The Universe is all of space and time and their contents including planets, stars, galaxies, and all other forms of matter and energy. In reality, the universe exists in space with no edge or borders. Stephen Hawking wrote about the history of time by describing the importance of space for the definition of time. He also talked about universal relative concepts like space and time, basic building blocks that make up the universe such as quarks and the fundamental forces that govern it such as gravity.
What is singularity? According to Stephen Hawking, the universe has not existed forever. Rather the universe and time itself had a beginning in the Big Bang about 15 billion years ago. The beginning of real time in well-defined space would have been a singularity at which the laws of physics would have broken down. He also added that the way the universe began would have been determined by the laws of physics, if the universe satisfied no boundary condition. This says that in the imaginary time direction, space-time is finite in extent but doesn’t have any boundary or edge.
How is the arrow of time defined? Hawking talked about real time and imaginary time. In the laws of science, imaginary time is described but real time is observed and experienced by humans. That is why time has a certain direction in space, notably from the past towards the future. The things that give time this property are the arrows of time.
What is the thermodynamic arrow of time? According to this, the arrow of time always starts from the state of higher order organized form, but overall disorderliness process in the world always increases as time passes. This is why we never see the broken pieces of a cup gather themselves together to form a whole cup. Even though human civilizations have tried to make things more orderly, the energy dissipated in this process has created more overall disorder in the universe.
Why is time considered as the memory factor of learning brain? The second arrow is the psychological arrow of time. Our subjective sense of time seems to flow in one direction, which is why we remember the past and not the future. Hawking said that our brain measures time in a way where disorder increases in the direction of time. We never observe it working in the opposite direction. In other words, the psychological arrow of time is intertwined with the thermodynamic arrow of time.
Why is the universe expanding or contracting? The third arrow of well-defined space is the cosmological arrow of time. This is the direction of time in which our universe is expanding and not contracting. Hawking believed that humans observe and experience the first two arrows of time in orderliness in which the universe would have to begin in a very smooth and orderly state. And then as it expanded, it became more disorderly. So the thermodynamic arrow agrees with the cosmological arrow.
The thermodynamic arrow of real-time space may go in the contracting phase of the universe and it will not be happened as strong contraction. Therefore, humans will experience these three arrows of time going in the same direction. However, because of the no boundary proposal for the universe, the universe will probably start to contract after a period of expansion. But it will probably not go backwards in time to a more smooth and orderly state.
Hawking postulates that humans have been living in the expanding phase of the universe. He thinks that intelligent life couldn’t exist in the contracting phase of the universe. Only the expanding phase of the universe is suitable for intelligent beings like humans to exist, because it contains a strong thermodynamic arrow. Hawking called this as the weak anthropic principle.
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