Introduction to Complementarity Principle
Complementarity is an interesting concept that was introduced by Neils Bohr in the year 1928. Neils Bohr introduced the complementarity principle or the concept of complementarity in one of the famous Como lectures. The concept of complementarity was not precisely descriptive in Bohr’s work, but whenever he elaborated the concept of complementarity it was fitting the explanation profoundly. Neils Bohr recognized the need for the mathematical formalism of quantum mechanics to be constructed in a rationally coherent conceptual framework if it were to serve as the core of an acceptable scientific theory.
Explain The Principle of Complementarity
As we already know, classical physics postulates that, at each instant of time, every elementary particle is located at some definite point or the position in space, and has a definite velocity, and hence corresponding definite momentum. On the other hand, in quantum physics, an elementary particle is represented by various distributions of possibilities, where the distributions in position and in momentum are related by Fourier transformation. This consequence explains that localization at a point in position space demands a complete lack of localization in momentum space and vice versa.
Because of these contradictory theories regarding quantum motion Bohr came up with the complementarity principle. He explains that the very nature of quantum theory eventually forces us to regard the claim of space-time coordination and the claim of causality, the union of which characterizes the classical theories, as complementary but exclusive features of the description, symbolizing the idealization of observation and definition respectively.
Bohr further explains that the theories of quantum mechanics are characterized by the acknowledgement of a fundamental limitation in the classical physical ideas when applied to atomic phenomena. The essence of atomic physics may be expressed in the so-called quantum postulate, which attributes to any atomic process an essential discontinuity, or rather individuality, completely new to classical theories and symbolized by Planck’s quantum of action. 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 so far arbitrary as it depends upon which objects are included in the system to be observed. Ultimately, every observation can obviously be reduced to our sense perceptions.
Bohr's Complementarity Principle
Now let us explain the principle of complementarity or Bohr’s complementarity principle. We know that the consequence of the uncertainty principle is both the wave and particle nature of the matter can not be measured simultaneously. In other words, we can not precisely describe the dual nature of light. Now suppose that an experiment is constructed in such a way that it is designed to measure the particle nature of the matter. This implies that, during this experiment, errors of measurement of both position and the time coordinates must be zero or absent, this in turns explains that the momentum, energy and the wave nature of the matter are completely unknown. Similarly, if an experiment is designed for measuring the wave nature of the particle, then the errors in the measurement of the energy and the momentum will be zero, whereas the position and the time coordinates of the matter will be completely unknown.
From the above explanation, we can conclude that, when the particle nature of the matter is measured or displayed, the wave nature of the matter is necessarily suppressed and vice versa. The inability to observe the wave nature and the particle nature of the matter simultaneously is known as the complementarity principle. It was first explained by Niels Bohr in the year 1928 and hence it is familiarly known as the Bohr’s Complementarity principle.
What Bohr explained or Bohr exact words were “In a situation where the wave aspect of a system is revealed, its particle aspect is concealed; and, in a situation where the particle aspect is revealed, its wave aspect is concealed. Revealing both simultaneously is impossible; the wave and particle aspects are complementary.”
Compactly stated, the essential idea here is that in theories of quantum physics the information provided by different experimental procedures that in principle cannot, because of the physical characteristics of the needed apparatus, be performed simultaneously, cannot be represented by any mathematically allowed quantum state of the system being examined. The elements of information obtainable from incompatible measurements are said to be complementary: taken together exhaust the information obtainable about the state. On the other hand, any preparation protocol that is maximally complete, in the sense that all the procedures are mutually compatible and are such that no further procedure can add any more information, can be represented by a quantum state, and that state represents in a mathematical form all the conceivable knowledge about the object that experiments can reveal to us.
Did You Know?
The introduction of quantum mechanics was one of the most controversial scenarios in physics history as it was about to violate many classical aspects. The correspondence principle is one such discovery. It was probably Einstein's new derivation of Planck's black-body radiation law (1916-17) that most directly inspired Bohr's formulation of the Correspondence Principle around 1918, which thereafter played such a large role in his attempts to understand quantum phenomena. Bohr's reliance on the correspondence principle seems to have been a principal motive for his distrust of the photon concept and related willingness to give up energy-momentum conservation to save the classical wave picture of electromagnetic radiation.
FAQs on Complementarity Principle
1. What is the complementarity principle in physics?
The complementarity principle, developed by Niels Bohr, states that a quantum object, like an electron, has pairs of properties that are mutually exclusive. This means you can't observe or measure both properties at the same time. The most common example is wave-particle duality, where an object can act as a wave or a particle, but never both simultaneously in the same experiment.
2. Who first proposed the complementarity principle and why?
The Danish physicist Niels Bohr introduced the complementarity principle in 1927. He proposed it to resolve the paradox of wave-particle duality. Instead of seeing the wave and particle models as contradictory, Bohr suggested they were two complementary aspects needed for a full description of quantum phenomena.
3. How does the double-slit experiment demonstrate complementarity?
The double-slit experiment is a classic example of complementarity in action.
- When you simply observe where electrons land on a screen after passing through two slits, they form an interference pattern, proving their wave nature.
- However, if you add a detector to see which slit each electron passes through, the interference pattern vanishes. The electrons then behave like individual particles, revealing their particle nature.
You can set up the experiment to observe one nature or the other, but you can never observe both at once.
4. What is the difference between the complementarity principle and Heisenberg's uncertainty principle?
While related, these two principles focus on different ideas.
- The complementarity principle is a broader, more philosophical concept about properties being mutually exclusive (e.g., you can see wave nature OR particle nature, not both).
- The Heisenberg uncertainty principle is a precise, mathematical statement. It says there is a fundamental limit to the accuracy with which you can simultaneously measure pairs of properties, such as a particle's position and its momentum.
In short, complementarity is about 'either/or' observation, while uncertainty is about the limits of precision in measurement.
5. Why is the idea of complementarity so important in quantum mechanics?
The complementarity principle is important because it provides a logical framework for understanding the non-classical behaviour of matter at the quantum level. It tells us that our everyday intuition doesn't apply and that we must accept that objects can have different, seemingly contradictory properties that are revealed only under different experimental conditions. It is a core idea for interpreting the results of quantum experiments.
6. Besides wave-particle duality, are there other examples of complementary properties?
Yes, wave-particle duality is just one example. Another key pair of complementary properties in quantum mechanics is the position and momentum of a particle. The more precisely you measure a particle's position, the less precisely you can know its momentum, and vice versa. This relationship is described mathematically by the Heisenberg uncertainty principle.
