Wavelength uncertainity for particle in a box

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Discussion Overview

The discussion revolves around the concept of wavelength uncertainty for a particle in a box, particularly focusing on the implications of wave functions, energy states, and the uncertainty principle in quantum mechanics. Participants explore the relationship between standing wave solutions and moving particles, as well as the implications for energy uncertainty.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that while the wave function for a particle in a box resembles a standing wave, the concept of a moving particle as a wave packet introduces uncertainty in wavelength and energy.
  • Others argue that the uncertainty principle indicates that the product of uncertainties in two observables cannot be zero, suggesting inherent energy uncertainty.
  • A participant clarifies that if a particle is in an eigenstate of the Hamiltonian, there is no uncertainty in energy, as the measurement will yield the corresponding eigenvalue with certainty.
  • Another participant notes that standing-wave solutions represent stationary states, where probability distributions do not change over time, contrasting with wave packets that exhibit uncertainty in energy, momentum, and wavelength.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between wave packets and energy uncertainty, with some asserting that eigenstates have no energy uncertainty, while others emphasize the uncertainty associated with superpositions of states.

Contextual Notes

The discussion includes assumptions about the nature of wave functions, the definitions of eigenstates, and the implications of the uncertainty principle, which remain unresolved.

Maharshi Roy
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For a particle in a box, we have been explained that the wave function is like a standing wave. Then we wrote:-
λ = 2L/n
where λ is the wavelength of the nth energy state. But a moving particle is considered as a wave packet and so does not have an unique wavelength. Then, how are we determining energy for nth state using this unique λ. There must be an uncertainity in energy too.
 
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Maharshi Roy said:
For a particle in a box, we have been explained that the wave function is like a standing wave.
More precisely, the possible wave functions are superpositions of these standing waves.

Maharshi Roy said:
There must be an uncertainity in energy too.
There is. That's why the uncertainty principle is so often written as the product of the uncertainty of two observables; the smaller the uncertainty in one the greater the uncertainty in the other, but usually neither is zero.
 
If your particle is prepared in an eigenstate of the Hamiltonian, there is no uncertainty in energy. You'll measure with 100% probability the corresponding eigenvalue of the Hamiltonian. Note that for a particle confined to a box, the energy eigenstates are true (square integrable) eigenstates. However, you should note that there is no momentum operator, and the position-momentum uncertainty relation sometimes qualitatively quoted in connection with the particle in a rigid box is questionable.
 
Maharshi Roy said:
But a moving particle is considered as a wave packet

The standing-wave solutions (energy eigenstates) of the particle in a box are not "moving particles" in the quantum-mechanical sense. Their probability distributions do not change with time. They are "stationary states."

If you construct a superposition of some number of energy eigenstates with different values of n, it will not be a stationary state. The probability distribution will "slosh" back and forth between the walls of the box. For such a state, the energy, momentum and wavelength are all uncertain.
 

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