The Copenhagen interpretation of quantum mechanics grew out of the discussions between Niels Bohr and Heisenberg in the late 1920s.
According to the Copenhagen interpretation which is the “standard” interpretation of quantum theory, it is meaningless to ascribe such properties as position, momentum, and energy to a microscopic particle like an electron in the absence of an observation or measurement. “The nature of reality in the Copenhagen interpretation is therefore essentially epistemological, that is all meaningful statements about the physical world are based on knowledge derived from observations”. According to John Archibald Wheeler “No elementary phenomenon is a phenomenon until it is a recorded phenomenon” (Wheeler, 1983, p. 184).
Quantum reality therefore differs profoundly from macroscopic reality, where a measurement or observation plays a pivotal role. But what constitutes an “observation”? Can a nonliving being make an observation?
The answer to these questions fell under the purview of philosophy at the time quantum mechanics was first developed. Several philosophical positions concerning reality emerged. The first, held by Einstein, is that objective existence of nature (or reality) occurs whether we know it or not. The second position, held by Bohr, is that reality has no meaning in absence of the measurement process (Copenhagen interpretation). These philosophical positions had their supporters, with Bohr’s position gradually becoming predominating among quantum physicists because of certain developments in quantum mechanics during the middle of the twentieth century, when the Irish physicists John Bell came up with a way of scientifically testing the hypotheses.
I want to emphasize that quantum mechanics, despite of its philosophical conundrums, is extremely precise in its mathematical predictions and works very well in predicting outcome of experiments. In fact, quantum mechanics has been repeatedly veriﬁed with exceedingly high accuracy and has never been contradicted experimentally. Yet despite its colossal success, there is still no consensus among physicists about what this theory says about the nature of reality. Most professional physicists, who use quantum mechanics in their everyday research, have accepted the Copenhagen interpretation in its face value, and choose not to worry about its philosophical dilemmas. But there are some who work with the foundations of quantum mechanics, believe that the Copenhagen interpretation really served to distinguish between a system that is truly quantum-mechanical and an observer who is essentially classical by postulating that these categories play a crucial role in the “fundamental architecture of reality”. It is nothing more than a useful approximation of some deeper underlying truth.
Recent developments in the foundations of quantum theory have shown that the various “interpretations” of quantum theory are not just obscure philosophical opinions as many physicists tend to believe. On the contrary, they seek to determine the correct interpretation of the formalism and identify the principles that lie beneath quantum theory. Why do we have a quantum world as opposed to a classical world? Of the many motivations for pursuing foundational research, one is the development of quantum technologies, such as quantum teleportation and quantum cryptography. A better understanding of the theory will facilitate how to harness of the power of non-classicality to identify and develop new and exotic technologies such as quantum computation that are not just distant dreams of a handful of futurists. They are steadily becoming reality, perhaps ushering a new dawn of technological breakthrough.