In the double-slit experiment, which is a fundamental experiment in quantum mechanics, the "observer" refers to any entity or system that interacts with or measures the quantum system under investigation. The observer does not have to be a conscious human being; it can be any macroscopic or microscopic entity capable of detecting or interacting with the particles or waves involved in the experiment.
In the basic double-slit experiment, particles such as electrons or photons are sent towards a barrier with two narrow slits. These particles exhibit wave-particle duality, meaning they can behave as both particles and waves. When particles are sent through the slits, they create an interference pattern on a screen behind the barrier, indicating wave-like behavior. However, if there is an observation or measurement made to determine which slit the particle passes through, the interference pattern disappears, and the particles behave more like individual particles or localized entities.
The crucial point is that any interaction or measurement made to determine the particle's path disrupts the interference pattern. This interaction can be done using various means, including physical detectors, cameras, or even indirect interactions that can leave a trace of which path the particle took. The important factor is that some information about the particle's behavior is extracted, causing a collapse of the wave function and destroying the interference pattern.
Therefore, in the double-slit experiment, the observer refers to the measurement apparatus or system that interacts with the particles or waves in a way that obtains information about their behavior. The observer does not need to be a conscious being; it can be a purely physical system or a measuring device. The act of observation, in this context, refers to the interaction between the quantum system and the measuring apparatus, which disturbs the system and affects its behavior.