The Sagnacs Effect

Presentation

Disclaimer: Due to the vastness of the topics covered, only the important themes are explained as understood (in the short interval.)

Overview

• Relativity• Newtonian Relativity• Einstein's Relativity• Ritz Theory of Emission• Relativity vs. Emission Theory• The Sagnac’s Effect• Ring Laser Gyros• Summary

Relativity

The fact that we cannot detect absolute motion is known as

Relativity.

Relativity

Video

Newtonian Relativity

• 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• Postulates: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

Transformation)

Newtonian Relativity

Newtonian Relativity

Newtonian Relativity

http://www2.is.wayne.edu/IST_2420_Web_Site/Newtonian_Relativity.htm

3 inertial frames:1. Carrier-Earth (land underneath)2. Car-Carrier3. Car-Base

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.

Einstein's Relativity

• 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

time.• Speed of light binds all frames of reference.

Relativity vs. Emission Theory

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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 unequal distances.• Interference occurs.

The Sagnac’s Effect

Video1 Video 2

Sagnac’s effect dismisses emission theory and strengthens the case for

special relativity.

Sagnac Effect (Uses): RLG

• The apparatus can be made to measures its own angular velocity with respect to the local inertial frame.

• Hence it is highly effective for use in inertial guidance systems.

• It is the basis for any Ring Laser Gyro

Ring Laser Gyro

Patterns

COUNT No. OF MOVING PARTS

Problem

• RLGs suffer from “lock-ins" at very slow rotation rates.• Is solved by automatic rotation of apparatus (Dithering)

Summary• Newtonian Relativity: Universal time and

Absolute space• 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)

THANK YOU

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Questions