There is a door in the laboratory. Behind it, a particle is fired at a wall with two slits. What happens next should be impossible. But it happens every time.
I. The Apparatus
Take a wall. Cut two parallel slits in it. Place a detection screen behind it. Fire a single electron toward the wall.
If you close one slit, the electron goes through the other and lands on the screen in a single point. Do this repeatedly, and the points cluster in a pattern — a single band behind the open slit. Expected. Classical. Normal.
Open both slits, and everything changes.
The electron, fired one at a time, does not land in two bands. It lands in a pattern of alternating stripes — bright bands where many electrons hit, dark bands where none do. The same pattern you get when two waves overlap and interfere.
But it was one electron. Fired alone. Through two open slits.
The electron, it seems, went through both slits simultaneously. As a wave. Then interfered with itself. Then landed as a particle.
II. The Orthodox Answer
For nearly a century, the official position of physics has been: don't ask.
The Copenhagen interpretation — named after Niels Bohr's institute — says that the electron does not have a position until it is measured. It exists in a cloud of probability. The act of observation "collapses" the cloud into a single point. Before measurement, there is no fact of the matter where the electron is.
This works mathematically. It predicts the interference pattern precisely. It is taught in every university. And it leaves most students with the quiet feeling that something is being swept under the rug.
Because the math works, but the story is incomplete. "Don't ask what happens between the slits and the screen" is not an explanation. It is a pact of silence.
III. The Deutsch Answer
David Deutsch proposes something different. Something that sounds like science fiction but follows from the logic of what we already know:
The particle does not go through both slits as a wave.
It goes through one slit — in one universe. And through the other slit — in another universe. Both events are real. Both universes exist. The interference pattern is the evidence of their interaction.
The bright bands are where the two versions of the particle arrive in phase — crest meeting crest. The dark bands are where they arrive out of phase — crest meeting trough, cancelling each other out.
What we call the "wave" is not a property of the particle. It is the shadow of a multiversal event projected onto our single timeline.
IV. The Frog and the Shadow
Deutsch offers an analogy. Imagine a frog sitting at the bottom of a pond. On the surface, two stones are dropped. The ripples spread, intersect, create a pattern of interference — alternating bands of high and low water.
The frog, looking up from below, sees a pattern of light and shadow on the surface. It has no concept of stones, or ripples, or interference. It only knows the pattern it sees.
We are the frog.
The double-slit experiment is our shadow on the surface. What we see is real — but it is not the whole story. The whole story involves events happening in parallel, across realities, that converge into the single pattern we observe.
This is not mysticism. It is the simplest explanation that accounts for what we see. The multiverse is not an addition to physics. It is what physics looks like when you stop pretending that the shadows are the whole of reality.
V. Why This Matters
If Deutsch is right — and the logic is compelling — then reality is not a single stage where events unfold. It is a branching structure, where every quantum event creates a divergence, and those divergences coexist.
You are not one person reading this sentence. You are a configuration of attention that happens to align with this particular branch. In another branch, you are reading something else. In another, you never clicked. In another, this paragraph does not exist because the universe that produced it never formed.
This is not a metaphor. This is the structure of reality, stated as directly as we know how to state it.
The multiverse is not a hypothesis. It is a consequence. The only question is whether you are willing to follow the logic to where it leads.
The particle does not decide. The universes do not compete. They coexist. What we call reality is the interference pattern of everything that is happening, everywhere, at once.
— Based on Chapter 1 of "The Fabric of Reality", David Deutsch