Lecture #9 Thurs 14 Feb 2008 What is Physics?

Physics is the study of the “fundamental” properties of matter and energy.

Fundamental field of physics: mechanics = motion of objects.

Today: classical mechanics (motion of objects on human scales of size, time, and speed)

Next week: relativity: motion of very fast and very massive objects and quantum mechanics: motion of very tiny objects

Physics has been dominated by different paradigms throughout history.

Paradigm: a higher narrative (metanarrative) for theories or even for the method of inquiry

Aristotle: universe is hierarchal – “natural place” Laws depend where you are

Newton: universal laws; “graph-paper universe” + no absolute center

Laws independent of position.

Relativity (Einstein): no absolute “rest frame” (no graph-paper)

Laws independent of motion

Quantum mechanics: universe evolves statistically

Aristotlean paradigm: A stratified or layered universe. Objects move to their “natural” place.. A focus on why and end results.

Challenge to Aristotlean paradigm: Trajectory of artillery. Motion of the stars, planets.

Newtonian paradigm: how do objects move? Forces and inertia.

Inertia: Objects in motion tend to stay in motion (in a straight line)

Objects at rest tend to stay at rest unless acted upon by an outside force

New mathematical tools: Cartesian coordinates: marriage of algebra and geometry

= “graph-paper”

The Newtonian paradigm: objects stay at rest/in (linear) motion unless acted upon by

outside force. Visualize a kind of grid (graph-paper) upon the world

Nature and mechanics of light

What is light? Newton did a lot of experiments on light, such as using prisms to get the spectrum. He believed light to be make of particles (“corpuscles”).

But Thomas Young (1773-1829) used an experiment to prove light has a wave nature. Light passing through a double slit shows an interference pattern much like water waves

... but if light is a wave, what is “waving?”

James Clerk Maxwell (1831-1879) suggested light is waves of electric and magnetic fields. The four Maxwell equations (1862) predicted other electro-magnetic waves such as radio waves (discovered by Heinrich Hertz in 1888).

One postulate: light travels in “luminiferous ether”. Luminiferous = “light bearing”

ether = heavenly substance (for celestial sphere). But no one has seen the ether.

Michelson and Morley expt (1887) used an “interferometer” to measure differences in velocities in two different directions simultaneously. It failed to detect the ether. So there is no ether!

Summary Newton viewed light as made of “corpuscles” (particles).

Young’s double-slit experiment exhibited interference, evidence for a wave nature for light.

What is waving? Maxwell: electric & magnetic fields.(Predicted radio waves, experimentally confirmed by Hertz.)

This in turn suggested a luminiferous ether, but the Michaelson-Morley experiment failed to detect the ether!