Causality and Space-time Description
What does Bohr say?
Bohr's second paper, "The Quantum Postulate and the Recent Development of Atomic Theory" (1927) may be regarded as the first Manifesto of Bohr's concept of "complementarity", and the so-called Copenhagen Interpretation of quantum mechanics. However, it is not easy to grasp what Bohr is trying to get at, by reading this paper alone. Bohr says as follows:
Notwithstanding the difficulties which ... are involved in the formulation of the quantum theory, it seems ... that its essence may be expressed in the so-called quantum postulate, which attributes to any atomic process an essential discontinuity, or rather individuality, completely foreign to the classical theories and symbolized by Planck's quantum of action
This postulate implies a renunciation as regards the causal space-time co-ordination of atomic processes.
This seems to be the main thesis, and Bohr is going to say that this "renunciation" does not mean the failure of physics, but, rather it can be saved by a new way of looking at descriptions of nature by means of physics; the new way is what Bohr calls "complementarity", which roughly means that two concepts are both necessary for describing nature, but when one is applied the other is excluded (we will come back to this later, with more specific examples). And when we realize that quantum mechanics requires the interpretation in terms of this complementarity, we have attained a new perspective for physics and human knowledge in general. However, this scenario seems to be entangled with another argument, the argument from the view of observation that any observation involves an interaction between an agent and the objects observed. On this, Bohr says as follows:
Now, the quantum postulate implies that any observation of atomic phenomena will involve an interaction with the agency of observation not to be neglected. Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to the phenomena nor to the agencies of observation. After all, the concept of observation is in so far arbitrary as it depends upon which objects are included in the system to be observed.
The trouble for most readers seems to be this. Bohr seems to be saying that this view of observation is basic, as far as atomic physics is concerned, and complementarity of causality and space-time co-ordination follows from this, as a consequence. Now, the contemporary readers, knowing something about the so-called the "observation problem" of quantum mechanics---while the wave function of a quantum system develops smoothly according to Schroedinger's equation, what is going to happen if an observation is made of this quantum system? Some abrupt change occurs, a definite result is obtained, and how do we explain this?---, may be easily misled, but Bohr is not addressing himself to this problem yet. Bohr seems to be providing some philosophical view, or Gedankenexperiment, for supporting his complementarity thesis. What we have to do is to extract definite elements of Bohr's view, as clearly as possible, and to reconstruct Bohr's overall view.
Causality and Space-time Coordination
Bohr's first example is the well-known particle-wave duality of light:
...the doubts regarding the validity of the superpositionh principle, on one hand, and of the conservation laws, on the other, have been definitely disproved through direct experiments. This situation would seem clearly to indicate the impossibility of a causal space-time description of the light phenomena. On one hand, in attempting to trace the laws of the time-spatial propagation of light according to the quantum postulate, we are confined to statistical considerations. On the other hand, the fulfilment of the claim of causality for the individual light processes, characterized by the quantum action, entails a renunciation as regards the space-time description.
Here, it is clear that Bohr's contrast is made between the following pairs:
propagation of light wave, space-time coordination vs. conservation laws through light quantum, causality
Maybe we should emphasize that Bohr is talking about laws governing such processes, not about particular descriptions of individual events. Take the two-slit experiment, for instance. This is a clear example of the propagation of light wave, and Bohr is saying, presumably, that the interference can be explained by superposition principle, and we have a definite law relating this phenomenon to space-time coordination of the process; however, if we regard light as a particle (photon), we have no means to trace its trajectory (i.e. trajectory in the sense of classical physics, satisfying the conservation laws), thus we have to renounce causality (embodied in conservation laws), and we have to satisfy ourselves only by statistical laws.
One the other hand, take the photo-electric effect, in which conservation laws are satisfied. Here, we certainly have a law satisfying the requirement of causality (an electron with such-and-such momentum is emitted because of this photon giving such-and-such momentum). But, this time, we have no means for predicting that such a process of causal action would take place at such-and-such place at such-and-such time; all we can do is to explain such an individual action, given that this action has taken place, accrding to a law of causality (conservation of momentum). When a photo-electric effect is produced, it is certainly at some place at some definite time but we have no means for telling this (in advance) according to a definite law; thus the explanation of photo-electric effect lacks what Bohr means by "space-time coordination".
Since Bohr already know (at this stage in 1927) de Broglie's ingenious idea of the wave mechanics for electrons, he can extend the same argument to the quantum treatment of matter. What follows is essentially a derivation of Heisenberg's uncertainty (indeterminacy) principle, augumented with abstruse philosophizing on measurement; and Bohr then begins to discuss the problem of measurement in quantum theory in section 3, coming closer to the "problem of measurements" as we now understand.
A New Turn: Indeterminacy Principle
However, we now have to notice that Bohr makes a new turn. Heisenberg's indeterminacy principle gives a quantitative relationship as regards indeterminacy or complementarity; the complementarity becomes a matter of degree. Recall that, Bohr's claim initially was that "if causality is satisfied, space-time coordination must be given up". With the indeterminacy principle, Bohr changes this claim into: "if causality is satisfied to such-and-such degree, space-time coordination must be given up to the corresponding degree". After all, causality and space-time coordination are compatible, both can be satisfied to a certain degree; but unlike the classical case, we have to renounce satisfying both to the maximal degree at the same time, and this, of course, is quite new in the quantum mechanics.
Recall What Bohr said at the Beginning
Let us recall a crucial statement Bohr made at the earlier part of the paper: "Now, the quantum postulate implies that any observation of atomic phenomena will involve an interaction with the agency of observation not to be neglected". Whether or not you can understand what Bohr is trying to get at, entirely depends on whether you question this statement. Why can Bohr say this, and what is the reason for this? If you felt this question and tried to extract Bohr's answser to this, then you must have come close to understanding the whole paper!
Bohr, N. Atomic Theory and the Description of Nature
See also a useful site on Microphysics, at Kyushu University: http://www2.kutl.kyushu-u.ac.jp/seminar/MicroWorld/MicroWorld.html