Christoph Schiller MOTION MOUNTAIN the adventure of physics – vol.iii light, charges and brains www.motionmountain.net Christoph Schiller M M e Adventure of Physics Volume III Light, Charges and Brains Edition ., available as free pdf at www.motionmountain.net Editio vicesima quinta. Proprietas scriptoris © Chrestophori Schiller primo anno Olympiadis trigesimae. Omnia proprietatis iura reservantur et vindicantur. Imitatio prohibita sine auctoris permissione. Non licet pecuniam expetere pro aliqua, quae partem horum verborum continet; liber pro omnibus semper gratuitus erat et manet. Twenty-h edition. Copyright ©  by Christoph Schiller, the rst year of the th Olympiad. is pdf le is licensed under the Creative Commons Attribution-Noncommercial-No Derivative Works . Germany Licence,whosefulltextcanbefoundonthewebsite creativecommons.org/licenses/by-nc-nd/./de, with the additional restriction that reproduction, distribution and use, in whole or in part, in any product or service, be it commercial or not, is not allowed without the written consent of the copyright owner. e pdf le was and remains free for everybody to read, store and print for personal use, and to distribute electronically, but only in unmodied form and at no charge. To Britta, Esther and Justus Aaron τ µο δαµονι Die Menschen stärken, die Sachen klären. PREFACE “ Primum movere, deinde docere.* ” Antiquity isbookiswrittenforanybodywhoiscuriousaboutnatureandmotion.Curiosity about how people, animals, things, images and space move leads to many adventures. is volume presents the adventures one encounters when exploring everything electric. e story ranges from the weighing of electric current to the use of magnetic elds to heal bone fractures and up to the understanding of the human brain. In the structure of physics, shown in Figure , motion due to electricity is the most fascinating aspect of the starting point at the bottom. Indeed, almost everything around us is due to electric processes. e present introduction to electricity, magnetism, light and the brain is the third of a six-volume overview of physics that arose from a threefold aim that I have pursued since : to present motion in a way that is simple, up to date and captivating. In order to be simple, the text focuses on concepts, while keeping mathematics to the necessary minimum. Understanding the concepts of physics is given precedence over using formulae in calculations. e whole text is within the reach of an undergraduate. In order to be up to date, the text is enriched by the many gems – both theoretical and empirical – that are scattered throughout the scientic literature. In order to be captivating, the text tries to startle the reader as much as possible. Read- ing a book on general physics should be like going to a magic show. We watch, we are astonished, we do not believe our eyes, we think, and nally we understand the trick. When we look at nature, we oen have the same experience. Indeed, every page presents at least one surprise or provocation for the reader to think about. Numerous interesting challenges are proposed. e motto of the text, die Menschen stärken, die Sachen klären,afamousstatementby Hartmut von Hentig on pedagogy, translates as: ‘To fortify people, to clarify things.’ Clar- ifying things – and adhering only to the truth – requires courage, as changing the habits of thought produces fear, oen hidden by anger. But by overcoming our fears we grow in strength. And we experience intense and beautiful emotions. All great adventures in life allow this, and exploring motion is one of them. Enjoy it! Munich,  November . * ‘First move, then teach.’ In modern languages, the mentioned type of moving (the heart) is called motivat- ing;bothtermsgobacktothesameLatinroot. Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net Copyright © Christoph Schiller June 1990–November 2012   Galilean physics, heat and electricity Adventures: sport, music, sailing, cooking, describing beauty and understanding its origin (vol. I), using electricity, light and computers, understanding the brain and people (vol. III). Special relativity Adventures: light, magnetism, length contraction, time dilation and E 0 = mc 2 (vol. II). Quantum theory Adventures: death, reproduction, biology, chemistry, evolution, enjoying colours and art, all high-tech business, medicine (vol. IV and V). Quantum theory with gravity Adventures: bouncing neutrons, under- standing tree growth (vol. V). Final, unified description of motion Adventures: understanding motion, intense joy with thinking, calculating couplings and masses, catching a glimpse of bliss (vol. VI). G c h, e, k PHYSICS: Describing motion with the least action principle. Quantum field theory Adventures: building accelerators, under- standing quarks, stars, bombs and the basis of life, matter, radiation (vol. V). How do everyday, fast and large things move? How do small things move? What are things? Why does motion occur? What are space, time and quantum particles? General relativity Adventures: the night sky, measu- ring curved space, exploring black holes and the universe, space and time (vol. II). Classical gravity Adventures: climbing, skiing, space travel, the wonders of astronomy and geology (vol. I). FIGURE 1 A complete map of physics: the connections are defined by the speed of light c,the gravitational constant G, the Planck constant h, the Boltzmann constant k and the elementary charge e. A   In my experience as a teacher, there was one learning method that never failed to trans- form unsuccessful pupils into successful ones: if you read a book for study, summarize every section you read, in your own images and words, aloud.Ifyouareunabletodo so, read the section again. Repeat this until you can clearly summarize what you read in your own images and words, aloud. You can do this alone in a room, or with friends, or while walking. If you do this with everything you read, you will reduce your learning and reading time signicantly. e most inecient learning method is to use a marker or to underline text: it wastes time, provides false comfort and makes the text unreadable. Nobody marking text is an Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net Copyright © Christoph Schiller June 1990–November 2012   ecient learner. Instead, by repeating every section in your own images and words, aloud, youwillsavetimeandmoney,enjoylearningfromgoodtextsmuchmoreandhatebad texts much less. Masters of the method can use it even while listening to a lecture, in a low voice, thus avoiding to ever take notes. U   Text in green, as found in many marginal notes, marks a link that can be clicked in a pdf reader. Such green links are either bibliographic references, footnotes, cross references to other pages, challenge solutions, or pointers to websites. Solutions and hints for challenges are given in the appendix. Challenges are classied as research level (r), dicult (d), standard student level (s) and easy (e). Challenges of type r, d or s for which no solution has yet been included in the book are marked (ny). F   is text is and will remain free to download from the internet. I would be delighted to receive an email from you at fb@motionmountain.net, especially on the following issues:  What was unclear and should be improved? Challenge 1 s  What story, topic, riddle, picture or movie did you miss?  What should be corrected? In order to simplify annotations, the pdf le allows adding yellow sticker notes in Adobe Reader. Alternatively, you can provide feedback on www.motionmountain.net/ wiki.Helponthespecicpointslistedonthewww.motionmountain.net/help.html web page would be particularly welcome. All feedback will be used to improve the next edi- tion. On behalf of all readers, thank you in advance for your input. For a particularly useful contribution you will be mentioned – if you want – in the acknowledgements, receive a reward, or both. Your donation to the charitable, tax-exempt non-prot organisation that produces, translates and publishes this book series is welcome! For details, see the web page www. motionmountain.net/donation.html. If you want, your name will be included in the sponsor list. ank you in advance for your help, on behalf of all readers across the world. A paper edition of this book, printed on demand and delivered by mail to any ad- dress, can be ordered at www.lulu.com/spotlight/motionmountain.Butaboveall,enjoy the reading! Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net Copyright © Christoph Schiller June 1990–November 2012 C   L ,      Fields:amber,lodestoneandmobilephones  •Howcanonemakelightning?  •Electriccharge  •Electriceldstrength  •Pumpingcharge  •What is electricity?  • Can we detect the inertia of electricity?  • Feeling electric elds  • Magnets and other magnetic materials  • Can humans feel magnetic elds?  • Magnetism and electricity  • How can one make a motor?  • Which currents ow inside magnets?  •Magneticelds  •Electromag- netism  • e invariants and the Lagrangian of electromagnetic elds  •e uses of electromagnetic eects  • How motors prove relativity to be right  • Curiosities and fun challenges about things electric and magnetic  • Hopping electrons and the biggest disappointment of the television industry  •Howdo nerves work?  • A summary: three basic facts about electricity    T      e rst eld equation of electrodynamics  •esecondeldequationofelec- trodynamics  •evalidityandtheessenceofMaxwell’seldequations  • Colliding charged particles  • e gauge eld – the electromagnetic vector poten- tial  • Energy and momenta of the electromagnetic eld  •eLagrangian of electromagnetism  • e energy–momentum tensor and its symmetries of motion  •Whatisamirror?  • What is the dierence between electric and magnetic elds?  • Could electrodynamics be dierent?  •ebrain:the toughest challenge for electrodynamics  • Challenges and fun curiosities about electrodynamics  • Summary   W? What are electromagnetic waves?  • Light as an electromagnetic wave  • Polarization and electromagnetic waves  • Light and other electromagnetic waves  • e slowness of progress in physics  •Anotherlookatelectromag- netic radiation  • How does the world look when riding on a light beam?  • Can one touch light?  • War, light and lies  •Whatiscolour?  •Fun with rainbows  • What is the speed of light? – Again  •Signalsandpredic- tions  • Aether good-bye  • Challenges and fun curiosities about light  • Summary on light    I    –  Ways to produce images  125 Light sources Why can we see each other? Black bodies and the temperature of light  •Limits to the concentration of light  •Measuringlightintensity •Otherlightand radiation sources  • Radiation as weapon  133 Images – transporting light Making images with mirrors  • Does light always travel in a straight line? – Refraction  • Bending light with tubes – bre optics  • 200 years too late – negative refraction indices  • Metamaterials  •Lightaroundcorners– diraction  • Beating the diraction limit  •Otherwaystobendlight • How does one make holograms and other three-dimensional images?  •Images through scanning  •Tomography  154 e eye and the brain: observing images Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net Copyright © Christoph Schiller June 1990–November 2012 [...]... Copyright © Christoph Schiller June 1990–November 2012 5 Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net 169 contents 12 318 Biblio graphy 342 Credits Acknowledgements 342 • Film credits 343 • Image credits 343 Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net Copyright © Christoph Schiller June 1990–November... Copyright © Christoph Schiller June 1990–November 2012 Ref 1 Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net W hat is light? The study of relativity left us completely in the dark, even though e had embarked in it precisely to find an answer to that question True, e have learned how the motion of light compares with that of objects We also learned... their joints This is due the coincidence Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net Measurement 32 1 electricit y and fields TA B L E 7 Searches for magnetic monopoles, i.e., for magnetic charges, in over 140 experiments Search Smallest magnetic charge suggested by quantum theory Search in minerals, from mountains to the deep ocean Search in meteorites... magnetic particles inside a cell; the cell then senses how these small built-in magnets move in a magnetic field The magnets are tiny, typically around 50 nm in size These small magnets are used to navigate along the magnetic Copyright © Christoph Schiller June 1990–November 2012 Can humans feel magnetic fields? Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net... fascination of fields more strongly, a deeper look into a few experimental results is worthwhile liquid electricit y, invisible fields and maximum speed 17 Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net using computer graphics and using seeds in oil Top: the field around a point or spherical charge; second row: two or three charges of different... there are two different types of electric charge is due to the French universal genius Charles Dufay (1698–1739) In a long and careful series of experiments Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net F I G U R E 5 Lightning: a picture taken with a moving camera, showing its multiple strokes (© Steven Horsburgh) liquid electricit y, invisible... measures is much higher than everything else we observe in our environment How can we measure the time nevertheless? And why did different researchers get very Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net metal cans 20 pendulum with metal ball 1 electricit y and fields on the roof in the hall F I G U R E 7 Franklin’s personal lightning rod – a copy... how it propagates are still a topic of research An introduction is given on page 169 Copyright © Christoph Schiller June 1990–November 2012 Electric charge Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net Challenge 5 s electric chime – is one of the many experiments that shows strikingly that charge can flow liquid electricit y, invisible fields... remains: no counter-example to charge conservation has as yet been observed Objects without electric charge are called neutral A charged object that is brought Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net confirm electric charge conservation: if rubbed fur is moved from the first pot to the second, the charge taken away from the first pot is transferred... conductors can be polarized; polarization occurs for single molecule up to whole stars Copyright © Christoph Schiller June 1990–November 2012 Electric charges Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net F I G U R E 9 Various electrometers: a self-made electrometer based on a jam pot, an ancient (opened) liquid electricit y, invisible fields and . www.lulu.com/spotlight/motionmountain.Butaboveall,enjoy the reading! Motion Mountain – The Adventure of Physics pdf file available free of charge at www.motionmountain.net. provide feedback on www.motionmountain.net/ wiki.Helponthespecicpointslistedonthewww.motionmountain.net/help.html web page would be particularly welcome.