MODERN PHYSICS (theories of relativity and quantum physics)

Physicists often refer to the theories of relativity and quantum physics as “modern physics,” to distinguish them from the theories of Newtonian mechanics and Maxwellian electromagnetism, which are lumped together as “classical physics.” As the years go by, the word “modern” seems less and less appropriate for theories whose foundations were laid down in the opening years of the 20th century. After all, Einstein published his paper on the photoelectric effect and his first paper on special relativity in 1905, Bohr published his quantum model of the hydrogen atom in 1913, and Schrödinger published his matter wave equation in 1926. Nevertheless, the label of “modern physics” hangs on.

All in all, two lines of investigation are truly “modern”, but at the same time have the most ancient of roots. They center around two deceptively simple questions:
What is the universe made of?
How did the universe come to be the way it is?
Progress in answering these questions has been rapid in the last few decades. Many new insights are based on experiments carried out with large particle accelerators. However, as they bang particles together at higher and higher energies using larger and larger accelerators, physicists come to realize that no conceivable Earth-bound accelerator that can generate particles with energies great enough to test the ultimate theories of physics. There has been only one source of particles with these energies, and that was the universe itself within the first millisecond of its existence.

In the 1930s, there were many scientists who thought that the problem of the ultimate structure of matter was well on the way to being solved. The atom could be understood in terms of only three particles—the electron, the proton, and the neutron. Quantum physics accounted well for the structure of the atom and for radioactive alpha decay.
The euphoria did not last. By the end of that same decade, there was discoveries of a host of new terms and a veritable flood of particles (such as muon, pion, kaon, and sigma), names that you should not try to remember. All the new particles are unstable; that is, they spontaneously transform into other types of particles according to the same functions of time that apply to unstable nuclei.

If you are temporarily bewildered, you are sharing the bewilderment of the physicists who lived through these developments and who at times saw nothing but increasing complexity with little hope of understanding. If you stick with it, however, you will come to share the excitement physicists felt as marvelous new accelerators poured out new results, as the theorists put forth ideas each more daring than the last, and as clarity finally sprang from obscurity.

The main message of this piece of writing is that, although we know a lot about the physics of the world, grand mysteries remain.