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Why does time move forward rather than backward?


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I found an outstanding article about theories regarding why time moves forward, and not backward:



It's some dense reading, but I've summarized it below to the best of my ability:


Time behaves in ways that defies our intuition. At the smallest scales, there is no meaningful "future" and no "past" (Credit: Edouard Taufenbach courtesy of Gallery C)


Newton's laws of motion still nag at cosmologists today. They describe the world we move through every day - but they also account perfectly well for a world in which people walk backwards, clocks tick back afternoon to morning and fruit soars up from the ground to its tree-branch. The Biggest Ideas in the Universe, by Sean Carroll, discusses the nature of time in his new book. Newton, Maxwell and Einstein's theories of the Universe have all worked just as well going forward in time as they do backwards. Part of the answer lies at the Big Bang nearly 14 billion years ago, in the Universe's eventual death. But how does a clear direction of time emerge from descriptions of the cosmos, which all lack their own arrow of time?


In Germany, 1865, the physicist Rudolf Clausius stated that heat cannot pass from a cold body to a hot one, if nothing else around them changes. Clausius came up with the concept he called "entropy" to measure this behaviour of heat. As Rovelli stresses in his book, this is the only basic law of physics that can tell apart the past from the future. When you zoom in to the level of one water molecule colliding and bouncing off another, the arrow of time disappears. At the very smallest scale, the phenomenon that produces heat – collisions of molecules – is time-symmetric. Rovelli: "From this step, from the fundamental microscopic vision of the world to the approximate description of the macroscopic world – this is where the direction of time comes in".


The low entropy of the Universe at the Big Bang is both an answer and an enormous question, according to cosmologist John Carroll. Carroll and his colleagues are trying to explain why the Universe had such a low entropy close to the big bang. "There's plenty of loopholes in the theory, plenty of aspects of it that are not completely baked – but I also think it is by far the best theory on the market," says Carroll. The Universe's low entropy past is a plausible source of time's arrow. Like most things that have a beginning, the arrow will also have an end.


As astrophysicist Katie Mack describes it, everything is decaying so much that all that's left is the waste heat of everything that ever existed in the Universe. "It only lasts for a little while," says Carroll.


Jenann Ismael, professor of philosophy at Columbia University, New York, is studying human experience of time. She says we intuitively understand and experience time as a series of arrows that form a core part of human experience. Human experience of the flow of time is built into our perception, she says. He looks at how we experience our experiences of time and relate them back to entropy. "I see no reason now to think that the kinds of arrows that are involved in human psychology are anything but ultimately rooted in the entropic arrow," he says.


His first target is causality, another element of the arrow of time as causes happen before their effects. "All of that is part of what I think of as the experience of passage, this idea that we experience every event as anticipated from the past, experienced in the present, remembered in retrospect," says Ismael. 

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