The Essentials of Niels Bohr

The behaviors of quantum systems that are manifested during the measurement are indispensible to the experimental arrangement that the quantum systems interact within. This implies the "impossibility of any sharp separation between the independent behaviors of atomic objects and their interaction with the measuring instruments which serve to define the conditions under which the phenomena appear"[Ref1, p18,26,32,].

"The quantum mechanical formalism permits the well-defined applications referring only to ... closed* phenomenon, [t]he word phenomenon ... refer[s] only to observations obtained under circumstances whose description includes an account of the whole experimental arrangement"[Ref1, p4]. This is the main distinguishing characteristic of micro nature according to Bohr.

As emphasized by Bohr, it is impossible to subdivide a quantum phenomenon into a sequence of physically unambiguous well-defined steps and "ascribe customary physical attributes" to them, i.e., the unambiguous use of space-time concepts for the description of each step. "In particular, it must be realized that - besides in the account of the placing and timing of the instruments forming the experimental arrangement - all unambiguous use of space-time concepts in the description of atomic phenomena is confined to the recording of observations which refer to marks on a photographic plate or to similar practically irreversible amplification effects like the building of a water drop around an ion in a cloud-chamber" [Ref1,p31].

In order to ascribe the unambiguous physical attributes to the dynamical evolution of object within the measuring instrument, we have to subdivide the phenomenon. "Any attempt of subdividing the phenomena will demand a change in the experimental arrangement". However, because of "introducing new possibilities of interaction between objects and measuring instruments"[Ref1,p18], the change of the experimental arrangement at the proper time and proper place will result to a new phenomenon. "Consequently evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomena exhausts the possible information about the objects"[Ref1,p18]. Therefore, "we are dealing with individual phenomena and that our possibilities of handling the measuring instruments allow us only to make a choice between the different complementary types of phenomena we want to study"[Ref1,p18].

* The italic emphasizes are made by the author of this paper.
Reference: “Quantum theory and measurement”, edited by J.A.Wheeler, W.H.Zurek, pp. 9-49, Princeton University Press, New Jersey, (1983).