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The Nature of Time

The Nature of Time. Henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality. Herman Minkowski, Space and Time (21 September 1908).

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The Nature of Time

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  1. The Nature of Time Henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality. Herman Minkowski, Space and Time (21 September 1908) The Disintegration of the Persistence of Memory, Salvador Dali 1954

  2. Two minute (your coordinate time!) problems: • A pair of flash bulbs are located at (t,x) coordinates (0s,3s) and (0s,9s). You are at (0s, 5s). You are all in the same inertial frame. The flash bulbs go off at t=1 s in your frame. Draw a spacetime diagram that represents this and describe when you see the flashes. • How (explain qualitatively) will your diagram change if the flash bulbs are in a frame moving at ¾ in the +x direction?

  3. Kinds of Time… • coordinate time: • is the time measured between clocks in the same inertial frame • coordinate time is frame-dependent! (analogy: coordinates) • proper time: • is the frame-independent value of time that all observers will agree upon. Proper time between a pair of events is the time measured by a single clock present at both events. (analogy: distance) • spacetime interval: • Is the proper time measured by an inertial clock present at both events. More about actually calculating this later. (analogy: path length)

  4. Motion affects clocks! • We are getting close to one of the most profound and counter-intuitive aspects of relativity • The end of simultaneity! Events which are simultaneous in one inertial frame will not be simultaneous from other (different) inertial frames • The Hafele-Keating experiment provides a graphic description of how motion affects time

  5. The Hafele-Keating experiment was performed in 1971 • Demonstrated that a moving clock lost synchronization with a “lab” or “stay-at-home” clock • Moving clocks “run slow”

  6. A Quick Look Ahead - The Photon Clock

  7. Spacetime Diagrams II

  8. Looking ahead… • The “loss” of simultaneity or “frame-dependence” is a fore-taste of what lies ahead. • Next class we will develop the Lorentz Transformations! • Next class we will develop the metric equation which will show us how coordinate time and proper time are related and will more fully illustrate that moving clocks tick slower than stationary ones!

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