The structure of the photon (here meaning the fact that it can fluctuate into an electron positron pair) is what leads to the speed of light being c and not something even larger. So your intuition is somewhat correct, except that the effect is "already taken into account" when we first learned what the speed of light was. Also, a photon can do this fluctuating even if it doesnt have a lot of energy, since the electron positron pair can be virtual - i.e. they have almost no mass and live only for an instant before annihilating back into a photon
Not a stupid question. However, I think that this process (whether it is included in calculations or not) shouldn't contribute to the dark matter issue. Electrons and positrons are "normal" matter and if they are anywhere near e.g. a star, they will contribute to the plasma around the star and therefore be visible. Also, even if it were not visible, my intuition is that the process is too rare to produce enough mass to account for the dark matter
Author here, thanks! I am not familiar with this website (science-news.co) so I cannot comment on the quality of the writing (or human origin). I agree with you that "prove" should be "predict" - they did not prove anything except that from a theory standpoint, this process can happen in quantum mechanics.
The sentence about lasers is also strange, I have no idea what is meant there. My only guess is that it might be trying to describe some laser experiment that uses lasers to "heat" a hohlraum to produce a field of photons. Then some other high energy photon beam is used to collide with the photons inside the hohlraum.
In classical electromagnetism photons (light) do not interact, period. But in quantum mechanics the photons can briefly fluctuate into electron + positron pairs which allow them to interact with each other. But the probability for interacting is very small AND you need enough energy to make the electron positron pair (for the Breit-Wheeler process), so it is very rare in practice.
I am not sure about the "time" the photons collide, but the interesting thing is that the Breit-Wheeler process is what determines the opacity of the universe - since high energy photons traveling through the universe can hit low energy photons from the cosmic microwave background and convert (disappearing) into an electron positron pair.
One of the authors here. You are correct, the famous SLAC result already demonstrated that light can be converted into matter, but they needed several photons >4, average of about 6 - that is the so called non-linear Breit-Wheeler (BW) process. This result is the original linear BW process where only 2 photons collide. The main difference is that for the linear BW you need much higher energy photons. Also, in this experiment we are able to measure the wavefunction of the two-photon system, which proves other aspects of the original predictions by Breit & Wheeler and others.
The Breit-Wheeler process (convert two photons into electron+positron) has been one of the most elusive fundamental processes in electromagnetism. This prediction came when quantum mechanics was first being developed in the early 1930s but was not observed for more than 80 years after its prediction, since high energy photons are hard to come by. In contrast, many of the other fundamental processes were discovered fairly quickly after their predictions (the photo electric effect was discovered even before its quantum interpretation was given by Einstein, for which he won the nobel prize)