A Brief History of Time | Chapter 1: The Picture of Our Universe | Summary
Have you ever pictured the universe in your mind? Ever wondered what does the universe looked like? Ever tried to figure out the mysteries of our cosmos? Well, there has always been the scope of the study of the universe. There have been theories and belief regarding our universe since ancient times. The great Greek philosopher Aristotle thought that earth was stationary and all the other objects in the universe orbited around the earth. The same concept was further elaborated by Ptolemy into a complete cosmological model. But all these were mere assumption. There was no scientific or mathematical evidence for this.
Newton gave the gravitational law and extended the same concept to give the explanation of elliptical orbits of moon and earth. According to the law of gravitation, stars just couldn’t stay at rest because of the force exerted by the other celestial objects. To this newton stated that if this happens, all the stars must fall at some point if there were the finite number of stars, distributed over a finite region of space, but this isn’t the case in reality! This means that there is the infinite number of stars distributed over the infinite region of space and hence almost cancelling out each other’s effect.
Certain attempts were made to state that at large distances, gravity, instead of being attractive in nature, was repulsive. However, such a system with such an equilibrium would be unstable, if the stars came got slightly nearer, the force of gravity will get stronger and thus colliding them whereas if they got little farther then the strong repulsive force will drive them further apart. Thus, due to this absurd logic, this theory failed.
Another German philosopher Heinrich Olber stated that in an infinite static universe, nearly every line of sight would end up on the surface of a star and so it could be expected that even the night sky should be as bright as the sun. Heinrich’s counter-argument was that light travelling from distant stars was dimmed by the intervening matter. However, if this was to hold true then the intervening matter should heat up and glow. The only way to avoid this conclusion would be to assume that stars had not been shining since eternity but had turned on at some finite time in the past and in that case either the absorbing matter might have not heated up to that level yet or the light from the distant stars might not have reached us until now.
Aristotle and other Greek philosophers didn’t believe in the creation of the universe. According to them, the universe had been there since eternity and will be there till perpetuity. Whereas, today, it is believed that the universe was created at BIG BANG and that’s what is believed to be the beginning of time.
[ A chapter’s summary from the great book A Brief History of Time -By Stephen Hawkins].
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