Particles and Waves

When is a particle not a particle? When it’s a wave. It’s called complementarity, and more specifically “wave-particle duality.” A subatomic particle, such as an electron, is both wave and particle simultaneously at the deepest level of reality, the level of the quantum realm. In our everyday world, it can show up only as a particle or a wave and not both, and how it shows up is restricted by the kind of experiment that is being conducted to detect it. It’s strange, but true that if an experimenter is seeking to explore the wave properties of a quantum particle, say an electron, then the electron shows up displaying its wave nature. If the experiment is designed to explore particles, the electron shows up with all the properties and dynamics of a tiny solid thing—a particle. Somehow it’s as if subatomic entities “know” what we are asking and so appear in our world in ways that best accommodate our questions. 

(An aside: We have to talk about language for a moment or you risk misunderstanding some of this discussion. Physicists commonly use the word “particle” as a matter of convenience when referring to a subatomic entity, even though they know that its quantum nature is such that it is both wave and particle at the same time. So don’t let the word “particle” throw you off. In addition, they talk about “observing” subatomic particles, but no one can ever see quantum particles—or anything about the quantum realm—directly. The quantum realm is called by some physicists a “shadow” realm, because quantum entities can only be detected indirectly through the “signatures” they leave behind in detectors. We can never see the actual particles themselves. So, when physicists, or we, talk about making an “observation”, what is meant is that a subatomic entity was measured or detected, and from that “observation” we can extract information about its properties.)

Back to particles that are waves, and waves that are particles. Particle-wave duality is an aspect of the quantum world that has been tested again and again, and always proved to be correct. (The seminal experiment was called the double-slit experiment. You can follow the link to learn more, or you can search yourself online or in physics books.) Another aspect of complementarity—which is a characteristic of the quantum realm first explained by two of the founding fathers of quantum mechanics, Neils Borh and Werner Heisenberg—is that we can’t know everything there is to know about a quantum particle at once. There is a limit to our knowledge. For example, if you know how fast a particle is going (its velocity), you can’t tell with one hundred percent certainty where it is (its position). And vice versa, if you know precisely where it is, you can’t be sure how fast it is moving. By knowing a lot about one aspect of a particle, you sacrifice some knowledge about other aspects of it. This is a fundamental overturning of classical, Newtonian physics. Theoretically, in the classical world—in our everyday world—Newtonian physics tells us that if we know all the initial conditions of a system, we can reveal everything there is to know about that system as it unfolds in time. Not so in the quantum universe.

Let’s use the example of driving a car to illustrate this point. According to classical physics, you can know where you (and your car) are and how fast you are moving instant to instant during your trip. But if you are driving a quantum car, when you look at the speedometer to determine your speed, you can’t know precisely where you are. And vice versa, if you look out the window to see where you are, you can’t know your precise speed. The real significance of this aspect of complementarity is that the quantum realm is broadcasting a message to our macro world—to us in this everyday world of matter—that there is always some “uncertainty” to what we can know about the quantum realm, which is why this aspect of quantum theory is called The Uncertainty Principle. (It’s also called Indeterminancy.)

Another strange aspect of the quantum world is that quantum entities take “quantum jumps,” or, in alternative lingo, “quantum leaps.” The word quantum is the plural of the word “quanta,” from the Latin quantus, which means “how much.” A quanta is a “packet” of energy. Particles can have only certain allowable energies; in other words, they come in only certain size packets. When an electron, for example, gains more energy (never mind how it happens), it doesn’t increase its energy in a smooth, continuous way; it “jumps” from one allowable energy level to another. To use a crude example, think of a stairway, with each stair representing a different possible energy level or “quanta.” If a particle that is sitting on the third step gains enough energy to jump up a step, what happens is that it simply blinks out of existence on the third step and reappears on the fourth step, never having traveled the space in between. It can never be halfway between steps, because that is not an allowable energy state. Quantum particles can also perform a feat called “quantum tunneling” that seems like magic. A quantum particle can go through a barrier, from one side of it to the other, but never travel in the space between. Again, it just disappears from one side of the barrier and appears on the other. Some researchers believe that quantum tunneling happens in our brains, in tiny structures called microtubules, and helps explain consciousness.

When a particle is in its “natural” state—meaning it is not being measured or observed via experiments in our macro world—then it is said to be in a state of “superposition,” which means it is in every possible state it can be all at once. That’s why it is said to be a potentiality. It embodies everything possible for it, in what is often described metaphorically as a fuzzy, smeared out cloud of probabilities. When the particle suddenly appears in one, definable state—because we have measured it in some way—then physicists say its “wave function” has collapsed.

What’s a wave function? The simplest way to explain it is that it’s a mathematical description of all the potentials of a quantum system. (As its name implies, it focuses on the wave aspects of a quantum entity or system, but it also provides a description of the possible states of the system.) Physicists say that the only way to detect a subatomic particle is to “collapse” its wave function. When that happens, the particle goes from a state of superposition (being everything it can be) to taking on specific properties (it is acting like a wave or like a particle, can be located at a certain position, is moving at a certain speed, and so on). How do you collapse the wave function? By observing the particle! Again, it’s as if quantum entities know when we are looking at them, and they show up and display themselves according to the experimental set-up. If you think it’s strange to think that a particle becomes “real” with the collapse of what amounts to an abstract mathematic representation of it, you are not alone. The debate continues in physics about whether wave functions have some kind of objective reality or whether they are abstract descriptions only. While scientists don’t really know what wave functions are, they know how to use them to describe and explore the quantum realm.

There are many other mind-bending aspects of quantum particles, but we’ll discuss only two more: entanglement and nonlocality.