Finding the probability density function given the eigenfunction

In summary, the problem is to find the probability density function given the eigenfunction, which is given by \psi=C\exp^({\frac{ipx}{\hbar}-\frac{x^2}{2a^2}}). The attempt at a solution involved squaring the function to get the probability density, but this resulted in a difficult integral. After trying to factorize the index and using change of variables, the student was unsure of what to do. However, they were able to make progress and found that the imaginary part vanishes when the function is squared. They then used this to simplify the function and integrate it from -\infty to \infty in order to find the normalisation constant. The final
  • #1
ypal
3
0

Homework Statement



I need to find the probability density function given the eigenfunction

Homework Equations



[itex]\psi=C\exp^({\frac{ipx}{\hbar}-\frac{x^2}{2a^2}})[/itex]

The Attempt at a Solution



I tried to square the function but that gave me a nasty integral that I could not solve. I also tried to factorise the index so I could've used change of variables for the Gaussian integral. I haven't come across such a question before so I am not quite sure what to do. Please help me.
Cheers
 
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  • #2
The probability density is given by
[tex]P(x)=|\psi(x)|^2.[/tex]
Here [itex]\psi[/itex] is the wave function of the particle. Calculate this square, and you'll find a pretty well-known probability distribution!
 
  • #3
I've realized my mistake...face palm -_- Thanks! But I can't make progress at this point.
Edit: I need the integral to find the normalisation constant, not for the actual density function. The thing that worries me is the x^2 and x term being together at the index.
 
Last edited:
  • #4
ypal said:
I need the integral to find the normalisation constant, not for the actual density function. The thing that worries me is the x^2 and x term being together at the index.
Show details of your attempt to perform the integral.
 
  • #5
ypal said:

Homework Statement



I need to find the probability density function given the eigenfunction

Homework Equations



[itex]\psi=C\exp^({\frac{ipx}{\hbar}-\frac{x^2}{2a^2}})[/itex]

The Attempt at a Solution



I tried to square the function but that gave me a nasty integral that I could not solve. I also tried to factorise the index so I could've used change of variables for the Gaussian integral. I haven't come across such a question before so I am not quite sure what to do. Please help me.
Cheers

Hey Guys!
I've made some progress and everything looks neat. It turns out the imaginary part vanishes when the function is squared(due to its conjugate). If someone could verify my work I'd appreciate it. Thanks!
[itex]\psi=C\exp^({\frac{ipx}{\hbar}-\frac{x^2}{2a^2}})[/itex]
[itex] |\psi(x)|^2=C^2 exp^{\frac{ipx}{\hbar}-\frac{x^2}{2a^2}} . exp^{\frac{-ipx}{\hbar}-\frac{x^2}{2a^2}}[/itex]
[itex] |\psi(x)|^2=C^2 exp^{-\frac{x^2}{a^2}}[/itex]
Let [itex]\beta=\frac{1}{a^2}[/itex], [itex] |\psi(x)|^2=C^2 exp^{-\beta x^2}[/itex]

Now I need to integrate this from [itex]-\infty[/itex] to [itex]\infty[/itex] and equate to 1 in order to find the normalisation constant.
[itex] C^{2}\int_{-\infty}^{\infty} exp^{-\beta x^2}=1[/itex]
[itex] C^{2}\sqrt{\frac{\pi}{\beta}}=1[/itex]
[itex] C^{2}=\sqrt{\frac{\beta}{\pi}}→ C=\frac{1}{\sqrt{a}\pi^{\frac{1}{4}}}[/itex]

I am new to Latex...so I'm trying my best :D
 

1. What is a probability density function (PDF)?

A probability density function (PDF) is a mathematical function that describes the probability distribution of a continuous random variable. It represents the relative likelihood of different values occurring within the given range of the variable.

2. What is an eigenfunction?

An eigenfunction is a special type of function that returns a scalar multiple of itself when acted upon by a linear operator. In the context of probability density functions, eigenfunctions represent the different possible states or outcomes of a system that can be measured.

3. How do you find the probability density function given the eigenfunction?

To find the probability density function given an eigenfunction, you need to solve for the coefficients or weights associated with each eigenfunction using the normalization condition. These coefficients represent the relative contributions of each eigenfunction to the overall probability distribution.

4. What is the normalization condition?

The normalization condition is a mathematical requirement that ensures the total probability of all possible outcomes of a system adds up to 1. In the context of probability density functions, it means that the integral of the PDF over the entire range of the variable must equal 1.

5. Can you give an example of finding the probability density function given the eigenfunction?

Yes, for example, if we have an eigenfunction represented by f(x) and we know its corresponding coefficient is c, the probability density function can be calculated as P(x) = c*f(x). The coefficient c can be determined by solving the normalization condition, which in this case would be c = 1/√(∫f(x)^2dx).

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