If a tree falls in the woods, does it make a sound? And even if it only produces waves... does "wave" have any mean in a "lifeless" universe?

The question of a falling tree in the woods, and the 'meaning' of waves in a lifeless universe, fundamentally differentiates between objective physical phenomena and subjective perceptual experience.

The Falling Tree: Sound as a Physical Process vs. Perceptual Event

When a tree falls, it physically creates pressure variations in the air or ground. This process involves a mechanical impact, vibrations, and the generation of acoustic waves that propagate at speeds like 343 m/s (at 20°C). From a purely physical, realist, or physicalist standpoint, these pressure variations are sound waves, regardless of whether a listener is present. Modern physics textbooks (e.g., Halliday & Resnick) define sound in terms of these objective pressure fluctuations. Only a tiny fraction (≈ 10⁻⁶ – 10⁻⁸) of the tree's energy converts to audible-frequency acoustic energy, with most manifesting as kinetic motion and heat.

However, if "sound" is defined as a perceptual event or subjective experience, then no, there is no sound without a listener. This phenomenalist or idealist stance, famously argued by George Berkeley ('to be is to be perceived') and reflected in John Locke's distinction between primary and secondary qualities, posits that the sensation of sound requires a conscious observer. Therefore, whether a falling tree "makes a sound" depends entirely on which definition of 'sound' is adopted: the physical process or the psychological event.

'Wave' in a Lifeless Universe: Objective Phenomenon vs. Subjective Meaning

The concept of a 'wave' in a universe devoid of life can also be viewed through two lenses:

1. Physical/Mathematical Meaning: A wave is a robust, observer-independent physical concept. It is a solution to a wave-type differential equation (e.g., acoustic wave equation, Maxwell's equations, Schrödinger equation), describing how a quantity (like pressure, electric field, or probability amplitude) varies in space and time. This definition is context-free and does not presuppose any life, cognition, or language. Examples include acoustic waves, electromagnetic waves, gravitational waves, and quantum probability waves. Even in a lifeless universe, fields or media would support these oscillations and fluctuations, carrying energy and information according to the laws of physics.

2. Semantic/Subjective Meaning: The word 'wave' itself, and any 'meaning' attached to it beyond its physical description, is a human linguistic label and convention. In a universe without sentient beings, the physical patterns and propagating disturbances would still exist, but there would be no mind to interpret them, assign labels, or ascribe philosophical significance. Meaning, in the sense of interpretation or symbolic significance (e.g., music, communication, information), arises from human cognition. Therefore, while the physical wave persists in a lifeless universe, its semantic meaning—the significance humans attach to it—would not.

Conclusion

In summation, a falling tree undeniably generates acoustic pressure waves, which are sound in a purely physical sense. Whether this constitutes "sound" in the everyday, experiential sense hinges on the presence of a perceiver. Similarly, physical waves exist objectively and follow natural laws in any universe, but the meaning (semantics, interpretation, significance) we attribute to them is an anthropocentric layer that requires consciousness.