The obvious question — and its shocking answer
Faced with the double slit, everyone asks the same reasonable thing: fine, the pattern looks wavy, but surely each electron really went through one slit or the other — so why not just look and find out? Put a tiny detector at the slits, note which one each electron passes, and settle the matter. It is the most natural experiment in the world to want to do. And its result is one of the most unsettling facts in physics.
The instant your detector can tell which slit each electron took — the instant you have what physicists call which-path information — the interference pattern vanishes. The bands dissolve, and the dots pile up into two plain clumps, exactly as ordinary little balls would. Switch the detector off, and the bands return. The electrons seem to know whether they are being watched, and they change their behaviour accordingly. Nothing in classical physics prepares you for this.
Why information is the culprit
To see why, recall what the interference pattern needed. It came from a single wave that went through both slits at once — a state of being in two situations blended together, which we call a superposition. The bands are the visible proof that the electron did not have a definite path; it was genuinely 'both' until it landed. The pattern is, in a sense, the universe's way of admitting it had not yet decided which slit.
Now demand which-path information. To record which slit the electron took, the detector must become correlated with it — the electron's path gets stamped onto something in the world. But the moment the path is recorded anywhere, the electron no longer is in a blended 'both' state; it has a definite slit. And with a definite path, the two overlapping waves needed for interference no longer both exist. No superposition, no overlap, no bands. You did not crudely disturb the electron — you forced it to commit to one history, and a committed electron is a plain particle.
A trade-off you cannot cheat
Read together, the two versions of the experiment reveal a hard rule. You can have the crisp interference pattern (full wave behaviour, no idea which slit) or you can have which-path knowledge (full particle behaviour, no pattern) — but never both at full strength at once. The more sharply you pin down the path, the more washed-out the bands become; ease off on the path and the bands sharpen back up. There is a smooth dial between the two, but its ends are mutually exclusive. Knowing the path and seeing the pattern are two payments drawn from the same limited account.
Physicists have tried every clever scheme to outwit this — and the cleverest of all is genuinely mind-bending. In a quantum eraser, you record the which-path information and then, before reading it, carefully destroy it so no one could ever recover which slit was used. The astonishing result: erase the information and the interference pattern comes back. It really is the availability of the knowledge, not any physical jostling of the electron, that decides whether the wave nature shows. The bookkeeping of information sits at the very heart of the result.
What it does — and does not — mean
It is tempting to leap to spooky conclusions — that a conscious mind is reaching out and changing reality, or that the electron is psychic. Resist both. "Observation" here does not require a human, a mind, or even a living thing; it means any physical process that records the path into the wider world. A detector that automatically logs the slit to a hard drive does the job even if no person ever reads it. The real lesson is calmer and stranger: in the quantum world, getting information about a system genuinely changes which possibilities can remain blended together.
Notice what this experiment quietly establishes: you cannot watch the wave behave as a wave. The very act of catching it being a particle (which slit?) forbids it from showing the wave's signature (the bands). Wave and particle are not just two faces — they are two faces that hide each other. That mutual hiding is no accident, and it is exactly the principle Niels Bohr built into the foundations of quantum theory, which the final guide takes up.