A few questions about photons

Question

I have a few questions about photons.

I know the emitting and absorbing of photons is related to quantum leaps. Does a quantum leap cause emitting/absorbing, or is it the other way around?
Do photons have a mass?
Are photons a unit of light energy, or do they just carry the energy?
Can photons be created or destroyed?

Questions 2 & 3 are a duplicate of this question. For Question 4, this question might be relevant. — Kyle Kanos, Sep 30 '14 at 00:57
You might also want to be careful using the phrase "Quantum leap", as it might have unintended alternative definitions ;) — Kyle Kanos, Sep 30 '14 at 00:58

score 1 · Accepted Answer · answered Sep 30 '14 at 01:50

Lets attempt some answers:

Both can happen, a quantum transtion can be associated with a photon exchange or a photon exchange can be associated with a quantum transition (this is just 2 ways to state the conservation of energy in these cases)
Photons do not have mass but they do have momentum. There are some approaches in physics which associate a virtual (relativistic) mass to the photon in order to formulate and solve some problems, however this approach is (strongly) debated
Photons are types of particles called bosons which represent the quantum of forces (in this case electromagnetic forces) as such they are carriers of energy and they have energy as well (it amounts to the same thing see point 1. above)
Yes photons can be created and destroyed, more correctly photons like other particles can be transformed into other particles (like a pair of electron-positron and vice-versa). One could say that photons were created (or destroyed) in these processes

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