Modern PhysicsRelativity

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Space is defined by measurements of length and depends on what “ruler” is used to measure the length.

Time is defined by measurements of time and depends on what “clock” is used to measure the time.

Events are defined as a certain place and time.

Events depend on what “rulers” and “clocks” are used to measure them.

What is an event?

Modern PhysicsRelativity

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Different cars in different places measure different distances to the bush.

The distance to the bush changes as the blue car moves.

Modern PhysicsRelativity

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Events that at one time are simultaneous are not simultaneous at other times.

The blue car sees the birds fly from the bush at the same time. (The events are simultaneous.)The red car does not.

If the blue car moves, simultaneous events are no longer simultaneous.

Modern PhysicsRelativity

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Just as with the drivers of the cars, measurements depend on who is observing.

Time and length will be different for different observers.

The choice of observer defines which measuring devices are used.

If the observer is not accelerating (not changing their speed OR direction), they are considered to be in an inertial reference frame.

What is an inertial reference frame?

Modern PhysicsRelativity

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The relativity postulate

The laws of physics are the same in all inertial reference frames.

The speed of light postulate

The speed of light in a vacuum, measured in any inertial reference frame, always has the same value c no matter how fast the source of light and the observer are moving with respect to one another.

This is what Einstein discovered.

Modern PhysicsRelativity

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Time dilation tells us that all moving clocks run slow.

The first question we must ask is “Moving with respect to what?”The answer is “Moving with respect to the observer.”

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Time as measured by a clock at rest with respect to the observer

Time as measured by a clock moving with respect to the observer

Speed between observer and the moving clock

Modern PhysicsRelativity

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BirdSeed

Lobs

Coyote uses obst

Actual time

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Ant on the bomb sees obst

Length contraction tells us that all moving lengths shrink.

Modern PhysicsRelativity

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Length contraction tells us that all moving lengths shrink.

BirdSeed

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Length as measured by an observer moving with respect to the thing being measured

Speed between observer and the thing being measured

Length as measured by an observer NOT moving with respect to the thing being measured

Modern PhysicsRelativity

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When an object is moving it has a mass given by

The energy gained by converting mass into energy is

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mm rest

2cmE rest

Mass and energy are really the same thing!

Modern PhysicsRelativity

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The velocity of a wave is given by the velocity of its peaks.

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Modern PhysicsRelativity

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The wave is the movement of its peaks not the movement of the medium that makes the peaks.

Modern PhysicsRelativity

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An object in motion can tell how fast it is moving with respect to the medium by counting waves.

Modern PhysicsRelativity

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Let’s see that again from Dr. Mike’s point-of-view.

Modern PhysicsRelativity

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Now let’s see it as he moves in the other direction.

Modern PhysicsRelativity

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If light is a wave, then the earth should be moving through its medium.

Modern PhysicsRelativity

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This is what Michelson should have seen.

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Modern PhysicsRelativity

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What Michelson saw was that the speed of the wave was the same no matter which way the earth moved.

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