Started reading “A Brief History of Time” by Stephen Hawking. It is an old book and I had always heard a lot about it. Finally got around to reading it.. This book definitely makes me feel little stupid because some of the things he talks about, I just don’t get it. Maybe it will take few more books on relativity to really understand or maybe life time is not enough to understand the work done by these really smart people. What fascinates me about theoretical physics is that the fact that the way it is explained in this book, it almost sounds like a philosophy rather than science, in fact it many of the early ideas/theories were put forward by philosophers. Maybe all different branches of science merge in one pure form of knowledge at certain point. First few pages, he talks about different theories or models that we had since as early as 340 BC to explain the nature of the universe. I just wanted to capture it in condensed version of it.
As Early as 340 BC, Greek philosopher Aristotle in his book, On the Heavens, talked about earth being a round sphere rather than flat plate. Three main observations made him think so….
- During eclipses of moon earth’s shadow on moon was always round, which would only be true of the earth was spherical.
- Different positions of north star from different positions of earth (Egypt and Greece). North start would appear directly over head at north pole Vs just over horizon when you go to Equator.
- Sails of the ship are seen before their hail when they come over the horizon
Aristotle also though that earth was stationary and whole universe, the sun, the moon, the stars and planets revolved around the earth.
This idea was elaborated by Ptolemy in 2nd century AD. He put forward model where earth stood at the center of 8 other spheres (each one for the sun, the moon and fixed stars as well as 5 planets known at the time). Although it was not universally accepted, it was adopted by Christian church because it left room outside those 8 spheres for heaven and hell.
In 1514, polish priest Nicholus Copernicus, proposed a model where the sun stood at the center and earth and other planets moved around it in circular orbit.
After a century, around 1609, death blow to Aristotelian/Ptolemaic theory came when German, Johannes Kepler and Italian, Galileo Galilei supported Copernicus’s model for universe. In that year, Galileo started observing sky using the telescope. Galileo observed that Jupiter’s moons moved around it and since to make the really revolve around earth, they would have to follow extremely complicated path. In stead it was much easier to use Copernicus model and say that everything does not revolve around earth.
At the same time, Kepler, modified Copernicus theory and said planets move around sun in Eclipse and not in circle. This took care of mismatch between Copernicus model and actual observation made of planetary motion. Though, having found this by accident, he did not reconcile the orbit path with believed force (magnetic force) that was causing the motion.
This explanation was provided by Sir Isaac Newton in his, Philosophiae Naturalis Principia Mathematica, using gravitation force. He not only put forward theory that explained planetary motion but also invented mathematics needed to analyze the same that we all know “lovingly” as calculus. Newton realized that, according to his theory, gravitations force will eventually make everything collapse to one point. in a letter in 1691, to Richard Bentley, another leading thinker, Newton argued that it would have happened if Universe was finite space with finite numbers stars but if universe was infinite space with infinite number of starts more or less uniformly distributed, then they would have one central place to fall to.
Another big question plaguing the great minds of all ages has always been when/why/how the universe was created. One argument for the necessity of have a reason/time/method for creation was to have “first cause”. Within universe if you explain every event has cause-effect relationship with another event then universe itself could only be explained in those terms.
St Augustine, in his City of God, accepted the date of 5000 BC as a date of creation of the universe according the book of genesis. Hawking mentions that is interesting that it is no so far away from 10000Bc, that is considered the end of last ice age and civilization really began. St Augustine also first introduced the idea that concept of time did not exist before the creation of universe. When asked “What did God do before he created universe”, he replied, Time was a property of the universe that God created and hence it did not exist before the universe was created”.
Aristotle and other Greek philosophers, believed that human race and its surrounding had existed for ever. The ancients had already considered the argument that history of civilization is more of a cycle that keeps on repeating that a one continuous progressive line.
In 1781, Philosopher, Immanuel Kant in his, Critique of Practical Reason (for some reason Hawking book calls it Pure instead of practical), called these questions antinomies because he though that there were compelling arguments for both side.
In 1929, Edwin Hubble, made a landmark discovery that distant galaxies are moving away, In other words, universe is expanding. this means at some point in time, all the matter in this universe was at a single point. This was the beginning of “Big Bang” theory.