Einstein-Bohr Debate

Alice C. Linsley

It is said that in the field of Physics two great minds represented the age-old determinism- indeterminism tension or paradox. They are Albert Einstein and Neils Bohr.

Einstein preferred the determinism of classical physics over the quantum physics of Bohr and Heisenberg in which Complementarity and Uncertainty dictate that all properties and actions in the physical world are to some degree non-deterministic. Einstein and Bohr had good-natured arguments over such issues throughout their lives.

Heisenberg and Bohr

Bohr was convinced that light behaved like both waves and particles, and in 1927, experiments confirmed the de Broglie hypothesis that matter like electrons also behaved like waves. Bohr conceived the principle of complementarity: that items could be separately analysed as having several contradictory properties, such as a wave or a stream of particles depending on the experimental framework – two apparently mutually exclusive properties – on the basis of this principle. Bohr summarized the principle as follows:

[H]owever far the [quantum physical] phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms. The argument is simply that by the word "experiment" we refer to a situation where we can tell others what we have done and what we have learned and that, therefore, the account of the experimental arrangements and of the results of the observations must be expressed in unambiguous language with suitable application of the terminology of classical physics.

This crucial point...implies the impossibility of any sharp separation between the behaviour of atomic objects and the interaction with the measuring instruments which serve to define the conditions under which the phenomena appear.... 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 phenomenena exhausts the possible information about the objects.[1]

For example, the particle and wave aspects of physical objects are such complementary phenomena. Both concepts are borrowed from classical mechanics, and measurements (e.g., the double-slit experiment) can demonstrate one or the other, but not both, phenomena at a particular moment. The principle of complementarity explains this as being due to the very nature of the measuring devices used. A measuring device may be designed to demonstrate either the particle or wave aspects, but the demonstration of one necessarily precludes the possibility of simultaneously demonstrating the other, because the object being measured is unavoidably affected by the measurement. It is impossible to design a measuring device that demonstrates both phenomena simultaneously not because of lack of creativity on the part of the experimenter, but simply because such a device is literally inconceivable. Moreover, Bohr implies that it is not possible to regard objects governed by quantum mechanics as having intrinsic properties independent of determination with a measuring device.

In Copenhagen in 1927 Heisenberg developed his uncertainty principle. The ultimate limitations in precision of property manifestations are quantified by the Heisenberg uncertainty principle and Planck units. Complementarity and Uncertainty dictate that all properties and actions in the physical world are therefore non-deterministic to some degree.

At a conference Como in September 1927 Bohr gave a presentation in which he demonstrated that the uncertainty principle could be derived from classical arguments, and without quantum terminology or matrices. Shortly before his death Bohr complained that no professional philosopher had grasped his understanding of complementarity.


Related reading:  Chance, Fortune, Determinism and Indeterminism