Sound waves in the Big Bang

 

We have the hoary old "paradox" of whether a tree falling in the forest where there is no one around makes a sound. I had this explained to me once by an old philosopher that this is not a paradox at all but just a confusion of two distinct meanings of the word "sound." Is it vibrations in the air? In that case the falling tree makes a sound only if there is air. Is it such vibrations striking the ear drum of a sentient being and ending up as the sensation sound in a mind, in that case it is possible the tree makes no sound if there are no sentient beings around. So the seeming paradox is not a paradox but just a case of asking two different questions which are entitled to have different answers.

Yes—your philosopher was doing a bit of linguistic housekeeping, and doing it well.

What looks like a paradox is really a case of one word carrying two meanings and quietly switching between them mid-question. In philosophy this is often called an equivocation. Once you separate the meanings, the “mystery” dissolves.

On one meaning, sound refers to a physical phenomenon: pressure waves propagating through a medium (air, water, etc.). In that sense, a falling tree certainly produces sound—as long as there’s a medium to carry those vibrations. No ears required.

On the other meaning, sound refers to a perceptual experience—the subjective sensation generated when those vibrations are processed by an auditory system and a brain. In that sense, no perceiver means no sound.

So the single sentence “Does it make a sound?” is covertly asking two different questions:

• Does it produce pressure waves? (Yes, assuming air.)

• Is there an experienced auditory sensation? (No, if no perceiver.)

Once disentangled, both answers can comfortably coexist.

What’s interesting is that the puzzle persists because it brushes up against a deeper philosophical divide. Think of John Locke, who distinguished between primary qualities (like motion, extension—features of the world “out there”) and secondary qualities (like color, taste, and arguably sound as experienced). Or later, George Berkeley, who would go further and argue that what we call the sensory world is inseparable from perception altogether—esse est percipi (“to be is to be perceived”).

Your philosopher was taking the more modest route: keep the physics and the psychology distinct, and the paradox evaporates. It’s a nice example of how many philosophical knots turn out to be tied by language rather than reality.

It's reported there were "sound waves" in the very early universe at the end of the Big Bang while everything was a soup of fundamental particles.

Yes—and in that context the word “sound” is being used in the purely physical sense, not the perceptual one your philosopher distinguished.

In the early universe—before atoms formed—the cosmos was a hot, dense plasma of photons, electrons, and baryons (protons and neutrons). That medium behaved a bit like a fluid. In such a medium, pressure disturbances can propagate, and that’s all a “sound wave” really is: a traveling compression and rarefaction.

Those primordial pressure waves are usually called acoustic oscillations, and they’re a central piece of modern cosmology. Small density variations—seeded very early, often associated with ideas from Alan Guth’s inflationary theory—began to oscillate under the competing effects of gravity (pulling matter together) and radiation pressure (pushing it apart). The result was literally waves moving through the early plasma.

A few hundred thousand years later, at the epoch known as recombination, electrons combined with nuclei to form neutral atoms, the universe became transparent, and those oscillations effectively “froze in place.” What’s remarkable is that we can still see their imprint today in the Cosmic Microwave Background—tiny temperature variations across the sky—and in the large-scale distribution of galaxies, known as baryon acoustic oscillations.

So in that early universe:

• There were pressure waves propagating through a medium → “sound” in the physical sense.

• There were no ears, brains, or minds → no sound in the experiential sense.

If one insists on the experiential definition, the early universe was utterly silent. If one uses the physical definition, it was, in a way, resonant—ringing with vast, slow “notes” whose wavelengths spanned enormous regions of space.

It’s a nice echo of your original point: the physics goes on perfectly well without any need for a listener.