Observer

A PyGRO object that defines a given observer in space-time. Useful to assign initial conditions to Geodesic objects.

class pygro.observer.ObserverView on GitHub

The Observer class represent PyGRO representation of physical observers in space-time. It is built by specifying either a frame or a co-frame uniquely identifying a tetrad in space-time. This can than be used to fire geodesics from the observer’s position giving a physical meaning to the intial values of the 4-velocity components for the integrated geodesic.

__init__(metric: Metric | None, x: Sequence[int | float], frame: list[str] | None = None, coframe: list[str] | None = None)View on GitHub

Initializes the Observer class. Accepts the following arguments:

Parameters:

• metric (Metric) – The Metric object to link to the GeodesicEngine and from which the geodesic equations and all the related symbolic quantities are retrieved. If not provided, the GeodesicEngine will be linked to the last initialized Metric.

• x (Sequence[Union[int, float]]) – The space-time position of the observer (must be a 4-dimensional array of numbers).

• frame/coframe (list[str]) – The symbolic definitions of either the observer’s frame or coframe in the tetrad formalism (accepts a list of four strings, one for each tetrad). See the tutorial Define a space-time Observer for more details.

convert_3vector(vector: Sequence[int | float], type: Literal['time-like', 'null'])View on GitHub

Converts a 3-vector in the reference frame of the Observer into a 4-vector in space-time. The user must specify the components of the 3-vector v and the desired 4-vector type that can either be time-like or geodesic.

Parameters:

• vector (Sequence[Union[int, float]]) – A 3-dimensional sequence of numbers corresponding to the components of the 3-vector in the Observer’s reference frame.

• type (Literal['time-like', 'null']) – The vector normalization. It accepts strings with either "time-like" (\(g_{\mu\nu}u^\mu u^\nu = -1\)) or "null" (\(g_{\mu\nu}u^\mu u^\nu = 0\)).

from_f1(theta_obs: float, phi_obs: float, type=typing.Literal['time-like', 'null'], v: float | None = None)View on GitHub

Returns a 4-vector corresponding to the initial 4-velocity for a time-like or null geodesic (depending on the type argument) fired with angles theta_obs and phi_obs from the \(f_1\) vector. See Define a space-time Observer for an illustrative example of this functionality.

Parameters:

• theta_obs (float) – The angle \(\theta_{\rm obs}\) in the local observer’s frame.

• phi_obs (float) – The angle \(\phi_{\rm obs}\) in the local observer’s frame.

• type (Literal['time-like', 'null']) – The vector normalization. It accepts strings with either "time-like" (\(g_{\mu\nu}u^\mu u^\nu = -1\)) or "null" (\(g_{\mu\nu}u^\mu u^\nu = 0\)).

• v (float) – The modulus of the spatial velocity of the geodesic. To be specified in the time-like case.

from_f2(theta_obs: float, phi_obs: float, type=typing.Literal['time-like', 'null'], v: float | None = None)View on GitHub

As from_f1() method, but for the vector \(f_2\).

from_f3(theta_obs: float, phi_obs: float, type=typing.Literal['time-like', 'null'], v: float | None = None)View on GitHub

As from_f1() method, but for the vector \(f_3\).