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24

Approaches to the Arrow of Time

Lecture no. 24 from the course: Mysteries of Modern Physics: Time

Approaches to the Arrow of Time

Taught by Professor Sean Carroll | 32 min | Categories: The Great Courses Plus Online Science Course

Use what you have learned in the course to investigate a range of different possibilities that explain the origin of time in the universe. Professor Carroll closes by presenting one of his favorite theories and noting how much remains to be done before conclusively solving the mystery of time.

Reviews

a********m
August 17, 2017
Another fascinating course by Sean Carroll! He's very ambitious to not only tackle the Universe - and the difficult abstract concept of Time's Arrow (direction) - but also to make it accessible (sort of) to a lay audience. Speaking of "layperson", even though that's the intended audience, I'd only recommend this course to people who are used to, at the very least, watching (and enjoying!) documentaries on topics such as quantum mechanics and relativity. To seriously tackle the physical concept of Time means bringing out a formidable armada of thermodynamics, classical physics, quantum mechanics, special relativity, general relativity and cosmology, with a sprinkling of string theory, etc. If most of those fields are new to you, you'll be in for a rough ride, in spite of Sean Carroll's super-human explaining skills! I was already familiar with most of those concepts, and my main issue with this course was a certain amount of repetitiousness (though understandable given the immense subject breath and the wish for accessibility to a determined lay audience!) Nonetheless, I sure appreciate how Sean Carroll brought it all together, with the patience and dogged determination of a star fictional detective. (The word "Mystery" in the title is appropriate!) In the detective-story analogy, please be aware that the "whodunit" is not fully revealed because, well, it's not fully understood. So, there is a certain letdown by the end. In particular, our own personal perception of time - probably the most fascinating aspect of time to us - cannot be full grasped because, oops, we don't understand how a physical system such as our brain develops consciousness. (Theoretical Neuroscience is my own research area.) The partial "whodunit" for the Arrow of Time, namely increasing entropy and the initial cosmological condition of low entropy is intellectually interesting, but may not be emotionally satisfying. Of course, that's not the professor's fault! Something else that I felt unsatisfying was a certain amount of "hand waving": for example, the collapse of the wave function, which at face value seems a prime suspect for an arrow of time, is summarily dismissed saying, "the arrow of time is not explained by the time-asymmetry of quantum mechanics. The time-asymmetry of quantum mechanics is explained by the arrow of time", together with brief reference to work in that area - but not a full explanation. Likewise, statements such the following one (taken from the accompanying booklet) beg for more explanation: "In the early universe, there were no black holes; there was just plasma and gas spread uniformly throughout the universe. Ordinarily, we think of that as a high-entropy state, but when gravity is important, things change." I realize, however, that the breath of the subject matter was staggering and that Sean Carroll was bending backwards to accommodate a general audience - in particular avoiding math - and so it's understandable that some elements just got a passing mention. Like any good "series", this course ends with a cliff-hanger: a speculative, but plausible mechanism by which a universe that has reached high, even maximum, entropy might spawn off a new "bubble universe". For "Season 2", we'll have to wait for more research! Meanwhile, I'll be dashing out to buy Sean Carroll's 2010 book, "From Eternity to Here: The Quest for the Ultimate Theory of Time": I bet it'll cast light on the parts I felt were too briefly touched upon in this amazing course!

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k********m
June 7, 2017

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t********m
February 21, 2017

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