In quantum mechanics, hidden variables refer to hypothetical underlying parameters or properties that could potentially explain the observed behavior of quantum systems. These variables are "hidden" because they are not directly observable or accounted for within the standard formalism of quantum mechanics.
The concept of hidden variables was proposed as a means to resolve certain philosophical and interpretational issues of quantum mechanics, such as the indeterminacy and randomness inherent in the theory. However, the prevailing interpretation of quantum mechanics, known as the Copenhagen interpretation, does not rely on hidden variables and considers quantum phenomena as fundamentally probabilistic.
The search for hidden variables has been an active area of research, but no conclusive evidence supporting their existence has been found to date. The famous Bell's theorem, formulated by physicist John Bell in the 1960s, provides a framework for testing whether quantum mechanics can be explained by local hidden variables. Experimental tests based on Bell's theorem, such as the Bell inequality tests, have been conducted, and the results have consistently favored the predictions of quantum mechanics over local hidden variable theories.
It is worth noting that the absence of evidence for hidden variables does not definitively prove their nonexistence, as there may be other forms of hidden variables or theories that have not yet been explored or tested. However, the prevailing consensus among physicists is that hidden variables are unlikely to provide a complete description of quantum phenomena, and the probabilistic nature of quantum mechanics appears to be an inherent feature of the universe.
In summary, the search for hidden variables in quantum mechanics has been pursued through theoretical investigations and experimental tests. So far, the results have supported the probabilistic nature of quantum mechanics without requiring the inclusion of hidden variables.