Read Online Human Universe by Professor Brian Cox - Free Book Online

solve at school on a damp autumn afternoon, then that may sound like a strange and abstract idea. But equations like Einstein’s field equations are much more complex animals. Recall that Einstein’s equations will tell you the shape of spacetime, given some distribution of matter and energy. That shape is known as a solution of the equations, and it is these solutions that contain the stories. The first exact solution to Einstein’s field equations was discovered in 1915 by the German physicist Karl Schwarzschild. Schwarzschild used the equations to calculate the shape of spacetime around a perfectly spherical, non-rotating mass. Schwarzschild’s solution can be used to describe planetary orbits around a star, but it also contains some of the most exotic ideas in modern physics; it describes what we now know as the event horizon of a black hole. The well-known tales of astronauts being spaghettified as they fall towards oblivion inside a supermassive collapsed star are to be found in Schwarzschild’s solution. The calculation was a remarkable achievement, not least because Schwarzschild completed it whilst serving in the German Army at the Russian Front. Shortly afterwards, the 42-year-old physicist died of a disease contracted in the trenches.

 
     
     
    There were two ways of
arriving at the truth;
I decided to follow them both.
    Georges Lemaître

 
    The most remarkable stories waiting to be found inside Einstein’s equations reveal themselves when we take an audacious and seemingly reckless leap. Instead of confining ourselves to describing the spacetime around spherical blobs of matter, why not think a little bigger? Why not try to use Einstein’s equations to tell us about all of spacetime? Why can’t we apply General Relativity to the entire universe? Einstein noticed this as a possibility very early in the development of his theory, and in 1917 he published a paper entitled ‘Cosmological Considerations of the General Theory of Relativity’. It’s a big step, of course, from thinking about someone falling off a roof to telling the story of the universe, and Einstein appears to have been uncharacteristically wobbly. In a letter to his friend Paul Ehrenfest a few days before he presented his paper to the Prussian Academy, he wrote ‘I have … again perpetrated something about gravitation theory which somewhat exposes me to the danger of being confined in a madhouse.’
    The universe modelled in Einstein’s 1917 paper is not the one we inhabit, but the paper is of interest for the introduction of what Einstein later came to view as a mistake. Einstein tried to find a solution to his equations that would describe a finite universe, populated by a uniform distribution of matter, and stable against gravitational collapse. At the time, this was a reasonable thing to do, because astronomers knew of only a single galaxy – the Milky Way – and the stars did not appear to be collapsing inwards towards each other. Einstein also seems to have had a particular story in mind; he felt that an eternal universe was more elegant than one that had a beginning, which left open the thorny question of a creator. He discovered, however, that General Relativity does not allow for a universe with stars, planets and galaxies to be eternal. Instead, his solution told the story of an unstable universe that would collapse inwards. Einstein tried to solve this unfortunate problem by adding a new term in his equations known as the cosmological constant. This extra term can act as a repulsive force, which Einstein adjusted to resist the tendency of his model universe to collapse under its own gravity. Later, he is famously said to have remarked to his friend George Gamow that the cosmological constant was his biggest blunder.
    As physicists began to search for solutions to Einstein’s equations, more and more possible universes were discovered. None, with the exception of Einstein’s universe and a universe without matter and