![\"i-8b87d28d7103d67b6d4a80283007845c-dec20_photon.png\"](\"http:\/\/scienceblogs.com\/principles\/wp-content\/blogs.dir\/467\/files\/2012\/04\/i-8b87d28d7103d67b6d4a80283007845c-dec20_photon.png\")
<\/p>\n<\/blockquote>\n
From the year, you can probably guess the guy responsible: Albert Einstein. Einstein realized that if you took Planck’s idea and ran with it, you could explain the photoelectric effect very neatly. Where Planck had viewed the quantized radiation as a fictitious property of the black-body emitting light, Einstein applied it to the light itself. Light was not a continuous wave, but a stream of “light quanta” (now called “photons”), little particles of light, each carrying an amount of energy equal to Planck’s constant (h<\/i>) multiplied by the frequency of the light (represented by a Greek letter nu (ν), for what reason I can’t say). You can also write this has Planck’s constant times the speed of light divided by the wavelength of the light, which is more convenient in some circumstances.<\/p>\n
So, why is this important?<\/p>\n
<\/p>\n
Well, why isn’t<\/em> it important? It was arguably the most revolutionary thing Einstein did in his career (Pais quotes Einstein calling it the only really revolutionary thing he did; relativity can be seen as simply putting existing physics on a more solid theoretical and philosophical foundation), and it was the one accomplishment specifically mentioned in his Nobel Prize citation<\/a>, awarded “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect.”<\/p>\n This is the step that puts the “quantum” in “quantum mechanics,” telling us that energy comes in discrete chunks, not a continuous range of values. It’s the theoretical explanation for the photoelectric effect, which in turn is the basis for all manner of light sensors– basically anything that converts a light signal into an electrical signal.<\/p>\n The discrete photon energy, when combined with Einstein’s relativity, also implies that photons carry momentum. This, in turn, has a number of technological applications: the scattering force that arises from atoms absorbing photons of light is the basis for laser cooling, which serves as the starting point for all manner of experiments investigating ultra-cold matter, including ultra-precise atomic clocks. At a much higher energy density, radiation pressure from light scattering plays a major role in making a hydrogen bomb work, and is used in laser-ignited fusion experiments as well.<\/p>\n And speaking of lasers, the operating principle of a laser is most easily understood using photons. And lasers are everywhere<\/em> these days– you wouldn’t be able to read this without the telecom lasers that carry the Internet over fiber-optic transmission lines. And while most laser technologies involve huge numbers of coherent photons, laser technology also makes it possible to make single-photon sources, and sources of entangled pairs of photons, which can be used to do quantum teleportation and quantum cryptography, and maybe someday form the backbone of a “quantum internet” carrying quantum information between quantum computers (and, presumably, quantum spam from quantum scammers in quantum Nigeria).<\/p>\n So, take a moment today to appreciate Einstein’s most revolutionary contribution to physics. And come back tomorrow for the next revolutionary development as we continue our countdown to Newton’s birthday.<\/p>\n","protected":false},"excerpt":{"rendered":" Yesterday’s equation was the first real result of quantum theory, Max Planck’s formula for the black-body spectrum. Planck never really liked the quantum basis of it, though, and preferred to think of it as just a calculational trick. It wasn’t until 1905 that anybody took the idea really seriously, leading to today’s equation: From the… Continue reading The Advent Calendar of Physics: Einstein’s Nobel<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[666,169,7,23,11],"tags":[],"class_list":["post-5957","post","type-post","status-publish","format-standard","hentry","category-advent","category-lasers","category-physics","category-quantum_optics","category-science","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/5957","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=5957"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/5957\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=5957"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=5957"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=5957"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}