tag:blogger.com,1999:blog-54391681799607871952024-03-19T16:27:32.679+10:00Condensed conceptsRuminations on emergent phenomena in condensed phases of matterRoss H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.comBlogger2367125tag:blogger.com,1999:blog-5439168179960787195.post-20465017825209425662024-03-19T16:26:00.001+10:002024-03-19T16:26:37.272+10:00A light conversation about condensed matter physics<p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;">Three weeks ago I did a local book launch for </span><a href="https://global.oup.com/academic/product/condensed-matter-physics-a-very-short-introduction-9780198845423?cc=au&lang=en&" style="color: #0078d7; font-family: times;" title="https://global.oup.com/academic/product/condensed-matter-physics-a-very-short-introduction-9780198845423?cc=au&lang=en&"><span style="color: #0563c1;">Condensed Matter Physics: A Very Short Introduction</span></a>.</p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><br /></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;">It was at a wonderful independent bookstore,<a href="https://avidreader.com.au/" target="_blank"> Avid Reader,</a> It is a vibrant part of the local community and has several author events every week.</span></p><div class="separator" style="clear: both; text-align: center;"><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm; text-align: left;"><br /></p></div><div class="separator" style="clear: both; text-align: center;"><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm; text-align: left;"><span style="font-family: times;">I had a conversation about the book with my friend,<span class="apple-converted-space"> </span><a href="https://en.wikipedia.org/wiki/Christian_Heim" style="color: #0078d7;" title="https://en.wikipedia.org/wiki/Christian_Heim">Dr Christian Heim</a>, an author, composer, and psychiatrist. My wife and daughter were surprised it was so funny. Most people loved it, but a couple of people thought it should have been more technical. I think that is not the point of such an event or of the <i>Very Short Introduction</i> series.</span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm; text-align: left;"><span style="font-family: times;"><br /></span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm; text-align: left;"><span style="font-family: times;">Here is <a href="https://drive.google.com/file/d/16KcG9M1l1AD2A9_Ex1neoQwWCPFeemZp/view?usp=sharing" target="_blank">a recording </a>of the conversation, including the Q&A with the audience afterwards.</span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm; text-align: left;"><br /></p></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEixoF9b_cX2wZBYvgQzLyVa-9vClNaMN4izC7jQySBo7XfuiQU8SsaphylVlRvol3HwJLQj-8bae9lt2mOifqqG2qmzAhW0sD10PaUBH3Ig4o3BE0VuBVvmA3_0d-a32urSbkkTSmJrx-Wn1NFz9USnCFZWMei7_Zpq8eSjEqax5LjcYrwTKKFD8iYc8v2u/s1638/IMG_20240226_183045800.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1234" data-original-width="1638" height="241" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEixoF9b_cX2wZBYvgQzLyVa-9vClNaMN4izC7jQySBo7XfuiQU8SsaphylVlRvol3HwJLQj-8bae9lt2mOifqqG2qmzAhW0sD10PaUBH3Ig4o3BE0VuBVvmA3_0d-a32urSbkkTSmJrx-Wn1NFz9USnCFZWMei7_Zpq8eSjEqax5LjcYrwTKKFD8iYc8v2u/s320/IMG_20240226_183045800.jpg" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgdteoY8xTAZHLPEhkD3qDS-rYeDRLoiubIx5kytlZJRuby36b6hA7YkKWssGM4FyVio4Mnc5gXRykuTrplEu5DvIsjuyR-IVtiLpPGaVgyLQtc_-Gx83fiflA8RnBdfJSoWPM1Ab19holU-6MChzCPO4bbWyJHNykI2ARYEFTCjvS1QOJqkngqwKCIX9pU/s1638/IMG_20240226_182112584.jpg" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" data-original-height="1234" data-original-width="1638" height="151" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgdteoY8xTAZHLPEhkD3qDS-rYeDRLoiubIx5kytlZJRuby36b6hA7YkKWssGM4FyVio4Mnc5gXRykuTrplEu5DvIsjuyR-IVtiLpPGaVgyLQtc_-Gx83fiflA8RnBdfJSoWPM1Ab19holU-6MChzCPO4bbWyJHNykI2ARYEFTCjvS1QOJqkngqwKCIX9pU/w200-h151/IMG_20240226_182112584.jpg" width="200" /></a><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgBuVnpCu46JJ6FZ8J-EhHKns7HdbU2m2gvvmJ7qWv5wdGT55k_QKLaLUFp99lZWavefWpLmlNfLFTUAiTTRpe-CN6Fb3J-tlNcuuWlUkWgvWGsR2g162TxSQ-aN1cw1jHW5cdwwOgBNaa9XN0hAiWdlUJ6ImURpiwu2tlAlLviGA7xxV5dUTkWrd0PI7YB/s1638/IMG_20240226_182817275.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1234" data-original-width="1638" height="151" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgBuVnpCu46JJ6FZ8J-EhHKns7HdbU2m2gvvmJ7qWv5wdGT55k_QKLaLUFp99lZWavefWpLmlNfLFTUAiTTRpe-CN6Fb3J-tlNcuuWlUkWgvWGsR2g162TxSQ-aN1cw1jHW5cdwwOgBNaa9XN0hAiWdlUJ6ImURpiwu2tlAlLviGA7xxV5dUTkWrd0PI7YB/w200-h151/IMG_20240226_182817275.jpg" width="200" /></a></div><div class="separator" style="clear: both; text-align: center;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiE5acB3jvwOxCGVSFtJJ9Q84RlUDd4VFlEp3GB8dLkKRG4-8hCqE-V42idJy-qIAmHWTD5uDY7mD1ZU9CcJxXe6E9IGR9mHDL7sqDphioJOVXT-ns1kaLf0HaMLHK9ARBFka1JU5fSoClBT8uIQ1GYSSUk0AmKduyw3Mk9vcyktKb7Z2gJ7dwD0dNqMnlM/s1638/IMG_20240226_183105639.jpg" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"></a><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjwhigu1VdwXNa6MnenhzrIDu2Qj1AWl2fpV93b8KJH_hY3ajHQAEHyGuwyTrzm7BaSbnboc6mu7l5Tzs8sdqLrXCI5ciJDr9HMjtsHHpel8xFu8BJyDW7Ts28awder8pa1GRxeAVYQy07Gc77BubgF9gfvl3L6LisDMmPWBTGuNmLOxVMuqr0kbpMurNNS/s2014/image1.jpeg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="2014" data-original-width="1512" height="200" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjwhigu1VdwXNa6MnenhzrIDu2Qj1AWl2fpV93b8KJH_hY3ajHQAEHyGuwyTrzm7BaSbnboc6mu7l5Tzs8sdqLrXCI5ciJDr9HMjtsHHpel8xFu8BJyDW7Ts28awder8pa1GRxeAVYQy07Gc77BubgF9gfvl3L6LisDMmPWBTGuNmLOxVMuqr0kbpMurNNS/w150-h200/image1.jpeg" width="150" /></a><img border="0" data-original-height="1234" data-original-width="1638" height="241" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiE5acB3jvwOxCGVSFtJJ9Q84RlUDd4VFlEp3GB8dLkKRG4-8hCqE-V42idJy-qIAmHWTD5uDY7mD1ZU9CcJxXe6E9IGR9mHDL7sqDphioJOVXT-ns1kaLf0HaMLHK9ARBFka1JU5fSoClBT8uIQ1GYSSUk0AmKduyw3Mk9vcyktKb7Z2gJ7dwD0dNqMnlM/s320/IMG_20240226_183105639.jpg" width="320" /><br /><div class="separator" style="clear: both; text-align: center;"></div></div></div>Many thanks to all the friends who came.<br /><br /><p></p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-49352448384221722882024-03-08T11:23:00.002+10:002024-03-08T11:23:23.667+10:00Emergence and the stratification of physics into sub-fields<div class="separator" style="clear: both;">The concept of emergence is central to understanding sub-fields of physics and how they are related, and not related, to other sub-fields.</div><div class="separator" style="clear: both;"><br /></div><div class="separator" style="clear: both;">The table below shows a stratum of sub-disciplines of physics. For each strata there are a range of length, time, and energy scales that are relevant. There are distinct entities that are composed of the entities from lower strata. These composite entities interact with one another via effective interactions that arise due to the interactions present at lower strata and can be described by an effective theory. Each sub-discipline of physics is semi-autonomous. Collective phenomena associated with a single strata can be studied, described, and understood without reference to lower strata.</div><div class="separator" style="clear: both;"><br /></div><div class="separator" style="clear: both;">Table entries are not meant to be exhaustive but to illustrate how emergence is central to understanding sub-fields of physics and how they are related to one another.</div><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgB-jZy2dbkd6YCnPq4PYSaXuqFrgfHhhAASLxRoVER-g-K6SBGlr84gHeDzCUPTxHT9hVg6po0Ud4Jvcz0NlVkPupevTRjluw3A92YA4DFozK5hg3BuHr7f64FDjEqlWWSPfrY_lXh7j_lo1-51x36JWmiyjsTTq7ONWsjx7OlFGXIhDrHLSCUjgA9dpFq/s5940/emergence.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="5940" data-original-width="4199" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgB-jZy2dbkd6YCnPq4PYSaXuqFrgfHhhAASLxRoVER-g-K6SBGlr84gHeDzCUPTxHT9hVg6po0Ud4Jvcz0NlVkPupevTRjluw3A92YA4DFozK5hg3BuHr7f64FDjEqlWWSPfrY_lXh7j_lo1-51x36JWmiyjsTTq7ONWsjx7OlFGXIhDrHLSCUjgA9dpFq/w453-h640/emergence.jpg" width="453" /></a></div><div class="separator" style="clear: both; text-align: left;">What do you think of the table? Is it helpful? Have you seen something like this before?</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">I welcome suggestions about entries that I could add.</div><br /><p></p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com3tag:blogger.com,1999:blog-5439168179960787195.post-58618685948020807812024-03-05T12:14:00.000+10:002024-03-05T12:14:03.204+10:00An illusion of purpose in emergent phenomena?<p> A characteristic of emergent phenomena in a system of many interacting parts is that they exhibit collective behaviour where it looks like the many parts are "dancing to the same tune". But who is playing the music, who chose it, and who conducts the orchestra?</p><p>Consider the following examples.</p><p>
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</p><p>1. A large group of starlings move together in what appears to be a coherent fashion. Yet, no lead starling is telling all the starlings how and where to move, according to some clever flight plan to avoid a predator. <a href="https://doi.org/10.1016/j.anbehav.2008.02.004" target="_blank">Studies of flocking</a> [murmuration] have shown that each of the starlings just moves according to the motion of a few of their nearest neighbours. Nevertheless, the flock does move in a coherent fashion <b>"as if"</b> there is a lead starling or air traffic controller making sure all the planes stick to their flight plan.</p><p>
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</p><p>2. You can buy a freshly baked loaf of bread at a local bakery every day. Why? Thousands of economic agents, from farmers to truck drivers to accountants to the baker, make choices and act based on limited local information. Their interactions are largely determined by the mechanism of prices and commercial contracts. In a market economy, no director of national bread supplies who co-ordinates the actions of all of these agents. Nevertheless, you can be confident that each morning you will be able to buy the loaf you want. The whole system acts in a co-ordinated manner <b>"as if"</b> it has a <b>purpose</b>: to reliably supply affordable high-quality bread.</p><p>
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</p><p>3. A slime mould spreads over a surface containing food supplies with spatial locations and sizes similar to that of the cities surrounding Tokyo. After a few hours, the spread of the mould has reorganised so that it is focussed on paths that are similar to the routes of the Tokyo rail network. Moulds have no brain or computer chip but they can solve optimisation problems, such as finding the shortest path through a complex maze. In nature, this problem-solving ability has the advantage that it allows them to efficiently locate sources of food and nutrients. Slime moulds act <b>"as if" </b>they have a brain.</p><p>A biologist Michael Levin discusses the issue of intelligence in very small and primitive biological systems in a recent article, <a href="https://doi.org/10.7551/mitpress/14642.003.0013" target="_blank">Collective Intelligence of Morphogenesis as a Teleonomic Process</a></p><p>[I first became aware of Levin's work through a podcast episode brought to my attention by Gerard Milburn. The relevant discussion starts around 36 minutes].</p><p>The emphasis on <b>"as if"</b> I have taken from <a href="https://en.wikipedia.org/wiki/Thomas_Schelling" target="_blank">Thomas Schelling</a> in the opening chapter of his beautiful book, <a href="https://www.goodreads.com/book/show/317333.Micromotives_and_Macrobehavior" target="_blank">Micromotives and Macrobehaviour.</a></p><p>He also mentions the example of Fermat's principle in optics: the path light takes as it travels between two spatially separated points is the path for which the travel time is an extremum [usually a minimum]. The light travels <b>"as if"</b> it has the purpose of finding this extremum. </p><p>[Aside: according to Wikipedia, </p><p></p><blockquote><span style="font-family: arial;">"Fermat's principle was initially controversial because it <b>seemed to ascribe</b> knowledge and intent to nature. Not until the 19th century was it understood that nature's ability to test alternative paths is merely a fundamental property of waves."</span></blockquote><p></p><p>Similar issues of knowledge/intent/purpose arise when considering the motion of a classical particle moving between two spatial points. It takes the path for which the value of the action [time integral of the Lagrangian along a path] has an extremal value relative to all possible paths. I suspect that the path integral formulation of quantum theory is required to solve the "as if" problem. Any alternative suggestions?</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-12360130061330854012024-02-27T16:40:00.004+10:002024-02-27T16:40:55.909+10:00Emergence? in large language models (revised edition)<p>Last year I wrote <a href="https://condensedconcepts.blogspot.com/2023/10/emergent-abilities-in-ai-large-language.html" target="_blank">a post about emergence in AI</a>, specifically on a <a href="https://arxiv.org/abs/2206.07682" target="_blank">paper claiming evidence</a> for a "phase transition" in Large Language Models' ability to perform tasks they were not designed for. I found this fascinating.</p><p>That paper attracted a lot of attention, even winning an award for the best paper at the conference at which it was presented.</p><p>Well, I did not do my homework. Even before my post, another paper called into question the validity of the original paper.</p><p><a href="https://arxiv.org/abs/2304.15004" target="_blank">Are Emergent Abilities of Large Language Models a Mirage?</a></p><p>Rylan Schaeffer, Brando Miranda, Sanmi Koyejo</p><p></p><blockquote><span style="font-family: arial;">we present an alternative explanation for [the claimed] emergent abilities: that for a particular task and model family, when analyzing fixed model outputs, emergent abilities <b>appear due to the researcher's choice of metric rather than due to fundamental changes in model behavior with scale</b>. Specifically, nonlinear or discontinuous metrics produce apparent emergent abilities, whereas linear or continuous metrics produce smooth, continuous predictable changes in model performance.</span></blockquote><p></p><blockquote><span style="font-family: arial;">... we provide evidence that alleged emergent abilities evaporate with different metrics or with better statistics, and may not be a fundamental property of scaling AI models.</span></blockquote><p>One of the issues they suggest is responsible for the smooth behaviour is </p><blockquote> <span style="font-family: arial;">the phenomenon known as neural scaling laws: empirical observations that deep networks exhibit power law scaling in the test loss as a function of training dataset size, number of parameters or compute </span></blockquote><p></p><p>One of the papers they cite on power law scaling is below (from 2017).</p><p><a href="https://arxiv.org/abs/1712.00409" target="_blank">Deep Learning Scaling is Predictable, Empirically</a></p><p></p><p>Joel Hestness, Sharan Narang, Newsha Ardalani, Gregory Diamos, Heewoo Jun, Hassan Kianinejad, Md. Mostofa Ali Patwary, Yang Yang, Yanqi Zhou</p><p>The figure below shows the power law scaling between the validation loss and the size of the training data set.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj6GuFwZ3dVvUKfDedT-ywMCuW16iQESZXz2Olpz8n0QqGciMZ33t4_sp6WEH1k1_uryGa9mMyrfLLC1aEW9E2iwOrsh2j8mMDOM-PcBnCxgRP_BHdzD-9PIt-pjfKeg17ZodX0SXXx8KAyVNjsygHbgbS5I4kUwGuqXXiFM32w1gvmi-0GeUvdneEIMLYm/s673/Untitled.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="510" data-original-width="673" height="303" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj6GuFwZ3dVvUKfDedT-ywMCuW16iQESZXz2Olpz8n0QqGciMZ33t4_sp6WEH1k1_uryGa9mMyrfLLC1aEW9E2iwOrsh2j8mMDOM-PcBnCxgRP_BHdzD-9PIt-pjfKeg17ZodX0SXXx8KAyVNjsygHbgbS5I4kUwGuqXXiFM32w1gvmi-0GeUvdneEIMLYm/w400-h303/Untitled.jpg" width="400" /></a></div>They note that these empirical power laws are yet to be explained.<br /><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">I thank Gerard Milburn for ongoing discussions about this topic.</div><br /><p><br /></p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-49856002713223992442024-02-16T09:54:00.001+10:002024-02-16T09:54:28.570+10:00Launching my book in a real physical bookshop<p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;">Physical bookstores selling physical books are in decline, sadly. Furthermore, the stores that are left are mostly big chains. Brisbane does have an independent bookstore,<a href="https://avidreader.com.au/" target="_blank"> Avid Reader,</a> in the West End. It is a vibrant part of the local community and has several author events every week.</span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><br /></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;">My daughter persuaded me to do a book launch, for <a href="https://global.oup.com/academic/product/condensed-matter-physics-a-very-short-introduction-9780198845423?cc=au&lang=en&" style="color: #0078d7;" title="https://global.oup.com/academic/product/condensed-matter-physics-a-very-short-introduction-9780198845423?cc=au&lang=en&"><span style="color: #0563c1;">Condensed Matter Physics: A Very Short Introduction (Oxford UP, 2023)</span></a><span style="color: black;"> </span></span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;"> </span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;">It is at Avid Reader on Monday, February 26, beginning at 6 pm.</span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;"><br /></span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;">Most readers of this blog are not in Brisbane, but if you are or know people who are please encourage them to consider attending.</span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;">The event is free but participants need to<a href="https://avidreader.com.au/pages/8980-RossHMcKenzie-CondensedMatterPhysicsAVeryShortIntroduction" target="_blank"> register</a>, as space is limited.</span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;"> </span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;">I will be in conversation about the book with my friend,<span class="apple-converted-space"> </span><a href="https://en.wikipedia.org/wiki/Christian_Heim" style="color: #0078d7;" title="https://en.wikipedia.org/wiki/Christian_Heim">Dr Christian Heim</a>, an author, composer, and psychiatrist. Like the book, the event is meant for a general audience.</span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;"><br /></span></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcNU87ArkOayS6lDmYq_bZLEZQO1bD66PzXb4ihuCeRi1axJ2rSZUEFC_vqoMlYe38IBbGj-_3QgpXfE_xW5p1ymQxvpZSAyATLJ0MSYhIuu5yhRCKY9OSfMWvWWuvc_DTHL4hOyHiVbOf91grwSIVhWz9RQGMuk5ZKD17NtI7I6wyBwZHyOScQyzCMK-0/s4000/IMG_20240212_134032181.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="4000" data-original-width="2250" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcNU87ArkOayS6lDmYq_bZLEZQO1bD66PzXb4ihuCeRi1axJ2rSZUEFC_vqoMlYe38IBbGj-_3QgpXfE_xW5p1ymQxvpZSAyATLJ0MSYhIuu5yhRCKY9OSfMWvWWuvc_DTHL4hOyHiVbOf91grwSIVhWz9RQGMuk5ZKD17NtI7I6wyBwZHyOScQyzCMK-0/w225-h400/IMG_20240212_134032181.jpg" width="225" /></a></div><p></p><p class="MsoNormal" style="caret-color: rgb(33, 33, 33); color: #212121; margin: 0cm;"><span style="font-family: times;"> </span></p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com1tag:blogger.com,1999:blog-5439168179960787195.post-35331793074024243462024-02-09T10:49:00.006+10:002024-02-09T10:49:00.130+10:00The role of effective theories and toy models in understanding emergent properties<p>Two of the approaches to the theoretical description of systems with emergent properties that have been fruitful are effective theories and toy models. These leverage our limited knowledge of many details about a system with many interacting components.</p><p><b>Effective theories</b></p><p>An effective theory is valid at a particular range of scales. This exploits the fact that in complex systems there is often a hierarchy of scales (length, energy, time, or number). In physics, examples of effective theories include classical mechanics, general relativity, classical electromagnetism, and thermodynamics. The equations of an effective theory can be written down almost solely from consideration of symmetry and conservation laws. Examples include the Navier-Stokes equations for fluid dynamics and non-linear sigma models in elementary particle physics. Some effective theories can be derived by the “coarse-graining” of theories that are valid at a finer scale. For example, the equations of classical mechanics result from taking the limit of Planck’s constant going to zero in the equations of quantum mechanics. The Ginzburg-Landau theory for superconductivity can be derived from the BCS theory. The parameters in effective theories may be determined from more microscopic theories or from fitting experimental data to the predictions of the theory. For example, transport coefficients such as conductivities can be calculated from a microscopic theory using a Kubo formula.</p><p>Effective theories are useful and powerful because of the minimal assumptions and parameters used in their construction. For the theory to be useful it is <i>not </i>necessary to be able to derive the effective theory from a smaller scale theory, or even to have such a smaller scale theory. For example, even though there is no accepted quantum theory of gravity, general relativity can be used to describe phenomena in astrophysics and cosmology and is accepted to be valid on the macroscopic scale. Some physicists and philosophers may consider smaller-scale theories as <a href="https://condensedconcepts.blogspot.com/2023/06/what-is-really-fundamental-in-science.html" target="_blank">more fundamental</a>, but that is contested and so I will not use that language. There also are <a href="https://doi.org/10.1016/j.shpsa.2022.01.014" target="_blank">debates </a>about how effective field theories fit into the philosophy of science.</p><p><b>Toy models</b></p><p>In his <a href="https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.89.040502" target="_blank">2016 Nobel Lecture</a>, Duncan Haldane said, “Looking back, … I am struck by how important the use of stripped down “toy models” has been in discovering new physics.” </p><p>Here I am concerned with a class of theoretical models that includes the Ising, Hubbard, Agent-Based Models, <a href="https://en.wikipedia.org/wiki/NK_model" target="_blank">NK</a>, <a href="https://condensedconcepts.blogspot.com/2019/04/the-emergence-of-social-segregation.html" target="_blank">Schelling</a>, and Sherrington-Kirkpatrick models. I refer to them as “toy” models because they aim to be as simple as possible, while still capturing the essential details of a particular emergent phenomenon. At the scale of interest, the model is an approximation, neglecting certain degrees of freedom and interactions. In contrast, at the relevant scale, effective theories are often considered to be exact because they are based on general principles.</p><p>Historical experience has shown that there is a strong justification for the proposal and study of toy models. They are concerned with a qualitative, rather than a quantitative, description of experimental data. A toy model is usually introduced to answer basic questions about <b>what is possible.</b> What are the essential ingredients that are sufficient for an emergent phenomena to occur? What details do matter? For example, the Ising model was introduced in 1920 to see if it was possible for statistical mechanics to describe the sharp phase transition associated with ferromagnetism. </p><p>In his book <a href="https://www.basicbooks.com/titles/scott-e-page/the-model-thinker/9780465094622/" target="_blank"><i>The Model Thinker </i></a>and online course <a href="https://www.coursera.org/learn/model-thinking" target="_blank">Model Thinking,</a> Scott Page has enumerated the value of simple models in the social sciences. An earlier argument for their value in biology was <a href="https://condensedconcepts.blogspot.com/2022/09/the-value-of-simple-models-for-complex.html" target="_blank">put by JBS Haldane</a> in his seminal article about “bean bag” genetics. Simplicity makes toy models more tractable for mathematical analysis and/or computer simulation. The assumptions made in defining the model can be clearly stated. If the model is tractable then the pure logic associated with mathematical analysis leads to reliable conclusions. This contrasts with the qualitative arguments often used in the biological and social sciences to propose explanations. Such arguments can miss the counter-intuitive conclusions associated with emergent phenomena and the rigorous analysis of toy models. Such models can show what is possible, what are simple ingredients for a system <i>sufficient </i>to exhibit an emergent property, and how a quantitative change can lead to a qualitative change. In different words, what details do matter? </p><p>Toy models can guide what experimental data to gather and how to analyse it. Insight can be gained by considering multiple models as that approach can be used to rule out alternative hypotheses. Finally, there is value in the adage, “all models are wrong, but some are useful.”</p><p>Due to universality, sometimes toy models work better than expected, and can even give a quantitative description of experimental data. An example is the three-dimensional Ising model, which was eventually found to be consistent with data on the liquid-gas transition near the critical point. Although, not a magnetic system, the analogy was bolstered by the mapping of the Ising model onto the lattice gas model. This success led to a shift in the attitude of physicists towards the Ising model. <a href="https://link.springer.com/article/10.1007/s00407-004-0088-3" target="_blank">According to Martin Niss, from 1920-1950,</a> it was viewed as irrelevant to magnetism because it did not describe magnetic interactions quantum mechanically. This was replaced with the view that it was a model that could give insights into collective phenomena. <a href="https://link.springer.com/article/10.1007/s00407-011-0086-1" target="_blank">From 1950-1965</a>, the view diminished that the Ising model was irrelevant to describing critical phenomena because it oversimplified the microscopic interactions.</p><p>Physicists are particularly good and experienced at the proposal and analysis of toy models. I think this expertise is a niche that they could exploit more in contributing to other fields, from biology to the social sciences. They just need<a href="https://condensedconcepts.blogspot.com/2013/07/how-to-not-break-into-new-field.html" target="_blank"> humility to listen to non-physicists</a> about what the important questions and essential details are.</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-45408332261077649632024-02-06T11:50:00.002+10:002024-02-06T11:50:32.568+10:00Four scientific reasons to be skeptical of AI hype<p>The hype about AI continues, whether in business or science. Undoubtedly, there is a lot of potential in machine learning, big data, and large language models. But that does not mean that the hype is justified. It is more likely to limit real scientific progress and waste a lot of resources.</p><p>My innate scepticism receives concrete support from an article from 2018 that gives four scientific reasons for concern.</p><p><a href="https://doi.org/10.1098/rsta.2018.0145" target="_blank">Big data: the end of the scientific method? </a></p><p>Sauro Succi and Peter V. Coveney</p><p>The article might be viewed as a response to a bizarre article in 2008 by Chris Anderson, editor-in-chief at Wired, <a href="https://www.wired.com/2008/06/pb-theory/" target="_blank">The End of Theory: The Data Deluge Makes the Scientific Method Obsolete</a></p><blockquote style="border: none; margin: 0 0 0 40px; padding: 0px;"><p style="text-align: left;"><span style="background-color: white; color: #333132; font-size: 26px;"><span style="font-family: arial;">‘With enough data, the numbers speak for themselves, correlation replaces causation, and science can advance even without coherent models or unified theories’.</span></span></p></blockquote><p><span face="Proxima Nova Subset, sans-serif" style="color: #333132;">Here are the four scientific reasons for caution about such claims given by Succi and Coveney.</span></p><p><span face="Proxima Nova Subset, sans-serif" style="color: #333132;"></span></p><blockquote><p><span style="font-family: arial;"><span face="Proxima Nova Subset, sans-serif" style="color: #333132;">(i)<span style="white-space: pre;"> </span></span><span face=""Proxima Nova Subset", sans-serif" style="color: #333132;">Complex systems are strongly correlated, hence they do not (generally) obey Gaussian statistics.</span></span></p></blockquote><p>The law of large numbers (central limit theorem) may not apply and rare events may dominate behaviour. For example, consider the power law decays observed in many complex systems. They are in sharp contrast to the rapid exponential decay in the Gaussian distribution. The authors state, "when rare events are not so rare, convergence rates can be frustratingly slow even in the face of petabytes of data."</p><blockquote><p><span style="font-family: arial;"><span face="Proxima Nova Subset, sans-serif" style="color: #333132;">(ii)<span style="white-space: pre;"> </span></span><span style="color: #333132;">No data are big enough for systems with strong sensitivity to data inaccuracies.</span></span></p></blockquote><p>Big data and machine learning involve fitting data to a chosen function, such as a "cost function" with many parameters. That fitting involves a minimisation routine which acts on some sort of "landscape." If the landscape is smooth and minima are well-separated and not separated by too large of maxima then the routine may work. However, if the landscape is rough or the routine gets stuck in some metastable state there will be problems, such as over-fitting.</p><blockquote><p><span style="font-family: arial;"><span style="color: #333132;">(iii)</span><span style="color: #333132; white-space: pre;"> </span><span style="color: #333132;">Correlation does not imply causation, the link between the two becoming exponentially fainter at increasing data size. </span></span> </p></blockquote><blockquote><p><span style="font-family: arial;"><span face="Proxima Nova Subset, sans-serif" style="color: #333132;">(iv)<span style="white-space: pre;"> </span></span><span style="color: #333132;">In a finite-capacity world, too much data is just as bad as no data.</span></span></p></blockquote><div>In other words, it is all about curve fitting. The more parameters used the less likely for insight to be gained. Here the authors quote the famous aphorism, attributed to von Neumann and Fermi, "with four parameters I can fit an elephant and with five I can make his tail wiggle."</div><div><br /></div><div>Aside: an endearing part of the article is the inclusion of tow choice quotes from C.S. Lewis</div><div></div><blockquote><div><span style="font-family: arial;">‘Once you have surrendered your brain, you've surrendered your life’ (paraphrased)</span></div><div><span style="font-family: arial;"><br /></span></div><div><span style="font-family: arial;"><span style="background-color: white; color: #333132; font-size: 26px;">‘When man proclaims conquest of power of nature, what it really means is conquest of power of </span><i style="background-color: white; box-sizing: border-box; color: #333132; font-size: 26px;">some</i><span style="background-color: white; color: #333132; font-size: 26px;"> men over other men’.</span></span></div></blockquote><p>I commend the article to you and look forward to hearing your perspective. Is the criticism of AI hype fair? Are these four scientific reasons good grounds for concern. </p><div><span style="background-color: white; color: #333132; font-family: "Proxima Nova Subset", sans-serif; font-size: 26px;"></span></div>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-67565193227208356142024-01-25T15:16:00.001+10:002024-01-25T15:16:26.341+10:00Emergence and the Ising model<p>The Ising model is emblematic of “toy models” that have been proposed and studied to understand and describe emergent phenomena. Although originally proposed to describe ferromagnetic phase transitions, variants of it have found application in other areas of physics, and in biology, economics, sociology, neuroscience, complexity theory, … </p><p><a href="https://www.quantamagazine.org/the-cartoon-picture-of-magnets-that-has-transformed-science-20200624/" target="_blank">Quanta magazine had a nice article </a>marking the model's centenary.</p><p>In the general model there is a set of lattice points {i} with a “spin” {sigma_i = +/-1} and a Hamiltonian</p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg3vBIZuZKpdVnqGXMKBxdmYQ_R0p3a3Wo4ucOHCIF-dku-Uwb5mEecwUYV27e_M_vIWv1ROURGlybpsFAMMVrZTWDfWNr-Hf2DkNVDg8UbR6kgoi5F7IPInzboDkQdhGFEmD0dCaMvoxN1qAbjOxfU4_14wPgGqFP-l7Cyxok6nqYwGPgeZX-kCQr7aqiJ/s521/Untitled.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="132" data-original-width="521" height="81" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg3vBIZuZKpdVnqGXMKBxdmYQ_R0p3a3Wo4ucOHCIF-dku-Uwb5mEecwUYV27e_M_vIWv1ROURGlybpsFAMMVrZTWDfWNr-Hf2DkNVDg8UbR6kgoi5F7IPInzboDkQdhGFEmD0dCaMvoxN1qAbjOxfU4_14wPgGqFP-l7Cyxok6nqYwGPgeZX-kCQr7aqiJ/s320/Untitled.jpg" width="320" /></a></div><div class="separator" style="clear: both; text-align: center;"><span style="text-align: left;">where h is the strength of an external magnetic field and J_ij is the strength of the interaction between the spins on sites i and j. The simplest models are where the lattice is regular, and the interaction is uniform and only non-zero for nearest-neighbour sites.</span></div><p></p><p>The Ising model illustrates many key features of emergent phenomena. Given the relative simplicity of the model, exhaustive studies since its proposal in 1920, have given definitive answers to questions often debated about more complex systems. Below I enumerate some of these insights: novelty, quantitative change leads to qualitative change, spontaneous order, singularities, short-range interactions can produce long-range order, universality, three horizons/scales of interest, self-similarity, inseparable horizons, and simple models can describe complex behaviour.</p><p>Most of these properties can be illustrated with the case of the Ising model on a square lattice with only nearest-neighbour interactions (J_ij = J). Above the critical temperature (Tc = 2.25J), and in the absence of an external magnetic field the system has no net magnetisation. Below Tc, at net magnetisation occurs. For J > 0 (J < 0) this state is ferromagnetic (antiferromagnetic).</p><p><b>Novelty</b></p><p>The state of the system below Tc is qualitatively different than that at very high temperatures or the state of a set of non-interacting spins. Thus, the non-zero magnetisation is <a href="https://condensedconcepts.blogspot.com/2023/02/what-is-emergence.html" target="_blank">an emergent property, as defined in this post</a>. This state is also associated with spontaneous symmetry breaking and more than one possible equilibrium state, i.e., the magnetisation can be positive or negative.</p><p><b>Quantitative change leads to qualitative change</b></p><p>The qualitative change associated with formation of the magnetic state can occur with a small quantitative change in the value of the ratio T/J, i.e., either by decreasing T or increasing J. Formation of the magnetic state is also associated with the quantitative change of increasing the number of spins from a large finite number to infinity. </p><p><b>Singularities</b></p><p>For a finite number of spins all the thermodynamic properties of the system are an analytic function of the temperature and magnitude of an external field. However, in the thermodynamic limit, these properties become singular at T=Tc and h=0. This is the critical point in the phase diagram of h versus T. Some of the quantities, such as the specific heat capacity and the magnetic susceptibility, become infinite at the critical point. These singularities are characterised by critical exponents, most of which have non-integer values. Consequently, the free energy of the system is not an analytic function of T and h.</p><p><b>Spontaneous order</b></p><p>The magnetic state occurs spontaneously. The system self-organises. There is no external field causing the magnetic state to form. There is long-range order, i.e., the value of spins that are infinitely apart from one another are correlated. </p><p><b>Short-range interactions can produce long-range order.</b></p><p>Although there is no direct long-range interaction between spins, long-range order can occur. Prior to Onsager’s exact solution of the two-dimensional model, many scientists were not convinced that this was possible.</p><p><b>Universality</b></p><p>The values of the critical exponents are independent of many details of the model, such as the value of J, the lattice constant and spatial anisotropy, and the presence of small interactions beyond nearest neighbour. Many details do not matter. This is why the model can give a quantitative description of experimental data near the critical temperature, even though the model Hamiltonian is a crude descriptions of the interactions in a real material. It can describe not only magnetic transitions but also transitions in liquid-gas, binary alloys, and binary liquid mixtures.</p><p><b>Three horizons/scales of interest</b></p><p>There are three important length scales associated with the model. Two are simple: the lattice constant, and the size of the whole lattice. These are the microscopic and macroscopic scale. The third scale is emergent and temperature dependent: the correlation length, i.e., the distance over which spins are correlated with one another. This can also be visualised as the size of magnetisation domains seen in Monte Carlo simulations. </p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgR-H_cvUH56yLaAwocn6U0MmcRb5usiTF50daqmefGWu5KJXYsWkpQ1zTvVMwQeaFEIHYjqHFVfM7A2wkyYHXVIgvFSNfpE8ami6wvjthRd6u3kbRcgk7MzqvJmJ1To4RlZCpHbMCXwibSyl_Pb5CtwMkLnSPC9ExDC3RYK-DpcUE2u1KXDD6Uj-Ms8gTl/s818/Picture%201.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="266" data-original-width="818" height="130" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgR-H_cvUH56yLaAwocn6U0MmcRb5usiTF50daqmefGWu5KJXYsWkpQ1zTvVMwQeaFEIHYjqHFVfM7A2wkyYHXVIgvFSNfpE8ami6wvjthRd6u3kbRcgk7MzqvJmJ1To4RlZCpHbMCXwibSyl_Pb5CtwMkLnSPC9ExDC3RYK-DpcUE2u1KXDD6Uj-Ms8gTl/w400-h130/Picture%201.png" width="400" /></a></div><p>The left, centre, and right panels above show a snapshot of a likely configuration of the system at a temperature less than, equal to, and greater than the critical temperature, Tc, respectively.</p><p>Understanding the connection between the microscopic and macroscopic properties of the system requires studying the system at the intermediate scale of the correlation length. This scale also defines emergent entities [magnetic domains] that interact with one another weakly and via an effective interaction.</p><p><b>Self-similarity</b></p><p>At the critical temperature, the correlation length is infinite. Consequently, rescaling the size of the system, as in a renormalisation group transformation, the state of the systems does not change. The system is said to be scale-free or self-similar like a fractal pattern. This is an example of self-organised criticality.</p><p><b>Inseparable horizons</b></p><p>I now consider how things change when the topology or dimensionality of the lattice changes or when interactions beyond nearest neighbours are added. This can change the relationships between the parts and the whole. Some details of the parts matter. Changing from a two-dimensional rectangular lattice to a linear chain the ordered state disappears. Changing to a triangular lattice with antiferromagnetic nearest-neighbour interactions removes the ordering at finite temperature and there are an infinite number of ground states at zero temperature. Thus, some microscopic details do matter.</p><p>The main point of this example is that to understand a large complex system we have to keep both the parts and the whole in mind. It is not either/or but both/and. Furthermore, there may be an intermediate scale, at which new entities emerge.</p><p><i>Aside:</i> I suspect heated debates about <a href="https://doi.org/10.1016/B978-0-08-097086-8.12225-1" target="_blank">structuralism</a> versus functionalism in social sciences, and the humanities are trying to defend intellectual positions (and fashions) that overlook the inseparable interplay of the microscopic and macroscopic that the Ising model captures.</p><p><b>Simple models can describe complex behaviour</b></p><p>Now consider an Ising model with competing interactions, i.e. the neighbouring spins of a particular spin compete with one another and with an external magnetic field to determine the sign of the spin. This can be illustrated with the an Ising model on a hexagonal close packed (hcp) lattice with nearest neighbour antiferromagnetic interactions and an external magnetic field. The lattice is frustrated and can be viewed as layers of hexagonal (triangular) lattices where each layer is displaced relative to one another.</p><p>This model has been studied by materials scientists as it can describe the many possible phases of binary alloys, AxB1-x, where A and B are different chemical elements (for example, silver and gold) and the Ising spins on site i has value +1 or -1, corresponding to the presence of atom A or B on that site. The magnetic field corresponds to the difference in the chemical potentials of A and B, and is related to their relative concentration.</p><p>The authors studied the Ising model on the hexagonal close-packed (hcp) lattice in a magnetic field. The authors are all from materials science departments and are motivated by the fact that the problem of binary alloys AxB1_x can be mapped onto an Ising model. <a href="https://journals.aps.org/prb/abstract/10.1103/PhysRevB.48.6767" target="_blank">A study of this model found rich phase diagrams</a> including 32 stable ground states with stoichiometries, including A, AB, A2B, A3B, A5B, and A4B3. Even for a single stoichiometry, there can be multiple possible distinct orderings (and crystal structures). Of these structures, six are stabilized by purely nearest-neighbour interactions, eight by addition of next-nearest neighbour interactions. The remaining 18 structures require multiplet interactions for their stability. </p><p>A second example is the <a href="https://condensedconcepts.blogspot.com/2020/11/the-devil-is-not-in-details.html" target="_blank">Anisotropic Next-Nearest Neighbour Ising (ANNNI) model</a>, which supports a plethora of ordered states, including a phase diagram with a fractal structure, known as the Devil’s staircase.</p><p>These two Ising models illustrate how relatively simple models, containing competing interactions (described by just a few parameters) can describe rich behaviour, particularly a <b>diversity</b> of ground states.</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-17055812277137361052024-01-19T11:10:00.000+10:002024-01-19T11:10:12.432+10:00David Mermin on his life in science: funny, insightful, and significant<p> <a href="https://en.wikipedia.org/wiki/N._David_Mermin" target="_blank">David Mermin</a> has posted a preprint with the modest title, <a href="https://arxiv.org/abs/2401.04711" target="_blank">Autobiographical Notes of a Physicist</a></p><p>There are many things I enjoyed and found interesting about his memories. A few of the stories I knew, but most I did not. He reminisces about his interactions with Ken Wilson, John Wilkins, Michael Fisher, Walter Kohn, and of course, Neil Ashcroft.</p><p>Mermin is a gifted writer and can be amusing and mischievous. He is quite modest and self-deprecating about his own achievements.</p><p>He explains why we should refer to the Hohenberg-Mermin-Wagner theorem, not Mermin-Wagner.</p><p>One of his Reference Frame columns in <i>Physics Today</i>, stimulated Paul Ginsbarg to start the arXiv.</p><p>I was struck by how Mermin's career belongs to a different era. The community was smaller and more personal. Doing physics was fun. Time was spent savouring the pleasure of learning new things and explaining them to others. Colleagues were friends rather than competitors. His research was curiosity-driven. This led to Mermin making significant contributions to quantum foundations. And, he only published about two papers per year!</p><p>Teaching was valued, enjoyable, and stimulated research. It was also a way to learn a subject, regardless of the level at which it was taught. For eight years, Mermin and Ashcroft spent half their time writing their beautiful textbook!</p><p>I look forward to hearing others' reflections.</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com4tag:blogger.com,1999:blog-5439168179960787195.post-8301507664557616262024-01-16T15:08:00.008+10:002024-01-19T09:38:17.519+10:00Wading through AI hype about materials discovery<p> Discovering new materials with functional properties is hard, very hard. We need all the tools we can from serendipity to high-performance computing to chemical intuition. </p><p>At the end of last year, two back-to-back papers appeared in the luxury journal Nature.</p><p><a href="https://www.nature.com/articles/s41586-023-06735-9" target="_blank">Scaling deep learning for materials discovery</a></p><p>All the authors are at Google. They claim that they have discovered more than two million new materials with stable crystal structures using DFT-based methods and AI.</p><p>On Doug Natelson's blog there are several insightful comments on the paper about why to be skeptical about AI/DFT based "discovery".</p><p>Here are a few of the reasons my immediate response to this paper is one of skepticism.</p><p>It is published in Nature. Almost every "ground-breaking" paper I force myself to read is disappointing when you read the fine print.</p><p>It concerns a very "hot" topic that is full of hype in both the science and business communities.</p><p>It is a long way from discovering a stable crystal to finding that it has interesting and useful properties.</p><p>Calculating the correct relative stability of different crystal structures of complex materials can be <a href="https://condensedconcepts.blogspot.com/2014/06/the-challenge-of-molecular-crystal.html" target="_blank">incredibly difficult.</a></p><p>DFT-based methods fail spectacularly for the low-energy properties of quantum materials, such as cuprate superconductors. But, they do get the atomic structure and stability correct, which is the focus of this paper.</p><p>It is a <a href="https://condensedconcepts.blogspot.com/2020/01/the-commercial-applications-gap.html" target="_blank">big gap</a> between discovering a material that has desirable technological properties to one that meets the demanding criteria for commercialisation.</p><p>The second paper combines AI-based predictions, similar to the paper above, with robots doing material synthesis and characterisation.</p><p><a href="https://www.nature.com/articles/s41586-023-06734-w" target="_blank">An autonomous laboratory for the accelerated synthesis of novel materials</a></p><p></p><blockquote><span style="font-family: arial;">[we] realized 41 novel compounds from a set of 58 targets including a variety of oxides and phosphates that were identified using large-scale ab initio phase-stability data from the Materials Project and Google DeepMind</span></blockquote><p></p><p>These claims have already been undermined by a preprint from the chemistry departments at Princeton and UCL.</p><p><a href="https://chemrxiv.org/engage/chemrxiv/article-details/65957d349138d231611ad8f7" target="_blank">Challenges in high-throughput inorganic material prediction and autonomous synthesis</a></p><p></p><blockquote style="font-family: arial;"><span style="font-size: x-small;">We discuss all 43 synthetic products and point out four common shortfalls in the analysis. These errors unfortunately lead to the conclusion that <b>no new materials have been discovered in that work</b>. We conclude that there are two important points of improvement that require future work from the community:</span> </blockquote><blockquote style="font-family: arial;"><span style="font-size: x-small;">(i) automated Rietveld analysis of powder x-ray diffraction data is not yet reliable. Future improvement of such, and the development of a reliable artificial intelligence-based tool for Rietveld fitting, would be very helpful, not only to autonomous materials discovery, but also the community in general.</span></blockquote><blockquote style="font-family: arial;"><span style="font-size: x-small;">(ii) We find that disorder in materials is often neglected in predictions. The predicted compounds investigated herein have all their elemental components located on distinct crystallographic positions, but in reality, elements can share crystallographic sites, resulting in higher symmetry space groups and - very often - known alloys or solid solutions. </span></blockquote><p><span style="font-family: times;">Life is messy. Chemistry is messy. DFT-based calculations are messy. AI is messy. </span></p><p><span style="font-family: times;">Given most discoveries of interesting materials often involve <a href="https://condensedconcepts.blogspot.com/2013/10/serendipity-remains-best-quantum.html" target="_blank">serendipity </a>or a lot of trial and error, it is worth trying to do what the authors of these papers are doing. However, the field will only advance in a meaningful way when it is not distracted and diluted by hype and authors, editors, and referees demand transparency about the limitations of their work. </span><span style="font-family: arial;"> </span></p><p><br /></p><p></p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com5tag:blogger.com,1999:blog-5439168179960787195.post-37546392552147060492024-01-05T15:22:00.000+10:002024-01-05T15:22:50.411+10:00Certain benefits of Bayes<p><span style="font-family: Times, "Times New Roman", serif;">Best wishes for the New Year! One thing I hope to achieve this year is an actual understanding of things "Bayesian".</span></p><p><span style="font-family: Times, Times New Roman, serif;">I am particularly interested because it gives a way to be more quantitative and precise about some of the intuitions that I use in science. For example, I tend to be skeptical of new experimental results (often hyped) that claim to go against well-established theories, regardless of how good the "statistics" of the touted result.</span></p><p><span style="font-family: Times, "Times New Roman", serif;"><span style="font-family: Times, "Times New Roman", serif;">In this vein, Phil Anderson argued that </span><a href="https://en.wikipedia.org/wiki/Bayes%27_theorem">Bayesian methods</a><span style="font-family: Times, "Times New Roman", serif;"> should have been used to rule out the significance of "discoveries" such as the 10 keV neutrino and the fifth force. In 1992 he wrote a </span><a href="https://physicstoday.scitation.org/doi/10.1063/1.2809482">Physics Today column on the subject.</a></span></p><p><span style="font-family: Times, "Times New Roman", serif;">An interesting metric for mathematical formula is the ratio of profound and wide implications to the simplicity of the formula and its derivation. </span><span style="font-family: Times, "Times New Roman", serif;">I suspect that <a href="https://en.wikipedia.org/wiki/Bayes%27_theorem" target="_blank">Bayes' formula for conditional probabilities</a> would win first place!</span></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgxnCMV9EKRiMIRCmOdrpb3Vm1bNYiVXOm8ilLziW9QSqiIFcAhYw__BjUDAR8Vwrv_bqYM-PJWYkjyjvQEca9xxgroApVmTI1QDfY3rfnroswYt_AVql6PUPP0MzGP8qGRb_zoIdHlznVL1Y09huz7Sr9xy6d6tDiaKnanztBlIJ6TbnFk5Q_-eAOE8OzS/s354/download.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="142" data-original-width="354" height="128" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgxnCMV9EKRiMIRCmOdrpb3Vm1bNYiVXOm8ilLziW9QSqiIFcAhYw__BjUDAR8Vwrv_bqYM-PJWYkjyjvQEca9xxgroApVmTI1QDfY3rfnroswYt_AVql6PUPP0MzGP8qGRb_zoIdHlznVL1Y09huz7Sr9xy6d6tDiaKnanztBlIJ6TbnFk5Q_-eAOE8OzS/s320/download.png" width="320" /></a></div><p></p><p><span style="font-family: Times, "Times New Roman", serif;">P(A|B) denotes the probability of A given B. </span></p><p><span style="font-family: Times, "Times New Roman", serif;">The proof takes about two lines. If you multiply both sides of the equation about by P(B) the identity holds because both sides of the equation are just different ways of writing P(A and B).</span></p><p><span style="font-family: Times, "Times New Roman", serif;">My first attempt to understand the applications and implications of Bayes was reading the relevant sections in Phil Nelson's beautiful book, <a href="https://www.physics.upenn.edu/biophys/PMLS2e/index.html" target="_blank">Physical Models of Living Systems</a>. There is a helpful section entitled, </span><span style="font-family: Times, "Times New Roman", serif;">"Bayes formula provides a consistent approach to upgrading our degree of belief in light of new data."</span></p><p><span style="font-family: Times, "Times New Roman", serif;">More recently, I found this wonderful and short video very helpful, as it clearly defines terms, uses graphical representations, and gives some concrete examples.</span></p><p>
<iframe allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen="" frameborder="0" height="235" src="https://www.youtube.com/embed/HZGCoVF3YvM?si=_tDmPC0ZsIDIh25s" title="YouTube video player" width="360"></iframe> </p><p><span style="font-family: Times, "Times New Roman", serif;">A Bayesian perspective highlights the importance of reporting negative results and is the basis of a seminal paper</span></p><p><span style="font-family: Times, Times New Roman, serif;"><a href="https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.0020124" target="_blank">Why Most Published Research Findings Are False</a> by </span><span style="font-family: Times, "Times New Roman", serif;">John P. A. Ioannidis</span></p><p><span style="font-family: Times, Times New Roman, serif;">A measure of the profundity of Bayes is that the S</span>tanford Encyclopedia of Philosophy has two articles on the topic</p><p><a href="https://plato.stanford.edu/entries/bayes-theorem/" target="_blank">Bayes Theorem</a></p><p><a href="https://plato.stanford.edu/entries/epistemology-bayesian/" target="_blank">Bayesian Epistemology</a></p><p><br /></p><p><br /></p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-37013603472465239992023-12-23T14:59:00.000+10:002023-12-23T14:59:00.451+10:00Niels Bohr on emergence<p>Until this week, I did not know that Bohr ever thought about emergence.</p><p><a href="https://en.wikipedia.org/wiki/Ernst_Mayr">Ernst Mayr </a>was one of the leading evolutionary biologists in the twentieth century and was influential in the development of the modern philosophy of biology. He particularly emphasised the importance of emergence and the limitations of reductionism. In the preface to his 1997 book, <a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674884694">This is Biology: the Science of the Living World</a>, Mayr recounts the development of his thinking about emergence.</p><p></p><blockquote><p><span style="font-family: arial;">At first I thought that this phenomenon of emergence, as it is now called, was restricted to the living world, and indeed, in a lecture I gave in the early 1950s in Copenhagen, I made the claim that emergence was the one of the diagnostic features of the of the organic world. </span><span style="font-family: arial;">The whole concept of emergence at the time was considered to be rather metaphysical. When Niels Bohr who was who was in the audience, stood up during the discussion, I was fully prepared for an annihilating refutation. However, much to my surprise, he did not at all object to the concept of emergence, but only to my notion that it provided a demarcation between the physical and the biological sciences. Citing the case of water whose "aquosity" could not be predicted from the characteristics of its two components, hydrogen and oxygen, Bohr stated that emergence is rampant in the inaminate world. (page xii).</span></p></blockquote><p><a href="https://condensedconcepts.blogspot.com/2015/09/quantum-biology-vitalism-of-bohr.html" target="_blank">Later in the book Mayr pillars Bohr for his support of vitalism,</a> including claims that vitalism has a "quantum" foundation.</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-88638702829123138772023-12-01T14:07:00.000+10:002023-12-01T14:07:08.349+10:00Very Short Introductions Podcast on Condensed Matter Physics<p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVP5gbtv_M7p5EzzTICdtfa1vXUU_BkZkjGLApiv1BGmLjsNU9vLs-sdgoZQXhzXHzHlVHAQIOneeDbHTgjKo8ltiA2IOKepmREBiw1LVseLRpLOBqvRtPSiTXJH_YvKBKruX6gCkdCnEiZCUrWKKYU5sp_g7qo13DdPGAJQwLeqBrD3e6YDpLA5CJj0sv/s1400/Ep%2077%20Cover.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1400" data-original-width="1400" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVP5gbtv_M7p5EzzTICdtfa1vXUU_BkZkjGLApiv1BGmLjsNU9vLs-sdgoZQXhzXHzHlVHAQIOneeDbHTgjKo8ltiA2IOKepmREBiw1LVseLRpLOBqvRtPSiTXJH_YvKBKruX6gCkdCnEiZCUrWKKYU5sp_g7qo13DdPGAJQwLeqBrD3e6YDpLA5CJj0sv/s320/Ep%2077%20Cover.png" width="320" /></a></div><p></p><div><span style="font-family: times;"><span style="caret-color: rgb(33, 33, 33); color: #212121; text-size-adjust: auto;">The<span class="Apple-converted-space"> </span></span><span class="outlook-search-highlight" data-markjs="true" style="caret-color: rgb(33, 33, 33); color: #212121; text-size-adjust: auto;">podcast</span><span style="caret-color: rgb(33, 33, 33); color: #212121; text-size-adjust: auto;"><span class="Apple-converted-space"> episode where I talk about my book just came out.</span></span></span></div><div><span style="font-family: times;"><span style="caret-color: rgb(33, 33, 33); color: #212121; text-size-adjust: auto;">It is available on a range of platforms, listed<span class="Apple-converted-space"> </span></span><a href="https://oxfordacademic.blubrry.net/subscribe-to-the-vsi-podcast/" style="color: #0078d7; text-size-adjust: auto;" target="_blank" title="https://oxfordacademic.blubrry.net/subscribe-to-the-vsi-podcast/">here</a><span style="caret-color: rgb(33, 33, 33); color: #212121; text-size-adjust: auto;">, including<span class="Apple-converted-space"> </span></span><a href="https://soundcloud.com/oupacademic/condensed-matter-physics-the-vsi-podcast-episode-77" style="color: #0078d7; text-size-adjust: auto;" target="_blank" title="https://soundcloud.com/oupacademic/condensed-matter-physics-the-vsi-podcast-episode-77">SoundCloud</a><span style="caret-color: rgb(33, 33, 33); color: #212121; text-size-adjust: auto;"><span class="Apple-converted-space"> and</span><span class="Apple-converted-space"> </span></span><a href="https://www.youtube.com/watch?v=Ut9ddYOu66E&list=PL3MAPgqN8JWhDt2ok5zoOotd7t9xUjccO&index=1" target="_blank">YouTube<span style="color: #212121;"><span style="caret-color: rgb(33, 33, 33);">.</span></span></a></span></div>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-88055665733320082512023-11-29T15:31:00.000+10:002023-11-29T15:31:02.749+10:00Emergence in nuclear physics<p>Nuclear physics exhibits many characteristics associated with emergent phenomena. These include a hierarchy of scales, effective interactions and theories, and universality.</p><p>The table below summarises how nuclear physics is concerned with phenomena that occur at a range of length and number scales. At each level of the hierarchy, there are effective interactions that are described by effective theories. Some of the biggest questions in the field concern how the effective theories that operate at each level are related to the levels above and below.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcbkG8-C95jFVLIBEm7AUhMkclcaN89FF-Hsz-wAbakqn7TDkVlw0SoYWxrMVdBPDmmwNan1KdoLlqeIKAzaG2iAY74HBqCzQN-gJllPJnVOD6zGH4rByRspWP6AH8Fg9o6PVRPP2vq31MYw-dYExVVIoEN8W2AA3djBz9WDeH_hiHEAX_qrGDVJym1owd/s4112/Untitled.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="2539" data-original-width="4112" height="248" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcbkG8-C95jFVLIBEm7AUhMkclcaN89FF-Hsz-wAbakqn7TDkVlw0SoYWxrMVdBPDmmwNan1KdoLlqeIKAzaG2iAY74HBqCzQN-gJllPJnVOD6zGH4rByRspWP6AH8Fg9o6PVRPP2vq31MYw-dYExVVIoEN8W2AA3djBz9WDeH_hiHEAX_qrGDVJym1owd/w400-h248/Untitled.jpg" width="400" /></a></div><p>Moving from the bottom level to the second top level, relevant length scales increase from less than a femtometre to several femtometres.</p><p>The challenge in the 1950s was to reconcile the liquid drop model and the nuclear shell model. This led to the discovery of collective rotations and shape deformations. The observed small moments of inertia were explained by BCS theory. <a href=" https://condensedconcepts.blogspot.com/2011/04/essence-of-nuclear-many-body-problem.html" target="_blank">Integration of the liquid drop and shell models </a>led to the award of the1975 Nobel Prize in Physics to Aage Bohr, Ben Mottelson, and Rainwater.</p><p>Since the 1980s a major challenge is to show how the strong nuclear force between two nucleons can be derived from Quantum Chromodynamics (QCD). The figure below illustrates how the attractive interaction between a neutron and a proton can be understood in terms of the creation and destruction of a down quark-antiquark pair. The figure is taken from <a href="https://physics.aps.org/articles/v10/72">here.</a></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj_rWRrsUxLWbWhbj_CRKIkL7MxikUiOaYSBHPtFsiufKeMIfzP3Ib4B9HvFGrKMwbEfqgf4TMDPLmg8www7StmyxOERGbGGFrV7mu9uACrLb46W5vzORn-3LH5hsK7Vq8aNhlA-sXLofl6m2pjRtYEN9BCrX51ZYRzj9d2Fn0ALRN2hNyfOki6WFUCyJcg/s800/e72_3.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="647" data-original-width="800" height="259" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj_rWRrsUxLWbWhbj_CRKIkL7MxikUiOaYSBHPtFsiufKeMIfzP3Ib4B9HvFGrKMwbEfqgf4TMDPLmg8www7StmyxOERGbGGFrV7mu9uACrLb46W5vzORn-3LH5hsK7Vq8aNhlA-sXLofl6m2pjRtYEN9BCrX51ZYRzj9d2Fn0ALRN2hNyfOki6WFUCyJcg/s320/e72_3.png" width="320" /></a></div><div class="separator" style="clear: both; text-align: left;"><div class="separator" style="clear: both; text-align: left;">An outstanding problem concerns the equation of state for nuclear matter, such as found in neutron stars. <a href="https://condensedconcepts.blogspot.com/2023/09/gravitational-waves-and-ultra-condensed.html" target="_blank">A challenge is to learn more about this from the neutron star mergers that are detected in gravitational wave astronomy.</a></div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both;">Characteristics of universality are also seen in nuclear physics. Landau’s Fermi liquid theory provides a basis for the nuclear shell model which starts from assuming that nucleons can be described in terms of weakly interacting quasiparticles moving in an average potential from the other nucleons. The BCS theory of superconductivity can be adapted to describe the pairing of nucleons, leading to energy differences between nuclei with odd and even numbers of nucleons. </div><div class="separator" style="clear: both;"><br /></div><div class="separator" style="clear: both;">Universality is also evident in the statistical distribution of energy level spacings in heavy nuclei. They can be described by random matrix theory which makes no assumptions about the details of interactions between nucleons, only that the Hamiltonian matrix has unitary symmetry. Random matrix theory can also describe aspects of quantum chaos and<a href="https://en.wikipedia.org/wiki/Freeman_Dyson#Quantum_physics_and_prime_numbers" target="_blank"> zeros of the Riemann zeta function relevant to number theory.</a></div><div class="separator" style="clear: both;"><br /></div></div><div><br /></div>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com1tag:blogger.com,1999:blog-5439168179960787195.post-6544337081332535942023-11-22T12:05:00.003+10:002023-11-22T17:35:37.638+10:00Shape memory alloys<p>Recently I <a href="https://www.madaboutscience.com.au/shop/nitinol-memory-wire-30cm-lengths-1.html" target="_blank">bought a small wire of NiTinol </a>to have fun with and use in demonstrations to kids. This video gives a spectacular demonstration and attempts to explain how it works. I did not know about their use in stents for heart surgery.</p><p>
<iframe allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen="" frameborder="0" height="235" src="https://www.youtube.com/embed/wI-qAxKJoSU?si=EYGbky5nAtfddshM" title="YouTube video player" width="360"></iframe>
<br /></p><p>I am still struggling to understand exactly how <a href="https://en.wikipedia.org/wiki/Shape-memory_alloy" target="_blank">shape-memory alloys</a> work. According to Wikipedia</p><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><p><span style="font-family: arial;">The shape memory effect occurs because a temperature-induced phase transformation reverses deformation...Typically the martensitic (low-temperature) phase is monoclinic or orthorhombic . Since t<b>hese crystal structures do not have enough slip systems for easy dislocation motion, they deform by twinning—or rather, detwinning</b>.</span></p><p><span style="font-family: arial;">Martensite is thermodynamically favored at lower temperatures, while austenite (B2 cubic) is thermodynamically favored at higher temperatures. Since these structures have different lattice sizes and symmetry, cooling austenite into martensite introduces internal strain energy in the martensitic phase. To reduce this energy, the martensitic phase forms many twins—this is called "self-accommodating twinning" and is the twinning version of <a href="https://en.wikipedia.org/wiki/Geometrically_necessary_dislocations" target="_blank">geometrically necessary dislocations.</a> </span></p></blockquote><p>In different words, I think the essential idea may be the following. In most metals large strains are accomodated by topological defects such as dislocations. These become entangled leading to work hardening and irreversible changes is macroscopic shapes. Shape memory alloys are different because of the low symmetry unit cell. The most natural defects are twinning domain walls and they are not topological and so their formation is reversible.</p><p>I am looking forward to reading the book chapter <a href="https://doi.org/10.1016/B978-0-12-814696-5.00014-9" target="_blank">Shape memory alloys </a>by Vladimir Buljak, Gianluca Ranzi</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhmju_I9diYLUSVJOcy8PXoOnPE-5CRal1SSsktFCX2eXTDS46YQoIyzDhPXIUlqbiWK7D8SnAVfAUhUV4IiLg39YgwHrX6besIKEbwR6PZJ6_ytJPKgszLFqMZpKQuxC7DvL1j0bmyQB4nEcl4kaErDdElPN5vG_m31kkpWNwF1JMOzmdit6SjgmxPkWiN/s1986/Untitled.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="719" data-original-width="1986" height="145" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhmju_I9diYLUSVJOcy8PXoOnPE-5CRal1SSsktFCX2eXTDS46YQoIyzDhPXIUlqbiWK7D8SnAVfAUhUV4IiLg39YgwHrX6besIKEbwR6PZJ6_ytJPKgszLFqMZpKQuxC7DvL1j0bmyQB4nEcl4kaErDdElPN5vG_m31kkpWNwF1JMOzmdit6SjgmxPkWiN/w400-h145/Untitled.jpg" width="400" /></a></div><br /><p><br /></p><p>Another fascinating phenomena that is related to shape-memory is "superelasticity", which I discussed in an <a href="https://condensedconcepts.blogspot.com/2021/08/springy-stringy-molecular-crystals.html" target="_blank">earlier post </a>on organic molecular crystals, and has <a href="https://pubs.rsc.org/en/content/articlelanding/2023/CS/D2CS00481J" target="_blank">recently been reviewed.</a></p><p>I welcome clarification of the essential physics.</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com2tag:blogger.com,1999:blog-5439168179960787195.post-35115990491984177652023-11-14T12:26:00.004+10:002023-11-14T12:58:49.047+10:00An emergentist perspective on public policy issues that divide<p>How is the whole related to the parts?</p><p>Which type of economy will produce the best outcomes: laissez-faire or regulated?</p><p>Can a government end an economic recession by "stimulus" spending? </p><p>What is the relative importance of individual agency and social structures in causing social problems such as poverty and racism?</p><p>These questions are all related to the first one. Let's look at it from an emergentist perspective, with reference to physics. </p><p>Consider the Ising model in two or more dimensions. The presence of nearest neighbour interactions between spins leads to emergent properties: long-range ordering of the spins, spontaneous symmetry breaking below the critical temperature, and singularities in the temperature dependence of thermodynamic properties such as the specific heat and magnetic susceptibility. Individual uncoupled spins have neither property. Even a finite number of spins do not. (Although, a large number of spins do exhibit suggestive properties such as an enhancement of the magnetic susceptibility near the critical temperature). Thus, the whole system has properties that are qualitatively different from the parts. </p><p>On the other hand, the properties of the parts, such as how strongly the spins couple to an external field and interact with their neighbours, influence the properties of the whole. Some details of the parts matter. Other details don't matter. Adding some interaction with spins beyond nearest neighbours does not change any of the qualitative properties, provided those longer-range interactions are not too large. On the other hand, changing from a two-dimensional rectangular lattice to a linear chain removes the ordered state. Changing to a triangular lattice with an antiferromagnetic nearest-neighbour interaction removes the ordering and there are multiple ground states. Thus, <b>some microscopic details do matter.</b></p><p>For illustrative purposes, below I show a sketch of the temperature dependence of the magnetic susceptibility of the Ising model for three cases: non-interacting spins (J=0), two dimensions (d=2), and one dimension (d=1). This shows how interactions can significantly enhance/diminish the susceptibility depending on the parameter regime.</p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhinhf2h31CR4XX1x_slbSwOnAHltGH4eCAOHLPkZr4GQqNw2ovX1AxLrUBD13c8eX6LvQ30gsmFPzz_EzYuKARkDVdIzXKoAKpZosA2lSiEQquyQkXkK9pXN1BDrEhXWIczI5C8ulXQj_gnLT7bkFsuRuoSDws1mhJuAsWeIlBNVCh2QAtL4rLv_9jSU64/s2250/Untitled.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1981" data-original-width="2250" height="282" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhinhf2h31CR4XX1x_slbSwOnAHltGH4eCAOHLPkZr4GQqNw2ovX1AxLrUBD13c8eX6LvQ30gsmFPzz_EzYuKARkDVdIzXKoAKpZosA2lSiEQquyQkXkK9pXN1BDrEhXWIczI5C8ulXQj_gnLT7bkFsuRuoSDws1mhJuAsWeIlBNVCh2QAtL4rLv_9jSU64/s320/Untitled.jpg" width="320" /></a></div><p></p><p>The main point of this example is to show that to understand a large complex system we have to keep both the parts and the whole in mind. In other words, we need both microscopic and macroscopic pictures. There are two horizons, the parts and the whole, the near and the far. There is a dialectic tension between these two horizons.<b> It is not either/or but both/and.</b></p><p>I now illustrate how this type of tension matters in economics and sociology, and the implications for public policy. If you are (understandably) concerned about whether Ising models have anything to do with sociology and economics, see my earlier posts about these issues. The first post introduced <a href="https://condensedconcepts.blogspot.com/2022/08/models-for-collective-social-phenomena.html" target="_blank">discrete-choice models that are essentially Ising models.</a> A second post discussed how these show how <a href="https://condensedconcepts.blogspot.com/2022/08/sociological-insights-from-statistical.html" target="_blank">equilibrium may never be reached</a> leading to the insight that local initiatives can "nucleate" desired outcomes. A third post, considered how <a href="https://condensedconcepts.blogspot.com/2022/08/hysteresis-hype-niches-nudges-and.html" target="_blank">heterogeneity can lead to qualitative changes</a> including hysteresis so that the effectiveness of "nudges" can vary significantly.</p><p>A fundamental (and much debated) question in sociology is the <a href="https://en.wikipedia.org/wiki/Structure_and_agency" target="_blank">relationship between individual agency and social structures</a>. Which determines which? Do individuals make choices that then lead to particular social structures? Or do social structures constrain what choices individuals make. In sociology, this is referred to as the debate between voluntarism and determinism. A middle way, that does not preference agency or structure, is <a href="https://en.wikipedia.org/wiki/Structuration_theory" target="_blank">structuration</a>, proposed by <a href="https://en.wikipedia.org/wiki/Anthony_Giddens" target="_blank">Anthony Giddens</a>.</p><p>Social theorists who give primacy to social structures will naturally advocate solving social problems with large government schemes and policies that seek to change the structures. On the other side, those who give primacy to individual agency are sceptical of such approaches, and consider progress can only occur through individuals, and small units such as families and communities make better choices. The structure/agency divide naturally maps onto political divisions of left versus right, liberal versus conservative, and the extremes of communist and libertarian. An emergentist perspective is balanced, affirming the importance of both structure and agency.</p><p>Key concepts in economics are equilibrium, division of labour, price, and demand. These are the outcomes of many interacting agents (individuals, companies, institutions, and government). Economies tend to self-organise. This is the "invisible hand" of Adam Smith. Thus, <a href="https://condensedconcepts.blogspot.com/2017/08/the-most-important-concept-in-economics.html" target="_blank">emergence is one of the most important concepts in economics.</a> </p><p>A big question is how the equilibrium state and the values of the associated state variables (e.g., prices, demand, division of labour, and wealth distribution) emerge from the interactions of the agents. In other words, what is the relationship between microeconomics and macroeconomics?</p><p>What are the implications for public policy? What will lead to the best outcomes (usually assumed to be economic growth and prosperity for "all")? Central planning (or at least some government regulation) is pitted against <i>laissez-faire. </i>For reasons, similar to the Ising and sociology cases, an emergentist perspective is that the whole and the parts are inseparable. This is why there is no consensus on the answers to specific questions such as, can government stimulus spending move an economy out of a recession? Keynes claimed it could but the debate rages on.</p><p>An emergentist perspective tempers expectations about the impact of agency, both individuals and government. It is hard to predict how a complex system with emergent properties will respond to perturbations such as changes in government policy. This is the "law" of <a href="https://en.wikipedia.org/wiki/Unintended_consequences" target="_blank">unintended consequences.</a></p><blockquote class="tr_bq"><span face=""arial" , "helvetica" , sans-serif" style="font-family: arial;">“The curious task of economics is to demonstrate to men <b>how little they really know</b> about what they imagine they can design.”</span></blockquote><p><a href="https://en.wikipedia.org/wiki/Friedrich_Hayek">Friedrich A. Hayek</a>, <i>The Fatal Conceit: The Errors of Socialism</i><br /></p><p>I think this cuts both ways. This is also reason to be skeptical about those (such as Hayek's disciples) who think they can "design" a better society by just letting the market run free.</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-46847755840123285072023-11-02T16:24:00.003+10:002023-11-02T16:24:52.902+10:00Diversity is a common characteristic of emergent properties <p>Consider a system composed of many interacting parts. I take the <a href="https://condensedconcepts.blogspot.com/2023/02/what-is-emergence.html" target="_blank"><b>defining</b> characteristic of an emergent property is <b>novelty</b>.</a> That is, the whole has a property not possessed by the parts alone. I argue that there are <a href="https://condensedconcepts.blogspot.com/2023/02/what-is-emergence.html" target="_blank">five other characteristics of emergent properties</a>. These characteristics are common but they are neither necessary nor sufficient for novelty.</p><p><i>1.<span style="white-space: pre;"> </span>Discontinuities</i></p><p><i>2.<span style="white-space: pre;"> </span>Unpredictability</i></p><p><i>3.<span style="white-space: pre;"> </span>Universality</i></p><p><i>4.<span style="white-space: pre;"> </span>Irreducibility</i></p><p><i>5.<span style="white-space: pre;"> </span>Modification of parts and their relations</i></p><p>I now add another characteristic.</p><p><i>6.<span style="white-space: pre;"> </span>Diversity</i></p><p>Although a system may be composed of only a small number of different components and interactions, the large number of possible emergent states that the system can take is amazing. Every snowflake is different. Water is found in 18 distinct solid states. All proteins are composed of linear chains of 20 different amino acids. Yet in the human body there are more than 100,000 different proteins and all perform specific biochemical functions. We encounter an incredible diversity of human personalities, cultures, and languages. </p><p>A related idea is that "simple models can describe complex behaviour". Here "complex" is often taken to mean diverse. Examples, how simple Ising models with a few competing interactions can describe <a href="https://condensedconcepts.blogspot.com/2020/11/the-devil-is-not-in-details.html" target="_blank">a devil's staircase of states</a> or the <a href="https://condensedconcepts.blogspot.com/2020/01/simple-model-hamiltonians-can-describe.html" target="_blank">multitude of atomic orderings found in binary alloys.</a></p><p>Perhaps the most stunning case of diversity is life on earth. Billions of different plant and animal species are all an expression of different linear combinations of the four base pairs of DNA: A, G, T, and C.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjF5S5LUqHggmBCifhyBcfOvRqHuhmj9lz2w7b3-nxJ4lb3n3-RW87jo5E1K_KSt8FfMG_tsbYjsQ6-mbwh_V3CjMCRpffpvGGAHEM7JzT5Np2gaFMIrQANQG93HUDswm3gtCQXLrsQyHhzAFEvevKXv_ndZ7pBt-3yxRHDMuuLEEVYY9Jstpv_hFJ43_zu/s960/392390_10151808953335251_1586116209_n.jpeg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="960" data-original-width="672" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjF5S5LUqHggmBCifhyBcfOvRqHuhmj9lz2w7b3-nxJ4lb3n3-RW87jo5E1K_KSt8FfMG_tsbYjsQ6-mbwh_V3CjMCRpffpvGGAHEM7JzT5Np2gaFMIrQANQG93HUDswm3gtCQXLrsQyHhzAFEvevKXv_ndZ7pBt-3yxRHDMuuLEEVYY9Jstpv_hFJ43_zu/w280-h400/392390_10151808953335251_1586116209_n.jpeg" width="280" /></a></div><p>One might argue that this diversity is just a result of combinatorics. For example, if one considers a chain of just ten amino acids there are 10^13 different possible linear sequences. But this does not mean that all these sequences will produce a functional protein, i.e., one that will fold rapidly (one the timescale of milliseconds) into a stable tertiary structure, and one that can perform a useful biochemical function. </p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-46010442730690932972023-10-24T08:57:00.005+10:002023-10-24T08:57:55.656+10:00Condensed matter physics in 15 minutes!<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiDxAJSy0TpO8ONCjt81L92cyO0cTDXkwtYv21BiH2lx-jkJwReNNv2brmPnbxBUDGI3rgasSoefJpxW8aZph5zIllkwzif8S2LAPN0n98iqHaoe_52x0zKCMWEb8Raqrvs9kz4e9jwo_n9R77At8-MFQMnsHkmqyl7npXeJEkQS1yjGpd9P16PkB4Dcp8i/s1400/VSI-Podcast-Cover-PowerPress.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1400" data-original-width="1400" height="200" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiDxAJSy0TpO8ONCjt81L92cyO0cTDXkwtYv21BiH2lx-jkJwReNNv2brmPnbxBUDGI3rgasSoefJpxW8aZph5zIllkwzif8S2LAPN0n98iqHaoe_52x0zKCMWEb8Raqrvs9kz4e9jwo_n9R77At8-MFQMnsHkmqyl7npXeJEkQS1yjGpd9P16PkB4Dcp8i/w200-h200/VSI-Podcast-Cover-PowerPress.png" width="200" /></a></div><p>Oxford University Press has a<a href="https://oxfordacademic.blubrry.net/subscribe-to-the-vsi-podcast/" target="_blank"> nice podcast on Very Short Introductions. </a></p><p>In each episode, an author of a specific volume has 10-15 minutes to introduce themself and answer several questions.</p><p>What is X [the subject of the VSI]?</p><p>What got you first interested in X?</p><p>What are the key aspects of X that you would like everyone to know?</p><p>The ones I have listened to and particularly liked are <a href="https://open.spotify.com/episode/0Dj7RxpDn9u9vXdz2IBIuF?si=rTqd1eMMR5ujbtDW2IxTVA" target="_blank">Infinity</a>, <a href="https://open.spotify.com/episode/7xgPIYrxGtNb8zEc0u5od7?si=4A-SZf1jQqa3SSBPqAMfDQ" target="_blank">Philosophy of Science</a>, <a href="https://open.spotify.com/episode/3EjHVXEV4dJCq6nwGAam5Y?si=gEm4C0RrT2-sNIkm-Fip0A" target="_blank">Evangelicalism</a>, <a href="https://open.spotify.com/episode/5L4c0c60jfHOCFJxcRifke?si=MHdctjCCSl2dafUZux3gdg" target="_blank">Development</a>, <a href="https://open.spotify.com/episode/34GqLhOv64DpiPBfHN2kUB?si=xPGlqaKMSQSDXsERGOfPGg" target="_blank">Consciousness</a>, <a href="https://open.spotify.com/episode/4pveSyfbuKScL5YblRl3XC?si=axyWMXqOTnifm8rAF6RTRg" target="_blank">Behavioural Economics,</a> and <a href="https://open.spotify.com/episode/5sATgGOBMsSbLiE6GoG7ZK?si=1r6arqEBQ0ag3PMJDsulUQ" target="_blank">Modern China.</a></p><p>Tomorrow, I am recording an episode for <a href="https://global.oup.com/academic/product/condensed-matter-physics-a-very-short-introduction-9780198845423?cc=au&lang=en&" target="_blank">Condensed Matter Physics: A Very Short Introduction.</a></p><p>Here is a practise <a href="https://drive.google.com/file/d/1rW4kO1s_ZmeXXfcQGniqaob7RC1c2Mcv/view?usp=sharing" target="_blank">version</a> of the audio and the draft text is below. </p><p>I welcome feedback.</p><p>VSI Podcast </p><p>I am Ross McKenzie. I am an Emeritus professor of physics at the University of Queensland in Brisbane, Australia. I have spent the past forty years learning, teaching, and researching condensed matter physics. I really love the Very Short Introduction series and so I am delighted to share my experience by writing Condensed Matter Physics: A Very Short Introduction.</p><p>What is condensed matter physics? It is all about states of matter. At school, you were probably taught that there are only three states of matter: solid, liquid, and gas. This is wrong. There are many more states such as liquid crystal, glass, superconductor, ferromagnet, and superfluid. New states of matter are continually, and often unexpectedly, being discovered. Condensed matter physics investigates how the distinct physical properties of states of matter emerge from the atoms of which a material is composed.</p><p>What first got me interested in condensed matter physics?</p><p>After I finished an undergraduate degree in theoretical physics in Australia in 1982, I would not have been able to answer the question, “what is condensed matter physics?”, even though it is the largest sub-field of physics. I then went to Princeton University in the USA to pursue a Ph.D. in and I took an exciting course on the subject and began to interact with students and faculty working in the field. </p><p>At Princeton was Phil Anderson, who had won a Nobel Prize in physics for work in condensed matter. At the time I did not appreciate his much broader intellectual legacy. In his recent biography of Anderson, Andrew Zangwill states “more than any other twentieth-century physicist, he [Anderson] transformed the patchwork of ideas and techniques formerly called solid-state physics into the deep, subtle, and intellectually coherent discipline known today as condensed matter physics.” Several decades later, my work became richer as Anderson gave me an appreciation of the broader scientific and philosophical significance of condensed matter physics, particularly its connection to other sciences, such as biology, economics, and computer science. When do quantitative differences become qualitative differences? Can simple models describe rich and complex behaviour? What is the relationship between the particular and the universal? How is the abstract related to the concrete?</p><p>So what are the key aspects of condensed matter physics that I would like everyone to know?</p><p>First, there are many different states of matter. It is not just solid, liquid, and gas. Consider the “liquid crystals” that are the basis of LCDs (Liquid Crystal Displays) in the screens of televisions, computers, and smartphones. How can something be both a liquid and a crystal? A liquid crystal is a distinct state of matter. Solids can be found in many different states. In everyday life, ice means simply solid water. But there are in fact eighteen different solid states of water, depending on the temperature of the water and the pressure that is applied to the ice. In each of these eighteen states, there is a unique spatial arrangement of the water molecules and there are qualitative differences in the physical properties of the different solid states.</p><p>Condensed matter physics is concerned with characterising and understanding all the different states of matter that can exist. These different states are called condensed states of matter. The word “condensed’’ is used here in the same sense as when we say that steam condenses into liquid water. Generally, as the temperature is lowered or the pressure is increased, a material can condense into a new state of matter. Qualitative differences distinguish the many different states of matter. These differences are associated with differences in symmetry and ordering.</p><p>Second, condensed matter physics involves a particular approach to understanding properties of materials. Every day we encounter a diversity of materials: liquids, glass, ceramics, metals, crystals, magnets, plastics, semiconductors, and foams. These materials look and feel different from one another. Their physical properties vary significantly: are they soft and squishy or hard and rigid? Shiny, black, or colourful? Do they absorb heat easily? Do they conduct electricity? The distinct physical properties of different materials are central to their use in technologies around us: smartphones, alloys, semiconductor chips, computer memories, cooking pots, magnets in MRI machines, LEDs in solid-state lighting, and fibre optic cables. Why do different materials have different physical properties? </p><p>Materials are studied by physicists, chemists, and engineers, and the questions, focus, goals, and techniques of researchers from these different disciplines can be quite different. The focus of condensed matter physics is on states of matter. Condensed matter physics as a research field is not just defined by the objects that it studies (states of matter in materials), but rather by a particular approach to the study of these objects. The aim is to address fundamental questions and to find unifying concepts and organizing principles to understand a wide range of phenomena in materials that are chemically and structurally diverse. </p><p>The central question of condensed matter physics is, how do the properties of a state of matter emerge from the properties of the atoms in the material and their interactions? </p><p>Let’s consider a concrete example, that of graphite and diamond. While you will find very cheap graphite in lead pencils, you will find diamonds in jewelery. Both graphite and diamond are composed solely of carbon atoms. They are both solid. So why do they look and feel so different? Graphite is common, black, soft, and conducts electricity moderately well. In contrast, diamond is rare, transparent, hard, and conducts electricity very poorly. We can zoom in down to the scale of individual atoms using X-rays and find the spatial arrangement of the carbon atoms relative to one another. These arrangements are qualitatively different in diamond and graphite.. Diamond and graphite are distinct solid states of carbon. They have qualitatively different physical properties, at both the microscopic and the macroscopic scale. </p><p>Third, I want you to know about superconductivity, one of the most fascinating states of matter. I have worked on it many times over the past forty years. Superconductivity occurs in many metals when they are cooled down to extremely low temperatures, close to absolute zero (-273 ºC). In the superconducting state, a metal can conduct electricity perfectly; without generating any heat. This state also expels magnetic fields meaning one can levitate objects, whether sumo wrestlers or trains. </p><p>The discovery of superconductivity in 1911 presented a considerable intellectual challenge: what is the origin of this new state of matter? How do the electrons in the metal interact with one another to produce superconductivity? Many of the greatest theoretical physicists of the twentieth century took up this challenge but failed. The theoretical puzzle was only solved 46 years after the experimental discovery. The theory turns out also to be relevant to liquid helium, nuclear physics, neutron stars, and the Higgs boson. New superconducting materials and different superconducting states continue to be discovered. A “holy grail” is to find a material that can superconduct at room temperature. </p><p>I find superconductivity even more interesting when considering quantum effects. By 1930 it was widely accepted that quantum theory, in all its strangeness, describes the atomic world of electrons, protons, and photons. However, this strangeness does not show itself in the everyday world of what we can see and touch. You cannot be in two places at the same time. Your cat is either dead or alive. However, condensed matter physicists have shown that the boundary between the atomic and macroscopic worlds is not so clear cut. A piece of superconducting metal can take on weird quantum properties, just like a single atom, even though the metal is made of billions of billions of atoms. It is in two states at the same time, almost like Schrodinger’s famous cat.</p><p>Fourth, condensed matter physics is all about emergence; the whole is greater than the sum of the parts. A system composed of many interacting parts can have properties that are qualitatively different from the properties of the individual parts. Water is wet, but a single water molecule is not. Your brain is conscious, but a single neuron is not. Such emergent phenomena occur in many fields, from biology to computer science to sociology, leading to rich intellectual connections. Condensed matter physics is arguably the field with the greatest success at understanding emergent phenomena in complex systems, particularly at the quantitative level. This is not because condensed matter physicists are smarter than sociologists, economists, or neuroscientists. It is because the materials we study are much “simpler” than societies, economies, and brains. </p><p>Finally, condensed matter physics is one of the largest and most vibrant sub-fields of physics. For example, in the past thirty years, the Nobel Prize in Physics has been awarded thirteen times for work on condensed matter. In the past twenty years, eight condensed matter physicists have received the Nobel Prize in Chemistry. </p><p>I hope I have sparked your interest in condensed matter physics. I invite you to learn more about why I consider this field of science significant, beautiful, and profound. </p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com1tag:blogger.com,1999:blog-5439168179960787195.post-87700743996161002972023-10-20T17:19:00.003+10:002023-10-20T17:23:45.085+10:00Opening the door for women in science<p> I really liked reading <a href="https://condensedconcepts.blogspot.com/2022/01/graduate-students-are-people.html" target="_blank">Transcendent Kingdom by Yaa Gyasi</a>. She is an amazing writer. I recently reread some of it for an extended family book club. Just check out some of <a href="https://www.goodreads.com/work/quotes/73528567-transcendent-kingdom" target="_blank">these quotes. </a></p><p>A colleague suggested I might like <a href="https://en.wikipedia.org/wiki/Lessons_in_Chemistry_(novel)" target="_blank">Lessons in Chemistry</a>, a novel by Bonnie Garmus. I have not read the book yet, but I have watched the first two episodes of the TV version on AppleTV. I watched the first episode for free.</p><p>The show contains a good mix of humour, love of science, and feminism. The chemistry dialogue seems to be correct. The show chronicles just how in the 1950s how awful life was for a young woman who aspired to be a scientist. Things have improved. But there is still a long way to go... </p>
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Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-36794144356473588382023-10-17T14:30:00.001+10:002023-10-17T14:30:59.915+10:00Could faculty benefit from a monastery experience?<p>A few months ago, the New York Times ran a fascinating Guest Opinion, <a href="https://www.nytimes.com/2023/05/25/opinion/college-students-monks-mental-health-smart-phones.html" target="_blank">Why Universities Should Be More Like Monasteries</a> by Molly Worth, a historian at University of North Carolina. She describes a popular undergraduate course, the "monk" class, at the University of Pennsylvania. </p><p></p><blockquote><span style="font-family: arial;">On the first day of class — officially called Living Deliberately — Justin McDaniel, a professor of Southeast Asian and religious studies, reviewed the rules. Each week, students would read about a different monastic tradition and adopt some of its practices. Later in the semester, they would observe a one-month vow of silence (except for discussions during Living Deliberately) and <b>fast from technology,</b> handing over their phones to him.</span></blockquote><p>This got me wondering about whether universities and funding agencies might experiment with <b>similar initiatives for faculty.</b> It might be <a href="https://condensedconcepts.blogspot.com/2016/08/aspen-versus-telluride.html" target="_blank">a bit like the Aspen Center for Physics </a>and Gordon Research Conferences were before the internet. Faculty would surrender their phones and have the internet disabled on their computers. For one week they would be required to spend their time reading, writing, and<b> thinking. </b>Exercise, daydreaming, doodling, and just having fun are to be encouraged. No administrative work or grant writing. The emphasis would be coming up with new ideas, not finishing off old work. For one hour per day, participants could meet with others and talk about their new ideas. I think it might be refreshing, reinvigorating, and highly productive (in the true sense of the word).</p><p>After trialling the program for a week, why not then try it for month-long periods.</p><p>These are the conditions under which Newton wrote the <i>Principia</i> and Darwin <i>The Origin of Species. </i>In their case, they did it over a period of several years.</p><p>What do you think?</p><p></p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com4tag:blogger.com,1999:blog-5439168179960787195.post-82798288039107321672023-10-13T12:35:00.000+10:002024-01-16T15:04:46.829+10:00Emergent abilities in AI: large language models<p>The public release of ChatGPT was a landmark that surprised many people, both in the general public and researchers working in Artificial Intelligence. All of a sudden it seemed<a href="https://en.wikipedia.org/wiki/Large_language_model" target="_blank"> Large Language Models</a> had capabilities that some thought were a decade away or even not possible. It is like the field underwent a "phase transition." This idea turns out to be more than just a physics metaphor. It has been made concrete and rigorous in the following paper.</p><p><a href="https://arxiv.org/abs/2206.07682" target="_blank">Emergent Abilities of Large Language Models</a></p><p>Jason Wei, Yi Tay, Rishi Bommasani, Colin Raffel, Barret Zoph, Sebastian Borgeaud, Dani Yogatama, Maarten Bosma, Denny Zhou, Donald Metzler, Ed H. Chi, Tatsunori Hashimoto, Oriol Vinyals, Percy Liang, Jeff Dean, William Fedus</p><p>They use the following definition, "Emergence is when quantitative changes in a system result in qualitative changes in behavior," citing Phil Anderson's classic <a href="https://www.science.org/doi/10.1126/science.177.4047.393" target="_blank">"More is Different"</a> article. [Even though the article does not contain the word emergence]. </p><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><p><span style="font-family: arial;">In this paper, we will consider a focused definition of emergent abilities of large language models: </span></p><p><span style="font-family: arial;"><i>An ability is emergent if it is not present in smaller models but is present in larger models.</i></span></p></blockquote><p>How does one define the "size" or "scale" of a model? Wei et al., note that "Today’s language models have been scaled primarily along three factors: amount of computation, number of model parameters, and training dataset size."</p><p>The essence of the analysis in the paper is summarised as follows.</p><p></p><blockquote><span style="font-family: arial;">We first discuss emergent abilities in the <i>prompting </i>paradigm, as popularized by GPT-3 (Brown et al., 2020). In prompting, a pre-trained language model is given a prompt (e.g. a natural language instruction) of a task and completes the response <b>without any further training or gradient updates to its parameters</b>. </span></blockquote><p>An example of a prompt is shown below </p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEigiwbt6eYScexESBBtcCGxmGxd-a9cpAc4-zTOYG4L2Y_ckQiSfwxwh28haJlD-rqVCzt6bMSi7Vst9XZXpuJxMhBT12W2aPcOhT-HqrZT_wn6F2Itqpg5Z18Z1gaiqS6TamnI2E10VudURqBQRYqX9Am2g90eXdDpkEWd2OprHDpPSZe3cSUyJpbaDRGu/s1439/Untitled.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="701" data-original-width="1439" height="156" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEigiwbt6eYScexESBBtcCGxmGxd-a9cpAc4-zTOYG4L2Y_ckQiSfwxwh28haJlD-rqVCzt6bMSi7Vst9XZXpuJxMhBT12W2aPcOhT-HqrZT_wn6F2Itqpg5Z18Z1gaiqS6TamnI2E10VudURqBQRYqX9Am2g90eXdDpkEWd2OprHDpPSZe3cSUyJpbaDRGu/s320/Untitled.jpg" width="320" /></a></div><blockquote><span style="font-family: arial;">Brown et al. (2020) proposed few-shot prompting, which includes a few input-output examples in the model’s context (input) as a preamble before asking the model to perform the task for an unseen inference-time example. </span></blockquote><p></p><blockquote> <span style="font-family: arial;">The ability to perform a task via few-shot prompting is emergent when a model has random performance until a certain scale, after which performance increases to well-above random. </span></blockquote><p></p><p>An example is shown in the Figure below. The horizontal axis is the number of training <a href="https://en.wikipedia.org/wiki/FLOPS" target="_blank">FLOP</a>s for the model, a measure of model scale. The vertical axis measures the accuracy of the model to perform a task, Modular Arithmetic, for which the model was not designed, but just given two-shot prompting. The red dashed line is the performance for a random model. The purple data is for GPT-3 and the blue for LaMDA. Note how once the model scale reaches about 10^22 there is a rapid onset of ability.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiew6rNxIRDWBhosCzRjJQsoFs5dmVE9LFTwmpOpTNRsleyG7OPXRmyszSsLrzLevg4O-T8S-mQFuFtudYc8mxL4SlpczG-GozGZOLU0mBvBPPu70NKkJgg1Ofh85Rmc2VUwzIrwzKIycWPimgdDhPr_O7G0KhNquYLRvK1ZXni2zC6bthl358HvUuf1m51/s1138/Untitled2.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1138" data-original-width="1013" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiew6rNxIRDWBhosCzRjJQsoFs5dmVE9LFTwmpOpTNRsleyG7OPXRmyszSsLrzLevg4O-T8S-mQFuFtudYc8mxL4SlpczG-GozGZOLU0mBvBPPu70NKkJgg1Ofh85Rmc2VUwzIrwzKIycWPimgdDhPr_O7G0KhNquYLRvK1ZXni2zC6bthl358HvUuf1m51/s320/Untitled2.jpg" width="285" /></a></div><div class="separator" style="clear: both; text-align: left;">The Figure below summarises recent results from a range of research groups studying five different language model families. It shows eight different emergent abilities.</div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgPiGXqRQQCGWor69OaCNm91dBVSAF6U1zpISnBYyzT8CjmI1W_KQxT7tkN8pX5Iaw1_1IcyQ53T6ijkXzNlSuIE2UdubeXCoratJn4N_l42XMq74zFjAvO6ofc-hnhonNQstOOD5QQv8zfomCm34aVKDhJZZzta1q1k6b1DrWnecUZZeFk_jtNxz2mDxBv/s3967/Untitled%2022.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="2791" data-original-width="3967" height="281" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgPiGXqRQQCGWor69OaCNm91dBVSAF6U1zpISnBYyzT8CjmI1W_KQxT7tkN8pX5Iaw1_1IcyQ53T6ijkXzNlSuIE2UdubeXCoratJn4N_l42XMq74zFjAvO6ofc-hnhonNQstOOD5QQv8zfomCm34aVKDhJZZzta1q1k6b1DrWnecUZZeFk_jtNxz2mDxBv/w400-h281/Untitled%2022.jpg" width="400" /></a></div><p>Wei et al., point out that "there are currently few compelling explanations for why such abilities emerge the way that they do".</p><p>The authors have encountered some c<a href="https://condensedconcepts.blogspot.com/2022/05/emergence-matters-in-nutshell.html" target="_blank">ommon characteristics of emergent properties</a>. They are hard to predict or anticipate before they are observed. They are often universal, i.e., they can occur in a wide range of different systems and are not particularly sensitive to the details of the components. Even after emergent properties are observed, it is still hard to explain why they occur, even when one has a good understanding of the properties of the system at a smaller scale. Superconductivity was observed in 1911 and only explained in 1957 by the BCS theory.</p><p>On the positive side, this paper presents hope that computational science and technology are at the point where AI may produce more exciting capabilities. On the negative side, there is also the possibility of <a href="https://arxiv.org/abs/2108.07258" target="_blank">significant societal risks</a> such as having unanticipated power to create and disseminate false information, bias, and toxicity.</p><p>Aside: One thing I found surprising is that the authors did not reference John Holland, a computer scientist, and his book, <a href="https://global.oup.com/academic/product/emergence-9780192862112?lang=en&cc=tr">Emergence.</a></p><p>I thank Gerard Milburn for bringing the paper to my attention.</p><p></p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-38114328752568495142023-09-28T16:36:00.000+10:002023-09-28T16:36:54.751+10:00Gravitational waves and ultra-condensed matter physics<p>In 2016, when I saw the first results from the LIGO gravitational wave interferometer my natural caution and skepticism kicked in. They had just observed one signal in an incredibly sensitive measurement. A lot of data analysis was required to extract the signal from the background noise. That signal was then fitted the results of numerical simulations of the solutions to Einstein's gravitational field equations describing the merger of two black holes. Depending on how you count about 15 parameters are required to specify the parameters of the binary system [distance from earth, masses, relative orientations of orbits, .... The detection events involve displacement of the mirrors in the interferometer by about 30 picometres!</p><p>What on earth could go wrong?!</p><p>After all, this was only two years after the <a href="https://en.wikipedia.org/wiki/BICEP_and_Keck_Array#BICEP2" target="_blank">BICEP2 fiasco</a> which claimed to have detected anisotropies in the cosmic microwave background due to gravitational waves associated with cosmic inflation. The observed signal turned out to be just cosmic dust! It led to a book, by the cosmologist Brian Keating, <a href="https://en.wikipedia.org/wiki/Brian_Keating#Losing_The_Nobel_Prize_(2018)" target="_blank">Losing the Nobel Prize: A Story of Cosmology, Ambition, and the Perils of Science’s Highest Honor</a></p><p>Well, I am happy to be wrong, if it is good for science. Now almost one hundred gravitational wave events have been observed and one event <a href="https://en.wikipedia.org/wiki/GW170817" target="_blank">GW170817</a> has been correlated with an x-ray observation.</p><p>But detecting some gravitational waves is quite a long way from gravitational wave astronomy, i.e, using gravity wave detectors as a telescope, in the same sense as the regular suite of optical, radio, X-ray, ... detectors. I was also skeptical about that. But it does not seem that gravity wave detectors are providing a new window into the universe.</p><p>A few weeks ago I heard a very nice UQ colloquium by <a href="https://research.monash.edu/en/persons/paul-lasky" target="_blank">Paul Lasky</a>, <i>What's next in gravitational wave astronomy?</i></p><p>Paul gave a nice overview of the state of the field, both past and future. </p><p>A key summary figure is below. It shows different possible futures when two neutron stars merge.</p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgJAfnECAejsZYvgH0eDQ7ueIx-B1bDt6Muk8xGz9hG65Kc9xo-nUslysPJ_mUMgdUkrbwiEzlN_U8U4cdDnv-96weq1LsqxxIe5Hza6jWXZD84AhHS3W5EU_TxvgPw94ZQpCfPPbSMtHcDDgnRfpnIj13bhE6uRmeusitSuK0YBC-mVQXjAOXrTjI4_hE9/s3062/Untitled%202.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1979" data-original-width="3062" height="259" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgJAfnECAejsZYvgH0eDQ7ueIx-B1bDt6Muk8xGz9hG65Kc9xo-nUslysPJ_mUMgdUkrbwiEzlN_U8U4cdDnv-96weq1LsqxxIe5Hza6jWXZD84AhHS3W5EU_TxvgPw94ZQpCfPPbSMtHcDDgnRfpnIj13bhE6uRmeusitSuK0YBC-mVQXjAOXrTjI4_hE9/w400-h259/Untitled%202.jpg" width="400" /></a></div><p>The figure is taken from the helpful review</p><p><a href="https://link.springer.com/article/10.1007/s10714-021-02831-1" target="_blank">The evolution of binary neutron star post-merger remnants: a review,</a> Nikhil Sarin and Paul D. Lasky</p><p>A few of the things that stood out to me.</p><p>1. One stunning piece of physics is that in the black hole mergers that have been observed the combined mass of the resulting black hole is three solar masses less than the total mass of the two separate black holes. <b>The resulting loss of mass energy (E=mc^2) of three solar masses is converted into gravitational wave energy within seconds.</b> During this time the peak radiant power was more than fifty times the power of all the stars in the observable universe combined!</p><p>I have fundamental questions about a clear physical description of this energy conversion process. First, defining "energy" in general relativity is <a href="https://iopscience.iop.org/article/10.1088/1361-6382/aad0ad" target="_blank">a vexed and unresolved question with a long history.</a> Second, is there any sense in which needs to describe this in terms of a quantum field theory: specifically conversion of neutron matter into gravitons?</p><p></p><p>2. <b>Probing nuclear astrophysics in neutron stars.</b> It may be possible to test the equation of state (relation between pressure and density) of nuclear matter. This determines the <a href="https://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%80%93Volkoff_limit" target="_blank">Tolman–Oppenheimer–Volkoff limit</a>; the upper bound to the mass of cold, non-rotating neutron stars. According to Sarin and Lasky</p><p><span style="font-family: arial;"></span></p><blockquote><span style="font-family: arial;">The supramassive neutron star observations again provide a <b>tantalising</b> way of developing our understanding of the dynamics of the nascent neutron star and the <b>equation of state of nuclear matter </b>(e.g., [37,121,127–131]). The procedure is straight forward: if we understand the progenitor mass distribution (which we do not), as well as the dominant spin down mechanism (we do not understand that either), and the spin-down rate/braking index (not really), then we can rearrange the set of equations governing the system’s evolution to find that the time of collapse is a function of the unknown maximum neutron star mass, which we can therefore infer. This procedure has been performed a number of times in different works, each arriving at different answers depending on the underlying assumptions at each of the step. The vanilla assumptions of dipole vacuum spin down of hadronic stars does not well fit the data [37,127], leading some authors to infer that quark stars, rather than hadronic stars, best explain the data (e.g., [129,130]), while others infer that gravitational radiation </span><span style="font-family: arial;">dominates the star’s angular momentum loss rather than magnetic dipole radiation (e.g [121,127]).</span></blockquote><p>As the authors say, this is a "tantalising prospect" but there are many unkowns. I appreciate their honesty. </p><span style="font-family: arial;"></span><p></p><p>3. <b>Probing the phase diagram of Quantum Chromodynamics (QCD)</b></p><p></p><div class="separator" style="clear: both; text-align: left;">This is one of my favourite phase diagrams and I used to love to show it to undergraduates.</div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSM-Gz8PoGM1BulKaLM6-MXjMLDtUGo4Kbj1aMlIPLEUrjLDY49WdXrwhZlJnxVxJZs5TxAbTzGbzgPKDNn2A7yo9KDm1nmUsFziSY6JZZINUB_6ffXce9JmWi1Vggty9xYyoZIKyHR6iIdRVXPhS7JgbBcXwF0Szy769jhkUj-fLg2W-hsQnqm12ONEXh/s850/Sketch-of-the-QCD-phase-diagram-in-the-plane-of-temperature-and-baryon-density.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="573" data-original-width="850" height="216" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSM-Gz8PoGM1BulKaLM6-MXjMLDtUGo4Kbj1aMlIPLEUrjLDY49WdXrwhZlJnxVxJZs5TxAbTzGbzgPKDNn2A7yo9KDm1nmUsFziSY6JZZINUB_6ffXce9JmWi1Vggty9xYyoZIKyHR6iIdRVXPhS7JgbBcXwF0Szy769jhkUj-fLg2W-hsQnqm12ONEXh/s320/Sketch-of-the-QCD-phase-diagram-in-the-plane-of-temperature-and-baryon-density.png" width="320" /></a></div><br />Neutron stars are close to the first-order phase transition associated with quark deconfinement.<p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhKBFnflQaO3xJm_7QJ98AX5grQuDIRGD0ur02pSWX89MVAaGxiE0ZCQ7nX-8pMZJcVzQ29lJrJ-RaGll2JJ37u-H4MveY9MvEyAgpQi5oZ_HBiYkLPonugRb7IUx-fJnWLPI6y9QLB_Gg7t6LIWvcX2Zd3nZNJPtKSenblJ5sF6t6OnNbdJwhBxKYASDZ4/s4000/IMG_20230908_113257887.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="2250" data-original-width="4000" height="180" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhKBFnflQaO3xJm_7QJ98AX5grQuDIRGD0ur02pSWX89MVAaGxiE0ZCQ7nX-8pMZJcVzQ29lJrJ-RaGll2JJ37u-H4MveY9MvEyAgpQi5oZ_HBiYkLPonugRb7IUx-fJnWLPI6y9QLB_Gg7t6LIWvcX2Zd3nZNJPtKSenblJ5sF6t6OnNbdJwhBxKYASDZ4/s320/IMG_20230908_113257887.jpg" width="320" /></a></div><p>When the neutron stars merge it may be that the phase boundary is crossed.</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com0tag:blogger.com,1999:blog-5439168179960787195.post-83087500421403054752023-09-14T12:56:00.000+10:002023-09-14T12:56:21.604+10:00Listing mistakes in Condensed Matter Physics: A Very Short Introduction<p>Someone told me that the day after your book is published you will start finding errors. They were correct.</p><p>Here are the first errors I have become aware of.</p><p>On Page 2 I erroneously state that diamond "conducts electricity and heat very poorly."</p><p>However, the truth about conduction of heat is below, taken from the opening paragraph of <a href="https://journals.aps.org/prb/abstract/10.1103/PhysRevB.80.125203" target="_blank">this paper.</a></p><p></p><blockquote><span style="font-family: arial;">Diamond has the highest thermal conductivity, L, of any known bulk material. Room-temperature values of L for isotopically enriched diamond exceed 3000 W/m-K, more than an order of magnitude higher than common semiconductors such as silicon and germanium. In diamond, the strong bond stiffness and light atomic mass produce extremely high phonon frequencies and acoustic velocities. In addition, the phonon-phonon umklapp scattering around room temperature is unusually weak.</span></blockquote><p></p><p>Figure 2 on page 4 has a typo. Diamond is "hard" not "hand".</p><p>On page 82 I erroneously state that for the superfluid transition, the "critical exponent alpha was determined to have a value of -0.0127, that is to five significant figures." The value actually has three significant figures. </p><p>I thank my engineering friend, Dave Winn, for pointing out the first and third errors.</p><p>Please do write other errors in the comments below. This will help with future revisions.</p>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com5tag:blogger.com,1999:blog-5439168179960787195.post-16528074738033453882023-09-11T13:40:00.002+10:002023-09-11T13:40:55.653+10:00Amazing things about Chandrasekhar's white dwarf mass limit <p>This is ultra-condensed matter physics!</p><p>In 1931, <a href="https://en.wikipedia.org/wiki/Subrahmanyan_Chandrasekhar" target="_blank">Subrahmanyan Chandrasekhar</a> published a seminal paper, for which he was awarded the Nobel Prize in 1983. He showed that a white dwarf star must have a mass less than 1.4 solar masses, otherwise it will collapse under gravity. White dwarfs are compact stars for which the nuclear fuel is spent and electron degeneracy pressure prevents gravitational collapse. </p><p>The blog Galileo unbound has<a href="https://galileo-unbound.blog/2019/01/07/chandrasekhars-limit/" target="_blank"> a nice post </a>about the history and the essential physics behind the paper.</p><p>There are a several things I find quite amazing about Chandrasekhar's derivation and the expression for the maximum possible mass. </p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhlvzz0-XA88QZsLXDoxGO-Vqyfc-MlPs74vcce54IMDnPGCXitfo_UCSxC9N4cUlcWkhcigbaxA7_03Vc1aTWNkGb7NnBEu1dj_gbj0G4C889orkoWcgGP-_W8ULQqnlGu-JFwMNRWYP9KFQ6NTg5f9NH5n_IuX5L9vLy4W8r7dmlgwzkix3brbfsjXFIw/s1042/Untitled.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="332" data-original-width="1042" height="127" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhlvzz0-XA88QZsLXDoxGO-Vqyfc-MlPs74vcce54IMDnPGCXitfo_UCSxC9N4cUlcWkhcigbaxA7_03Vc1aTWNkGb7NnBEu1dj_gbj0G4C889orkoWcgGP-_W8ULQqnlGu-JFwMNRWYP9KFQ6NTg5f9NH5n_IuX5L9vLy4W8r7dmlgwzkix3brbfsjXFIw/w400-h127/Untitled.jpg" width="400" /></a></div><div class="separator" style="clear: both; text-align: left;">m_H is the mass of a proton. M_P is the Planck mass. The value of the mass limit is about 1.4 solar masses.</div><p><b>Relativity matters</b></p><p>If the electrons are treated non-relativistically then there is no mass limit. However, when the star becomes dense enough the Fermi velocity of the electrons approaches the speed of light. Then relativistic effects must be included.</p><p><b>A macroscopic quantum effect</b></p><p>Degeneracy pressure is a macroscopic quantum effect. The expression above involves Planck's constant.</p><p><b>Quantum gravity</b></p><p>The mass formula involves the Planck mass, M_P. On the one hand, this phenomena does not involve quantum gravity because there is no quantisation of the gravitational field. On the other hand, the effect does involve the interplay of gravity and quantum physics.</p><p><b>A "natural" scale</b></p><p>Formula that involve <a href="https://en.wikipedia.org/wiki/Planck_units" target="_blank">Planck scales</a> usually represent scales of length, energy, mass, time, and temperature that are "unreal", i.e., they are vastly different from terrestial and astrophysical phenomena. For example, the temperature of the <a href="https://en.wikipedia.org/wiki/Hawking_radiation" target="_blank">Hawking radiation </a>from a black hole of one solar mass is 60 nanoKelvin. </p><p>In contrast, the limiting mass is on the same scale as that of our own sun!</p><p><b>It agrees with astronomical observations</b></p><p><a href="https://doi.org/10.1111/j.1365-2966.2006.11388.x" target="_blank">Determinations of the masses of hundreds of white dwarfs</a> show most have a mass of about 0.5 solar masses and the highest observed value is 1.3 solar masses.</p><div><br /></div>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com2tag:blogger.com,1999:blog-5439168179960787195.post-53998364433336216552023-09-07T11:42:00.005+10:002023-09-07T11:42:49.672+10:00Hollywood and a Physical Review paper<p> I am not sure I have seen this before. If you watched the movie Oppenheimer, you may have noticed that a one point a student excitedly showed Oppenheimer the latest issue of <i>Physical Review</i> and the following image flashed across the movie screen.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg1eqOhyOj0rm1wUigWt8xFydlFFIX5qtIClGZan-8faSzKjE2U3upGU3upvDILhijNw-a8nkq3AaUgwZuYY9sK83y9dNcpN8hols-w90WcYS3qpWoZ5_pLetZLf716vcAzvxH5m3e9KFol36FGBnonnFCXZTCWQtnpF26YwEQwbWNcpuJTwe46SrnEMhO6/s4123/Untitled.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="2369" data-original-width="4123" height="230" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg1eqOhyOj0rm1wUigWt8xFydlFFIX5qtIClGZan-8faSzKjE2U3upGU3upvDILhijNw-a8nkq3AaUgwZuYY9sK83y9dNcpN8hols-w90WcYS3qpWoZ5_pLetZLf716vcAzvxH5m3e9KFol36FGBnonnFCXZTCWQtnpF26YwEQwbWNcpuJTwe46SrnEMhO6/w400-h230/Untitled.jpg" width="400" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><div class="separator" style="clear: both; text-align: left;"><a href="https://journals.aps.org/pr/abstract/10.1103/PhysRev.56.455" target="_blank">On Continued Gravitational Contraction</a></div><div class="separator" style="clear: both; text-align: left;">J. R. Oppenheimer and H. Snyder</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;"><div class="separator" style="clear: both;">A beautiful blog post just appeared on 3 Quarks Daily,</div><div class="separator" style="clear: both;"> </div><div class="separator" style="clear: both;"><a href="https://3quarksdaily.com/3quarksdaily/2023/09/september-1-1939-a-tale-of-two-papers.html" target="_blank">September 1, 1939: A Tale Of Two Papers</a></div><div class="separator" style="clear: both;">by <a href="http://wavefunction.fieldofscience.com/" target="_blank">Ashutosh Jogalekar</a></div><div class="separator" style="clear: both;"><br /></div><div class="separator" style="clear: both;">The post describes the scientific and historical significance of the paper, including how it attracted no interest for twenty years, being eclipsed by a paper in the same issue of Physical Review.</div><div class="separator" style="clear: both;"><br /></div><div class="separator" style="clear: both;"><div class="separator" style="clear: both;"><a href="https://journals.aps.org/pr/abstract/10.1103/PhysRev.56.426" target="_blank">The Mechanism of Nuclear Fission</a></div><div class="separator" style="clear: both;">Niels Bohr and John Archibald Wheeler</div></div><div class="separator" style="clear: both;"><br /></div><div class="separator" style="clear: both;">Have you ever seen a Hollywood movie that explicitly showed the page of a scientific journal article.</div></div></div>Ross H. McKenziehttp://www.blogger.com/profile/09950455939572097456noreply@blogger.com1