We’ve talked about what quantum means, but what does “quantum mechanics” mean?
Quantum mechanics is a very general set of rules governing the physical world that was developed starting in 1925. The year 2025 was chosen as the International Year of Quantum Science and Technology because it marks 100 years of quantum mechanics. We’ve talked elsewhere about what quantum means; the mechanics part refers to a systematic set of rules that can be widely applied to describe how things move and change.
Do “quantum mechanics” and “quantum theory” mean the same thing?
These terms are often used interchangeably, but a conceptual and historical distinction can be made between them. Historians usually trace the beginning of quantum theory to the year 1900. This is the first time a quantum hypothesis – in this case, that energy came in countable pieces – was introduced in trying to understand a physical phenomenon. It became clear this was a useful hypothesis, but there was disagreement at the time about what its physical significance was. In the period from 1900 to 1925, other physical phenomena were explained using this and other quantum hypotheses. This was a period of quantum theory, sometimes now called the “old quantum theory,” but it was before there was quantum mechanics.
Then what changed to go from quantum theory to quantum mechanics?
In the 1900-1925 period, there was no consistency in how and when to apply these quantum hypotheses to explain experiments and make predictions. Sometimes they seemed to work spectacularly well, which gave many people confidence that there must be something to the idea. But many other times, scientists tried to use these hypotheses to model or predict things, and the model didn’t make any sense, or the predictions were wrong. The point is that there was no systematic way of applying quantum theory ideas to different physical systems. A systematic method would be a “mechanics.”
And this systematic method was developed in 1925?
The groundwork for it, yes. The basic framework and some general sets of principles to follow took a few years to sort out in order to be able to apply them systematically to a wide range of problems. People are even now still working to revise and extend this framework, but many of the core pieces of quantum mechanics were put in place in 1925. The term “quantum mechanics” started to be widely used in the 1920s to describe these systematic rules. It was also a phrase that distinguished this new mechanics from what’s now called “classical mechanics.”
What is “classical mechanics”?
Classical mechanics, or sometimes just “mechanics,” is the framework for describing the motion of massive objects that was initially developed in the 17th century. This framework is a set of general rules that can be used to describe how planets orbit the sun or the rate at which a dropped object falls to the ground.
These would be ideas like “to every action, there is an equal and opposite reaction” and other rules of motion?
Yes, exactly. The rules of classical mechanics are still very useful and often easier to use than those of quantum mechanics, but quantum mechanics is an even broader theory that, in many scientists’ assessments, supersedes the rules of classical mechanics. One way to put it is that by the end of the 19th century, scientists thought they had a good, systematic theory for how matter moved around – that’s classical mechanics – and a good, systematic theory for how light worked – this is the electromagnetic wave description of light. However, there were a number of puzzles in trying to understand how light and matter interacted with each other. In the period from 1900-1925, some of these puzzles seemed to be solved using quantum ideas, but there was no systematic understanding of how light and matter interacted in all cases.
And quantum mechanics provided a systematic way for understanding how light and matter interact?
Not only did quantum mechanics provide a full description of how light and matter interact, but in doing so it dramatically revised our understanding of light and matter and the rules governing each of them. The earlier “classical” rules governing matter and light were revealed to be only approximations of a richer, quantum description of matter, light, and their interactions.
Written by Paul Cadden-Zimansky, Associate Professor of Physics at Bard College and a Global Coordinator of IYQ.
IYQ mascot, Quinnie, was created by Jorge Cham, aka PHD Comics, in collaboration with Physics Magazine.
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Illustrations: Solmar Varela
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