Section 2.1 Introduction to Quantum Mechanics
In the early 1900s, a series of experiments and theoretical breakthroughs dramatically changed our understanding of how the universe works, including our conceptions of space and time, predictability versus randomness, and the limits imposed on measurement at the atomic scale. Not only did these developments — referred to as modern physics — significantly overturn and revise the known laws of physics, they also became the foundation for a complete understanding of the foundations of chemistry (and therefore biology as well), and they led to a series of significant applications that have already resulted in an explosion of modern technology, e.g., semiconductor physics and modern electronics; computer technology; communications (including cell phones); numerous medical diagnostic devices, surgical techniques and radiation therapies; new and significantly enhanced forms of microscopy; ultra-precise navigation devices; and nuclear power generation and weaponry.
And the technological applications of modern physics will continue well into the future. In particular, there is a significant on-going research effort into the development of nanotechnological devices that will likely revolutionize the fields of medicine and engineering during the next 50 years. Imagine, for instance, nanometer-scale devices that could be programmed to search and destroy cancer cells or repair internal injuries at the cellular scale within the human body. This idea is currently speculative (as of the year 2021), but successful development of something like this — which would almost certainly require an understanding of principles of modern physics — could revolutionize cancer treatment in a way that will cause society to look back at chemotherapy and radiation treatment the way we currently look back at the use of leeches as medical “devices” in the middle ages. Other potential future quantum applications include quantum computing and encryption; replacements for semiconductor switches based on new graphene materials or quantum optical devices; or even molecular and biological electronic devices based on DNA.
In PHYS 211, you learned about one of the two pillars of modern physics, Einstein's Theory of Relativity, which extends classical (Newtonian) physics to systems that travel at speeds approaching the speed of light and also sheds important light on the nature of matter and energy. In this unit, you will learn about the other pillar of modern physics: the theory of quantum mechanics. Quantum effects are most noticeable on a microscopic scale, yet quantum behavior has critical effects on the macroscopic world that we see around us every day. As an example, atoms wouldn't be possible without quantum principles, so all matter around us would be dramatically different without this subatomic behavior (actually, there wouldn't be any “us”). And we have developed techniques that enable us to manipulate the subatomic world in ways that have significant technological applications, including much of what forms the basis of modern chemistry, chemical engineering, materials science, and electronics.
What we've learned about the microscopic world is that it is unlike anything we can picture! Things in the microscopic world, such as electrons or protons or electric fields, can sometimes act as particles and sometimes act as waves. Nothing in our macroscopic world is like this, so it is not easy to build simple mental pictures for it. To give you a sense of how strange this is: when a “particle” such as an electron is “acting as a wave” it does not have a definite position! Its very existence is spread out in some manner over a region of space.
So be it. It's one of the greatest triumphs of science that we have developed the experimental and theoretical tools to uncover the behavior of nature even when our fundamental intuition can no longer aid us. The theory of quantum mechanics, as we shall see, is necessarily somewhat abstract. But it is very much a physical theory, well-grounded in experimental evidence.
To be an educated, scientifically aware citizen in the 21st century, it is essential to have a grounding in the basic laws of quantum mechanics.