Modern Physics

1
Modern Physics
"Colliding Black Holes"CreditNational Center
for Supercomputing Applications (NCSA)

• Fred Raab,
• LIGO Hanford Observatory

2
Physics near the end of the 19th century

• Maxwell had shown that electricity and magnetism
  were different faces of a single force
• Light was understood to be an electromagnetic
  wave and other, invisible electromagnetic waves
  were known to be produced by oscillating currents
• Gravity was the only other known force
• But some riddles remained

3
Why were there missing colors in sunlight?

• Since Newtons studies of the splitting of light
  into a rainbow, or spectrum, scientists were
  interested in the study of color and its origins.
  Fraunhofer, using exacting techniques to make
  high-resolution spectra, discovered many narrow
  slices of color were missing from the spectrum of
  sunlight. Finding out why this occurred would
  lead to the revolutionary advances in 20th
  century science.

4
What are stars and how do they shine?

• Annie Jump Cannon studied photographic images of
  stars and looked for clues in the colors of
  stars. She developed a classification scheme for
  stars by color that is still used today and
  became crucial to understanding the life cycles
  of stars.

5
What is the medium that carries light?

• All other known waves traveled through something,
  but light from the stars appeared to move through
  empty space
• This seemed incredible and it was imagined that
  space was suffused with some ethereal medium that
  carried light waves, but was so thin it caused no
  mechanical drag on planets

6
An important clue, seen as a huge success

• Studying the light given off by elements burned
  in flames and by sparks in gases, it was noticed
  that the colors came out in sharp slices of
  color. Above is the series of visible spectral
  lines from hydrogen, named the Balmer series,
  after a Swiss high-school chemistry teacher who
  worked on spectra.

7
An important clue, seen as a failure

• Albert Michelson, inventor of methods for
  ultrahigh precision spectroscopy, and his
  chemistry colleague Morley, devise a measurement
  to detect the ether.
• It failed to detect the ether, even though it was
  designed to measure motion of the earth through
  the ether to about 10 precision

8
Annus Mirabilis, 1905

• Einstein, age 26, working at Swiss Patent Office
  in Bern, published a series of papers that
• Laid the foundation for the quantum theory of
  light
• Ended a millenia-old debate on whether atoms were
  real
• Introduced a theory of space and time, called
  relativity
• Discovered the equivalence of matter and energy,
  enshrined in the worlds most famous equation,
  Emc2.
• These papers launched a revolution that led to
  most of 20th-century science and technology

9
Rapid progress in quantum theory

• Danish physicist, Neils Bohr produces a model of
  the atom that explains the Balmer series of
  hydrogen and the stability of matter. It relies
  on the fact that, at the atomic scale, the
  fundamental particles of matter behave like waves

10
Back in the heavens

• Cecilia Payne, the first woman in America to earn
  a Ph.D. in astronomy, argues that the missing
  colors in stellar spectra are caused by
  absorption by cooler gases in the atmospheres of
  stars. Using this realization to study the
  composition of stars, she determines that stars
  are mostly made from hydrogen and a small amount
  of helium