Common Misconceptions: Is an electron a particle or a wave?

By Christopher Moore

Electromagnetic radiation (or light) sometimes behaves as a particle and sometimes it behaves as a wave. The same observations are made with regular particles like electrons. This apparent contradiction has led many to question the foundation of modern science, specifically quantum theory. How can something be A in one instance and be B in another? The answer is: that something is neither A or B.

First off, a little history. In around 1805 Thomas Young performed an experiment that has come to be called Young’s Double-slit experiment. Wikipedia summarizes the experiment as follows:

The double-slit experiment or two-slit experiment consists of letting light diffract through two slits producing fringes on a screen. These fringes or interference patterns have light and dark regions corresponding to where the light waves have constructively and destructively interfered.

Young was attempting to determine whether light was made up of particles or whether it was made up of waves. That he observed an interference pattern lead him to the conclusion that light was a wave. It wasn’t until the early 20th century that experiments began to suggest that light was made of particles. Einstein’s photoelectric effect is an example of light “behaving” like a particle.

So in some instances light behaves like a wave, but in others light behaves like a particle. And to further complicate the matter, Young’s experiment can be performed using beams of electrons or atoms. So particles seem to act like waves sometimes as well!

In 1961, Claus Jönsson of the University of Tübingen performed the Young experiment with a steady stream of electrons, and in 1974 the experiment was carried out “one electron at a time” at the University of Milan by Pier Giorgio Merli.

Here are the results (whether done with electrons or light):

If either slit is covered, the individual photons hitting the screen, over time, create a pattern with a single peak. But if both slits are left open, the pattern of photons hitting the screen, over time, again becomes a series of light and dark fringes. This result seems to both confirm and contradict the wave theory. On the one hand, the interference pattern confirms that light still behaves much like a wave, even though we send it one particle at a time. On the other hand, each time a photon with a certain energy is emitted, the screen detects a photon with the same energy.

A remarkable refinement of the double-slit experiment consists of putting a detector at each of the two slits, to determine which slit the photon passes through on its way to the screen (If the photon or electron passes through only one slit - which it must do, as, by definition, a photon or an electron is a quantum, or “packet” of energy which cannot be subdivided - then logically it cannot interfere with itself and produce an interference pattern). When the experiment is arranged in this way, the fringes disappear.

This is truly remarkable and seems to be completely contradictory. What this suggests is that light (or electrons) behave like waves unless we start counting them, and then they behave like particles. Observing things affects their behavior it would seem. The Young experiment applied to particles directly resulted in the development of quantum physics.

Some consider this a glaring contradiction, and therefore doubt the validity of quantum theory. They suggest that one thing cannot exhibit two very different behaviors based on the circumstance. Unfortunately for them, the experiment is very reproducible. Quantum theory doubters base their arguments on a misconception of what may really be going on: they continue to view the world through Newtonian glasses, where particles are particles and waves are waves.

One of the moderators on our forum puts it this way:

We call something a particle if it behaves in a particle-like manner. But what does that mean? What is particle-like behavior? Similarly, we say that something is a wave if it exhibits wave-like behavior. But what do we mean by that? What is wave-like behavior? Whatever your answers, they will be drawn from pre-20th Century (“classical physics”) constructs for describing the physical world that we observed. They are models originally distilled and codified from early observation and experiment.

A model is only as good as the accuracy of its results, and when investigating very small particles, classical models and descriptions no longer hold. An electron will behave differently depending on the circumstances. The common misconception is that this is contradictory, that something has to be either a particle or a wave. It is not contradictory. The wave-particle duality “problem” is only an issue if you continue to wear a Newtonian mental straightjacket and believe that only one of two types of behavior can be valid. Those who found an argument against quantum theory on this premise need merely be pointed to experimental data.

Martin once again:

Perhaps a reasonable analogy might be a coin: It has two sides (heads and tails). When you flip the coin, it will land showing either the heads side or the tails side, but not both. You would not be at all perplexed by this: Even though sometimes you see the heads side but other times you see the tails side, you have no doubt that you’re looking at a coin. The coin is then analogous to a “wavicle” of light, which has two mutually exclusive properties: sometimes it shows its “wave” behavior, sometimes its “particle” behavior.

Our Newtonian blinders confuse us. We are conditioned to think in terms of hard particles (like baseballs) and soft waves (like water ripples). This is what we see all around. It is easy to assume that smaller objects should exhibit similar behavior, but that assumption is absolutely false. George Gamow coined the term “wavicle” to describe quantum entities that exhibited both wave and particle behaviors. The conclusion that can be drawn from the data is that an electron is neither a wave or a particle. It is neither A or B. It is something else completely.

It is a wavicle.

---

Common Misconceptions: Force

By Christopher Moore

I will begin writing short posts each week about common misconceptions students have about basic ideas in physics. This week I’d like to address the concept of force.

Most students think they know what a force is after finishing an introductory course in physics. But most students carry away several misconceptions about the concept.

In a typical class, force is introduced as a push or a pull. Although, in a sense this is correct, this view of force leads to several misconceptions. By viewing force as a push, students connect the concept of force with the actual act of pushing.

Teacher: If John pushes Jason to the ground, then who is using force?

Student: John, of course.

A force is not an act or a tangible thing, or a property of an object. A force is an interaction between two objects. It is correct that the act of pushing involves a force, but the act itself is not the force. Beginning a discussion on force by describing the concept as an interaction may result in a different exchange between teacher and student:

Teacher: If John pushes Jason to the ground, then who is using force?

Student: Well, in a sense both use force. But force isn’t actually a thing that can be “used” so your question is poorly formed. The force is the interaction between both Jason and John.

Refering to force as a push or pull also leads to misconceptions about Newton’s Third Law. A typical exchange might go as follows:

Teacher: If a book that weights 10 N sits on a desk, what is the equal and opposite reaction to the weight.

Student: The table pushes up on the book with 10 N of force.

Teacher: How many forces are involved?

Student: There are two forces. The book pushes down on the table and the table pushes up on the book.

Although the student’s statements are correct, they do not correctly answer the teacher’s questions. By treating force as a push or a pull, the student can naturally read Newton’s Third Law as: “For every force there is an equal and opposite force.” This is incorrect. It is: “for every action there is an equal and opposite action.” Believeing force to be an “action” leads to this misconception. Force is an interaction between two objects, and Newton’s Second Law describes this interaction quantitatively. Newton’s Third Law couples the objects, and descibes force qualitatively.

A student well versed in force-as-interaction may answer as follows:

Teacher: If a book that weights 10 N sits on a desk, what is the equal and opposite reaction to the weight.

Student: Weight is an interaction between an object and the Earth, so if the Earth exerts 10 N of force on the book, then the book must exert 10 N of force on the Earth.

Teacher: Why is that so?

Student: Because they are the same force. The Earth “pulling” the book is the same interaction as the book “pulling” the Earth. Force does not causes an equal and opposite force. The force causes an equal and opposite action. Newton’s Third Law simply says that a force is an interaction between two objects. So if that interaction affects one of the objects, then it has to affect the other.

Teacher: How many forces are involved?

Student: For a book on a table there are two forces. The interaction between the book and the Earth and the interaction between the book and the table.

Another common misconception about force is the belief that a force is an inherent property of an object. This misconception usually arises due to a failure to adequetly distinguish between mass and weight. And example is as follows:

Teacher: An elephant weights 2000 N. How much force does the elephant have?

Student: 2000 N.

Our well trained student would answer differently:

Teacher: An elephant weights 2000 N. How much force does the elephant have?

Student: That’s a stupid question. An elephant can’t have force. Force is an interaction between two objects. The interaction between the Earth and the elephant can be quantified as 2000 N. But an elephant does not have weight.

Issac Assimov in The History of Physics describes force as a double headed vector, that is an interaction. Students need to be taught the concept of force correctly to avoid such common misconceptions. Force is much more than a push or pull.