Stable 2c/3 signal receiver
On 23 January 2005, Noam Elkies found a collision of 8 gliders with an LWSS that could repeatably create an input signal travelling at two thirds of the speed of light (2c/3) in the "transmitter" end of Dean Hickerson's stable diagonal 2c/3 "signal track" from 18 March 1997.
The new signal-inserting collision is shown on the left edge of the pattern below; the rest of the pattern is a stable 2c/3 signal receiver constructed by Dave Greene on February 6.
One remaining open problem is the construction of a similar stable pattern to produce the "transmitter" collision from a single input signal.
Update: a first draft "transmitter" has been built -- see this weblog entry.