Relativity

Historical Development 1600s Newton discovered his laws of mechanics Applied to a wide variety of problems over the next two decades Worked well Late 1800s Maxwell’s equations explained the physics of electromagnetism and light Early 1900s Relativity Quantum Mechanics

Types of Relativity Special relativity Concerned with objects and observers moving at a constant velocity Topic of this chapter General relativity Applies to situations when the object or the observer is accelerated Has implications for understanding gravitation

Relativity The term relativity arises when a situation is described from two different points of view When the railroad car moves with a constant velocity, Ted and Alice see different motions of the ball Section 27.1

Reference Frames A reference frame can be thought of as a set of coordinate axes Inertial reference frames move with a constant velocity The principle of Galilean relativity is the idea that the laws of motion should be the same in all inertial frames For example, adding or subtracting a constant velocity does not change the acceleration of an object and if Newton’s Second Law is obeyed in one inertial frame, it is obeyed in all inertial frames Section 27.1

Interpretation by Ted and Alice Ted observes the ball’s motion purely along the vertical direction Alice sees the ball undergo projectile motion with a nonzero displacement in both the x- and y-directions Ted would think the ball’s horizontal velocity is zero, but Alice would disagree Both agree that the ball’s acceleration is downward with a magnitude g Both agree the ball’s horizontal acceleration is zero Both agree that the only force acting on the ball is the force of gravity and that Newton’s second law is obeyed Section 27.1

Galilean Relativity and Light According to Maxwell’s equations, the speed of light is a constant He also showed the speed of light is independent of the motion of both the source and the observer Assume Ted is moving with a constant velocity v relative to Alice when he turns on a flashlight Newton’s mechanics predict that speed of the light wave relative to Alice should be c + v According to Maxwell’s theory, Ted and Alice should both observe the light wave to move with speed c

Galilean Relativity and Light, cont. Galilean Relativity and electromagnetism predict different results for observers in different inertial frames Experiments showed that Maxwell’s theory was correct The speed of light in a vacuum is always c Galilean relativity for how the speed of light depends on the motion of the source is wrong Section 27.1

Special Relativity Einstein developed a theory to analyze the Ted and Alice situation called special relativity His work was not motivated by any particular experiment He suspected the speed of light is the same in all reference frames Maxwell was correct He then worked out what that implies for all the other laws of physics The basics of the theory were stated in two postulates about the laws of physics For fast-moving objects Newton’s theory breaks down and Einstein’s theory gives a correct description of motion in this regime Section 27.2

Postulates of Special Relativity All the laws of physics are the same in all inertial reference frames The speed of light in a vacuum is a constant, independent of the motion of the light source and all observers Section 27.2

Postulates – Details First postulate is traced to the ideas of Galileo and Newton on relativity The postulate goes further than Galileo because it applies to all physical laws Not just mechanics The second postulate is motivated by Maxwell’s theory of light This is not consistent with Newton’s mechanics The postulates will lead to a new theory of mechanics that corrects and extends Newton’s Laws Section 27.2

More About Light Our everyday experience with conventional waves cannot be applied to light Light does not depend on having a conventional material medium in which to travel A light wave essentially carries its medium with it as it propagates In the electric and magnetic fields The lack of a conventional medium was surprising and hard to reconcile with conventional intuition Section 27.2

Inertial Reference Frames Inertial reference frames play an important role in special relativity A definition of what it means to be inertial is needed The modern definition of an inertial reference is one in which Newton’s First Law holds You can test for an inertial frame by observing the motion of a particle is zero If the particle moves with a constant velocity, the reference frame is inertial Newton’s other laws should also apply in all inertial frames Section 27.2

Earth as a Reference Frame Since the Earth spins about its axis as it orbits the Sun, all points on the Earth’s surface have a nonzero acceleration Technically, a person standing on the surface of the Earth is not in an inertial reference frame However, the Earth’s acceleration is small enough that it can generally be ignored In most situations we can consider the Earth to be an inertial reference frame Section 27.2

Light Clock The two postulates lead to a surprising result concerning the nature of time A light clock keeps time by using a pulse of light that travels back and forth between two mirrors The time for the clock to “tick” once is the time needed for one round trip: 2ℓ / c Section 27.3

Moving Light Clock The clock moves with a constant velocity v relative to the ground From Ted’s reference frame, the light pulse travels up and down between the two mirrors Section 27.3

Moving Light Clock, cont. The time for the clock to make one tick as measured by Ted is Alice sees the light pulse travel a longer distance The speed of light is the same for Alice as for Ted Because of the longer distance, according to Alice the light will take longer to travel between the mirrors Section 27.3

Moving Clock, Alice’s Time For Alice, the time for one tick of the clock is The time for Ted is different from the time for Alice The operation of the clock depends on the motion of the observer Section 27.3

Moving Clocks Run Slow Alice’s measures a longer time than Ted Postulate 1 states that all the laws of physics must be the same in all inertial reference frames Therefore the result must hold for any clock Special relativity predicts that moving clocks run slow This effect is called time dilation For typical terrestrial speeds, the difference between Δt and Δt o is negligible Section 27.3

Time Dilation When the speed is small compared to c, the factor is very close to 1 Approximations given in Insight 27.1 may be used in many terrestrial cases Section 27.3

Speeds Greater the c If the value of the speed is greater than the speed of light, Δt / Δt o will be imaginary Speeds greater than the speed of light have never been observed in nature Experiments have shown that the time dilation predicted by special relativity is correct The result applies to all clocks, even biological ones Section 27.3

Proper Time The time interval Δt o is measured by the observer at rest relative to the clock This quantity is called the proper time The time interval measured by a moving observer is always longer than the proper time The proper time is always the shortest possible time that can be measured for a process, by any observer Section 27.3

Twin Paradox An astronaut, Ted, visits a nearby star, Sirius, and returns to Earth Sirius is 8.6 light-years from Earth Ted is traveling at 0.90 c Alice, Ted’s twin, stays on Earth and monitors Ted’s trip Section 27.3

Twin Paradox, Times Alice measures the trip as taking 19 years Ted’s body measures the proper time of 8.3 years Alice concludes that Ted will be younger than she is Ted calculates the Earth (and Alice) move away from him at 0.90 c Ted concludes Alice will age 8.3 years while he ages 19 years Ted concludes that Alice will be younger than he is Section 27.3

Twin Paradox, Resolution Time dilation appears to lead to contradictory results Alice’s analysis is correct She remains in an inertial frame and so can apply the results of special relativity Ted is incorrect He accelerates when he turns around at Sirius Special relativity cannot be applied during this time spent in an accelerating frame Section 27.3

Time Dilation and GPS Each GPS satellite contains a very accurate clock The satellite clocks are moving in orbit, so they experience time dilation They run slow by about 7µs per day To accurately determine a position, the effect of time dilation must be accounted for Section 27.3

Simultaneity Two events are simultaneous if they occur at the same time Ted is standing the middle of his railroad car He moves at a speed v relative to Alice Two lightning bolts strike the ends of the car and leave burn marks on the ground which indicate the location of the two ends of the car where the bolts strike Section 27.4

Simultaneity, cont. Did the two lightning bolts strike simultaneously? According to Alice She is midway between the burn marks The light pulses reach her at the same time She sees the bolts as simultaneous According to Ted The light pulse from at B struck before the bolt at A Since he is moving toward B The two bolts are not simultaneous in Ted’s view Section 27.4

Simultaneity, final Simultaneity is relative and can be different in different reference frames This is different from Newton’s theory, in which time is an absolute, objective quantity It is the same for all observers All observers agree on the order of the events Section 27.4

Length Contraction Alice marks two points on the ground and measures length L o between them Ted travels in the x-direction at constant velocity v and reads his clock as he passes point A and point B This is the proper time interval of the motion Section 27.5

Length Contraction, cont. Distance measured by Alice = L o = v Δt Distance measured by Ted = L = v Δt o Since Δt ≠ Δt o, L ≠ L o The difference is due to time dilation and The length measured by Ted is smaller than Alice’s length

Proper Length Ted is at rest Alice moves on the meterstick with speed v relative to Ted Ted measures a length shorter than Alice Moving metersticks are shortened The proper length, L o, is the length measured by the observer at rest relative to the meterstick

Length Contraction Equation Length contraction is described by When the speed is very small, the contraction factor is very close to 1 This is the case for typical terrestrial speeds Section 27.5

Proper Length and Time, Review Proper time is measured by an observer who is at rest relative to the clock used for the measurement Proper length is measured by an observer who is at rest relative to the object whose length is being measured Section 27.5