When an object is so massive that even light, which has no mass, cannot escape the well, the object is known as a ‘black hole’. The more massive the object, the more it curves spacetime, and the steeper the gravitational well. Scientists in the LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration and the Virgo Collaboration discovered evidence for gravitational waves from a pair of merging black holes in 2016. These ripples are known as gravitational waves. 8.3.3 Gravitational wavesĪs massive objects move through spacetime, they change its curvature and this change will dissipate from the objects at the speed of light, creating ‘ripples’ in spacetime like ripples on a pond. These effects were first observed by the astronomers Dennis Walsh, Robert Carswell, and Ray Weymann at the Kitt Peak National Observatory in the United States in 1979. The Swiss astronomer Fritz Zwicky first considered using galaxy clusters as gravitational lenses in 1937. Eddington showed that light is deflected around the Sun in 1919. Gravitational lensing magnifies and brightens an image and it’s also possible for the same image to be projected more than once as light is deflected in different directions. This effect is known as gravitational lensing, and it can affect the shape of an event’s light cone (discussed in Chapter 7), allowing light to travel into previously forbidden regions. The curvature of spacetime means that the path of light is deflected around massive objects. This effect is more noticeable the closer a planet is to the Sun, and so this explained why these effects were first observed in Mercury’s orbit. The curvature of spacetime forces planets to orbit in open ellipses that are rotating. This has caused images of the galaxies to be enlarged and duplicated in a process known as gravitational lensing.Ĩ.3 Consequences of general relativity 8.3.1 The precession of Mercury The light from distant galaxies is affected by the curvature of spacetime caused by a massive cluster of galaxies - Abell 2218. Einstein’s theory predicted that the gravitational force of the Sun can cause starlight to deflect by up to 1.75 arc seconds (0.0005°) and Eddington confirmed this. This means that if the glare of the Sun were blocked, like it is during an eclipse, he would be able to see stars that should be behind it. Eddington knew that if mass curves spacetime, then light would travel in a curved path as it approaches a massive object like the Sun. 8.2 Confirmation of general relativityĪrthur Eddington confirmed general relativity after the 1919 solar eclipse. These ideas were formalised by the British astronomer Edward Milne in the 1930s, and verified by NASA’s WMAP (Wilkinson Microwave Anisotropy Probe), which launched in 2001. Homogeneity assumes that our observations are representative of the whole universe, and isotropy means that the universe is the same in whichever direction we look. This assumes that the universe is homogeneous and isotropic when averaged over very large scales. In order to apply his theories to the universe as a whole, Einstein applied the cosmological principle. This meant general relativity predicted that the path of light is bent by heavy objects, like the Sun. The shortest path, which may be curved, is known as a geodesic. Light moves through curved spacetime taking the shortest possible path, however, the shortest path across a curved surface is not necessarily a straight line. This means that the curvature of space and hence the force of gravity is invariant. General relativity shows that observers in any frame will agree on how spacetime is curved by objects and hence their gravitational field, whether they are moving relative the object or not. A tensor contains more than two properties, which may be written in a matrix - numbers or symbols that are arranged in rows and columns. Velocity, for example, is a vector as it represents speed in a given direction. The energy-momentum tensor is the source of the gravitational field, just as mass is the source of the gravitational field in Newton’s equations.Ī tensor is like a vector, which contains two properties. g μ ν is the metric tensor for Minkowski space, which describes all the intrinsic properties of the spacetime manifold, including time periods, distances, volumes, and the curvature, and finally, T μ ν is the energy-momentum tensor, which describes the distribution of matter. R μ ν is the Ricci tensor, which describes the relationship between Euclidean and non-Euclidean geometry.
Here, G μ ν is the Einstein tensor, which describes the curvature of spacetime.