In the last two post I introduce the classes formalTensor (which has been not that useful) and Tensor, the latter include a series of attributes.

Now is time to get started with GR 😛

# Metric Class

I left this class just as Sergei defined, it looked weird to me at the beginning, but again I decipher it 😉 with the help of **Reinteract**, b.t.w., those who want to play around with Reinteract, it’s in Ubuntu repositories, so just type in your console,

`$ sudo apt-get install reinteract`

Ok. Continuing… here is the code,

class Metric(Tensor): '''Represents a metric. Note that coordinates now MUST be provided''' def __init__(self,coord,rank=(0,2),sh=(-1,-1),symbol='g'): self.coord=coord super(Metric,self).__init__(symbol,rank,sh,coords=coord) def invert(self): '''Find the inverse of the metric and store the result in a Metric object self.inverse''' '''Create a unit matrix of dimension dim''' temp = sympy.eye(self.dim) '''Assign the values of the metric to temp''' for key in self.components.keys(): id = tuple(np.abs(key)) temp[id] = self.components[key] '''invert the matrix with inv() from sympy''' inv = temp.inv() '''convert the matrix in a dictionary''' inverse = self._dictkeycopy(self.components) for i in range(self.dim): for j in range(self.dim): inverse[i,j] = inv[i,j] self.inverse = Metric(self.coord,rank=(2,0),sh=(1,1),symbol='g_inv') self.inverse.components = inverse

## Inherit…

The first I notice, as a non-expert programmer (or non-programmer at all 😛 ) was the word **Tensor** inside the brackets of the Metric class. This means that *Metric* is a *Tensor*… just as a *Tensor* is an *Object*. Thus, all methods defined in the Tensor class are applicable to the Metric class.

## Coordinate system needed

The __init__ method define the rank, shape and symbol of the metric, which are always the same. However, the set of coordinates must be given.

#### Example of implementation

Coordinated should be entered like a *tuple*, and the name of coordinates MUST be “declared” as sympy.Symbol

t = Symbol('t') r = Symbol('r') th = Symbol('theta') ph = Symbol('phi') g = Metric((t,r,th,ph))

Given the this data, the *Metric* class calls the *Tensor* class and create a covariant rank 2 tensor. And we should give the components. For example, the easiest Schwarzschild metric (the metric is easier than the name!!!),

g[0,0] = -(1 - 2*M/r) g[1,1] = 1/(1 - 2*M/r) g[2,2] = r**2 g[3,3] = r**2*(sin(th))**2

WAIT!!!! Don’t forget to declare the mass parameter, so, again…

M = Symbol('M') g[0,0] = -(1 - 2*M/r) g[1,1] = 1/(1 - 2*M/r) g[2,2] = r**2 g[3,3] = r**2*(sin(th))**2

… Sorry guys, it’s again too late! and is already Monday!

Ok, tomorrow we’ll continue with the inverse metric 😉

Get GRmodule.py

and the proof-file.py.

Cya tomorrow!

Dox

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