Difference Between Accidental Degeneracy And Normal Degeneracy

Degeneracy in quantum mechanics refers to a peculiar phenomenon where different quantum states share the same energy level under a given Hamiltonian. This concept, central to understanding the complexities of atomic and molecular physics, branches into two distinct types: accidental degeneracy and normal degeneracy. Each type plays a critical role in the study and applications of quantum mechanics, affecting everything from the atomic spectra we observe to the stability of quantum systems.

Accidental degeneracy occurs without symmetry-based reasons, often resulting from specific mathematical properties of the system, while normal degeneracy is directly tied to the symmetries in the physical system. For example, in an atom, electrons may occupy the same energy level because their states are symmetrically similar, showcasing normal degeneracy. Conversely, accidental degeneracy might arise in less straightforward scenarios, influenced by the unique characteristics of the quantum system rather than by symmetry.

Understanding these two forms of degeneracy enhances our grasp of quantum behavior, influencing how we interpret experimental data and theoretical models. The distinctions between them are subtle yet profound, providing key insights into the foundational principles governing the quantum world.


What is Degeneracy?

In the realm of quantum mechanics, degeneracy refers to a condition where two or more quantum states of a system have the same energy level. This phenomenon is pivotal for interpreting various physical behaviors and properties, particularly in atomic and molecular physics. Quantum states, though potentially different in their physical or quantum numbers, can end up having equal energy levels, thereby sharing degeneracy.

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Defining Accidental Degeneracy

Accidental degeneracy occurs when quantum states have identical energy levels without symmetry reasons or apparent connection through physical properties. This form of degeneracy is not predicted by standard symmetry arguments and is often considered a mathematical curiosity within quantum mechanics. It arises under specific, often unique, conditions of the quantum system’s external or internal parameters.

Defining Normal Degeneracy

Normal degeneracy, on the other hand, is influenced directly by the symmetries inherent to the system. For example, in an atom, the spatial symmetry can lead to several states having the same energy due to these symmetrical properties. Normal degeneracy is predictable based on the symmetrical attributes of the Hamiltonian governing the system.

Causes and Context

Causes of Normal Degeneracy

Normal degeneracy typically stems from the symmetrical nature of a physical system. In quantum systems:

  • Spatial symmetry: Identical energy levels arise from symmetrical electron configurations in atoms or molecules.
  • Time-reversal symmetry: Situations where the physical properties of states are the same even when the direction of time is reversed.
  • Interaction symmetries: Certain interactions within the system that maintain symmetry can also lead to degenerate states.

Causes of Accidental Degeneracy

Accidental degeneracy is influenced by less predictable factors and often involves:

  • Mathematical peculiarities: Unique solutions to the system’s wave equations that result in the same energy for different states.
  • Parameter specifics: Specific values of parameters within the quantum system that coincidentally result in degeneracy without symmetry considerations.


Normal Degeneracy in Atomic Orbitals

In atomic physics, normal degeneracy is often illustrated through the electron configurations in atoms. For instance:

  • Hydrogen atom: All orbitals with the same principal quantum number 𝑛n have the same energy, regardless of the orbital angular momentum quantum number 𝑙l.
  • Carbon atom: Electrons in the 2p orbital are threefold degenerate due to their equal energy in a symmetrical field.

Accidental Degeneracy in Molecular Structures

Accidental degeneracy is less common but can be intriguing when it occurs. For example:

  • Benzene molecule: Certain vibrational modes in benzene exhibit degeneracy not directly attributable to the molecular symmetry.
  • Complex organic molecules: Occasionally, accidental degeneracy appears in molecular orbital energies, resulting from specific alignments and bonding scenarios.

Implications in Physics

Impact on Quantum Mechanics

The concept of degeneracy is crucial in quantum mechanics as it influences both the theoretical framework and experimental analysis of quantum systems. Degeneracy helps explain phenomena such as:

  • Quantum superposition: Degenerate states can superpose, resulting in observable physical states that are combinations of the basic states.
  • Energy stability: Degeneracy can indicate a higher stability in certain energy states, as systems tend to occupy these degenerate states more frequently.
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Understanding these aspects is essential for advancing quantum computing and precision measurement technologies.

Relevance in Spectroscopy

In spectroscopy, degeneracy plays a key role in determining the intensity and pattern of spectral lines. The degeneracy of energy levels affects the transition probabilities between states, which in turn influences the spectra produced by substances under study. This makes degeneracy a fundamental concept in:

  • Atomic absorption and emission spectroscopy: Helps predict and explain the lines seen in atomic spectra.
  • Molecular vibrational spectroscopy: Understanding the degeneracy of vibrational modes aids in interpreting infrared and Raman spectra.

Mathematical Framework

Mathematical Representation of Degeneracy

The mathematical treatment of degeneracy involves solving the Schrodinger equation, where degenerate states are solutions that have the same energy eigenvalue. The Hamiltonian operator, which represents the total energy of the system, plays a central role in these calculations. Key considerations include:

  • Eigenvalues and eigenvectors: Degenerate states are characterized by having identical eigenvalues but different eigenvectors in the solution set of the Hamiltonian.
  • Symmetry operations: These are used to simplify the Hamiltonian and reveal underlying degeneracies.

Solving for Degenerate States

To solve for degenerate states, physicists use techniques such as:

  • Perturbation theory: This method adjusts the Hamiltonian slightly to lift the degeneracy, allowing for the detailed study of closely related energy states.
  • Group theory: A powerful mathematical tool used to analyze the symmetries of a system and predict the occurrence of degeneracy.

Comparison and Contrast

Key Differences Summarized

While both types of degeneracy, accidental and normal, result in states having the same energy, their causes and implications vary significantly:

  • Origin: Normal degeneracy arises from symmetries, while accidental degeneracy does not.
  • Predictability: Normal degeneracy is predictable based on symmetry considerations; accidental degeneracy is often unexpected.
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Similarities in Physical Systems

Despite their differences, both types of degeneracy share common features:

  • Energy equivalence: Both lead to situations where different states share the same energy.
  • Quantum mechanical implications: Both influence the quantum behavior of systems in similar ways, such as affecting the density of states at particular energy levels.

Case Studies

Historical Examples of Accidental Degeneracy

One notable historical example involves the hydrogen atom, where the accidental degeneracy of the energy levels was a critical aspect in the development of quantum mechanics. Despite lacking the complex symmetries of larger atoms, the hydrogen atom exhibits a high degree of degeneracy in its excited states.

Notable Cases of Normal Degeneracy

Normal degeneracy is often observed in crystalline solids where the symmetrical arrangement of atoms leads to degenerate electronic bands. This is crucial for understanding the electronic properties of materials and is extensively studied in the field of solid-state physics.


What is Quantum Degeneracy?

Quantum degeneracy refers to a condition where two or more quantum states of a system have the same energy. This can occur under specific conditions, reflecting the fundamental symmetries and mathematical properties of the system.

How does Accidental Degeneracy occur?

Accidental degeneracy occurs when two or more states possess the same energy without any apparent symmetry reasons. This type of degeneracy is often a result of unique conditions or mathematical coincidences within the quantum system.

What causes Normal Degeneracy?

Normal degeneracy is typically caused by inherent symmetries within the quantum system. For example, in atomic physics, electrons in symmetric orbital configurations might share the same energy levels, reflecting this type of degeneracy.

How do Accidental and Normal Degeneracy impact Quantum Mechanics?

Both types of degeneracy have significant implications in quantum mechanics, influencing theoretical predictions and experimental observations. They affect the spectral lines observed in atomic spectra and are essential in understanding the stability and behavior of quantum systems.


The concepts of accidental and normal degeneracy provide profound insights into the quantum mechanical behavior of physical systems. These phenomena not only deepen our understanding of the atomic and molecular structures but also challenge and refine theoretical frameworks in physics. By studying the differences and applications of each, researchers continue to uncover the nuanced behaviors of quantum systems, advancing both theoretical knowledge and practical applications.

In conclusion, the exploration of degeneracy in quantum mechanics highlights the intricate balance of symmetry, energy, and mathematical properties that govern the quantum world. As research progresses, the insights gained from understanding these degeneracies will undoubtedly contribute to the advancements in technology and science, pushing the boundaries of what is currently known.

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