So What Actually Is Quantum Physics?

Quantum Physics:
Fields, Waves, and Particles
Multi-Particle Waves

You may have heard the saying, “No one understands quantum physics.” This is, quite frankly, a lie. When Richard Feynman said the quote, he did not mean no one understood the theory; it is mathematically robust, and makes the most precise predictions out of any theory in science. What Feynman meant is that no one understands its implications on the underlying metaphysical structure of reality, i.e. which interpretation of it is correct. If we are just talking about the physical theory, experts in the field understand quantum physics very well, and I believe you can too. That’s why I’ve started this series explaining quantum physics in a streamlined top-down approach.

What you get out of this series will be up to your expectations. If you go into this with a “can’t understand” mindset, you will not be able to understand it. Failure is a self-fulfilling prophecy. You can understand quantum physics. It may be difficult, and you may not get it on the first read-through, but with enough persistence and perhaps some help from other resources, you can get to the point where you can brag to your friends that you know quantum physics.

Unless you took quantum physics classes, what you know of quantum physics is probably wrong

There are two main things holding people back from understanding quantum physics. First, its reputation for being incomprehensible, which, as I just mentioned, is false. Second, most people’s exposure to quantum physics comes from either science fiction movies or spiritual gurus, both of which use “quantum” as a modern substitute for “magic.” Stories and mystics want to invoke alternative histories, portals to other realms of existence, psychic powers, time travel, and all kinds of uncommon phenomena.

Back in the day, people were more generally open to the existence of the supernatural, and we could get away with calling it magic. If a story had a magic mirror that took curious wanderers to a bizarre world, a reader might entertain the notion that such a mirror might exist somewhere in the reaches of the world untouched by modern society. Nowadays, a magic mirror would be seen as a children’s fancy, not something to be taken seriously in adult fiction. A quantum mirror, on the other hand, crosses the boundary back into the fringes of believability, and it feels like it may be invented someday, or perhaps already has been by some alien civilization out in the universe somewhere.

A timeline-hopping quantum mirror as seen in the Stargate SG-1 episode, “There But for the Grace of God.”

Background Knowledge: Fields and Waves

Before we get to quantum physics, we need to lay a foundation of supporting knowledge, so that the concepts of quantum physics will come more naturally. To start off, let’s look at the concept of a field. In everyday language, a field is a wide open area of land, usually covered by a certain kind of plant or combination of plants. A physical field is similar. It is anything that fills all of space and has some numerical value everywhere.

Temperature, for example, is a field. Pick any spot in the universe, and it has a temperature. Gravity is a field. Pick any spot in the universe, and the total gravity from all masses will have a single direction and strength. The same is true for the electric field. Any spot in the universe has a direction and strength of the electric field from all charged particles added together.

You might have been taught in physics classes that charged objects each create their own electric field, which interacts with other charged particles to cause static electric forces. Similarly, magnets create magnetic fields. However, it is more correct to say there is one electromagnetic field throughout all the universe, and charged particles and magnets create perturbations in this single field. Just like the rest of us, scientists tend to use the language that is most useful for the problem at hand, not necessarily what is most true.

Next, let’s talk about waves in the fields. For an easy example, think about tossing rocks into a lake. From where the rock lands, ripples spread out. The water does not move horizontally, just up and down. Also, aside form the initial splash, the surface doesn’t break.

Physical fields can have waves too. Just like the water’s surface, physical fields don’t “break.” When a point in a field is perturbed, it tugs on all the points around it, and is tugged back in return. This causes a ripple through the field as each point is tugged and tugs on the points after it in turn. Unlike the water’s surface, which is two-dimensional, physical fields fill all three dimensions of space.

What direction is the field pulled in? After all, the surface of a lake is pulled in the third dimension, upward and downward. Fields, on the other hand, are not pulled in any dimension. Rather, they are pulled in the level of their strength. Heat doesn’t have a direction, it has a temperature. A wave of heat is a front of increasing temperature. An electromagnetic wave, also known as light, is a wave of increasing and decreasing electric and magnetic field strength.

Now that we understand fields and waves, we are ready to get quantum.

Definition of Quantum: Limited Allowable Quantities

The word, “quantum,” seems mysterious. But it has a simple meaning: A quantity is quantized if it has a limited number of possible values. A quantum is one of those values. Okay, maybe that doesn’t sound so simple at first glance, but once we start seeing examples it will start to make sense.

One type of quantum is a number of pixels. There is no smaller piece of visual information your screen can produce than a pixel. It cannot display half a pixel, and it cannot display a pixel and a half; the number of pixels it can display is limited to the natural numbers (0, 1, 2, 3, …). Therefore, a natural number of pixels is a quantum of computer graphics display.

Pixels are also a nice example of degeneracy. Degeneracy is when there are multiple possible states for a given quantum level. Again, examples will make this clear. If there are 0 pixels, there is only one state: off. 0 pixels is non-degenerate. For 1 pixel, there is also one state: on. At the 2-pixel level, however, there are two states: one on top of the other, and beside one another. The 2-pixel level has a degeneracy of 2. The 3-pixel level has a degeneracy of 6, as is shown in the diagram below.

Particles: Excitations in Quantum Fields

Now that we have all the background knowledge we need, we can start to learn quantum physics. Let’s begin by putting aside all our notions of matter and particles, and imagine a vast region of empty space with only fields inside it. These fields can interact with one another, but only in certain amounts at once; in other words, the fields are quantized. They are quantum fields.

These fields, like the fields we discussed before, can have waves. But for each wavelength, there is a smallest possible excitation; if you try to put less energy into a quantum field than its smallest possible excitation, nothing will happen.

Image source

In the electromagnetic field, the smallest possible excitation is called a photon. In the electron field, the smallest possible excitation is an electron. In the quark fields, the smallest possible excitations are quarks. What we are getting at here is that fundamental particles are the smallest possible chunks of wave in quantum fields. All forms of matter and energy in the universe are made up of these field excitations and their interactions with one another.

Image source

Let’s look at this in more detail. Imagine a place where the electron field is zero. The field is there, there just aren’t any electrons. Now we put energy into a point on the field, perhaps by shining a high-energy photon through it. If the photon has enough energy, at least twice the amount of energy of an electron’s mass as given by E=mc², then there is a chance it will interact with the electron field, giving up its energy, and creating two particles, an electron and an anti-electron.* If the light pulse has less energy than that, it will not give any energy to the electron field, because an electron is the smallest possible excitation of the electron field.

If the photon has a lot more energy than necessary to create an electron/anti-electron pair, it will stimulate the second quantum level of the electron field, creating a muon/anti-muon pair. If its energy is anywhere in between, the excess is given to the electron and anti-electron as a burst of speed.

Particles or Waves? The Wave Function

Most of us picture particles as infinitesimally small dots that zip around bumping into things, and waves as ripples that spread out to unlimited size and affect everything they touch. When it comes to fundamental particles, however, both of these pictures are incorrect. Fundamental particles are something new; they move like waves, and interact like particles.

First, let’s talk about the wave part, using electrons as a case study. In empty space, an electron spreads out as a wave in the electron field. As the wave approaches something it can interact with, the probability it will interact correlates with the amplitude of the wave. Where the wave is highest and lowest, the electron has the highest probability of interacting, and where the wave crosses zero, the electron has no probability of interacting.

Now we don’t need any math for the concepts we discuss today, but we should at least mention the Schrödinger equation, because it is an icon of quantum physics. You don’t need to be able to solve the equation, or even understand it, but you should be able to recognize it when you see it.

The most important part of the Schrödinger equation is the wave function, ψ. The wave function is a mathematical representation of particle waves, their peaks and valleys, and how they move through space and time.

Every behavior of quantum physics, from the double slit experiment to the quantum eraser behave as expected when the correct values are fed into this equation. It also explains, when understood, why many values are quantized rather than continuous.

Particles or Waves? Exclusive Interactions

As I mentioned above, fundamental particles spread out like waves, but interact like particles. As a wave, it has a probability of interacting everywhere, correlated with its amplitude at that point. But when it interacts, it interacts fully at one location. At that moment, it becomes impossible for any other part of the wave to interact. This is called the collapse of the wave function. After the interaction, the particle once again spreads out as a wave from the point of interaction, ready for its next interaction.

This is where the interpretations come in. The reason people say “no one understands quantum physics,” is because no one knows what happens to the parts of the wave function that don’t interact. If they just disappear, it is called the Copenhagen interpretation. If the interaction causes a split in the universe, and every part of the wave function interacts in one of the branches, it is called the Many-Worlds interpretation. There are other interpretations too, but those are the main contenders.

Summary and Conclusion

Let’s recap what we’ve learned today. Space is filled with substance-like things called fields, including the electromagnetic field, the electron field, and others. Particles are quanta of excitation in these fields (smallest, second-smallest, third-smallest, and perhaps more). These particles behave like waves until they interact, whereby they interact all at once. This causes the wave to collapse and spread out again from the point of interaction.

That’s the basics of the basics in a nutshell. It’s not enough to understand most experiments and technology that use quantum physics—that will have to wait for part two—but it should be enough that when you hear the word “quantum,” you know it relates to subatomic wave-particles and discrete levels of smallest-possible things. It has absolutely nothing to do with love, telepathy, willpower, perception, or anything like that. That’s just misusing the word “quantum” as a substitute for magic.

Next time in the quantum physics series, we will talk about multi-particle waves, finishing up the foundational knowledge necessary to understand quantum physics-based experiments and technology, and taking a glimpse into how atoms work. I hope to see you then!

*Another name for an anti-electron is a positron. For reasons we may talk about in a future blog post, in order for a particle of matter to be created in a quantum field, a particle of antimatter must also be created.