Can a photon be split? 
*

*A photon is pure energy, zero mass.

*Different photons have different wave length and carry a different amount of energy.


Now, it seems logical that a high energy photon can be split into 2 low energy photons.
I don't know if this could happen via some natural decay or under an external influence or not at all.
Please explain, thank you.
P.s. this is not a duplicate of the previous similar question about splitting a photon. The other question presents a photon as a particle, where as this one considers a photon as energy. I think this leads to different lines of thought.
 A: A free photon cannot split into two or more photons. You can’t conserve energy, momentum, and angular momentum.
By the way, saying that “a photon is pure energy” is meaningless, unless all you mean is it doesn’t have mass-energy. There is no such thing as “pure energy”; energy comes in different kinds, none of them “purer” by any sensible criterion than others. Also, a photon has more properties than just zero mass and nonzero energy, such as momentum and a particular spin value, so a photon has energy but, in terms of what it is, it’s not just energy.
A: The two answers are correct.
There is no natural decay  of the photon due to conservation of momentum and energy. If it split into two photons their added four vectors would have an invariant mass.
External influences, as the example given by S.McGrew , can give two new photons which add up to the energy of the incoming  photon, within the uncertainty principles of the case. Conservation of momentum is taken up by the whole system giving the two photons: crystal+input photon, crystal+output photons. The mass of the crystal is very much larger than the invariant mass of the two photons, similar to conservation of momentum for a ball hitting a wall.
A: Yes, you are right.  Spontaneous parametric downconversion is one way that a high - energy photon can be converted into two low-energy photons.
