Axiomatic Approach

Although the two Schrödinger equations form an important part of quantum mechanics, it is possible to present the subject in a more general way. Dirac gave an elegant exposition of an axiomatic approach based on observables and states in a classic textbook entitled The Principles of Quantum Mechanics. (The book, published in 1930, is still in print.) An observable is anything that can be measured—energy, position, a component of angular momentum, and so forth. Every observable has a set of states, each state being represented by an algebraic function. With each state is associated a number that gives the result of a measurement of the observable. Consider an observable with N states, denoted by ψ1, ψ2, . . ., ψN, and corresponding measurement values a1, a2, . . ., aN. A physical system—e.g., an atom in a particular state—is represented by a wave function Ψ, which can be expressed as a linear combination, or mixture, of the states of the observable. Thus, the Ψ may be written as
Ψ = c1 Ψ1 + c2 Ψ2 +... + cN ΨN.  (10)
For a given Ψ, the quantities c1, c2, etc., are a set of numbers that can be calculated. In general, the numbers are complex, but, in the present discussion, they are assumed to be real numbers.
The theory postulates, first, that the result of a measurement must be an a-value—i.e., a1, a2, or a3, etc. No other value is possible. Second, before the measurement is made, the probability of obtaining the value ais c12, and that of obtaining the value a2 is c22, and so on. If the value obtained is, say, a5, the theory asserts that after the measurement the state of the system is no longer the original Ψ but has changed to ψ5, the state corresponding to a5.
A number of consequences follow from these assertions. First, the result of a measurement cannot be predicted with certainty. Only the probability of a particular result can be predicted, even though the initial state (represented by the function Ψ) is known exactly. Second, identical measurements made on a large number of identical systems, all in the identical state Ψ, will produce different values for the measurements. This is, of course, quite contrary to classical physics and common sense, which say that the same measurement on the same object in the same state must produce the same result. Moreover, according to the theory, not only does the act of measurement change the state of the system, but it does so in an indeterminate way. Sometimes it changes the state to ψ1, sometimes to ψ2, and so forth.
There is an important exception to the above statements. Suppose that, before the measurement is made, the state Ψ happens to be one of the ψs—say, Ψ = ψ3. Then c3 = 1 and all the other cs are zero. This means that, before the measurement is made, the probability of obtaining the value a3 is unity and the probability of obtaining any other value of a is zero. In other words, in this particular case, the result of the measurement can be predicted with certainty. Moreover, after the measurement is made, the state will be ψ3, the same as it was before. Thus, in this particular case, measurement does not disturb the system. Whatever the initial state of the system, two measurements made in rapid succession (so that the change in the wave function given by the time-dependent Schrödinger equation is negligible) produce the same result.
The value of one observable can be determined by a single measurement. The value of two observables for a given system may be known at the same time, provided that the two observables have the same set of state functions ψ1, ψ2, . . ., ψN. In this case, measuring the first observable results in a state function that is one of the ψs. Because this is also a state function of the second observable, the result of measuring the latter can be predicted with certainty. Thus the values of both observables are known. (Although the ψs are the same for the two observables, the two sets of a values are, in general, different.) The two observables can be measured repeatedly in any sequence. After the first measurement, none of the measurements disturbs the system, and a unique pair of values for the two observables is obtained.

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## FAQs on Axiomatic Approach - Modern Physics for IIT JAM

 1. What is the axiomatic approach in physics?
Ans. The axiomatic approach in physics is a method of constructing theories based on a set of self-evident principles, called axioms. These axioms serve as the foundation for the entire theoretical framework and are used to derive various laws and equations in physics.
 2. How does the axiomatic approach differ from other approaches in physics?
Ans. The axiomatic approach in physics differs from other approaches, such as the empirical or phenomenological approach, by emphasizing the use of logical reasoning and deduction. Instead of relying solely on experimental observations, the axiomatic approach starts with a small set of fundamental axioms and derives the rest of the theory through logical deductions.
 3. What are the advantages of using the axiomatic approach in physics?
Ans. The axiomatic approach in physics offers several advantages. Firstly, it provides a rigorous and systematic framework for developing theories, ensuring logical consistency and coherence. Secondly, it allows for the derivation of new results and predictions based on the axioms, providing a deeper understanding of the underlying principles. Lastly, the axiomatic approach promotes simplicity and elegance in theories, as unnecessary assumptions can be eliminated through logical deductions.
 4. Are there any limitations to the axiomatic approach in physics?
Ans. Yes, there are limitations to the axiomatic approach in physics. One limitation is that the choice of axioms is subjective and can vary between different researchers or schools of thought. This subjectivity can lead to different theories that are equally consistent with the axioms. Additionally, the axiomatic approach may not be suitable for all areas of physics, especially those that involve complex and chaotic systems where it is difficult to establish a small set of fundamental axioms.
 5. Can the axiomatic approach be applied to all branches of physics?
Ans. While the axiomatic approach can be applied to many branches of physics, its applicability may vary depending on the nature of the phenomena being studied. The axiomatic approach is most commonly used in fundamental theories, such as classical mechanics or quantum mechanics, where a small set of axioms can capture the essence of the physical laws. However, in branches like astrophysics or complex systems, where phenomena are influenced by numerous variables and interactions, the axiomatic approach may be less effective and other approaches may be more suitable.

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