1. Chapter 26 Relativity © 2006, B.J. Lieb
Some figures electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Giancoli, PHYSICS,6/E © 2004.
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2. Galilian-Newtonian Relativity
Relativity deals with experiments observed from different reference frames.
Example: Person drops coin from moving car
In reference frame of car: coin is at rest and falls straight down
In “earth” reference frame, coin is moving with initial velocity and follows projectile path.
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3. Inertial Reference Frames
Inertial Reference Frame: one in which Newton’s First Law (Law of Inertia is valid.)
A reference frame moving with constant velocity with respect to an inertial reference frame is an inertial frame.
If the car is moving with constant velocity relative to the earth, it is an inertial reference frame.
Accelerated or rotating reference frames are noninertial reference frames.
The earth is approximately inertial.
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4. Relativity Principle
“the basic laws of physics are the same in all inertial reference frames”
This principle was understood by Galileo and Newton.
Space and time are absolute– different inertial reference frames measure the same length, time etc.
All inertial reference frames are equivalent– there is no preferred frame.
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5. Special Theory of Relativity
Einstein asked the question “What would happen if I rode a light beam?”
Would see static electric and magnetic fields with no understandable source.
Understanding electromagnetic radiation requires changing E and B fields.
Concluded that:
no one could travel at speed of light.
No one could be in frame where speed of light was anything other than c.
No absolute reference frame
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6. Postulates of Special Theory of Relativity
First: The laws of physics have the same form in all inertial reference frames
Second: Light propagates through empty space with a definite speed c independent of the source and observer
This means that an observer trying to catch a light beam and moving at 0.9c will measure the speed of that light as c and an observer on earth will also measure c.
In order for this to be true, observers must differ on measurements of distance and time
The special theory of relativity deals with reference frames that move at constant velocity with respect to an inertial reference frame.
The General Theory deals with accelerated reference frames and is primarily a theory of gravity.
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7. Relativistic Clock
In the above clock, light is reflected back and forth between two mirrors and timer counts “ticks”
This is an ideal clock because of special properties of light
An observer at rest with respect to the clock concludes that the time for a tick is
In order to study time dilation, we will places this clock in a spaceship moving past earth.
 t0 is called proper or rest time because clock is at rest in spaceship (note, we don’t call it “correct” time)
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8. Time dilation
Spaceship moves by earth at speed v. (both observers agree that speed is v.)
Observer on earth sees light move distance
per tick.
Observer on earth sees spaceship moving
Observer on earth writes this equation for c
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9. Time dilation
The formulas on the previous slides can be combined to give
Clocks moving relative to an observer are measured by that observer to run more slowly (as compared to clocks at rest).
Clock is in spaceship so this measures the proper or rest time Δt0.
It is often convenient to write v as fraction of c, thus v = 3.0x107 m/s is written v = 0.10 c.
We call this effect time dilation because the time in the moving reference frame is always longer than the time in the proper reference frame
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10. Time dilation factor Consider how this depends on v v (m/s) v (c)
 t0 (sec)
 t (sec)
2.00x105 c
3.00x106 c
3.00x107 c
2.00x108 c
2.97x108 c
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11. Length Contraction
A spaceship passes Earth and continues on to Neptune
Earth and spaceship observers disagree on time, and they also disagree as to length L (distance to Neptune)
Since earth is at rest, it measures the “proper” length L0.
Both observers agree on the relative velocity v, so we can use the time dilation to derive length contraction equation
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12. Length Contraction
The length of an object is measured to be shorter when it is moving relative to the observer than when it is at rest.
This contraction is only in the direction of the velocity. The drawing shows the changed shape of a picture when a person moves by horizontally.
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13. Example 26-1 (7) Suppose you decide to travel to a star that is 85 light-years away at a speed that tells you the distance is only 25 light-years. How many years would it take you to make the trip?
We determine the speed from the length contraction. The light-year is a unit of length. The rest or proper length is L0 = 85 ly and the contracted length is L = 25 ly.
which gives
We then find the time from the speed and distance:
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14. Example 26-2: A muon is an elementary particle with an average lifetime of 2.2 μs. A muon is produced in the earth’s atmosphere with a velocity of 2.98 x108 m/s.
What is the distance traveled by the muon before it decays as measured by observers in the muon’s rest frame? We note that the muon’s reference frame is the proper frame for time and the earth is the proper frame for distance.
What is the lifetime of the muon as measured by observers in the earth reference frame?
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15. Example 26-2 (continued): A muon is an elementary particle with an average lifetime of 2.20 μs. A muon is produced in the earth’s atmosphere with a velocity of 2.98 x108 m/s.
What is the distance traveled by the muon as measured in the earth reference frame?
Use the above results to calculate the velocity of the muon in each reference frame.
Notice that observers in the two reference frames disagree as to distance and time but agree as to velocity.
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16. Four-Dimensional Space-Time
Since observers in different reference frames often do not agree on time measurements as well as length measurements, time is treated as a coordinate in our coordinate system along with the three spatial coordinates X,Y and Z. Thus we speak of a four-dimensional space time.
Time
The red line represents a light ray and the blue line represents the “world-line” of an object through space-time.
Space
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17. Mass Increase
The mass of an object increases as the speed of the object increases:
Mass increase is observed in particle accelerators
m0 is the “rest mass”, the mass as measured in a reference frame at rest with respect to the object.
This mass increase can be seen as the reason that objects can’t travel at the speed of light.
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18. Relativistic Kinetic Energy
Einstein used the work-energy theorem to derive a new, relativistic equation for kinetic energy
When v << c, this equation can be approximated by
KE = (1/2) m v2,
so this equation can still be used for non-relativistic speeds.
The equation for kinetic energy is the difference between two quantities which have units of energy
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19. Total Energy
This is the total energy of the object. It implies that there is energy in mass and mass can be converted into energy and energy can be converted into mass
This is the rest energy of the object. We can thus write the total energy as
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20. Units of Mass-Energy
Earlier in the course we defined the electronvolt (eV) as a unit of energy where 1 eV = 1.6 x 10-19J.
We will also use keV (103) and MeV (106)
From E=mc2, we can solve for m = E / c2 and thus MeV / c2 is a unit of mass.
Example: electron mass is
me = MeV/c2
It is useful to use c2 = MeV / u.
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21. Additional Relativistic Equations
The Special Theory of Relativity is a revision of the laws of mechanics for objects with velocity close to the speed of light.
The relativistic equations all can be approximated by the non-relativistic equations when v << c.
For problems where v  0.10 c, the relativistic correction is less than 1 percent and the non-relativistic equations are used.
Momentum:
Energy-Momentum:
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22. ( One eV is the energy gained by electron accelerated through 1.0 V.
Example An electron is accelerated through an electrical potential difference of 2.00x106 V. Calculate the mass energy, kinetic energy, total energy and veloctiy of the electron.
( One eV is the energy gained by electron accelerated through 1.0 V.
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23. Mass of electron = me = 0.00054858 u = 0.511 MeV/c2
Example Neutrons outside of the nucleus are unstable and decay with a half-life of 10.4 mins into a proton, electron and a neutrino. If a neutron decays at rest, calculate the energy released by this decay. This energy is shared by the proton, electron and neutrino.
Note: The neutrino mass is only a few eV/c2 so we will assume it is zero. Table 30-1 on page 838 contains the masses we need.
Mass of electron = me = u = MeV/c2
Mass of proton = mp = u = MeV/c2
Mass of neutron = mn = u = MeV/c2
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24. (slight difference due to round-off)
Alternative Solution
Example Neutrons outside of the nucleus are unstable and decay with a half-life of 10.4 mins into a proton, electron and a neutrino. If a neutron decays at rest, calculate the energy released by this decay. This energy is shared by the proton, electron and neutrino.
Solution in MeV/c2:
me = MeV/c2
mp = MeV/c2
mn = MeV/c2
(slight difference due to round-off)
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