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Time is an arrow

Seeing Things Different . The Last Century

Before the 20th century, Newton\'s laws belonged to the basis of physics. But at the end of the 19th century there were discovered two conflicts, with thermodynamics and electro-magnetism, that ultimately led to the formulation of first the theory of relativity and then the quantum theory. Einstein told us that space and time were part of a four-dimensional space-time and both relative. He was the first to count these to physics instead of accepting it as simply "existing". When it was later discovered that the universe is expanding, scientists quickly realised that everything had to start existing at a certain point in the past, known as the "Big Bang".

. The Beginning of Time

At the Big Bang the universe\'s density was infinite. Under such conditions all the laws of science, and therefor all ability to predict the future, would break down. If there were events earlier than this time, then they could not affect what happens at the present time. Their existence can be ignored because it would have no observational consequences. One may say that time had a beginning at the big bang, in the sense that earlier times simply would not be defined.

. Sensing Time

We are creatures in time and this has a very great effect on how we think about time and the temporal aspect of what is real.
The psychological time is very much different from the physical one. It seems that we are not able to perceive too short events, and that our brain manipulates our perceptions before they become conscious. Based on experiments, psychologists therefore suggest that our consciousness is a whole bunch of parallel processes.
People seem to sense time in a very subjective way, a fact that is in conflict with an universal time. In other cultures, like the Aborigines, there is not even a clear distinction between past, present and future. But the latter vanished in physics too, with the invention of relativity.
Many religions and philosophers believe in a cyclic time, and they are consistent with some scientific theories. Laplace first realised that when everything is predictable, knowledge of the moment is enough to know the situation in every moment, also in the future, thus making time obsolete.

. The Arrows of Time

With the laws of thermodynamics physicists then realised that the universe is developing towards maximal entropy, or chaos. This made the perpetuum mobile impossible and put an end to Newton\'s linear time. It also brought an arrow of time into physics. Based on Boltzmann´s work Poincaré then proved the possibility of a cyclic universe, but with cycles being incredibly long.
Religions like to believe in creation, which cannot be proven to be wrong. The P\'an Ku myth of China\'s third century describes the time before creation like that:

In the beginning, was the great cosmic egg. Inside the egg was chaos , and floating in chaos was P \' an Ku, the divine Embryo.

In India\'s ninth century, the Mahapurana, one of the most important books was written. In this book, the beginning of time is described as follows:

If God created the world, where was He before Creation?... Know that the world is uncreated, as time itself is, without beginning and end.

It is not entirely impossible that we all live only since some minutes ago, created with memories of past times. Seen out of this respect, the flow of time can never be proven, and time itself may as well be an illusion.

. Which Direction?

The increase of disorder or entropy with time is one example of what is called an arrow of time, something that distinguishes the past from the future, giving a direction to time. There are at least three different arrows of time.
First, there is the thermodynamic arrow of time, the direction of time in which disorder or entropy increases. Then, there is the psychological arrow of time. This is the direction in which we feel time passes, the direction in which we remember the past but not the future. Finally, there is the cosmological arrow of time. This is the direction of time in which the universe is expanding rather than contracting.
The psychological arrow is essentially the same as the thermodynamic arrow, so that the two would always point in the same direction.

. Intelligent Life

The no boundary proposal for the universe predicts the existence of a well defined thermodynamic arrow of time because the universe must start off in a smooth and ordered state. And the reason why we observe this thermodynamic arrow to agree with the cosmological arrow is that intelligent beings can exist only in the expanding phase of our universe.
However, a strong thermodynamic arrow is necessary for intelligent life to operate. In order to survive, human beings have to consume food, which is an ordered form of energy, and convert it to heat which is a disordered form of energy. Thus intelligent life could not exist in a contracting phase of the universe. This is the explanation of why we observe that the thermodynamic and cosmological arrows of time point in the same direction.
The contracting phase will be unuitable because it has no strong thermodynamic arrow of time.

. A fourth Arrow?

Some scientists interpose a fourth arrow, an arrow that helps to explain the causal asymmetry. We use \"cause\" to mark the earlier and \"effect\" to mark the later of a pair of events which are related this way. Cause-effect relation is itself asymmetric - that is, that causes and effects can be distinguished in some way.
Scientists are everything else but the same opinion in which direction they themselves should research to find answers to the questions the arrows and the asymmetry of time pose to us. In the respect of the increasing disorder in the course of time, James Thurber was right as he said:

\"It is better to know some of the questions than all of the answers.\"

Because the more answers we find the more questions arise.


Up to the beginning of our century people believed in an absolute time.
Newton considered time to be moving like a straight arrow, which unerringly flies forward toward its target. Nothing could deflect or change the course of this arrow once it was shot. Einstein, however, abandoned the idea of an absolute time and showed that time was more like a mighty river, moving forward but often meandering through twisting valleys and plains created through matter on a space-time surface. The presence of matter or energy might momentarily shift the direction of the river, but overall the river\'s course was smooth: It never abruptly ended or jerked backward.
However, successors like Kurt Gödel or Louis Tamburino showed that the river of time could be smoothly bent backward into a circle. Rivers, after all, have eddy currents and whirlpools. In the main, a river may flow forward, but at the edges there are always side pools where water flows in a circular motion.



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