Stable Glider Reflectors

In October 1996 Paul Callahan constructed the the first stable glider reflector in Life. This is a collection of still lifes which, when hit by a glider coming along a certain path, bounces the glider at either 180 degrees or 90 degrees to its original path without itself being permanently damaged in any way.

Paul's original stable reflector actually bounces the glider in several directions at once. Most later stable reflectors do the same, simply because all of them rely on an initial reaction which doesn't quite work, and (except in the case of Dave Greene's reflector) a Herschel is generated and moved around to fix the damage - and most known Herschel conduits release at least one glider. Unwanted gliders can be removed with eaters.

Paul's reflector was large and slow, although improved somewhat by Dean Hickerson the following month. (For a review of stable reflectors see an article I wrote some time ago for Paul. That article is now a bit out of date and contains a couple of minor historical errors, but should still provide a reasonable introduction to the subject.) In April 1997 Dieter Leithner offered a prize of $100 for the first person to find a stable glider reflector that fits in a 50×50 square, and in January 1999 Alan Hensel added $100 of his own to this prize. The $200 prize was won in April 2001 by Dave Greene, who based his reflector on the initial reaction from Paul's old reflector, adding extra still lifes to convert the resulting active region into a glider that travels back to delete the unwanted beehive. This reflector is shown below, with a glider entering at top left which will be reflected back in the direction from which it came.

44x32 stable glider reflector

Dave Greene's reflector is a 180-degree reflector, which is less useful than a 90-degree reflector, so he has offered a $50 prize for the first 90-degree stable glider reflector that fits in a 50×50 box, and a further $50 for the first that fits in a 35×35 box. At present the smallest known 90-degree stable glider reflector is the 81×62 one that I found in November 1998. It uses an initial reaction found a few days earlier by Paul Callahan, together with Dave Buckingham's 77-step and 112-step Herschel conduits. It is shown below with a glider entering at top left which will be reflected backwards and at right angles in both directions.

81x62 stable glider reflector

Any claim for the $50 prizes should be addressed to Dave Greene (e-mail: In the event of any dispute over the prize money his decision is final.

(Note. Clicking on a picture on this page should load the corresponding pattern into your Life player, provided you have one such as Life32 or MCell that can handle rle files and is set as the default application for files with .lif extension. Alternatively you can download a zip file (2K) that contains the patterns in three different formats, at least one of which should work with your software.)

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This page is maintained by Stephen Silver
Last updated: 2001 Jun 2
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