A Herschel stopper is a method of cleanly suppressing a Herschel signal with an asynchronous boat-bit, discovered by Dean Hickerson in November 1996. In the infobox pattern, a ghost Herschel marks the location where an output signal will appear in cases where the boat-bit is not present. The first Herschel travels through the Herschel stopper site unaffected, but after the glider produces a boat-bit, the next Herschel collides with the boat and the two are mutually annihilated without affecting any of the nearby catalysts. Other boat-bit locations that allow for clean suppression of a Herschel are also known.
The term "Herschel stopper" is also occasionally used to refer to any mechanism that cleanly suppresses a Herschel. These are usually stable conduits that allow the Herschel's first natural glider to escape, so they are more commonly classified as converters. See SW-2.