Carl S. Helrich - The Quantum Theory―Origins and Ideas: A Historical Primer for Physics Students

If quantum mechanics hasnt profoundly shocked you, you havent understood it yet.

Niels Bohr

Any of us who has taught quantum mechanics has tried to convey to our students something of the shock the quantum theory brought to the physics community. This we do in a standard fashion by outlining the experiments that pointed to the failures of classical thinking. This presentation follows the pattern our professors used before they got down to the real objective of the course, which was to present the theory itself.

Of course, our students must also learn the mathematical basis of the quantum theory. Our objective is to show our students how to represent results in terms of sets of eigenfunctions, rather than position and momentum. In my experience, however, students rarely give up the classical sense that position and momentum are properties of the electron. They are normally very interested in the quantum theory and even want to be shocked. But we cannot help them understand how disturbing the twentieth century was if we do not ourselves fully understand what happened.

This book is what I wish had been on my bookshelf as I began each course I taught on quantum mechanics. I was an undergraduate student of engineering at Case when Martin J. Klein was there. So I was familiar enough with his work to realize that there was a lot I did not know. But I also realized that acquiring a fair understanding of the transition in our thinking from classical physics to the quantum theory was going to require an effort, something I kept putting off.

Eventually, I decided to write a text on quantum mechanics with a serious introductory chapter. When that introductory chapter began to get rather long I asked an editor at Springer Nature if this might be acceptable as a book. And as you can see, her answer was affirmative. This book is the result.

My original intention was to provide students with a broader picture of what happened so that they would understand that those critical experiments and ideas did not simply appear from nowhere. Why did the worlds leading metrological laboratory focus so much effort on blackbody radiation? And why were electrical discharges in rarefied gases once central in physics? Human personalities also became important as physics developed. Our students should understand this because they are also actors in a continuing drama as we try to understand how things came about.

Because I am writing for students preparing to enter the first course in quantum mechanics, I have assumed the usual level of mathematics. The reader should also be familiar with the classical mechanics and the Maxwell electrodynamics.

Of course, there are also experiments, which are central to the story. I hope to have conveyed something of the experimental genius, dogged hard work, elation, and occasionally even despair that has been so important in the story.

I could see no honest way around beginning with the search for a realistic atomic theory, which had its origins in the thought of the Presocratic Greek philosophers. The atomic structure of matter gradually became a central issue in the eighteenth and nineteenth centuries, and it was behind J. J. Thomsons experiment to identify cathode rays. His primary interest was actually in a rather complex atomic model.