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IUPUI AST 105 - Photon

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AST 105 1st Edition Lecture 24Outline of Past Lecture (#24 from 4/16)I. Physics of the Small II. Point at Which Physics Becomes High Energy III. The 4 Forces Outline of Current Lecture (#25 from 4/21) Topic: Particle Physics I. PhotonsII. WeinbergIII. Goal of Particle PhysicsIV. Matter Dominated UniversePhoton- Mediator of EM force- 2 electrons feel a repulsion due to EM force this repulsion is mediated by a photon - We describe this interaction using a Feynman Diagram  Each segment is a piece of an equationo quantum electro dynamics (QED) Weinberg- In the early 1970’s, Steven Weinberg recognized that the W- Boson behaves like a photon- We know about neutron decay and that it is mediated by the weak force; neutron is only completely stable when buried inside nucleus - Recognized that he could reproduce neutron decay w/ a Feynman diagram that looked a tad bit different than the original - W- Boson acts just like a photon; force mediator here - Hence, the weak-force, which is mediated by the W- boson could be combined w/ the EM force, which is mediated by the photon These 2 forces are combined to form what we call the electroweak force These 2 forces become one when the energies are about the same as the mass of the W- boson (about 90 GeV)- When the Big Bang occurred, the temp. of the universe was very hot and the energies were above 90 GeV. Only when the universe cooled did the forces break apart into the 4 that we havetoday These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.The Goal of Particle Physics is to develop a complete theory that incorporates these 4 forces into a single force.- The current framework for uniting the forces is superstring theory  We do know that superstring theory (also known as M-theory, Brain theory) has built in a quantum theory of gravity - We want to explain everything. o We want a theory that explains the universe. It would be nice if the theory could predictthe fundamental constants. This idea, that we can explain everything, is called reductionism.  (Reductionism: break down all the complexity we see in the universe)Matter Dominated Universe- After big bang, up to about 300,000 yrs after big bang, the density of photons exceeded the density of matter (baryonic) This means that photons had a greater effect than matter- We had a radiation dominated universe- As the universe cools and expands, photon density drops below baryonic density Hence, from 300,000 years to today matter dominated


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IUPUI AST 105 - Photon

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