The last century of the second millennium began with a revolution that dramatically changed scientists' understanding of the basic properties of matter and energy. A new understanding that energy and matter were equivalent and that, at the submicroscopic level, the rules that governed their behavior were entirely different than those of the larger world amended Newton's laws of physics.

This new understanding arose from a radical new theory of light that many scientists initially found to be unbelievable. While eighteenth century science treated light as a particle, 19yh century science treated it as a wave. Twentieth century science went a step further and determined that light was, in fact, both particle AND wave.

In 1900, German physicist Max Planck published a controversial theory proposing that atoms did not release their energy in a continuous flow, as scientists thought, but in discrete packets he called quanta (singular, quantum). While the physics community at large seemed unimpressed by Planck's theory and unsure of its implications, one German theoretical physicist--Albert Einstein--carried Planck's concept a step further.

In a paper published in 1905, Einstein proposed that light is composed of energy "particles" that, under most circumstances, behave like a wave. Using this concept, he did something traditional physics had not been able to do; he successfully explained the photoelectric effect, for which he received the 1921 Nobel Prize for Physics.

Outrageous and contradictory as it seemed, quantum theory revolutionized physics because it successfully explained physical phenomena at the level of the atom, something Newtonian physics couldn't do. Although its effects may not be observable in the larger world, quantum theory played an integral role in the development of new technologies that significantly influenced this century.

Advances in microscopy also gave scientists tools for exploring the realm of the extraordinarily tiny. In 1931, Ernst Ruska developed components for the first electron microscope, which he built in 1933 and for which he was awarded the Nobel Prize for Physics in 1986. The principle of the microscope was to use a focused beam of electrons, which behaved like waves with a very short wavelength, instead of a visible light source. This dramatically increased resolution and allowed scientists to view objects too small to be seen with a light microscope.

With increasingly powerful telescopes, astronomers continued to identify more objects in the sky, from asteroids to distant galaxies. In 1930, Pluto was added to the inventory of known planets in the solar system.

Astronomers began to move beyond identifying and cataloging objects in the sky, developing theories of cosmology to explain the development of the universe. In 1912, American astronomer Vesto Slipher observed that the lines in the spectrum of all galaxies are shifted toward the red frequencies of the light spectrum. In 1929, another American astronomer, Edwin Hubble, proposed that this shift meant that the universe is expanding at a constant rate (Hubble's constant). This opened debate about whether the universe was going to expand forever or would begin to contract at some point, in a series of endless expansions and contractions.

While the relatively new medium of radio grew in popularity and availability during the first three decades, another wireless medium was in development. In 1926, A Scottish engineer, John Baird, demonstrated the first working prototype for television—radio with a view.

1900 - 1933 1900 Max Planck (Germany) theorizes that electromagnetic radiation is emitted in discrete packets of energy called quanta. 1902 Philipp E.A. Lenard (Germany) performs experiments on the photoelectric effect and finds that there is a threshold frequency that must be achieved in order to produce the effect. Light frequencies below the threshold will not produce the photoelectric effect. 1902 Annie Jump Cannon (USA) publishes the first volume of the Henry Draper Catalog of stars, which classifies stars by stellar magnitudes (surface temperature). A total of nine catalogs are published by 1924, listing more than 225,000 stars, 300 of which were discovered by Cannon. 1905 Albert Einstein (Germany) publishes a paper on the photoelectric effect, presenting the idea that light radiation consists of packets of energy (later called photons). He also publishes four other papers that revolutionize twentieth century physics. 1906 Charles Barkla (England) polarizes X rays (selecting X-ray waves that vibrate in the same plane), demonstrating that X rays are transverse waves like other electromagnetic radiation, such as light. 1912 Vesto Melvin Slipher (USA) observes that the lines in the spectrum of all galaxies are shifted toward the red spectral region. 1913 Neils Bohr (Denmark) completes his theory of atomic structure, stating that the absorption and emission of light by an atom happens when an electron moves from one orbital energy state to another. The light is absorbed or emitted in discrete amounts, or quanta, equal to the energy gained or lost by the electrons. 1924 Louis de Broglie (France) develops a revolutionary theory of electron waves, proposing that particles of matter may behave like waves under certain conditions. 1924 Indian physicist Satyendra Nath Bose publishes a paper clarifying the relationship between waves and particles. This will lead to collaboration with Albert Einstein and a theory called Bose-Einstein statistics. 1926 Using an ingenious experiment with an eight-sided rotating mirror and a vacuum tube spanning two mountain peaks, Albert A. Michelson (USA) makes a more precise measurement of the speed of light. This betters his previous measurements for which he was awarded a Nobel Prize in 1907. 1926 John Logie Baird (Scotland) publicly demonstrates a fully working prototype of an electomechanical television, electrically transmitting moving images. 1928 Indian physicist Chandrasekhara Raman observes that when light passes through a transparent substance, some of the light is deflected and changes in wavelength. This will eventually be called Raman scattering, a result of the Raman effect. 1928 Edward H. Synge publishes a series of articles that first conceptualized the idea of an ultra-high resolution optical microscope. Synge's proposal suggested a new type of optical microscope that would bypass the classical diffraction limit. 1929 Edwin Powell Hubble (USA) discovers that the universe is expanding at a constant rate, which will eventually be referred to Hubble's constant. 1930 American astronomer Clyde W. Tombaugh discovers the planet Pluto by examining photographs he had taken at the Lowell Observatory in Flagstaff, Arizona. 1930 Bernhard Schmit (Estonia) invents the Schmit telescope, which uses a spherical mirror instead of a parabolic reflector, and a correcting plate as the telescope aperture. 1930 While studying flaws in diffraction gratings, Frits Zernike (Holland) discovers the phase contrast principle that allows him to view the internal structure of transparent objects. The different materials making up the object have different refractive indices, which makes it possible to illuminate them in a way that they are visible. 1931 Ernst Ruska (Germany) builds the first electron lens, an electromagnet that could focus a beam of electrons just as a lens focuses a beam of light. By 1933, he uses several electron lenses in a series to make the first electron microscope. 1932 Edwin H. Land (USA) announces the development of the first polarizing filter made out of synthetic material and secures the trademark name "Polaroid." 1932 RCA (Radio Corporation of America) demonstrates the first all-electronic television based on the iconoscope, which was patented by Vladimir Zworykin in 1923, and a receiver equipped with cathode ray tube.

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