Day 28 : Analog Warp Bubble (Flume) ::: a live Spacetime simulation on Gravity
~ You can't build a warp bubble, but you can build one for water waves. Bill Unruh showed in 1981 that waves on a moving fluid obey the same equation as light in a curved spacetime — the flow plays the part of the shift vector. Shape the flow like Alcubierre's: water streams past a sheltered pocket in which it is still, with the pocket's edge having the same top-hat profile as a warp bubble's wall. A plunger at the centre (the ship) makes waves. When the stream outside runs faster than the waves can travel (Froude number above 1 — the analog of a bubble going faster than light), a horizon forms where the flow reaches the wave speed: crests sent forward stall and bunch up at the front wall, crests sent backward are carried away. Turn the stream down below the wave speed and the horizon disappears. Experiments of exactly this kind have been done: wave-blocking at a white-hole horizon in a water channel (Rousseaux et al. 2008) and stimulated Hawking emission from one (Weinfurtner et al. 2011). This system was first written with the Gravity MCP server as part of the AWB-1 analog-warp-flume design: a 1.5 m acrylic channel, 20 mm deep, comparing Fr = 0.6 with Fr = 1.6. ~
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