A Brief History of Time

A Brief History of Time

Arrows of time Term Analysis

Stephen Hawking suggests there are three arrows of time. First, there is the thermodynamic arrow, which relates to entropy. This arrow points in the direction of disorder increasing—for example, whole glasses smashing into pieces, rather than smashed glasses forming into whole ones. The psychological arrow of time relates to the thermodynamic arrow as our memories form in the same direction of growing disorder. While our brain, or a computer’s, may become more ordered with the creation of memories, that order is outweighed by the heat, a disordered form of energy, emitted into the universe during the process. These two arrows always point in the same direction. The third arrow is the cosmological arrow of time, which is the direction the universe is expanding or contracting, meaning that arrow is not always pointing the same way as the other two.
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Arrows of time Term Timeline in A Brief History of Time

The timeline below shows where the term Arrows of time appears in A Brief History of Time. The colored dots and icons indicate which themes are associated with that appearance.
Chapter 9
Human Curiosity and Ingenuity Theme Icon
Entropy, a concept defined in the second law of thermodynamics, directs the first, thermodynamic, arrow of time. Second is the psychological arrow of time, which is the direction we feel... (full context)
Human Curiosity and Ingenuity Theme Icon
...suggests the no boundary universe model and the weak anthropic principle explain why these three arrows all point the same way, and why they exist at all. The thermodynamic arrow determines... (full context)
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The thermodynamic arrow relies on the law of entropy, which states disorder becomes more likely as time goes... (full context)
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For the psychological arrow of time, the process of making memories creates more order internally, but the energy used... (full context)
The Search for a Unifying Theory of the Universe Theme Icon
Human Curiosity and Ingenuity Theme Icon
...might also have been lumpy and disordered. If the universe was completely disordered, the thermodynamic arrow might well point the opposite way from the cosmological arrow, but that is not what... (full context)
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...to clump, forming galaxies, stars, and people. In this way disorder increased, creating the thermodynamic arrow of time, pointing the same way as the cosmological arrow of time, or the universe’s... (full context)
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...then arises as to whether disorder decreases as the universe begins to collapse—would the thermodynamic arrow reverse as the cosmological arrow does? At first Hawking believed so. He thought the universe... (full context)
Human Curiosity and Ingenuity Theme Icon
...and so Hawking changed his mind. Disorder ought to continue to increase when the cosmological arrow reverses and the universe begins to contract. (full context)
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Hawking wondered, if disorder always increases, and the psychological arrow follows the thermodynamic one, then why does the cosmological arrow happen to point toward expansion... (full context)
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At the turn of the contracting phase there would be no strong thermodynamic arrow as the universe would be in almost total disorder. Yet life requires the thermodynamic arrow,... (full context)
The Search for a Unifying Theory of the Universe Theme Icon
Human Curiosity and Ingenuity Theme Icon
...science do not distinguish between the forward and backward direction of time, but the three arrows of time do. Human understanding has created order in a small corner of an ever-more... (full context)