ELECTROMAGNETISM

 

Light

 

Thomas Young’s Crucial Two-Slit Experiment

·         Thomas Young (1773-1829)

·         Investigated interference patterns in waves

·         Showed that light behaves like waves

·         Augustin Fresnel developed a mathematical wave theory of light

·         Young’s theory ill received because it contradicted Newton

 

 

If waves, then waves of what?

·         Aether?

·         Problems:

·         The wave pattern of light is of a transverse wave:

·         Particles of light travelling through empty space would produce longitudinal waves (like sound waves)

 

The Aether

 

Naturphilosophie

·         German philosophical movement in XIX

·         Sought unity in nature

·         Postulated a single unifying force that would explain everything

·         Hans Christian Oersted showed that an electric current deflected a magnet tangentially

·         Michael Faraday showed that an electric current could be induced with a moving magnet

 

Maxwell’s Wave Equations

·         James Clerk Maxwell (1831-1879)

·         Maxwell put Faraday’s descriptions into a mathematical form

·         Accounted for electrical and magnetic phenomena as waves that move at the speed of light

·         Maxwell: electricity, magnetism, and light are wave disturbances of the Aether

·         The Aether then becomes a necessary concept

 

The Michelson-Morley Experiment

·         Sought to measure the Earth’s motion through the Aether

·         Let:

·         t = time required for light to travel back and forth over a path stationary to the Aether

·         t’ = time to travel same path when path is moving through the Aether (parallel to itself)

·         v = velocity of path through the Aether

·         c = velocity of light

·         Text Box: Then, the expected difference in times is given by:

 

 

Explaining Michelson and Morley’s Negative result

n      The Michelson-Morley Interferometer detected no change in interference patterns no matter which way they aimed their interferometer

n      Text Box: That is they found that t = t’

n      Solving their equation, this implies that the velocity through the Aether is zero:

 

Logical Structure of the Argument

            H = The Aether is motionless in the universe and the earth moves through it

T = Light will appear to travel at different speeds when measured by instruments travelling at different speeds through the Aether (i.e. in different directions on earth).

H ŕ T (That is, if H is true so is T)

 

T is False.

Therefore H is false (modus tollens)

 

Then, what about H is false?

·         Maybe the earth really is motionless and the heavens move.

·         Maybe the earth drags the Aether around with it.

·         Maybe H is correct and T is set up wrongly.

·         Maybe the Interferometer shrinks in the direction of its motion by just enough to make the Aether wind undetectable

 

Ernst Mach and Positivism

·         We can only know what we can verify with out senses.

·         Science should use no concepts that cannot be measured experimentally.

·         For example, behavioural psychology banished notions such as thought, consciousness, the will, the mind, etc., since they could not be verified.