Lecture03 - Physics 344 Foundations of 21st Century...

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Physics 344 Foundations of 21 st Century Physics: Relativistic and Quantum Systems Instructor: Dr. Mark Haugan Office: PHYS 282 [email protected] TA: Dan Hartzler Office: PHYS 7 [email protected] Grader: Fan Chen Office: PHYS 222 [email protected] Office Hours: If you have questions, just email us to make an appointment. We enjoy talking about physics! Reading: Sections 1.1 through 1.5 and Chapter 2 in Six Ideas that Shaped Physics, Unit R. Help Session: A schedule conflict is forcing us to shift the start and finish times for the session by half an hour. For the rest of the semester it will run from 2:00 – 4:00pm on Thursdays.
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Inertial Coordinate Systems When we predicted the relativistic velocity of an electron exiting a particle accelerator we did so partly to emphasize the role that an inertial coordinate system plays whenever we use fundamental principles to explain or predict something. When we considered how to measure a relativistic electron’s velocity we did so partly to illustrate and emphasize that measuring the spacetime coordinates which represent where and when something happens in an inertial coordinate system is a bit trickier than you might have thought. We developed a conceptual model of an inertial reference frame as a lattice of rulers and synchronized clocks to deal with the subtle problem of measuring the times at which events at different locations occur. x y z When we select a location as an origin, coordinate axes and an instant as a zero of time we define an inertial coordinate system that assigns a unique set of spacetime coordinates x , y , z and t to every event.
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Clock Synchronization and Simultaneity One natural way to synchronize the clocks in our model inertial coordinate system is to preset each one to an appropriate time and to start them when they receive a flash of light emitted by the clock at the origin. If we set the clock at the origin to read t = 0 sec and start it when the flash is emitted, we must preset the clock at the location represented by spatial coordinates x , y and z to the time d/c , where is the clock’s distance from the origin, for it to be properly synchronized by this process. 2 2 2 d x y z = + + Notice that for our synchronization procedure to work it must be the case that the speed of light in vacuum has the same unchanging value c for light propagating in all directions away from the clock at the origin that emits the synchronizing pulse. x y z Events are simultaneous when the synchronized clocks at their locations read the same time when they occur.
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So far, we have justified believing this to be true by appealing to our well-tested fundamental principles of electromagnetic physics, Maxwell’s equations. They predict light propagates in just that way. For the same synchronization procedure to work
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This note was uploaded on 12/09/2011 for the course PHYS 344 taught by Professor Garfinkel during the Fall '08 term at Purdue University.

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Lecture03 - Physics 344 Foundations of 21st Century...

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