oh, and we call it "Nuu", a few story-writers say that

at the beginning ... Hell was in there

for the science that place is also known as Plank Era,

a way to describe 10^−43 seconds after the singularity

(Big Bang) a way to say that our physics laws are not applicable there

therefore the science is more interested in what happens to particles a time-quantium after

say n*10^−44 seconds after the singularity, n={1,2,3,.....}

and with n>0 we have time, we have space, we have sub particles, we have our physics laws ...

and we can represent a quantum of electromagnetic radiation or sub particles representing a quantum of interaction in between, we don't actually know *what happens* with graviton, a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory, but we are happy to know what can happen when two photons collide, and more happy to know that a photon can carry energy proportional to the radiation frequency, and happy to know that it can have zero rest mass, and that it can't go faster than the light speed

and we are so *happy* because, even if we are still don't know anything, we can be sure that all the interactions it can have are updated every 10^-44 second in time and 1,616 × 10^−35 meters in space

we are so happy with photons that our theories have options to predict what happens when two photons collide

- option a) like electromagnetic waves, they will interfere with each other and keep their wave nature
- option b) like particles, they will bounce like classical balls

but we have also a new theory called "Quantum electrodynamics" which tells us that two photons cannot couple directly, that leaves us with classical electromagnetism, which tells us that electromagnetic waves pass through each other without any interference.

therefore we also have

- option c) two photons interact through higher-order processes -> there is a quite small probability amplitude for two photons to get absorbed in, and two photons be emitted by, e.g. an fermion-anti-fermion virtual pair, or the QED(1) for the process(photon + photon) = electron + positron

(1) in order to respect the energy conservation, this process is only possible if the sum of the energy of the photons is above twice the electron mass, therefore in the center of mass frame of the di-photon system, the photons need to have at least ~511KeV.