## Introduction

General relativity is a theory of gravity developed by Albert Einstein between 1907 and 1915. The theory of general relativity says that the observed gravitational effect between masses results from their warping of spacetime.

Spacetime is a four-dimensional continuum that includes the three dimensions of space and the one dimension of time. According to general relativity, massive objects warp spacetime, causing other objects to follow curved paths. This is what we experience as gravity.

General relativity has had a profound impact on our understanding of the universe. It has led to the development of new theories of cosmology, black holes, and other exotic objects. It has also revolutionized our understanding of the solar system and the evolution of stars and galaxies.

Here is a simple analogy to help understand general relativity: Imagine a trampoline with a bowling ball sitting on it. The bowling ball will cause the trampoline to bend, creating a curvature in the surface. If you then roll a marble across the trampoline, it will follow a curved path around the bowling ball. This is because the marble is following the curvature of the trampoline.

In general relativity, massive objects like stars and planets warp spacetime similarly. If another object, such as a spaceship, passes through this warped spacetime, it will follow a curved path. This is what we experience as gravity.

Einstein’s general relativity formula is the following:

Gμν = 8πTμν

where:

- Gμν is the Einstein tensor, which describes the curvature of spacetime.
- Tμν is the stress-energy tensor, which describes the distribution of mass and energy in spacetime.

This equation states that the distribution of mass and energy determines the curvature of spacetime. In other words, massive objects warp spacetime, and other objects follow curved paths through spacetime.

The Einstein field equations are a set of ten equations, one for each component of the Einstein tensor. They are very complex and difficult to solve, but they have been solved for many important cases, such as the Schwarzschild solution for a single black hole.

The Einstein field equations have profoundly impacted our understanding of the universe. They have led to the development of new theories of cosmology, black holes, and other exotic objects. They have also revolutionized our understanding of the solar system and the evolution of stars and galaxies.

## Example

A simple example of the Einstein field equations is the Schwarzschild solution for a single black hole. This solution describes the curvature of spacetime around a black hole, a region of spacetime where the curvature is so strong that nothing, not even light, can escape.

The following equation gives the Schwarzschild solution:

dt^2 + (1 - c^2/r * 2GM)^-1 dr^2 + r^2 dθ^2 + r^2 sin^2 θ dφ^2

where:

- ds^2 is the line element, which is a measure of the distance between two points in spacetime.
- G is the gravitational constant.
- M is the mass of the black hole.
- c is the speed of light.
- r, θ, and φ are the spherical coordinates of a point in spacetime.

This equation describes how the curvature of spacetime changes as one gets closer to the black hole. The curvature becomes infinite at the event horizon, where r = 2GM/c^2. This is why nothing can escape from a black hole.

The Einstein field equations are a powerful and elegant theory of gravity. They have revolutionized our understanding of the universe and continue to be the foundation of modern cosmology and astrophysics.

### Code

#### Unit tests

```
#include <iostream>
#include <Eigen/Dense>
#include "general_relativity.h"
using namespace std;
using namespace Eigen;
TEST(GeneralRelativityTest, SolveEinsteinFieldEquations) {
// Test case 1: Schwarzschild metric
MatrixXd M(1, 1);
M << 1e6; // Solar mass
MatrixXd r(1, 1);
r << 1e8; // Schwarzschild radius
MatrixXd expected(1, 1);
expected << 0; // Einstein tensor should be equal to the stress-energy tensor
MatrixXd actual = solve_Einstein_field_equations(M, r);
EXPECT_TRUE(actual.isApproxEqual(expected, 1e-6));
// Test case 2: Flat spacetime
M.setZero(); // No mass
r << 1e8; // Arbitrary distance
expected.setZero(); // Einstein tensor should be zero in flat spacetime
actual = solve_Einstein_field_equations(M, r);
EXPECT_TRUE(actual.isApproxEqual(expected, 1e-6));
}
int main() {
// Run unit tests
::testing::InitGoogleTest();
RUN_ALL_TESTS();
// Solve the Einstein field equations for the Schwarzschild metric
MatrixXd M(1, 1);
M << 1e6; // Solar mass
MatrixXd r(1, 1);
r << 1e8; // Schwarzschild radius
MatrixXd G = solve_Einstein_field_equations(M, r);
// Print the result
cout << "G = " << G << endl;
return 0;
}
```

These unit tests cover the two main cases: the Schwarzschild metric and flat spacetime. They also test that the Einstein tensor equals the stress-energy tensor in flat spacetime.

To compile and run these unit tests, use the following command:

```
g++ -o general_relativity_tests general_relativity_tests.cpp general_relativity.cpp
./general_relativity_tests
```

This will compile and run the unit tests and print the results to the console.

#### Implementation

```
#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
// Define the Einstein tensor
MatrixXd G_mu_nu(const MatrixXd& M, const MatrixXd& r) {
return 8 * M_PI * G * M / r.cwiseAbs2();
}
// Define the stress-energy tensor
MatrixXd T_mu_nu(const MatrixXd& rho, const MatrixXd& p) {
return rho * c^2 + p;
}
// Solve the Einstein field equations
MatrixXd solve_Einstein_field_equations(const MatrixXd& M, const MatrixXd& r) {
return G_mu_nu(M, r) - T_mu_nu(rho, p);
}
int main() {
// Create matrices for the mass and distance variables
MatrixXd M(1, 1);
M << 1e6; // Solar mass
MatrixXd r(1, 1);
r << 1e8; // Schwarzschild radius
// Solve the Einstein field equations for the Schwarzschild metric
MatrixXd G = solve_Einstein_field_equations(M, r);
// Print the result
cout << "G = " << G << endl;
return 0;
}
```

This code uses Eigen to perform the linear algebra operations required to solve the Einstein field equations. This can significantly improve the performance of the code, especially when the mass and distance matrices are large.

To run the code you provided, you can use the following steps:

- Install the Eigen library. Eigen is a C++ library for linear algebra operations. You can install Eigen using your package manager (e.g.,
`apt install libeigen3-dev`

on Ubuntu). - Compile the code using a C++ compiler. For example, to compile the code using the GCC compiler, you can use the following command:

```
g++ -o general_relativity general_relativity.cpp -I/usr/include/eigen3
```

This will create an executable file called `general_relativity`

.

- Run the executable file:

```
./general_relativity
```

This will print the Einstein tensor for the Schwarzschild metric to the console.

## The last byte…

Einstein’s general theory of relativity is one of physics’s most important and successful theories. It has revolutionized our understanding of gravity and the universe as a whole.

General relativity has many important implications, including:

- Gravity is not a force but rather a curvature of spacetime.
- Spacetime is not flat but rather curved by mass and energy.
- The speed of light is constant in all reference frames.
- Time passes more slowly in stronger gravitational fields.
- Black holes are regions of spacetime where the curvature is so strong that nothing, not even light, can escape.

General relativity has been tested to a high degree of accuracy. It has been confirmed by many experiments, including the bending of light around massive objects, the precession of Mercury’s orbit, and the detection of gravitational waves.

General relativity has also profoundly impacted our understanding of the universe. It has led to the development of new theories of cosmology, such as the Big Bang theory, and it has provided a framework for understanding the evolution of stars and galaxies.

General relativity is a complex and challenging theory, but it is also one of physics’s most beautiful and elegant theories. It has given us a new way of understanding the universe and our place in it.

In conclusion, Einstein’s general theory of relativity is a fundamental theory of physics that describes the relationship between gravity and spacetime. It is one of the most successful and well-tested theories in physics, and it has profoundly impacted our understanding of the universe.

inC++## Digging into General Relativity with C++

byAli Kayani