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Relativity Relativity

H2: Concepts and postulates of special H2: Concepts and postulates of special relativityrelativity

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Inertial frames of Inertial frames of referencereference

Inertial reference frames are reference frames Inertial reference frames are reference frames in which Newton’s laws holdin which Newton’s laws hold

From Newton’s 1From Newton’s 1stst law: law: If an object experiences no net force, the If an object experiences no net force, the

object either remains at rest or continues object either remains at rest or continues with constant speed in a straight linewith constant speed in a straight line

The Earth is not an inertial frame (it The Earth is not an inertial frame (it rotates) but is close enough to be rotates) but is close enough to be considered an inertial frameconsidered an inertial frame

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Inertial frames of Inertial frames of referencereference

Inertia is a property of matterInertia is a property of matter It is that property of matter which opposes It is that property of matter which opposes

changes in velocity changes in velocity So, one must centre one's physics reasoning So, one must centre one's physics reasoning

on these thoughts: on these thoughts: An object's velocity will not change all on its own.An object's velocity will not change all on its own. Pushes, or pulls, are necessary to change an Pushes, or pulls, are necessary to change an

object's velocity. object's velocity.

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Inertial frames of Inertial frames of referencereference

There are several ways to describe an inertial There are several ways to describe an inertial frame. Here are a few descriptions:frame. Here are a few descriptions: An inertial frame of reference is a frame of reference An inertial frame of reference is a frame of reference

with constant velocity. with constant velocity. An inertial frame of reference is a non-accelerating An inertial frame of reference is a non-accelerating

frame of reference. frame of reference. An inertial frame of reference is a frame of reference An inertial frame of reference is a frame of reference

in which the law of inertia holds. in which the law of inertia holds. An inertial frame of reference is a frame of reference An inertial frame of reference is a frame of reference

in which Newton's laws of motion hold. in which Newton's laws of motion hold. In an inertial frame of reference no fictitious forces In an inertial frame of reference no fictitious forces

arise. arise.

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Postulates of the Special Postulates of the Special Theory of RelativityTheory of Relativity

All motion is relative, not to any stationary All motion is relative, not to any stationary hitching post in the universe, but to arbitrary hitching post in the universe, but to arbitrary frames of reference.frames of reference.

A familiar experience to a passenger on a train A familiar experience to a passenger on a train who looks out his window and sees the train on who looks out his window and sees the train on the next track moving by his window.the next track moving by his window.

The important point: If you were in a train with The important point: If you were in a train with no windows, there would be no way to no windows, there would be no way to determine whether the train was moving with determine whether the train was moving with uniform velocity or was at rest. uniform velocity or was at rest.

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First of Einstein 's First of Einstein 's postulates of the special postulates of the special theory of relativity: theory of relativity: All laws of nature are the same in all uniformly All laws of nature are the same in all uniformly

moving frames of referencemoving frames of reference. . Any number of experiments can be devised to Any number of experiments can be devised to

detect accelerated motion, but none can be detect accelerated motion, but none can be devised, according to Einstein, to detect a state of devised, according to Einstein, to detect a state of uniform motion. uniform motion.

Therefore absolute motion has no meaning. Therefore absolute motion has no meaning. No experiment, mechanical or electrical or No experiment, mechanical or electrical or

optical, has ever revealed absolute motion. optical, has ever revealed absolute motion. That is what the first postulate of relativity means. That is what the first postulate of relativity means.

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Second postulate in his special Second postulate in his special theory of relativitytheory of relativity: :

The speed of light in free space has the The speed of light in free space has the same measured value for all observers, same measured value for all observers, regardless of the motion of the source or regardless of the motion of the source or the motion of the observer; that is, the the motion of the observer; that is, the speed of light is a constant.speed of light is a constant.

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To illustrate this statement, consider a To illustrate this statement, consider a rocket ship departing from the space rocket ship departing from the space

stationstation A flash of light traveling at 300,000 A flash of light traveling at 300,000

kilometers per second, or c, is kilometers per second, or c, is emitted from the station.emitted from the station.

An observer in the rocket sees the An observer in the rocket sees the flash of light pass her at the same flash of light pass her at the same speed cspeed c

If a flash is sent to the station from If a flash is sent to the station from the moving rocket, observers on the the moving rocket, observers on the station will measure the speed of station will measure the speed of the flash to be c. the flash to be c.

All All observers who measure the observers who measure the speed of light will find it has the speed of light will find it has the same value c. same value c.

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Simultaneity:Simultaneity:

An interesting consequence of Einstein's An interesting consequence of Einstein's second postulate occurs with the concept second postulate occurs with the concept of of simultaneity.simultaneity.

Two events are simultaneous if they Two events are simultaneous if they occur at the same time. occur at the same time.

Consider, for example, a light source in Consider, for example, a light source in the exact center of the compartment of a the exact center of the compartment of a rocket ship. rocket ship.

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From the point of view of the observer who travels with the compartment, light From the point of view of the observer who travels with the compartment, light from the source travels equal distances to both ends of the compartment and from the source travels equal distances to both ends of the compartment and

therefore strikes both ends simultaneously.therefore strikes both ends simultaneously.

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The events of light striking the front and back of the compartment are not simultaneous from The events of light striking the front and back of the compartment are not simultaneous from the point of view of an observer in a different frame of reference. Because of the ship’s the point of view of an observer in a different frame of reference. Because of the ship’s motion, light that strikes the back of the compartment doesn’t have as far to go and strikes motion, light that strikes the back of the compartment doesn’t have as far to go and strikes

sooner than light strikes the front of the compartmentsooner than light strikes the front of the compartment