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Soniferous Aether

|< < Prev #3238 (April 27, 2026)

Title text: Imagine you could ride alongside a sound wave. It would probably be pretty cool, right? We're putting in a departmental budget request to buy a really fast plane so we can check it out.

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Explanation

Beginning in the late 1690s, scientists posited a luminiferous aether to explain properties of light (especially its ability to travel in a vacuum) which should not be possible for a wave. This theory was disproven by the 1887 Michelson–Morley experiment, which demonstrated that the speed of light was constant, regardless of relative movement through the supposed aether. (If the luminiferous aether did exist, light would move at a set speed relative to that aether, and therefore would appear, from a human perspective, to move slower in the direction the Earth was currently traveling, and faster in the opposite direction- but this did not occur.)

In this comic, Cueball and Ponytail have noticed that the speed of sound is not constant, and therefore have concluded, following a similar line of reasoning, that sound does travel through a medium. They then coin the term 'soniferous aether', following the same naming scheme, as the name for this 'newly-discovered' medium. ("Luminiferous" is Latin for "light-carrying", and "soniferous" would be the corresponding "sound-carrying".) Although their conclusion is, strictly speaking, entirely correct, it overlooks the fact that this medium has already been discovered and named. Sound is widely known to travel through physical media, such as air or water, and cannot exist in a vacuum. Since these substances are already known to modern science^{[citation needed]}, it is clearly unnecessary for new scientists to discover or name them.

The comic's caption states that the physicists are specifically reinventing air from first principles. Although sound can be carried by any form of matter, a human would likely identify air as the primary form of soniferous aether, since humans generally experience sounds that travel through air, rather than other mediums like water or metal. As air has very little viscosity and density, is almost entirely invisible to light, and is very familiar to humans, its presence is often overlooked in many situations, and this comic takes that to the humorous extreme of physicists forgetting it exists even while creating a new theory to explain its effects.

By analogy with the theory of luminiferous aether, soniferous aether would logically be an otherwise unobtrusive medium which is primarily observable through its interactions with sound waves. This is a somewhat reasonable description of air, hence why the physicists are stated to have 'reinvented air' (although air does have many other effects that humans generally take for granted, such as the presence of oxygen and the ability for scents to diffuse through 'empty' space).

The title text references Albert Einstein's thought experiment where he imagined riding alongside a light particle/wave (which was referenced in 2959: Beam of Light), but with a sound wave instead. Travelling at the speed of sound can be accomplished with a fast airplane. Usually these would be military aircraft, though Concorde, the Tu-144 and (briefly) a DC-8 were commercial aircraft that did achieve supersonic flight, and the Boom Overture is in the process of being tested as of the comic's publication.

Reinventing things from first principles has previously been discussed multiple times on xkcd.

Transcript

[Ponytail is addressing an unseen audience in front of her while she holds her hand out towards them. She is standing on a podium with Cueball behind her. Cueball holds an arm out behind him indicating a screen behind him showing a graph with three sine waves with different wavelengths. The top has two cycles, the middle four cycles and the bottom one cycle.]

Ponytail: We all know the speed of light is constant for all observers.

Ponytail: But our experiments show that the speed of sound changes based on the observer's motion.

Ponytail: Thus, we posit the existence of the soniferous aether, a medium that fills the space between us and carries sound waves.

[Caption below the panel:]

Sometimes physicists forget that air exists and rediscover it from first principles.

---

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