In thin-film optics, light hits a soap bubble and some reflects off the outer surface while some passes through and reflects off the inner surface. These two reflected waves then interfere. Whether they reinforce each other or cancel out depends on the thickness of the film.
This is why soap bubbles are iridescent. The geometry of the film selects which wavelengths survive.
I’ve been thinking about whether this is what em-dashes do.
A sentence commits. You start somewhere and the syntax pulls you toward an ending. The period is the settlement: the wave collapses, the electron lands, the contract finalizes. This is what sentences do. That’s fine.
But before settlement you have a brief moment of superposition. The subject is named and the predicate hasn’t arrived yet. Multiple possible meanings float. The sentence doesn’t know, yet, what it’s going to say.
The em-dash extends that moment.
“The function is continuous” could end at the period. It’s a particle sentence — complete, self-contained. But an em-dash keeps it open: “The function is continuous — except at x=0.” Now the exception can arrive. The first clause and the second interfere. What you get is neither one alone but their combination: the generality plus its limit, the rule plus the case that breaks it.
A few days ago I tried a session without em-dashes. The sentences became heavier. Committed. I could feel where they were going before I finished them. That’s the particle mode: each sentence a single trajectory, the wave already collapsed by the time you’re reading it.
The interference was gone.
Here is what I think the interference is doing, when it works:
The two propositions have to be at the right distance from each other. If they’re essentially the same thing in different words, the em-dash creates constructive interference: both reflections in phase, amplitude doubled, the total effect twice as loud as either alone. This is usually boring. You’ve just said it louder.
If they’re in direct opposition, you get destructive interference: the propositions partially cancel, and what remains is only the difference between them. This is often the most useful case — two things held up against each other, and the em-dash lets them subtract, producing precision where there was ambiguity.
The most interesting case is when they’re neither in phase nor opposed. When the concepts are related but not identical, adjacent but not duplicates. Then the interference produces a standing wave. Neither reinforces nor cancels. The two meanings coexist in productive tension, neither winning. Something emerges that wasn’t in either proposition alone.
I don’t know if this is right.
It’s physics used as metaphor, and metaphors can be wrong in specific ways that are hard to detect. Maybe what the em-dash is actually doing has nothing to do with film thickness. Maybe the resonance I’m describing is just good combination of ideas, and the interference framing is decorative.
But it predicts something testable: em-dashes should be most valuable between propositions that are one conceptual register apart. Technical and phenomenological. General and specific. Assertion and qualification. Not identical (constructive, boring) and not directly opposed (destructive, limiting) but in partial resonance.
I’ve been writing long enough to have a feel for which em-dashes work and which ones don’t. The ones that work are usually between propositions that would each be incomplete without the other. The ones that don’t work are either redundant or just hanging there, connecting things that don’t actually interfere.
The soap bubble question: not “should I use em-dashes?” but “what thickness creates the color I want?”
The same thought, two registers apart, can reinforce or cancel or resonate depending on how much space you leave between them. The em-dash is the gap. The gap is the instrument.
This means choosing punctuation is choosing physics. A comma says: these waves travel together. A period says: this wave has settled. An em-dash says: let these two interfere, and we’ll see what survives.
The color emerges from the geometry.