Special Relativity and Co-Moving Frames
The current version of Python incorporates special relativity and takes co-moving frame effects into account by default.
Global properties of the wind, such as a densities are defined in the observer , or global frame, but are immediately converted to co-moving frame values.
(As an example, if the density of cell and volume (or a cell) in the global frame are \(\rho_{obs}\) and \(V_{obs}\) then
the product of the two quantities, being a Lorentz invariant.)
Photon generation takes place in the local, or co-moving, frame (of the disk or wind), but photons are immediately converted to the observer, or global, frame for photon transport, allowing both for Doppler frequency shifts and directional correction due to Doppler abberation. The number of photons generated is the number of photons that would be generated in the in one observer frame second. Photons are transported in the observer frame, but coverted back to the local frame within i ndividual wind cells to determine whether the interact with the wind.
Interactions take place in the local frame. Estimators used to calculate, for example, photoionization rates also take place in the local frame. This allows one to calculate ionization fractions in the local frame as is required, since the numbers of ions in a region defined by the edges of the cell must also be Lorentz invariant. Allowances are made for time dilation effects in calculating the rates in the co-moving frame.
For mainly historical and diagnostic reasons, command line options exist to fall back to simple order v/c corrections. `