Friday, November 27, 2015

quantum mechanics - Factor 2 in Heisenberg Uncertainty Principle: Which formula is correct?


Some websites and textbooks refer to $$\Delta x \Delta p \geq \frac{\hbar}{2}$$ as the correct formula for the uncertainty principle whereas other sources use the formula $$\Delta x \Delta p \geq \hbar.$$ Question: Which one is correct and why?


The latter is used in the textbook "Physics II for Dummies" (German edition) for several examples and the author also derives that formula so I assume that this is not a typing error.


This is the mentioned derivation:




$\sin \theta = \frac{\lambda}{\Delta y}$


assuming $\theta$ is small:


$\tan \theta = \frac{\lambda}{\Delta y}$


de Broglie equation:


$\lambda = \frac{h}{p_x}$


$\Rightarrow \tan \theta \approx \frac{h}{p_x \cdot \Delta y}$


but also:


$\tan \theta = \frac{\Delta p_y}{p_x}$


equalize $\tan \theta$:


$\frac{h}{p_x \cdot \Delta y} \approx \frac{\Delta p_y}{p_x}$



$\Rightarrow \frac{h}{\Delta y} \approx \Delta p_y \Rightarrow \Delta p_y \Delta y \approx h$


$\Rightarrow \Delta p_y \Delta y \geq \frac{h}{2 \pi}$


$\Rightarrow \Delta p \Delta x \geq \frac{h}{2 \pi}$



So: Which one is correct and why?




No comments:

Post a Comment

classical mechanics - Moment of a force about a given axis (Torque) - Scalar or vectorial?

I am studying Statics and saw that: The moment of a force about a given axis (or Torque) is defined by the equation: $M_X = (\vec r \times \...