The Janus Point Book Summary - The Janus Point Book explained in key points
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The Janus Point summary

Julian Barbour

A New Theory of Time

4 (122 ratings)
17 mins
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    The Janus Point
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    The laws of nature don’t distinguish between past and future.

    Picture a diver leaping off a platform. She straightens her body and splashes into the water. Spray fills the air. Now imagine the same scene played in reverse. The diver emerges out of the water, and spray appears to be sucked back into the pool. Cause and effect have switched places. Time has flowed in the wrong direction.

    As this example shows, time, as we experience it, has arrows. We are used to living in a world where everything flows in the same direction: forward. Humans, animals, and stars all age and die –⁠ they never grow younger. 

    Clearly, the direction of time is important to life. So it must be somehow enshrined in the laws of nature, right? Actually, as it turns out, it isn’t.

    The key message here is: The laws of nature don’t distinguish between past and future.

    Let’s think back to those arrows of time. There are many of them, but among the most significant is what’s called equilibration. It’s a process that leads to an equilibrium, and it’s easy to see in action –⁠ just take a glass of water, stick your finger in it, and twirl it around. After you take your finger out, the water will very quickly return to its original stillness. 

    Note, though, that this process never happens in reverse. The water never becomes disturbed spontaneously. Our diver will never emerge backward out of the water. These phenomena are said to be time-asymmetric

    But not everything in the world is like this. Imagine a video in which two identical billiard balls collide on a perfectly smooth table. Say you showed that video to someone who’d never seen it before. You could play the original, or you could play it in reverse, and they wouldn’t be able to tell which ball actually moved first.

    At the microscopic level, all the laws of nature are time-reversal symmetric. We, however, are used to asymmetry, to a clear cause and effect, to a past and a future. But why? Why doesn’t the world behave like balls on a pool table? 

    Well, most physicists would tell you that it’s because of the second law of thermodynamics, which states that entropy always increases. And there’s your answer: the “direction” of time results from this increase. 

    But the author believes that there’s another explanation.

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    What is The Janus Point about?

    The Janus Point (2020) is a provocative, new take on the origins of time and the fate of the universe. Today, most physicists believe that the universe as we know it began with the big bang. But there may be a different possibility – that the big bang wasn’t the beginning of time, but merely a very special point in the history of our cosmos.

    Who should read The Janus Point?

    • Big-picture thinkers who love exploring fundamental questions about the universe
    • People who are fascinated – but may sometimes feel intimidated – by physics
    • Anyone who enjoys delving into complex and abstract theories

    About the Author

    Julian Barbour is a theoretical physicist and former visiting professor at the University of Oxford. He is the author of the best-selling book The End of Time, in which he lays out the theory that time is merely an illusion.

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