# Can a photon be split? [duplicate]

1. A photon is pure energy, zero mass.
2. 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.

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.

Yes, you are right. Spontaneous parametric downconversion is one way that a high - energy photon can be converted into two low-energy photons.

• I think we should stress interaction. OP seems to ask about a particle like decay – Alchimista Oct 29 '19 at 9:32

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.

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.

• Anna, I’m now questioning my own answer. Can’t you satisfy energy-momentum conservation by having a photon split, say, into two, each with half the energy and half the momentum? If so, there is another reason why this doesn’t happen, such as angular momentum conservation. – G. Smith Oct 29 '19 at 5:55
• @G.Smith if a lone photon would split that will be some perpendicular momentum, an angle, between the two photons. That would immediately give an invariant mass to the sum of the four vectors and therefore a rest system for the photon which is not allowed as it goes with velocity c. – anna v Oct 29 '19 at 5:59
• Collinear splitting again would violate c – anna v Oct 29 '19 at 6:02
• I guess you could satisfy the conservation laws if they split such that $p=p_1+p_2$ but they are both still going the same direction ($\hat{\bf p}_1=\hat{\bf p}_2$). But the two photons would have zero relative velocity in every reference frame, so it's not much of a split... The two photons would have different frequencies though, so it might be observable as decoherence? – Mario Carneiro Oct 29 '19 at 13:03