Non-constant wavelength of particles Is it possible for a particle's wavelength to change with respect to time? I'm thinking of a massless particle like a photon, but as it evolves through time its wavelength changes. I'm aware photons wavelengths do change from the expansion of spacetime, but I'm thinking on a smaller scale.
EDIT
Is it possible to have a particle that has its wave length change in time? So imagine a particle that is created with a wave length of say 500nm then it decays to infinity or something as it evolves through time.
 A: The wavelength of a photon does not mean a single  photon is spread in space. At our level of experiments and theories photons and the other elementary particles are point particles at (x,y,z) at time t, described by four vectors, whose "length" gives the invariant mass of the particle.


In the case of photons, the invariant mass is zero, but energy and momentum are there, and the energy is given by $hν$  where $ν$ is the frequency of light that can be produced by accumulating photons of such energy, see this simple experiment to get some intuition on how photons behave.
Now four vectors transform from kinematic frame to kinematic frame according to the Lorentz transformations, and depending on the frame the energy of the photon can be larger or smaller.
So the answer  is that the energy of the photons changes according to the kinematic  frame it is observed  in, and thus the wavelength of the possible light beam constructed of such energy photons would change .
A: The photon wavelength may vary for a photon propagating in a non-uniform medium - with variable $n(\vec{r})$, the frequency being constant.
