2 lines
12 KiB
HTML
2 lines
12 KiB
HTML
|
<html><head><title>Introduction to Feynman Diagrams</title><style type="text/css">ol{margin:0;padding:0}.c3{max-width:468pt;background-color:#ffffff;padding:72pt 72pt 72pt 72pt}.c1{height:11pt}.c2{text-decoration:underline}.c0{direction:ltr}.title{padding-top:24pt;line-height:1.15;text-align:left;color:#000000;font-size:36pt;font-family:"Arial";font-weight:bold;padding-bottom:6pt}.subtitle{padding-top:18pt;line-height:1.15;text-align:left;color:#666666;font-style:italic;font-size:24pt;font-family:"Georgia";padding-bottom:4pt}li{color:#000000;font-size:11pt;font-family:"Arial"}p{color:#000000;font-size:11pt;margin:0;font-family:"Arial"}h1{padding-top:24pt;line-height:1.15;text-align:left;color:#000000;font-size:18pt;font-family:"Arial";font-weight:bold;padding-bottom:6pt}h2{padding-top:18pt;line-height:1.15;text-align:left;color:#000000;font-size:14pt;font-family:"Arial";font-weight:bold;padding-bottom:4pt}h3{padding-top:14pt;line-height:1.15;text-align:left;color:#666666;font-size:12pt;font-family:"Arial";font-weight:bold;padding-bottom:4pt}h4{padding-top:12pt;line-height:1.15;text-align:left;color:#666666;font-style:italic;font-size:11pt;font-family:"Arial";padding-bottom:2pt}h5{padding-top:11pt;line-height:1.15;text-align:left;color:#666666;font-size:10pt;font-family:"Arial";font-weight:bold;padding-bottom:2pt}h6{padding-top:10pt;line-height:1.15;text-align:left;color:#666666;font-style:italic;font-size:10pt;font-family:"Arial";padding-bottom:2pt}</style></head><body class="c3"><h1 class="c0"><a name="h.39yx9t902a8r"></a><span>Feynman diagrams</span></h1><p class="c1 c0"><span></span></p><p class="c0"><span>A Feynman diagram is a picture that shows the interaction between particles. By studying Feynman diagrams, you can learn more about how particle physics really works. Physicists also use them to help them calculate the properties of different particle interactions. </span></p><p class="c1 c0"><span></span></p><p class="c0"><span>A simple Feynman diagram looks like this:</span></p><p class="c1 c0"><span></span></p><p class="c0"><span>[picture of e.g. electron-electron scattering Feynman diagram]</span></p><p class="c1 c0"><span></span></p><p class="c0"><span>Feynman diagrams are made up of straight lines, that represent fermions, and wiggly or dashed lines that represent the different kinds of bosons. These lines represent the particles’ journey through space. The horizontal direction represents time, and the vertical direction represents position.</span></p><p class="c1 c0"><span></span></p><p class="c0"><span>fermion: [straight line]</span></p><p class="c0"><span>photon: [picture of photon propagator]</span></p><p class="c0"><span>gluon: [picture of gluon propagator]</span></p><p class="c1 c0"><span></span></p><p class="c0"><span>Three (or sometimes four) different lines can meet to form a “vertex”, which is nothing but an intersection point. When this happens, the particles can interact with each other. For example, they might cause each other to change direction, such as when two electrons repel each other due to their negative charge. This is shown in the diagram above. We can see that when the two electrons interact via their electric charges they exchange a photon. This is because the photon is the force carrying boson for the electromagnetic interaction.</span></p><p class="c1 c0"><span></span></p><p class="c0"><span>Before we explore Feynman diagrams further, we need to take a short excursion to talk more about antimatter. We have seen that, for example, an anti-electron (usually called a positron) is exactly like an electron but with a positive charge rather than a negative charge. Now it turns out that a positron can also be thought of as an electron travelling backwards through time! This is a bit mind-bending, but it is absolutely true. Can you see how a positive charge travelling forwards through time is the same as a negative charge travelling backwards through time?</span></p><p class="c1 c0"><span></span></p><p class="c0"><span>Now we are ready to return to Feynman diagrams. One co
|