Chapter 1

What Is Quantum Physics, Anyway?

IN THIS CHAPTER

Laying a physics foundation

Identifying key elements of quantum physics

Understanding the scale of quantum physics

Thinking about experiments and their results

Throughout the twentieth century, quantum physics transformed our world. Humanity went from a species that questioned whether atoms existed to one that harnessed the power of the atom. Humans also used the understanding of atoms and subatomic particles to create desktop computers out of microscopic transistors - feats made possible by quantum physics.

Homes across the globe are powered by streams of electrons, a subatomic particle that had been discovered but was barely understood before the rise of quantum physics. The first computers stored information on physical cards, but then transitioned to using magnetism, on both hard drives and floppy diskettes. For a time, humanity stored information on compact disks that machines read by using lasers (also a product of quantum physics). Now, a common storage medium is a solid-state drive (SSD) built of semiconductors (another quantum physics outcome), and most people carry micro supercomputers (called smartphones) in their pockets.

In this chapter, I provide a high-level discussion of the transformation from the classical to quantum understanding of matter and energy. I talk briefly about the world before the discovery of quantum physics and then introduce the key features that physicists discovered when they first began exploring the quantum nature of reality. I discuss why people don't see these quantum effects in their everyday lives, and how improvements in technology allow them to first see and then expand on these understandings to grow their knowledge of physics.

The Classics: Pre-Quantum Physics

At its core, physics is the scientific study of the fundamental elements of physical reality: matter and energy. As you scale up the physical structure, and this matter and energy takes the form of chemicals mixed together or, say, a squirrel, the scientific study becomes chemistry and biology. But if you're talking about the baseline study of matter and energy, that means you're talking about physics.

Any scientific field, of course, has a lot of sub-disciplines. If you are studying the squirrel, for example, you aren't just studying biology, but also zoology (the study of animals). If you're studying how an acorn grows into a tree, then that would be biology but also botany (the study of plants).

If you're studying the path of an acorn hurled from a tree by an angry squirrel, well, that's physics. But it's also the specific field of kinematics. Physics includes many sub-fields, including thermodynamics (the study of heat energy), optics (the study of light), and electromagnetism (the study of electricity and magnetism).

Throughout this book, I assume that you, the reader, have a general understanding of the basic ideas of classical physics. Chapter 2 focuses on many of the disciplines of classical physics that study matter and energy in different forms and structures. And as these studies became more detailed, they left questions that laid the foundation, at the end of the nineteenth century, for the discovery of an entirely new field of physics - quantum physics.

What Makes Physics Quantum?

Quantum physics refers to a series of discoveries from the first half of the twentieth century and the scientific explanations related to those discoveries. The insights from these explanations revolutionized the understanding of matter and energy at the smallest scale and caused a transformation in the fundamental way that physicists describe and think about the physical reality of these structures. I cover these revolutionary discoveries and experiments in detail in Chapter 3.

But what are the key insights that make quantum physics different from the physics that came before it? A couple of major differences are central to understanding how quantum physics differs from classical physics.

• Quantization: Physical quantities are measured in discrete units, packets, or quanta that cannot be broken down any further.
• Uncertainty and probability: Systems have inherent uncertainty built into them.

The bulk of this book explores how these two ideas interact with each other and show up in quantum physical systems, and the implications that arise from them. Note: These implications often seem counterintuitive.

The main misperception that you need to overcome in studying quantum physics is, to put it bluntly, that you actually understand how the universe operates.

In quantum physics, when your hand rests on a table, you aren't looking at two solid physical objects. You are looking at two fields of particles interacting together in a particular way. Both are made up mostly of vast, empty space, but they somehow still push against each other. Thinking of this situation as two solid surfaces pressing against each other isn't wrong, it just doesn't represent the activity going on at the quantum mechanical level. The solid surfaces that you see and feel are an outcome of all of the more fundamental quantum physical interactions.

A Matter of Scale, or a Scale of Matter

Part of the reason physicists took so long to figure out these quantum elements of physical reality is that the elements become apparent only at extremely small scales. In their normal lives, people go around interacting with large, macroscopic systems.

It's worth noting what the word large means in this context. A grain of sand is estimated to contain anywhere from 1 quintillion to 100 quintillion atoms. That estimate translates to more than 100,000,000,000,000,000,000 (1020) atoms in a single grain of sand - or about as many atoms in a grain of sand as there are stars in the universe. And a grain of sand is so large that you don't need to take quantum physics into account to figure out how it behaves.

These large systems (relatively speaking) have the quantum effects washed out. So, although each individual atom involves quantum uncertainty, when you look at the full 100 quintillion atoms in the grain of sand, all of those quantum uncertainties cancel each other out. When viewed as a whole, the quantum uncertainty that remains on the grain of sand is completely irrelevant.

Physicists began to notice quantum physics only after they could look at a single atom or, even more precisely, after they began to look inside of a single atom - for example, when they examined electrons within an atom or the structure of the atomic nucleus. This tiny level was where quantum behaviors really became evident.

Quantum physics discoveries enabled physicists to finally begin to understand what was going on inside of atoms. The modern understanding of atomic structures is entirely built upon the understanding of quantum physics, even though the macroscopic physical structures that come out of those atoms - whether a grain of sand, a squirrel, or a planet - don't exhibit the same quantum behaviors that you can see when looking at their smallest pieces.

Measurements and Observables: How Scientists Know Quantum Physics Is True

Quantum mechanics is a means of carefully analyzing a quantum physical situation and describing the observable outcomes of measuring a quantum mechanical experiment. Although some key insights guide the field, quantum mechanics is driven largely by the fact that the equations that are used work. The equations are complex and messy; they required years of mathematical and physical study to fully understand them. But when you do figure out how these equations work, they give you information that matches with the observable output of an experiment.

Because quantum physics relies on behavior that people don't experience in their day-to-day world, one of the biggest challenges for those studying quantum physics is learning how to rely on abstract understandings that are inconsistent with their natural intuitions.

Studying classical physics is a cakewalk by comparison because people have natural intuitions that are completely consistent with classical physics. Many a child can toss a ball at the right angle to be caught by another child. (Not me, necessarily, but many other children could do it.)

Doing the right tests

Researchers know that, to show whether anything in science is true, the explanation must match with experimental outcomes. In many cases, the findings of experimental results are fairly consistent with your intuitions, but in some cases, accepting the findings involves realizing when your intuitions are wrong.

Almost everything learned in quantum physics experimentation involves people realizing that their intuitions are wrong.

To use a historical example, thinkers going back to before the ancient Greeks believed that heavier objects fell at faster rates than lighter objects. This is a very intuitive thing to believe and is probably still the guess most children would make about how things fall.

Not only is this idea intuitive, but it is even supported by simple experiments. If you have two balls of exactly the same size, but one is made of lead and one is made of foam, the lead ball (when dropped in a simple experiment) is going to...