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Category: Particle-wave duality

Does matter act simultaneously like a particle and a wave?

A quantum wave (running through the red mesh) interacts with a detector screen (green film) and creates a particle (yellow/orange spot). If this image were accurate, the wave would disappear simultaneously with the appearance of the particle. [Image source: stills from Fermilab video by Dr. Don Lincoln, “Quantum Field Theory” (in the public domain) Jan. 14, 2016; Quantum Field Theory.]

When it’s not interacting, the matter is in a superposition of many possible states. A superposition is more than one wave on top of another in the same spacetime. That’s really the definition for a superposition in ordinary (classical) physics. For example, we can have a superposition of sound waves in air either reinforcing each other or flattening out each other.

Click here to see video: Waves go into superposition & out, animation. For Slow Motion: Click Settings (gear icon), Speed. [Set speed for .25.]

In the case of a quantum superposition, the waves are not in a known medium. Their physical nature is not understood or at least there’s no consensus among physicists as to their physical nature. The superposition represents the mathematical idea that there are many possibilities for properties of the particle. Let’s say, for example, we’re interested in the position of an electron. Until observed, the electron is in a superposition of many possible positions.

The mathematical equation which describes this superposition looks like an equation for a sound wave or a water wave in classical physics. So, the superposition state is called the “wave state” of the quantum particle. The equation (the famous Shrodinger Wave Equation) tells us the probabilities of where we will find the electron as a particle if measured. In the meantime, until measured, it’s a set of possible positions.

Quantum superposition (on left) and particles forming objects in spacetime (on right). The superposition is described by an equation (the “wavefunction”) derived from the Shrodinger Wave Equation. Upon interaction with parts of the physical universe (observation/measurement), the superposition instantaneously becomes the particles forming the objects that we perceive in spacetime. This is called the “collapse of the wavefunction.” [Image source: David Chalmers and Kelvin McQueen, “Consciousness and the Collapse of the Wave Function” http://consc.net/slides/collapse…]

The superposition is described by an equation that looks like a wave equation, but also the superposition state acts like a wave. For example, physicists and biologists now believe that the superposition state is important in photosynthesis. To create sugar in photosynthesis, a photon excites an electron in chlorophyll. Then, the electron needs to find the right spot in the plant leaf (the “reaction center”) to interact with. The electron finds the right spot much faster than a particle is capable of traveling; it seems to be able to check out many parts of the leaf at the same time, as a spread-out wave could.

This is also described as the electron being in more than one place at the same time or exploring all possible paths to the reaction center simultaneously.Then, the electron gets itself to the right spot, gives the reaction center a particle of energy, and helps to make a molecule of sugar.

So, the electron, in its superposition wave-state, surveys and travels through the leaf. Upon interacting in the reaction center, it creates a particle of energy. This wonderful trick is described in a video. The Magical Leaf: The Quantum Mechanics of Photosynthesis

Are electrons waves or particles?

The accompanying video demonstrates how an electron can be both a particle and a wave. (The video has two unfortunate errors in it, which I’ll point out.) The video shows how different kinds of objects, including an electron, act when they speed towards a barrier perforated by two slits. Then, it shows the pattern the objects form on a detection screen after passing through the slits in the barrier. This is the famous Double Slit Experiment. Here’s what’s going on in this video, step by step:

How Particles Act

Double Slit Experiment showing wave-particle duality. [Animation by: http://toutestquantique.fr/]
Double Slit Experiment
Figure (1) Double slit experiment—pebbles land in two clumps on detection screen. (Image source: Dan Hooper, (in Public Domain) http://saturdaymorningphysics.fnal.gov/wp-content/uploads/2017/02/Hooper-II-2017.pdf p. 35)

“Particle” shows ordinary particles, let’s say pebbles. Particles are separate individual little things that at any one moment are in a tiny, very localized position. So, individual pebbles shoot through one slit or the other. Then, the pebbles hit the detection screen.

Here’s where the video goes off the rails. It shows a pattern on the detection screen of random dots all over the screen. Your common sense would tell you that the pebbles should form two clumps on the detection screen, one behind each slit. Real experiments show that your common sense is correct. Figure (1) by Fermilab is a more accurate depiction of what the detection screen should look like.


How Ordinary Waves Act

“Wave,” in the video, shows an ordinary wave, let’s say a water wave. The water wave is spread out, so it goes through both slits. On the far side of the barrier, two waves emerge, one from each slit. The two waves interfere with each other. They form the crisscross pattern of ripples that we see if we throw two stones into a pond. This crisscross

Double slit experiment
Figure (2) Water waves pass through two slits and create an “interference pattern.” [Image source: Dan Hooper, Fermilab http://saturdaymorningphysics.fnal.gov/wp-content/uploads/2017/02/Hooper-II-2017.pdf p. 36]
pattern of ripples hits the detection screen and forms a striped pattern.

Figure (2) clearly shows how a water wave creates the crisscross “interference pattern” and marks the detection screen with a striped pattern. This is just like in the video. This striping is the signature pattern of waves interacting. When physicists see this pattern, they think “waves.”

To summarize, here are the differences from particle behavior: the wave is spread-out; goes through both slits, not just one or the other; forms two interacting waves on the far side of the barrier; and forms a striped pattern on the detection screen.




How Electrons Act

“Quantum object” shows a subatomic particle, for example, our electron. It doesn’t act at all like an ordinary particle such as a pebble. At first, it acts more like a wave. It’s spread out and goes through both slits. It emerges as two different waves on the far side of the barrier, and these interfere with each other. The two waves form the same crisscross pattern that ordinary waves form.

But upon hitting the detection screen, the wave “collapses.” The electron wave hits the screen in one tiny spot as if it were a particle. The experiment is run over and over. One at a time, electrons flow wave-like through the barrier and collapse at the detection screen, each time hitting one tiny spot, that is, suddenly turning into a particle. Over time, a pattern on the detection screen emerges. It’s the striped pattern—the signature pattern of two waves interacting! Somehow the particles which hit the screen “know” where to land on the detection screen such that over time, they collectively seem to show the influence of the two interacting waves.

The Quantum Wave vs. Ordinary Wave

Even though the electron acts in certain ways like a wave, there are significant differences between the wave of a quantum particle and an ordinary wave like a water wave. The electron type-wave is called a “quantum wave.” An ordinary wave is called a “classical wave.” The mathematical equations which describe the properties of a quantum wave and a classical wave are very different. While quantum waves share some similarities of behavior with classical waves, for example, creating a striped pattern on the detection screen, quantum waves also act significantly differently. Quantum waves and classical waves differ in both their mathematical descriptions and in their behavior.

Here’s a brief listing of differences between a quantum wave and a classical wave (for more detail see the article in this encyclopedia on wave):

  • As shown in the video, the quantum wave collapses when it hits the detection screen and lands on it as a particle. This is called the “collapse of the wave function.” An ordinary wave retains its wave nature when it hits the detection screen.
  • The amplitude of a quantum wave is proportional to the probability that the quantum particle will be detected in a specific position. In contrast, the amplitude of a classical wave is proportional to the wave’s strength.
  • The equation of a quantum wave can include imaginary numbers. These are numbers that include the square root of negative 1. As no number times itself is a negative number, imaginary numbers do not refer to anything that has physical reality. The equations for classical waves do not include imaginary numbers and describe physically real things.
  • It is when an electron is in the quantum wave state, rather than in its particle state, that it displays quantum weirdness: superposition (being in more than one place at the same time), entanglement (behaving in an instantaneously correlated manner with an electron as far as across the universe), quantum tunneling (appearing on the other side of a barrier despite having insufficient energy to cross the barrier), and other weirdnesses. Classical waves, of course, do none of these things.

Wave-Particle Duality

collapse of the wave function
Figure (3) Collapse of the wave function. [Image source: David Chalmers and Kelvin McQueen, “Consciousness and the Collapse of the Wave Function” http://consc.net/slides/collapse.pdf]
The ability of electrons and other quantum particles to act like both a wave and a particle is called “wave-particle duality.” I’ve found that the Transactional Interpretation of quantum mechanics is able to make some sense of wave-particle duality.* This interpretation proposes that when the electron is in its wave-like state, it is not in our physical reality. That is, it’s not in spacetime but is in an underlying level of reality that we can call “Quantumland.” This level of reality, while not observable by us, is lawful in that it follows the laws of quantum mechanics. It underlies and determines the probabilities of what occurs in our spacetime.

*The Transactional Interpretation is explained in lay terms without math in  Ruth E. Kastner, Understanding Our Unseen Reality, Solving Quantum Riddles; Imperial College Press, 2015, London.

How does the electron enter our physical reality? It interacts with something physical that is made up of lots of particles—a “macroscopic object” like a detection screen. Upon interacting with the screen, it’s suddenly a particle. This is called the “collapse of the wave function.” Since, the electron always becomes a particle as soon as it interacts with a macroscopic object, we can never observe it in its wavy state. We’re like King Midas. He could never feel his daughter’s soft hand because she turns to gold the moment that he touches her. We can never observe the wavy state of an electron because the wave function collapses to a particle when we interact with it sufficiently to perceive it.

Figure (3) depicts Quantumland on the left, the collapse of the wave function, and the resultant objects in everyday spacetime on the right. This depiction is a gross simplification because the collapse from wave to particle does not occur at the scale of entire objects like homes and picnicking families. Instead electrons, quarks, and other quantum particles are continually moving from their wavy states, interacting with others, and collapsing to particles. When they collapse, the entire atom and molecule collapses with them. Then, quantum particles revert to their wavy state, collapse again, and on and on. So, at any one moment, many of the atoms and molecules of an object are in their wavy state and many are collapsed down to particles.

Add an Observer

This brings us to the final part of the accompanying video, “Add an Observer.” This part of the video shows the electron wave approaching the barrier. But this time, there’s a detector at the barrier watching which slit the electron goes through. This could be a human with Superman vision or a Geiger counter or another device. The device interacts with the electron sufficiently to determine which slit, so the electron collapses down to a particle and goes through only one slit.

Once past the barrier, the electron, freed from interaction, reverts to its wavy state. Upon interaction with the detection screen, it again collapses down to a particle and lands as a tiny localized dot. Again, the video incorrectly shows that after repeated runs of the experiment, random dots cover the detection screen with no particular pattern. Experimental results show that the resulting pattern on the detection screen is, instead, two clumps as shown in Figure (1).

Consciousness and Wave Function Collapse

The role of consciousness in the collapse of the wave function has had a controversial history. In the early days of development of quantum mechanics, many of the founding fathers of the field contemplated the possibility that consciousness played a role in collapsing the wave function. Over time, this view was rejected. Considerable work was done starting in the 1950’s on the theory of decoherence. This is the theory that interaction with macroscopic particles cause collapse of the wave function. While this is a well-accepted theory, experiments in recent years also point to the possibility that consciousness can also cause collapse of the wave function. For further discussion, see the article in this encyclopedia collapse of the wave function.

In Summary 

In summary, the electron is definitely a particle when it hits the detection screen. And at other times, it’s a wave. But it’s not a physical wave like a water wave or sound wave. It’s a wave that follows the laws of quantum mechanics.

How do we know a quantum particle is in a superposition if detecting the particle will destroy the superposition?

It’s not possible to KNOW that the particle is in a superposition of states since we can’t observe the superposition. The superposition idea is trying to explain what must be happening in the real world given that Schrodinger’s Wave Equation works. Schrodinger’s Wave Equation (and later upgrades like the equations of Quantum Electrodynamics) are very successful at predicting the results of quantum physics experiments. The Copenhagen Interpretation,  the original interpretation of Schrodinger’s Wave Equation, describes the reality underlying the equation as a superposition.

Quantum physicists are in a situation similar to chemists before atoms could be observed with powerful microscopes. (See this article for information on our current ability to observe atoms.) Chemists predicted the results of their experiments on the assumption that atoms exist. They used atomic behavior in their calculations very successfully. Yet, for a century, from the 1800’s into the 1900’s, they had no hope of observing atoms. Some might have said that even, in principle, observation would be impossible. Similarly, many quantum physicists describe the state of quantum particles prior to detection as a superposition because doing so helps them to understand what is going on. They probably have even less hope of detecting a superposition than did chemists regarding observing an atom.

However, interestingly, I just ran across this article on a proposed 2018 experiment that may help them take a peek into the world of the superposition.

Alternatives to the Superposition Idea

The concept of the superposition is part of the Copenhagen Interpretation of quantum mechanics. But there are other explanations, for example, the de Broglie-Bohmian Interpretation, the Many Worlds interpretation, the Transactional Interpretation, and many others. Many don’t require the superposition idea.

The Superposition Idea Is Workable

The superposition idea, whether it will survive in the long run, is useful. Take the example of a photon of sunlight in photosynthesis. Plants are able to use the red photons from the sun. Red photons zip through the cells of a leaf to the “reaction center” where they provide the energy for photosynthesis, that is, the production of sugars.

The trouble is that, once absorbed by a molecule of chlorophyll, the photon must find its way through a maze of cells to find the reaction center, where it will contribute its energy. If the photon used the ordinary strategy of wandering through the maze, its energy would be lost long before it reached the reaction center. But, in fact, biologists have found that plants are able to use almost every red photon for sugar production. How?

Quantum equations to the rescue! Scientists can use quantum equations to describe what the photon actually does, which is different from wandering through a maze of cells. But what is the photon actually doing physically? The Copenhagen Interpretation is that while in a superposition, the photon is experiencing a superposition of all paths. (Richard Feynman would say that it is traveling all possible paths.) Then, it enters physical reality having selected the fastest path. Here is a 4-minute video describing this:

And here is an academic paper which gives a more technical description: Quantum mechanics explains efficiency of photosynthesis

One way to look at this is that the equations show that the photon is in a superposition. However, that’s only one look at it, the Copenhagen Interpretation way. The same equations appear in other interpretations of quantum physics and predict the same results for the red photon. There’s lots of information about these interpretations in books and on the web, starting with this Wikipedia article: Interpretations of quantum mechanics – Wikipedia