# In QM why can wavefunctions be written as $\psi (x,t)= Ae^{i(kx-\omega t+\phi)}$?

In my textbook, a wave traveling in the positive $x$-direction can be described using $$\psi(x,t)=Ae^{i(kx-\omega t+\phi)}.$$

I understand that the equation for a classical wave can be extrapolated by solving the differential equation associated with simple harmonic motion, giving us $\psi(x,t)=A\cos(kx-\omega t +\phi)$. I know Euler's identity is $e^{ix}=\cos(x)+i\sin(x)$. It would seem that we could simply equate $\psi(x,t)=A\cos(kx-\omega t +\phi)$ with $\Re (Ae^{i(kx-\omega t+\phi)})$ by omitting the $i\sin(kx-\omega t +\phi)$ term. My book seems to equate $\psi(x,t)=A\cos(kx-\omega t +\phi)$ to the entire complex version of the wavefunction $\psi (x,t) = Ae^{i(kx-\omega t+\phi)}$ with the stipulation that you can simply extract the real portion of the function when needed. It seems like these two functions should not be able to be equated for reasons described above. Is this a typo or am I not understanding this correctly?

Also, why in QM do we need to include the imaginary terms associated with the $i\sin(kx-\omega t +\phi)$ to get an accurate picture of the wave? Can the above relation only hold because it is derived from a wave associated with simple harmonic motion with classical not quantum waves?

• The imaginary part is needed to get the probability distribution since $\mathbb{P}[x]=\psi(x)\psi^*(x)$. With regards to equating $e^{i\theta}$ to $\cos(\theta)$ some authors just assume that it is understood that they are always taking $\Re (e^{i\theta})$. May 28, 2018 at 0:28

A general quantum amplitude has both a real and an imaginary part. Without the imaginary part, there can be no interference between orthogonal states. The cosine function only gives a real number, so it's not possible to represent a general quantum state with only a real number. Another way to put this is that a complex number can be represented by a 2-component vector, while a real number is represented by a single component vector. The full quantum mechanical description requires a 2 component object.

• There can never be interference between orthogonal states. This does not explain much, it just restates the facts. Aug 20, 2019 at 19:06
• Just restating what @my2cts said just to emphasize that there can be no interference between orthogonal states, period. That is to say with or without the imaginary part in the amplitude. Feb 1, 2022 at 21:41
• I really don't understand what you guys mean, consider a particle with definite spin x+, $\psi=|x+\rangle \propto |z+\rangle + |z-\rangle$. If you apply an operator which only acts on one of the $z$ states, e.g. split with stern-gerlach apply magnetic field to one leg and recombine, you'll start rotating $|x+\rangle$ into $|x-\rangle$. In other words, the two orthogonal z states cause an oscillation in measurements of orthogonal states of $x$. This is also discussed in this interferometer writeup Feb 3, 2022 at 9:15

$\psi (x,t) = Ae^{i(kx-\omega t+\phi)}$ is a solution of the free particle Schrödinger equation, but $\psi(x,t)=Acos(kx-\omega t +\phi)$ is not.

• How can the cosine equation satisfy the wave equation $\frac {\partial^2 \psi(x,t)}{ \partial x^2}$ - $\frac {1}{v^2} \frac {\partial^2 \psi (x,t)}{\partial t^2}=0$ but not the Schrodinger equation if you get the Schrodinger equation using the wave equation with substitutions of the de Broglie equations?
– Joe
May 28, 2018 at 0:42

As stated in the answer by @my2cts, $$\psi(x,t)=Ae^{i(kx-\omega t+\phi)}$$ is the solution of the free particle Schrödinger equation, but $$\psi(x,t)=A\cos(kx-\omega t+\phi)$$ is not, even if it is a solution to wave equation $$\frac{\partial^{2}\psi(x,t)}{\partial x^{2}}-\frac{1}{v^{2}}\frac{\partial^{2}\psi(x,t)}{\partial t^{2}}=0$$ because Schrödinger equation has the form $$\frac{\partial^{2}\psi(x,t)}{\partial x^{2}}\propto\frac{\partial\psi(x,t)}{\partial t}$$. Therefore, the solution to Schrödinger equation is a linear combination of sines and cosines (as you can show), which is represented as the imaginary exponential function by Euler's identity.