• Also known as classical relativity
• Defined in terms of reference frame.
• A reference frame in which Newton’s Laws hold is called
an inertial frame
1. There exists an absolute space ( Newton Laws true)
2. All inertial frames share a universal time.
• Time runs at the same rate in all frames (Galilean
3 inertial frames:
1. Carrier-Earth (land underneath)
CONCLUSION: Nothing (in terms
motion-speed) is constant, every
thing is relative to some other
inertial frame of reference
Ritz Theory of Emission
• A competing theory for the theory of relativity.
• It obeys the central principal of relativity : No fixed
frame of reference.
• But claims: what if the light travels at a speed X with
reference to the source?
• Hence it tries to combine the laws governing
electrodynamics and mechanics.
• It is a theory of the structure of spacetime.
• It is based on two postulates:
1. Laws of physics remain the same for all observers
(who are in uniform motion relative to each other)
2. Speed of light is independent of the speed of any
other object, even its source.
• Postulate two contradicts Ritz Emission theory.
• Important consequences include Relativity of
simultaneity, Time Dilation, Energy Equivalence.
Einstein's Relativity vs. Newton
• There is no universal time
• Every inertial frame has its own independent
• Speed of light binds all frames of reference.
Relativity vs. Emission Theory
Emission theory was not able to explain the behavior
of stars moving across our field of vision.
The Sagnac’s Effect
• A phenomenon observed during interference of
waves when the source is rotated.
• A beam of light is split in two.
• Beams travel in opposite direction
• When apparatus is stationary both beam have an
equal distance to cover.
• Due to rotation of the apparatus both beams face
• Interference occurs.
The Sagnac’s Effect
Video1 Video 2
Sagnac’s effect dismisses emission
theory and strengthens the case for
Sagnac Effect (Uses): RLG
• The apparatus can be made to measures its
own angular velocity with respect to the local
• Hence it is highly effective for use in inertial
• It is the basis for any Ring Laser Gyro
• RLGs suffer from “lock-ins" at very slow rotation rates.
• Is solved by automatic rotation of apparatus (Dithering)
• Newtonian Relativity: Universal time and
• Einstein Relativity: Independent time and
inertial frames, concept of spacetime fabric.
• Ritz Emission theory: Light moving with a
speed RELATIVE to its source (rejected)
• Sagnac’s effect: Interference of two beams of
light due to rotation.
• RLG: Working and Problem (solved)