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A reflector is a stable or oscillating pattern that can reflect some specific type of spaceship (usually a glider) without suffering permanent damage. The first known reflector was the pentadecathlon, which can function as a 180-degree glider reflector as in relay. Other examples include the buckaroo, the twin bees shuttle and some oscillators based on the traffic jam reaction. Glider guns can also be made into reflectors, although these are mostly rather large.

In September 1998 Noam Elkies found some fast small-period glider reflectors based on an arrangement of a block, boat and eater 1 that can be used in conjunction with a suitable sparker to create a 90-degree glider reflector. Pipsquirter 1, pipsquirter 2, and figure eight work for periods 6, 7, and 8, respectively. A more complicated construction is required for a p5 reflector (which, as had been anticipated, soon led to a true period 55 gun - see Quetzal). A variant of the pentadecathlon can also be used to construct a period 15 reflector in this way.

Stable reflectors

Main article: Stable reflector

Stable reflectors are special in that, if they satisfy certain conditions, they can be used to construct oscillators of all sufficiently large periods. It was known for some time that stable reflectors were possible (see universal constructor), but no one was able to construct an explicit example until Paul Callahan did so in October 1996.

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