A lightspeed bubble is a type of negative spaceship traveling through the zebra stripesagar. The center of the bubble is simple empty space, and the length and/or width of the bubble can usually be extended to any desired size.
Below is a small stabilized section of agar containing a sample lightspeed bubble, found by Gabriel Nivasch in August 1999. The bubble travels to the left at the speed of light, so it will eventually reach the edge of any finite patch and destroy itself and its supporting agar.
An open problem related to lightspeed bubbles was whether large extensible empty areas could be created whose length was not proportional to the width (as it must be in the above case, due to the tapering back edge). This was solved in February 2017 by Arie Paap; a simple period-2 solution is shown below.