system
Given the following post, respond as if you're commenting on Reddit:
human
When a photon is created, does it accelerate to c or does it instantly reach it?

Sorry if my question is really stupid or obvious, but I'm not a physicist, just a high-school student with an interest in physics. And if possible, try answering without using too many advanced terms. Thanks for your time!
gpt
This is a cool question with a complicated answer, simply because there is no framework in which you can actually sit down and calculate an answer for this question.

The reason why know that photons travel at "c" is because they are massless. Well, but a photon is not really a particle in the classical sense, like a billiard ball. A photon is actually a quantized excitation of the electromagnetic field: it's like a ripple that propagates in the EM field.

When we say that a field excitation is massless, it means that if you remove all the interactions, the propagation is described by a wave equation in which the flux is conserved - this is something that you don't understand now but you will once you learn further mathematics. And once the field excitation obeys this wave equation, you can immediately derive the speed of propagation - which in this case is "c".

If you add a mass, then the speed of propagation chances with the energy that you put in. But what happens if you add interactions? 

The answer is this: classically, you could in principle try to compute it, and for sure the interaction would change the speed of propagation. But quantum mechanically, it's impossible to say exactly what happens "during" an interaction, since the framework we have for calculating processes can only give us "perturbative" answers, i.e.: you start with states that are non-interacting, and you treat interactions as a perturbation on top of these. And all the answers we get are those relating the 'in' with the 'out' states, they never tell us anything about the intermediate states of the theory - when the interaction is switched on.