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Phy107Lect17

# Phy107Lect17 - From last time Galilean Relativity Laws of...

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From last time… • Galilean Relativity – Laws of mechanics identical in all inertial ref. frames • Einstein’s Relativity – All laws of physics identical in inertial ref. frames – Speed of light= c in all inertial ref. frames • One consequence Simultaneity: events simultaneous in one frame will not be simultaneous in another.

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Concept test An elevator freely falling in the Earth’s gravitational field is NOT an inertial reference frame because A. It is not moving at zero velocity. B. It is not moving at constant velocity. C. It is not moving with constant acceleration
Time and simultaneity • What does it mean for two things to happen at the same time? I do not define time, space, place, and motion, as being well known to all. — Newton If you do not ask me what is time, I know it. When you ask me, I cannot tell it. — St. Augustine

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Simultaneity and relativity • When are two events simultaneous? – Easy if they are at the same spatial location. – Different spatial locations require some synchronization, which requires signal transmission between locations.
Analogy with sound • Suppose you hear two loud shots about 1/2 second apart. • Did they occur at the same time? •Obviously not. •But suppose you find out one of the shots was fired closer to you than the other. •Sound travels at 340 m/s. •The sound pulse closer to you would arrive first, even if they were fired at the same time.

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But everyone can agree • If you know your distance from the shots, you can easily determine if they were simultaneous. • And everyone will agree with you, after doing the same correction for distance. • You might even come up with a definition Event ( x 1 , t 1 ) is simultaneous with event ( x 2 , t 2 ) if sound pulses emitted at t 1 from x 1 and at t 2 from x 2 arrive simultaneously at the midpoint between x 1 and x 2 .
Relativistic simultaneity • Einstein came up with a similar definition for relativistic simultaneity. • Simultaneity of separated events: Event ( x 1 , t 1 ) is simultaneous with event ( x 2 , t 2 ) if light signals emitted at t 1 from x 1 and at t 2 from x 2 arrive simultaneously at the midpoint between x 1 and x 2 BUT , due to the constancy of the speed of light we are forced into the conclusion that not everyone will agree that two events are simultaneous (occurred at the same time).

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Simultaneity and relativity, cont Means there is no universal, or absolute time.
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Phy107Lect17 - From last time Galilean Relativity Laws of...

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