1900: We know the distances to 70 of the nearest stars. There are no galaxies except for the Milky Way, although some astronomers do believe that some of the fuzzy "nebulae" seen in the night sky are separate, very distant galaxies. The universe is unchanging. Newton's laws are used to explain all physical phenomena. The elements that have been identified are believed to be built of indivisible atoms that are the "essence" of the element. Physicists believe there is little more to be discovered in their field. However, there are a few nagging issues, such as the behavior of light, which will spur a revolution in the field as the new century begins.

Here are a few steps on the science odyssey from then to now, including links to activities and databank entries on this site.

• In 1900, Max Planck explains that energy is released by matter in discrete packets, which he calls quanta. The quantum theory explains why a heated object emits only light of specific colors, and this sets the stage for new theories about the structure of the atom.
  

  Discoveries: Planck discovers the quantum nature of energy
  People: Max Planck
  

• In 1905, Albert Einstein shakes up the Newtonian view of the world when he publishes his special theory of relativity.
  

  Discoveries: Einstein publishes the special theory of relativity
  People: Albert Einstein
  

• In 1911, Ernest Rutherford describes an atom as a nucleus, containing positively charged particles called protons, surrounded by a cloud of negatively charged electrons. In 1913, Niels Bohr states that electrons circle the nucleus at specific quantum levels, providing further explanation of Planck's theory.
  

  You Try It: Atom Builder
  Discoveries: Rutherford and Bohr describe atomic structure
  People: Ernest Rutherford
  People: Niels Bohr
  

• In 1912, Henrietta Leavitt creates the Cepheid period-luminosity chart, which will later serve as the first cosmic yardstick, allowing astronomers to accurately measure the distance to far-away objects.
  

  Discoveries: Leavitt discovers a correlation between Cepheids' period and luminosity
  People: Henrietta Leavitt
  

• In 1915, Einstein goes on to tackle greater questions of time, space, and gravity in his general theory of relativity.
  

  "That's My Theory!" with special guest Albert Einstein
  Discoveries: Einstein announces the general theory of relativity
  People: Albert Einstein
  

• In 1924, Edwin Hubble finds a Cepheid in the Andromeda Nebula, allowing him to measure the distance to the nebula. He finds it to be so far from earth that in fact Andromeda is a separate galaxy, and our Milky Way is only one small part of the universe.
  

  Discoveries: Hubble identifies a new galaxy
  People: Edwin Hubble
  

• In 1926, Erwin Schrödinger publishes his wave equations, providing mathematical explanations of Bohr's theories of quantum atomic structure. The following year, Werner Heisenberg states his uncertainty principle, which begins to explain the strange working of the subatomic world, a world that behaves unlike anything we can observe in our everyday life.
  

  Discoveries: Heisenberg states the uncertainty principle
  People: Werner Heisenberg
  People: Erwin Schrödinger
  

• That same year, 1927, Georges LeMaitre develops a theory of the origins of the universe, which becomes knows as the "big bang."
  

  Discoveries: Big bang theory is introduced
  

• In 1929, Edwin Hubble finds that very distant stars appear to be moving away from earth at a speed proportional to their distance.
  

  Discoveries: Hubble finds proof that the universe is expanding
  People: Edwin Hubble
  

• In 1931, Ernest Lawrence invents the cyclotron, which allows physicists to study the behavior of atomic particles accelerated at very high speeds. This inaugurates an age of "big science."
  

  Discoveries: Lawrence invents the cyclotron
  

• In 1932, James Chadwick discovers that the nucleus of an atom is composed of protons and neutrally charged particles he calls neutrons.
  

  You Try It: Atom Builder
  Discoveries: Chadwick discovers the neutron
  

• In 1942, as part of the Manhattan Project working to develop an atomic bomb, Enrico Fermi creates the first controlled nuclear reaction by bombarding an atom of uranium with an accelerated neutron.
  

  Discoveries: Fermi creates controlled nuclear reaction
  

• In 1945, an atomic bomb is first exploded in New Mexico and another is soon dropped on Japan.
  

  Discoveries: The first atomic bomb is detonated
  People: J. Robert Oppenheimer
  People: Niels Bohr
  

• In 1964, Murray Gell-Mann proposes that subatomic particles such as protons and neutrons are composed of even smaller particles, which he calls "quarks." The existence of quarks is proven ten years later.
  

  You Try It: Atom Builder
  People: Murray Gell-Mann
  

• In 1965, Arno Penzias and Robert Wilson hear an annoying hiss in their satellite antenna, which they identify as cosmic microwave radiation left over from the big bang.
  

  Discoveries: Penzias and Wilson discover cosmic microwave radiation
  

• In 1967, Jocelyn Bell finds a message from a distant star, the first identified pulsar in the universe
  

  On the Edge: Little Green Men
  Discoveries: Bell and Hewish discover pulsars
  People: Jocelyn Bell
  

• In 1967, Steven Weinberg continues the search to replace the laws of Newton with new explanations that explain the behavior of all forces, including those at the subatomic level.
  

  People: Steven Weinberg
  

• In 1976, Thomas Kibble suggests that the uneven distribution of matter in the universe can be explained by the existence of cosmic strings formed shortly after the big bang.
  

  Discoveries: Cosmic string theory introduced
  

• Today, physicists like Jim Gates continue to explore the mathematical workings of the subatomic world.
  

  People: Jim Gates
  

Today: With the aid of radio telescopes and the Hubble Space Telescope, launched in 1990, astronomers can now see the birth of galaxies, find planets orbiting distant stars, and probe 15 billion light years to the edge of the known (and expanding) universe. We have identified and continue to study numerous cosmic phenomena such as pulsars, quasars, black holes, and dark matter. At the other extreme, a "zoo" of subatomic particles has been studied, and much of their behavior is understood. But no unifying theory can yet explain all physical forces, and no one has proven what the ultimate building block of matter might be.