickness of a pane is no longer measured in centimetres or inches, it's measured in days, weeks, or years.

Imagine what you can do with it. If you've got a pane that's twelve hours thick, you can put it outside during the day, take it inside at night, and have sunlight around the clock! Or you could take panes several years thick, place them somewhere scenic, and allow them to soak up the natural beauty for a while. Later, you harvest them, hang them in your city house's window frames, and you've got an instant view of the mountains! (Until all the light from the mountains has passed through, at which point, you're looking out at the street below, only several years ago.)

Imagine also the scope for crime. As a crooked glazier, you could craftily replace someone's window with 24 hour thick slow glass, and they won't see you stealing their car until a day later. Think of the perfect alibis you could manufacture!

But what about surveillance? Drop a tiny shard of slow glass in your competitor's boardroom, then have your spy in the janitorial staff vacuum it up a couple of days later. Pop it under a microscope, and voila: revealed are those trade secrets!

Slow Glass is one of the most memorable truly original science fiction concepts. Shaw struck to the emotional heart of it in "Light Of Other Days", which was justly nominated for the Hugo and Nebula, and has gone on to become one of the most anthologised SF stories ever. He developed it further in the fixup novel Other Days, Other Eyes (1972), which leads the reader through stories on all of the themes noted above, and builds up to the conclusion of an almost dystopian world of ubiquitous surveillance.

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