It's an old idea:
"Sir William Thomson (later Lord Kelvin) published his proposal for an ideal foam, intended as a model for the ether of space ..."
“We may regard the ether as a continuous incompressible elastic solid, permeated by a system of spherical vortices… or, more simply, as a foam of equal cells.”(1887)
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Lord Kelvin (William Thomson), in his 1901 Baltimore Lectures on Molecular Dynamics and the Wave Theory of Light*, discussed the luminiferous aether—a hypothetical medium proposed to propagate light waves—as a highly elastic solid capable of supporting transverse waves without viscosity. To illustrate the aether's paradoxical properties (rigid enough for shear waves like light, yet fluid-like in other respects), he referenced an experiment involving soap foam in a tube. This was originally performed by physicist Oliver Lodge in the 1890s but was popularized by Kelvin in his lectures as a tangible analogy.
Key Details of the Experiment
• Setup: A long, narrow glass tube (about 1-2 meters long and 1-2 cm in diameter) is filled with soap foam, essentially a dense lattice of tiny soap bubbles packed together. The ends of the tube are sealed with rubber stoppers or caps to contain the foam.
• Procedure: One end of the tube is fixed rigidly, while the other is twisted (torsioned) sharply by hand or a mechanism. The twist propagates as a shear wave through the foam, causing the bubbles to distort and slide past each other elastically.
• Observations:
- The foam transmits the torsional motion with remarkable fidelity: the free end of the tube rotates almost exactly in sync with the twisted end, even after many cycles, showing high elasticity and rigidity in shear.
- There's minimal energy loss or "damping" (viscosity), so the wave doesn't dissipate quickly—unlike in a viscous fluid like water or oil.
- Under compression (e.g., squeezing the tube lengthwise), the foam behaves more fluidly, compressing easily without much resistance.
• Duration and Scale: The foam's structure holds up for repeated trials (hundreds of twists), but eventually, air diffusion or bubble bursting degrades it.
Why It Matters for the Aether
Kelvin used this to model the aether's ideal qualities:
• Rigidity without Viscosity: Light waves require a medium that's "solid" in shear (to support transverse oscillations) but not dissipative (to allow undamped propagation over cosmic distances). Soap foam mimics this: its bubble network provides elastic resistance to twisting but allows low-friction sliding.
• Contrast to Other Media: Compare it to a rubber rod (too rigid and prone to fatigue) or a liquid (too viscous). The foam's cellular structure bridges the gap, hinting at how the aether might be a "sponge-like" continuum of vortices or gyrostatic elements in Kelvin's vortex atom theory.
• Limitations: Kelvin noted the analogy wasn't perfect—the foam has some compressibility and eventual breakdown—but it vividly demonstrated that such a medium was physically plausible.
This experiment influenced early 20th-century debates on aether models before relativity supplanted the concept.
