# L6.5 Semiclassical approximation and local de Broglie wavelength | Summary and Q&A

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February 14, 2019
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MIT OpenCourseWare
L6.5 Semiclassical approximation and local de Broglie wavelength

## TL;DR

The semiclassical approximation, also known as the WKB approximation, relates classical physics to quantum wave functions by considering slowly-varying spatial coefficients. It involves the concept of the Broglie wavelength and requires the understanding of the local momentum and wave number.

## Key Insights

• 🦾 The semiclassical approximation, also known as the WKB approximation, is a method for solving differential equations with slowly-varying spatial coefficients in quantum mechanics.
• 🦾 The concept of the Broglie wavelength is important in determining when quantum mechanical effects become significant compared to classical behavior.
• 👋 The local momentum and wave number in the semiclassical approximation provide information about the behavior of the wave function at different positions.
• 👋 The phase factor in the wave function is related to the probability current and can be used to understand the behavior of the wave function in terms of surfaces of constant phase.
• 🦾 The semiclassical approximation requires the understanding of concepts from classical mechanics and introduces complexities due to the presence of units.

## Transcript

PROFESSOR: So, WKB approximation, or semiclassical approximation. So this is work due to three people-- Wentzel, Kramers, and Brillouin-- in that incredible year, 1926, where so much of quantum mechanics was figured out. As it turns with many of these discoveries, once the discoveries were made, people figured out that somebody did them before. And... Read More

## Questions & Answers

### Q: What is the semiclassical approximation in quantum mechanics?

The semiclassical approximation, or the WKB approximation, is a method used to solve differential equations with slowly-varying spatial coefficients in quantum mechanics. It relates classical physics to quantum wave functions.

### Q: What is the Broglie wavelength and its significance in the semiclassical approximation?

The Broglie wavelength is the wavelength associated with a particle in quantum mechanics. In the semiclassical approximation, it determines whether quantum mechanical effects are important based on its size relative to the physically relevant sizes of the problem. When the Broglie wavelength is much smaller, the particle behaves more like a classical particle.

### Q: How is the local momentum defined in the semiclassical approximation?

The local momentum is defined as the momentum a particle would have when it is at a specific position in space. In the semiclassical approximation, it is related to the gradient of the phase of the wave function and is represented by the symbol p(x).

### Q: What is the significance of the phase factor in the wave function in the semiclassical approximation?

The phase factor in the wave function, represented by the symbol S, determines the probability current in the semiclassical approximation. The current is proportional to the gradient of the phase, and the current density is orthogonal to the surfaces of constant phase.

## Summary & Key Takeaways

• The video discusses the semiclassical approximation, also known as the WKB approximation, in quantum mechanics.

• The approximation deals with differential equations with slowly-varying spatial coefficients.

• The concept of the Broglie wavelength and its relationship to the physically relevant sizes of a problem is explored.