Special Theory Of Relativity

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1. SPECIAL THEORY OF
RELATIVITY

2. •Showed that Newton’s ideas of absolute
space and absolute time were mistaken.
•Implied that matter and energy were
interconvertible.

3. •Waves travel through a medium ( water,
air)
•Electromagnetic waves must travel
through an undetectable medium in space
called an ether.

4. •Should be a difference if light travels in the
direction of the Earth’s motion through the
ether compared to right angles to it.
•Michelson-Morley experiment showed no
difference.
•Hence no ether.

5. Motion
•Motion is measured relative to a fixed
point called a Frame of Reference
•All bodies have inertia so resist any
change to their motion
•A spaceship in deep space ( minimal
gravitational effects) would provide a
frame of reference where a body would
remain stationary or with a constant
velocity unless a force was applied.

6. •A Frame of Reference where Newton’s
first law of motion is obeyed ,is called an
•Inertial Frame

7. •An Inertial Frame can have a
constant velocity but cannot have
an acceleration.

8. •Any constant velocity of the spaceship is
shared by everything inside it.
•If you were inside, you could not detect
this velocity.

9. •If you are concerned by the acceleration of
gravity, it is legitimate to think of an inertial
frame on the Earth’s surface concerned
only with horizontal forces or movements.
•No limit to the number of these restricted
frames which can move relative to each
other.

10. Key Ideas to Einstein’s Special
Theory of Relativity
•Newton’s idea of an inertial frame
•Maxwell’s discovery that all types of
electromagnetic radiation must travel at
the same speed.

11. First Postulate of the special theory
of Relativity
•Einstein took Newton’s idea that the laws
of motion apply to any inertial frame.
•Extended it to include all the laws of
physics.

12. Second Postulate of the special
theory of relativity
•The speed of all electromagnetic radiation
is a fundamental law and applies to all
inertial frames.
•It is constant however it is measured from
any inertial frame.

13. •As the speed is constant ,an ether no
longer makes any sense.
•As Speed = Distance / Time
•And there is constant relative velocity
between any two inertial frames, then
measurements by observers moving
relative to each other must give different
values of length and time.

14. •We need thought experiments to look at
these ideas.

15. Time Dilation
A
B

16. •If you are observer A and sent a pulse of
light out at the same time as B.
•B’s light would appear to travel further
between reflections than A’s.
•So B’s clock would appear to be running
slower.
•Likewise B would think A’s was running
slower.

17. •The time interval between two events that
occur in a particular inertial frame is longer
when measured by someone moving
relative to it than someone inside the
frame.
•This is time dilation and affects any type of
clock.

18. •Life span of Muons can be increased by
accelerating them to 99.7% of the speed
of light.
•The high proportion that reach the Earth’s
surface when created in the upper
atmosphere from cosmic rays, supports
this.

19. Length contraction
•The length of a train can be compared to
the length of a tunnel.
•Lamp x comes on as the train emerges
•Lamp y comes on as the other end enters.
•Observer A is by the track mid way in the
tunnel.
•Observer B is on the train at mid point.

20. •Y X
A
B

21. •Observer A sees both lights come on
together
•Observer B sees X come on before Y as
he is travelling towards X and away from Y
•This indicates that the train has left the
tunnel before the end went in. So B thinks
the train is longer than the tunnel.

22. • √ 1 – v
2
c
2

23. General Theory of Relativity
•Einstein wanted to link his theory to
Newton’s Law of gravitation
•Solved some of its problems.
•E.g. the orbit of Mercury