Modern Physics

This isn’t Newton’s Physics!

Democritus – 400 BC

First known person to advance the idea of “atoms” as building blocks of matter.

Dalton – 1800

First name: John Established the likelihood that

elements consist of basic structural units (like Democritus’ atoms).

Michelson - 1879

First name: Albert Measured the speed of light

(accurately) for the first time.

“Luminiferous Ether”

It was known that light was a wave (interference, diffraction, etc), yet it was able to travel through space which apparently contained no continuous mass. It was, therefore, assumed that some undetectable mass existed in space, the luminferous ether.

Michelson/Morley - 1887 Michelson, in combination with

Edward Morley, conducted a famous experiment that bears their names to detect the luminiferous ether.

No ether was detected

Hertz - 1887 First name: Heinrich Discovered the Photoelectric Effect

(accidentally). Shining light on certain metals

caused an electrical current.

Thompson – 1897

First name: JJ (Joseph John) Discovered the existence of

electrons as particles. Formerly known as “cathode rays”.

Planck - 1900 First name: Max Radiating objects (black-body) emit specific

colors regardless of temperature. EM theory predicted an “ultraviolet catastrophe”.

Planck noted that blending two separate (and apparently unrelated) theories resolved the problem.

Concluded that light must act like particles (photons) which contained fixed (quantized) amounts of energy related to their frequency (E=hf). Referred to as the Quantum Theory.

Einstein - 1905

First name: Albert Published his Theory of Special Relativity Measurement of time wouldn’t be the same

for all observers, but would instead be related to their relative motions.

Energy and mass were interchangeable (E=mc2)

Thompson – 1905(?) “Plum-pudding” Atomic Model

Millikan - 1910 First name: Robert Measured the charge on the

electron in the famous experiment that bears his name.

Rutherford – 1911 First name: Ernest Gold Foil experiment

Gold Foil Experiment:

Rutherford’s Conclusions:

Most positive charge and mass is in a small, centralized region (nucleus).

A consequence of this is that there must be positive particles (protons).

Electrons are “outside” the nucleus. Suggested that neutral particles

consisting of proton plus electron might also be found (in the nucleus).

Bohr - 1913

First name: Niels Followed up Rutherford’s experiment by

suggesting a “planetary” model for the atom (electrons as “planets”; nucleus as “sun”).

“Orbits” had specific energies. He successfully predicted the energy

levels for hydrogen atoms.Demo

Einstein - 1915

Explained the photoelectric effect by using Planck’s Quantum Theory.

Ironically, this explanation led to the widespread acceptance of Planck’s Quantum Theory, and Einstein spent much of his later life seeking to circumvent its statistical nature (“GOD doesn’t play dice”).

Photoelectric Effect Energy is required to release electrons

from a photoemissive metal (ionization energy).

If light is a wave, then increasing brightness should release electrons.

Photoelectric Effect (cont.) Energy is required to release electrons

from a photoemissive metal (ionization energy).

If light is a photon, then increasing frequency should release electrons.

Millikan - 1915 Verified Einstein’s explanation of

the photoelectric effect Determined the value of Planck’s

constant

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Einstein - 1916

Published his General Theory of Relativity which led to the prediction of the existence of black holes, gravity-warped space, gravity-deflected light, etc.

Compton - 1922

First name: Arthur Photons colliding with electrons

caused the electrons to be deflected, and the process conserved both energy and momentum.

Photons act as if they have mass.

de Broglie - 1924

First name: Louis Suggested that just as light waves

could behave like particles (photons), particles could similarly behave like waves, with wavelength related to momentum.

=h/mv

Schroedinger - 1926

First name: Erwin Developed wave equations to define

particles (electrons). Solutions to the wave equations defined the characteristics of the particles.

Led to the Electron Cloud Model of the atom and removed the limitations inherent in Bohr’s model of the atom.

Heisenberg - 1927

First name: Werner Published his Uncertainty Principle.

It was impossible to determine both the momentum AND position of a particle/wave.

Measuring affects the measurements.

Chadwick – 1932

First name: James Discovered the neutron

Gell-Mann and Zweig - 1964

First names: Murray and George Proposed that nucleons were

composed of smaller particles (quarks)

Friedman and Kendall - 1968

First names: Jerome and Henry Provided the first experimental

evidence for the existence of quarks.