Many astronomers resent the interstellar medium. It blots out their view of many interesting objects, such as the alleged black hole at the centre of our Galaxy. Daedalus sees it more positively.

He points out that all known galaxies are rotating. They spin, not like a wheel but like a bath-tub vortex. The matter near the centre travels faster than that further away — as Kepler would dictate. Yet their collective motion is still too wheel-like for comfort. To fit it to gravitational theory, astronomers must assume that each galaxy contains not only visible stars, but a vast outer ballast of invisible material. This is the mysterious ‘dark matter’.

But Daedalus recalls that the viscosity of a gas is independent of its pressure. The tenuous interstellar medium should thus be about as viscous as normal air. On this basis, the vortex-like appearance of spiral galaxies is no surprise. A galaxy is indeed a vast bath-tub vortex, and its outer regions are rotated, not by the gravity of dark matter, but by viscous drag from the fast-spinning centre. He even muses that the vortex itself may be maintained by the inflow of stars and gas as they go down the awful ‘plug-hole’ of the central galactic black hole.

What happens to the energy viscously absorbed by the stirred interstellar medium? Daedalus calculates its Reynolds’ number as well inside the turbulent region. Its energy will descend a whole hierarchy of swirls and eddies on its way to heat. This explains the puzzling heterogeneity of the medium, and the amazing temperatures it can attain — some parts reach 106 K, far hotter than any stellar surface. Furthermore, as in a bath-tub vortex, the turbulence must release a rude noise in some extreme low-frequency band. This will propagate as infrasound through the interstellar medium.

Infrasound originating nearby should be easiest to observe. Tenuous as it is, the interstellar medium must have a slight optical density. As its waves pass between us and the stellar background, the stars should dim and brighten slightly with its changing density. Their frequency will be very low — a cycle a year or even longer. Daedalus feels they could be best detected by continuous photon counting, for years on end, of specific star clusters (to discount individual stellar fluctuations). Data will trickle in very slowly; but who can say what advances might come from the new discipline of acoustic astronomy?