{"id":674,"date":"2014-05-20T22:48:16","date_gmt":"2014-05-20T10:48:16","guid":{"rendered":"https:\/\/blog.waikato.ac.nz\/physicsstop\/2014\/05\/20\/seeing_in_the_dark\/"},"modified":"2014-05-20T22:48:16","modified_gmt":"2014-05-20T10:48:16","slug":"seeing-in-the-dark","status":"publish","type":"post","link":"https:\/\/blog.waikato.ac.nz\/physicsstop\/2014\/05\/20\/seeing-in-the-dark\/","title":{"rendered":"Seeing in the dark"},"content":{"rendered":"<p> No, nothing to do with carrots and vitamin A I&#039;m afraid.&nbsp;<\/p>\n<p> With dark evenings and mornings with us now :(, Benjamin&#039;s become interested in the dark. It&#039;s dark after he&#039;s finished tea, and he likes to be taken outside to see the dark, the moon, and stars, before his bath. &quot;See dark&quot; has become a predictable request after he&#039;s finished stuffing himself full of dinner. It&#039;s usually accompanied by a hopeful &quot;Moon?&quot; &nbsp;(pronounced &quot;Moo&quot;) to which Daddy has had to tell him that the moon is now a morning moon, and it will be way past his bedtime before it rises.&nbsp;<\/p>\n<p> I haven&#039;t yet explained that his request is an oxymoron. How can one see the dark? Given dark is lack of light, what we are really doing is not seeing. But there&#039;s plenty of precedence for attributing lack of something to an entity itself, so &#039;seeing the dark&#039; is quite a reasonable way of looking at it. &nbsp;<\/p>\n<p> One can talk about cold, for example. &quot;Feel how cold it is this morning&quot;. It is heat, a form of energy, that is the physical entity here. Cold is really the lack of heat, but we&#039;re happy to talk about it as if it were a thing in itself. Another example: Paul Dirac in 1928 interpreted the lack of electrons in the negative energy states that arise from <a href=\"http:\/\/en.wikipedia.org\/wiki\/Dirac_equation\" target=\"_blank\">his description of relativistic quantum mechanics<\/a> as being <a href=\"http:\/\/en.wikipedia.org\/wiki\/Dirac_equation#Hole_theory\" target=\"_blank\">anti-electrons<\/a>, or positrons. In fact, this was a <u>prediction<\/u> of the existence of anti-matter &#8211; the discovery of the positron didn&#039;t come until latter. &nbsp;<\/p>\n<p> In semiconductor physics, we have &#039;holes&#039;. These are the lack of electrons in a valence band &#8211; a &#039;band&#039; being a broad region of energy states where electrons can exist. If we take an electron out of the band we leave a &#039;hole&#039;. This enables nearby electrons to move into the hole, leaving another hole. In this way holes can move through a material. It&#039;s rather like one of those slidy puzzles &#8211; move the pieces one space at a time to create the picture. Holes are a little bit tricky to teach to start with. Taking an electron out of a material leaves it charged, so we say a hole has a positive charge. That&#039;s a bit confusing &#8211; some students will usually start of thinking that holes are protons. Holes will accelerate if an electric field is applied (because they have positive charge) and so we can attribute a mass to the hole. That&#039;s another conceptual jump. How can the lack of something have a mass? Holes, because they are the lack of an electron, tend to move to the highest available energy states not the lowest energy states. Once the idea is grasped, we can start talking about holes as real things, and that is pretty well what solid-state physics textbooks will do. It works to treat them as positively charged particles. It&#039;s easy then to forget that we talking about things that are really the lack of something, rather than something in themselves.&nbsp;<\/p>\n<p> A more recent example is being developed in relation to mechanics of materials as part of a <a href=\"http:\/\/www.royalsociety.org.nz\/programmes\/funds\/marsden\/\" target=\"_blank\">Marsden-funded<\/a> project by my colleage Ilanko. He&#039;s working with <a href=\"http:\/\/researchcommons.waikato.ac.nz\/handle\/10289\/5779\" target=\"_blank\">negative masses and stiffnesses on structures<\/a> &#8211; as a way of facilitating the analysis of the vibrational states and resonances of a structure (e.g. a building). By treating the lack of something as a real thing, we often can find our physics comes just a bit easier to work through.&nbsp;<\/p>\n<p> So seeing the dark is not such a silly request, after all.<\/p>\n<p> &nbsp;<\/p>\n<p> &nbsp;<\/p>\n<p> &nbsp;<\/p>\n<p> &nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>No, nothing to do with carrots and vitamin A I&#039;m afraid.&nbsp; With dark evenings and mornings with us now :(, Benjamin&#039;s become interested in the dark. It&#039;s dark after he&#039;s finished tea, and he likes to be taken outside to see the dark, the moon, and stars, before his bath. &quot;See dark&quot; has become a [&hellip;]<\/p>\n","protected":false},"author":20,"featured_media":837,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[11,7,12,25,17,13],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v22.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Seeing in the dark : Physics Stop<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blog.waikato.ac.nz\/physicsstop\/2014\/05\/20\/seeing-in-the-dark\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Seeing in the dark : Physics Stop\" \/>\n<meta property=\"og:description\" content=\"No, nothing to do with carrots and vitamin A I&#039;m afraid.&nbsp; With dark evenings and mornings with us now :(, Benjamin&#039;s become interested in the dark. 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