Lecture Four

Attempts to Save

Ether Hypothesis

Status

Michelson-Morley experiment shows

null result.

Attempts to Save Etherbefore

Special Relativity

1. Contraction Hypothesis

2. Ether-Drag Hypothesis

3. Modify Electrodynamics

Contraction Hypothesis

•Fitzgeral (1892)

•Lorentz

Length is contracted in the direction of motion.

Demolishedby

Kennedy-Thorndike

experiment (1932)

Ether-Drag Hypothesiscontradicted with

• Stellar aberration(Bradley 1727)

• Fizeau convection coefficient(Fresnel 1817, Fizeau 1857)

Stellar Aberration

Stellar Aberration

Stellar Aberration

Fizeau Experiment

Fizeau Experiment

velocity of light v in a medium of refractive index n moving with a velocity vw

Modify Electrodynamics

Emission Theories The velocity of a light wave is

connected with the motion of the source rather than with an ether.

Modify ElectrodynamicsEmission Theories

• The velocity of light is c relative to the original source.

• This velocity is independent of the state of the medium transmitting the light.

Emission Theoriesare contradicted with

• de Sitter observations on binary stars

• Michelson-Morley experiment using extraterrestrial light source

Conclusions• The speed of light is the same in

all inertial systems, independent of the relative motion of source and observer.

• A relativity principle applicable both to mechanics and to electrodynamics.

Conclusions• Galilean transformations must be

replaced.

• The laws of mechanics which were consistent with Galilean transformations needs to be modified.

Special Relativity

Einstein 1905

“On the Electrodynamics of Moving Bodies”

Two Postulates

•Principle of Relativity

•Principle of Constancy of Speed of Light

Principle of Relativity

• include all laws of physics•no measurements to designate an inertial system as intrinsically stationary or moving

Principle of Relativity

•no experiment entirely within an inertial system can tell the motion with respect to any other.

Principle of Constancyof Speed of Light

•consistent with Michelson-Morley experiment

Experimental Basis

Program of the Theory

•nature of time

•transformation keeping velocity of light constant

•Lorentz transformation

Einstein• In his early life none of his

“elders” recognized his genius.• Until almost thirty he had

never seen a real theoretical physicists, “except in the mirror!”

Einstein• statistical mechanics• atomic nature of matter• special relativity• principle of equivalence• light quanta hypothesis• specific heat of solids

Einstein 1905•Quantum theory of light•Brownian motion•Special Relativity

Einstein•not in the mainstream•nature of time•abandoned Newton’s universal time