What the experiment actually compares
In a double-slit experiment, particles such as electrons are sent toward a barrier with two narrow openings. When no usable information distinguishes which opening each particle passed through, repeated detections build an interference pattern: a striped distribution associated with wave-like behavior.
Add a detector that can record the particle’s path, and that pattern disappears. The key change is not that someone looks at the apparatus. It is that the particle becomes physically correlated with the detector, leaving a record that can distinguish the two routes.
A measurement is a physical interaction
A detector must interact with what it measures. It might absorb or scatter a particle, or register a signal in a material. In quantum terms, the particle and detector become linked: different paths leave different possible detector states. Once the path information is available in the surrounding physical system, the two alternatives no longer combine to produce the same interference pattern.
That interaction need not be dramatic. A detector made from silicon can register a particle by converting its arrival into an electrical signal. Silicon has no awareness; its importance is that its physical state changes in a way that can carry information about the path.
Why “observer” causes confusion
In everyday language, an observer is a person who notices something. In discussions of this experiment, the word often means a measurement process or device. Treating the two meanings as interchangeable can make the result sound as if human attention creates the outcome.
The experiment instead shows how acquiring path information changes the conditions for interference. It does not demonstrate that a mind can choose what becomes real. A useful way to picture the distinction is that the detector leaves a physical trace—whether or not anyone later reads it. That trace, not a conscious glance, is what makes the two routes distinguishable.
