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What is the difference between a photon and a quantum?

[This article is under construction.] 

A photon is a type of quantum. So, it’s like the difference between a blue jay (photon) and a bird (quantum). If this answers your question, you can stop reading. If you want to know more about what a photon and a quantum really are, read on.

A Photon Is a Bit of Light

A photon is a bit of light. To see how light can be divided into bits, photons, it’s necessary to understand some things about light. Light travels as a wave, an electromagnetic wave. An electromagnetic wave is an electrical wave and a magnetic wave traveling together.

As the electrical wave rises and falls, it creates a magnetic wave and then, the magnetic wave rises and falls, creating an electrical wave, and on and on.  This video shows how electrical and magnetic waves create each other: https://www.youtube.com/watch?v=1SQV9kBN_b4

When physicists speak of “light,” they mean any kind of electromagnetic wave. This includes visible light,  the kind that we see with our eyes. But electromagnetic waves also include X-rays, ultraviolet rays, infrared, microwaves, radio waves, and many others. Any type of electromagnetic wave can also be called “radiation.” The difference between the various types of electromagnetic radiation or waves is their wavelength (see image above).

The red “spring” at the top of the accompanying image shows the spectrum of electromagnetic waves from radio waves on the left to gamma rays on the right.

What about photons?

When light, that is, an electromagnetic wave, strikes an object, it immediately collapses into tiny bits of energy. Each of these bits is a photon. It’s as if an ocean wave hits a rock, and shatters into a gazillion tiny droplets. Each “droplet” of the light wave is a photon, and each carries a bit of the energy of the light wave. If the light wave were to hit a piece of film, we would be able to see all the tiny spots of light. Each photon creates a bit of the photo, usually a small fraction of a pixel. Together, they form the image.


So, what is a quantum?

Subatomic particles travel as waves, as we have seen for electromagnetism. When these waves strike an object, they collapse into bits. In the case of electromagnetism, the bits are called photons. But the more general term is “quanta,” or in the singular, “quantum.” There are other types of waves at the subatomic level besides electromagnetic waves. For example, electrons travel as waves as do neutrinos, and other subatomic particles. Quanta are the bits that various subatomic waves create when they interact with objects.

I should clarify that the waves don’t physically shatter when they hit objects. It’s more like they interact with the objects and due to the laws of quantum physics, the waves transform into tiny energy-bearing particles, that is, quanta. And, if I really wanted to be accurate, waves at the subatomic level are not like ocean waves or sound waves. Their physical nature is still under debate, with one possibility being that they are no more a mathematical description of a wave. But this is diving deeper into quantum physics than is useful here.

Why does the Born Rule predict quantum probabilities?

There’s both a mathematical explanation and an explanation based on the nature of reality.

First, the mathematical explanation: Let’s take the example of the Double Slit Experiment. A laser shoots photons one-at-a-time through the two slits of a

Double Slit Experiment shooting one photon at a time. [Image source: modification of https://en.wikipedia.org/wiki/Double-slit_experiment]

screen towards a photographic plate. The wave function is the equation that describes the behavior of the

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]

photon. The amplitudes calculated for the wave by the wave function are proportional to the probability of the photon being detected in any particular position on the photographic plate.

The mathematical expressions for the wave amplitudes often include complex numbers (numbers that include the square root of negative 1). We cannot visualize such a number because what number multiplied times itself equals negative 1? There is no such number. We label this non-number as i and just don’t try to imagine what amount i really represents. Even though i does not describe anything that we’re familiar with in the physical universe, both mathematicians and physicists have found it useful to work equations which include i, that is, complex numbers.

period of a wave
Amplitude is a key property of waves. The horizontal axis is time. [Image source: modification of Kraaiennest – CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4038226. Retrieved Sept. 21, 2017.]

While neither water waves nor sound waves nor any other wave that we might observe in the physical universe have amplitudes that can be measured by complex numbers, the wave function often calculates complex number amplitudes for quantum waves. This is a clue that quantum  waves do not operate in the spacetime of our physical universe. In the Copenhagen Interpretation, the original and conventional interpretation of quantum mechanics, it’s not clear where they operate. The status of their reality is an enigma wrapped in a mystery. A bit later in this answer, another interpretation will provide more information on the issue.

But returning to the wave function in the Copenhagen Interpretation. Max Born (1882-1970) was the quantum physicist who first realized that the amplitude of the quantum wave predicts the probability of detecting a particle in a particular position. But this creates a problem. What if the  amplitude includes a complex number?

A probability cannot be expressed using complex numbers. Probabilities are expressed as positive numbers ranging from 0% to 100%. That is, we can say that there’s a 50% chance when tossing a coin of getting heads. Or a 0% chance that every moment of the day will be fun. And a 100% chance that a human being will eventually die. But to say that the chances of an event are the square root of negative 1 makes no sense.

Max Born (1882-1970), developer of the Born Rule. [Image source: Public Domain, https://en.wikipedia.org/wiki/Max_Born]

Born solved the problem by multiplying the amplitude of the wave by its complex conjugate. This squares the square root of negative 1, yielding simply negative 1. The result is probabilities calculated by the wave function are quite nice. They range from 0% to 100%.

This calculation is the Born Rule. Experimental results show that the Born Rule is accurate in calculating quantum behavior. So, not only does the rule cancel out the troublesome complex numbers, it accords with empirical results. The Born Rule is an integral part of the Copenhagen Interpretation.

However, the Born Rule does not explain what is happening in the physical universe that requires that we multiply the wave amplitude by its complex conjugate. Nor does any other part of the Copenhagen Interpretation provide such an explanation. The Transactional Interpretation does. While it would be too lengthy to fully explain this interpretation here, an example gives a taste:

Let’s say take a look again at the Double Slit Experiment with a laser emitting a single photon towards a photographic plate. Until it arrives at the photographic plate, it’s a quantum wave that physicists can calculate a wave function for. The wave function identifies the possible positions where the quantum wave could deposit its energy on the photographic plate. So far, this is like the Copenhagen Interpretation. But the Transactional Interpretation adds something new. It says that if the photon is to land on the photographic plate, an electron in the plate must take action to absorb it. Electrons in the plate must send out their own waves. When the emitting wave and the receiving wave interact, the photon transfers energy to the electron, and takes its place in physical reality at a position on the plate.

The wave function of the absorbing electron in the plate has an amplitude that fits well with the amplitude of the photon: it’s the complex conjugate of the photon’s amplitude.

But why multiply the two amplitudes? This is how we find the probability of two events occurring, in this case both the photon heading towards a particular position on the plate and an electron in that position absorbing it. When we calculate the probability of any two events, we multiply the two probabilities. For example, the chances of a baby being born a girl with brown eyes is: 1) the probability of being a girl (about 48%) times (2) the probability any baby having brown eyes (about 80%). We multiply 48% times 80% and get 38%. There’s a 38% chance that a baby born anywhere in the world will be a girl with brown eyes.

The emitting wave and the absorbing wave have to interact if the photon is to land in any particular position. We multiply the probability of the photon landing in a particular position (the complex number describing the amplitude) times the amplitude of the receiving wave at that position on the plate (the complex conjugate).

The Transactional Interpretation is based on Absorber Theory developed by Richard Feynman and John Wheeler in the late 1930’s. It was fully developed as an interpretation of quantum mechanics by John Cramer in the 1980’s and further developed by Ruth E. Kastner. Kastner has written technical papers on the Transactional Interpretation and also a book for laypeople, Understanding Our Unseen Reality. I highly recommend the book because it provides a coherent and understandable explanation of the physical meaning quantum mechanics.

What are some good online resources that can help me understand physics better?

The Physics Classroom and Khan Academy both have excellent free on-line courses on physics, starting from the beginning. They’re step-by-step and give practice problems. On Khan Academy, if you sign in, it will keep track of your progress. It also gives “awards” like badges for good progress. Both websites are excellent. Khan Academy is a little more interactive. I have used both. To really gain understanding and certainty, I’ve done lessons on the same subject on both websites.

When I didn’t understand something in a particular lesson, I found videos on Youtube. I typed the subject into the Youtube search bar, for example, “inertia.” Then, I just started watching videos until I finally got it.

It’s important to understand all the jargon as it comes up. When terms came up I wasn’t sure of, like “mass,” I watched Youtube videos on the subject and/or googled the physics meaning. When googling, I often asked for images. Finding visuals really helps.

I’m writing definitions of physics terms in an on-line encyclopedia. It focuses on quantum physics, but many of the terms, like acceleration, are shared with classical physics. Classical physics is the first physics that you learn on websites like the PhysicsClassroom and KhanAcademy.org.

Can quantum mechanics be understood? Does it make logical sense?

Quantum mechanics is logical. By logical, I mean that the assumptions, the principles, the equations, and the empirical data all fit together. They don’t contradict each other.

However, quantum mechanics (QM) does not fit with the assumptions and principles of classical physics (physics prior to 1900 that we learned in high school). QM is the description of the quantum world. Classical physics is the description of the macroscopic world—the world of tables, chairs, apples, etc. The two worlds are described by two different systems of assumptions, principles, equations, and empirical data.

If we attempt to view the quantum world while retaining classical assumptions and principles, the quantum world seems full of paradoxes. For example, classical physics is based on the unspoken assumption that when an object changes position from Point A to Point B, it traverses the distance in between. This assumption is violated in the double slit experiment of quantum mechanics (QM). Another assumption of classical physics is that if one knows the initial conditions of a system, one can calculate its evolution through time. Due to the true randomness in the behavior of individual quantum particles, QM violates this assumption.

Classical physics tells us that if we apply a specific force to a billiard ball, we can predict exactly where it will roll. This is called the billiard ball model of physics.

Neither system is particularly intuitive. Newton’s First Law isn’t intuitive: no forces are needed to maintain an object at a constant velocity. Or gravity – Newtonian gravity is action-at-a-distance. Neither Newton nor we are able to describe the underlying nature of physical reality such that action-at-a-distance occurs.

But the principles of classical physics fit together into a self-consistent logical system. Classical physics was also consistent with experimental data until the late 1800’s. That’s when scientists started investigating atoms and the interiors of atoms. And, that’s when they need QM.

QM can be seen as describing a sublevel of reality which operates on different assumptions from those of the macroscopic world.

The green film represents ordinary reality as we perceive it with our senses. The red grid represents the quantum world, a sublevel to our reality. A wave travels through the quantum world (red grid) and creates a particle (dot in the green film colored orange or blue) in our perceived reality. [Image source: stills from Fermilab video by Dr. Don Lincoln, “Quantum Field Theory” (in the public domain) Jan. 14, 2016; See the video below.]

The quantum world is a sublevel of reality in the same sense that computer programmers work at a sublevel of a video game. Before they key the program into the computer and see the “macroscopic” world that they’ve created, they follow rules different from those that the characters in the video game follow. The programmers can program a character to exit screen left and enter screen right—no need to traverse the distance in between. The programmers can correct an action in an earlier “frame”–no need to go back in time—they just re-type some symbols.

Later, when the game is actually playing on the screen, the characters follow different rules, more like those of our macroscopic world. For example, a character in a video game can’t correct one of her actions by going back in time (unless it’s a sci fi game).

[For more detail on the idea of the quantum world as a sublevel of reality, see this excellent short video by Fermilab. Also see the entry for quantum field theory in the quantum physics encyclopedia for laypeople QuantumPhysicsLady.org.]

Some interpretations of QM are better than others at logical descriptions of reality. The Copenhagen Interpretation, the original interpretation, doesn’t even try. The slogan of this interpretation has come to be known as “Shut up and calculate!” In other words, physicists use the highly useful math of QM in developing things like computer technology but don’t worry about the implications for the nature of reality. They know that the implications are self-consistent if we confine our attention to the quantum world; but they’re not consistent with the laws of Newton that describe our experiences in the macroscopic world of tables and chairs.

The Transactional Interpretation* does well at providing a logical description of reality. In describing QM as describing a sublevel of reality, I’ve relied on this interpretation.

In short, QM follows its own logic. As long as we don’t make assumptions based on our everyday experience or on classical physics, QM makes logical sense of experimental results.

* See the book: Ruth E. Kastner, Understanding Our Unseen Reality.

Why study quantum physics?

Here’s why I began studying quantum physics. I wondered: How does this universe work? What underlies molecules and atoms? How is reality created?

Quantum physics is part of the answer—a huge part. But the trouble is, physicists don’t understand how quantum particles create the solid objects that our senses perceive. After all, quantum particles are just vibrations in what appears to be huge quantities of empty space.

Many physicists are unperturbed by this question. They use the mathematics of quantum physics for running experiments or for developing technologies, and they leave the Big Questions alone. However, some physicists/mathematicians have gone ahead and speculated about the Big Questions.

How does quantum physics explain our perceptions of the world?

One speculation of particular interest to me is that Information Theory can cast light on this question. Information Theory reduces the universe to mathematical patterns. It reduces the vibrations of quantum particles to the mathematical equations which calculate the vibrations. These equations describe changes in matter and energy, what physicists call “evolutions.” The equations are not just static descriptions like the formula for the composition of water: H2O.

The entire universe can be seen as an intermeshing of equations, one supplying data to another, each equation being influenced by others. The physicist, Max Tegmark, wrote the book Our Mathematical Universe on this premise. Another good book on the subject is Programming the Universe by one of the inventors of the quantum computer, Seth Lloyd.

Information Theory is illuminating. But there’s a big piece of the puzzle that’s still missing. How do mathematical equations become subjective experience? We experience colors, sounds, tastes, and other sensations as if they were out in the world. But, actually, these are our subjective experiences of electrical impulses in the brain. After all, our skulls don’t have holes in them to let the world in. The only thing going on in our brains are electrical impulses.

"The Matrix" raining code
Raining computer code, as in the film, “The Matrix.” Are equations generating digits that our consciousness turns into sensations? [Image source: By Jamie Zawinski – screenshot, MIT, https://en.wikipedia.org/wiki/Matrix_digital_rain; retrieved Nov. 29, 2018]
electrical impulses in neurons
Electrical impulses traveling through brain cells somehow allow us to experience sensations. [Image source: Looie496 created file, US National Institutes of Health, National Institute on Aging created original – Public domain; https://en.wikipedia.org/wiki/Neuron]

Here’s where my brain starts to hurt—our brains are, themselves, describable as mathematical equations interacting with each other. And the electrical impulses that shoot through the brain are describable as mathematical equations. And these electrical impulses arise from sense organ stimulations that are describable as mathematical equations. These stimulations arises from interactions with the external world (which, of course, are describable as mathematical equations).

But, how exactly, do we experience electrical impulses traveling through the brain as colors, sounds, tastes, and so on? How do mathematical equations become subjective experience?

The quantum physicist, Amit Gswami in The Self-Aware Universe, suggests how this happens. He proposes that our consciousness codes equations into the images, sounds, smells, and tastes of our subjective experience. In other words, the world is really in our minds. Or possibly, there is one mind, and we’re all tuned into it, each one of us experiencing it somewhat differently due to our own unique filters. The traditional Buddhist view has things to say about this.

Is consciousness fundamental?

Plato (428-348 BC) [Image source: Public domain; https://en.wikipedia.org/wiki/Plato]

This view, “Idealism,” depends on the assumption that consciousness is fundamental and matter derivative. Of course, the current scientific view that consciousness arises from brain matter, is exactly the reverse.

Max Planck
Max Planck (1858-1947) [Image source: Public Domain, https://commons.wikimedia.org/w/index.php?curid=20429172]

Some philosophers and physicists throughout history have espoused Idealism. Plato, considered one of the founding fathers of Western thought, was an Idealist. And so was the grandfather of quantum physics, Max Planck. Planck said in 1931:

“I regard consciousness as fundamental. I regard matter as derivative from consciousness. We cannot get behind consciousness. Everything that we talk about, everything that we regard as existing, postulates consciousness.”

Rene Descartes proposed another view called “Dualism” in the 1600’s. He saw both matter and consciousness as fundamental but also as very different substances. Dualism formed one of the basic unspoken assumptions of the worldview that I grew up with and probably most Westerners have grown up with. It is the worldview that most Westerners who aren’t paid to think, unthinkingly adopt.

Descartes dualism
Descartes (1596-1650) thought consciousness and matter were two different fundamental substances that met in the pineal gland of the brain (the epiphysis). [Image source: Public domain; https://en.wikipedia.org/wiki/Mind]

Starting in the middle of the 20th Century, mainstream scientists began rejecting both Idealism and Dualism. They adopted Materialism. This view is that matter is fundamental and consciousness arises from the brain. These days, some scientists, like Amit Gswami, are beginning to question Materialism. Philosophers like David Chalmers are working out the logic of Materialism and pointing out some logical difficulties that it raises. The precepts of quantum physics, in particular the precept that fundamental particles are nothing more than vibrations, can be seen to undermine the foundations of Materialism.

What is the difference between theoretical quantum physics and experimental quantum physics?

Short answer: Theoretical physics is one of two branches of physics: theoretical and experimental. Like almost all other types of physics, quantum physics has both a theoretical physics branch and an experimental physics branch.

Explanation: Experimental physicists do experiments to find out how accurately physics theories describe the real world. For example, they shoot atoms and bits of atoms around in particle accelerators. They measure the speed of light when it travels through water versus empty space, and so on.


particle accelerator at CERN
Particle Accelerator at CERN in Switzerland [Source: By User:Freerk – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=455724] 

Theoretical physicists don’t do experiments. They read articles and books about physics experiments. They think. And they play with a lot of mathematical equations on black boards or white boards. Then, if these equations accord with experimental results, they publish them along with explanatory text in physics journals.

Albert Einstein
Albert Einstein at the Blackboard, 1921 [Source: https://en.wikipedia.org/wiki/Albert_Einstein; in Public Domain]

The job of theoretical physicists is to develop mathematical equations and verbal explanations that describe the results of physics experiments. Ideally, theoretical physicists also use their explanations and mathematical equations to predict experimental results. Albert Einstein and Stephen Hawking were both theoretical physicists. They did not do experiments. They studied, thought, and developed new theories and equations.

Now to turn to quantum physics. Quantum physics is the field of physics in which physicists learn about certain types of unusual behavior of particles smaller than atoms but sometimes the size of atoms or even larger. They focus on the “quantum state.” This is the state in which these tiny particles can be in many positions at the same time and where they can act as both waves and particles. Many quantum physicists are theoretical physicists. But many others do experiments.

This is the same in most other fields of physics. For example, physicists who study the physics of heavenly bodies, astrophysicists, can be either theoreticians or experimentalists. The experimentalists often use telescopes for their work. The theoreticians write equations, these days, on white boards.

Before the mid-1900’s, most physicists were both experimentalists and theoreticians. For example, Isaac Newton, who founded modern physics, did many famous experiments. For example, he shot beams of light through prisms. But he also was one of the inventors of calculus, which he used when developing theories which explained the results of his experiments on the motion of objects. Isaac Newton developed the modern type of physics theory—ones which are expressed using mathematical equations. As an example of a mathematical theory, Einstein’s equation, e = mc2, is his theory of the relationship between the quantity of energy and of mass.

Today, few, if any, physicists do both theoretical and experimental work. Both fields require extremely specialized knowledge and skills. But theoreticians and experimentalists work closely with each other to push physics forward.

What does “measurement” mean in quantum mechanics?

In quantum mechanics, when physicists measure a property of a quantum particle, like a photon, they are really saying that they detect one of its properties. The key to understanding why “measurement” is of such consequence in the quantum world is understanding that detection creates a fundamental change in a particle’s condition.

For example, let’s take the detection of a photon. Here’s a highly simplified experiment: we hold an extremely dim lamp in front of a photographic plate. It’s so dim that it emits only one photon at a time. The photon exposes the plate in one spot and makes a mark there. Once the photon hits the plate, we know the position of the photon—it’s a certain distance from each edge of the plate. So, we have detected it and, specifically, we have detected its position. Quantum physicists would say that we have “measured” its position.

This might seem just like measuring anything in the macroscopic world. For example, let’s say that a biologist uses a microscope to detect the position of a living bacterium in a living human cell on a slide. She measures the distance of the bacterium from each edge of the cell.

Here’s the difference: prior to the biologist measuring the position of the bacterium, the bacterium was happily swimming about the cell. After the measurement, same thing: happily swimming bacterium.

What was the photon doing before the measurement, before it hit the photographic plate? In our physical universe, it wasn’t doing anything because it hadn’t yet made an appearance in our universe.

Measurement converts the particle from a superposition. 

So, did the measurement create the photon? No. Prior to hitting the plate, the photon was in a la-la land of being in all possible positions in the photographic plate and maybe elsewhere as well. This condition of being in many possible positions is a “superposition.” A superposition of a particle is a state that we never observe in the physical universe. Have you ever seen an object occupy many positions at the same time? Neither have I. But prior to measurement, the photon was in every possible position that the lamp could have shot it to.

The condition of being in a superposition is also called a “quantum state.” It is the condition in which the weird behavior of quantum mechanics occurs. One way to look at this condition is to see it as a sublevel of reality underlying our everyday physical universe reality. This is how the Transactional Interpretation of quantum mechanics describes the quantum world.

body builder, Arnold Schwarzenegger
Arnold Schwarzenegger, body builder, 1974 [Image source: Public domain; https://en.wikipedia.org/ wiki/Bodybuilding]

Two levels of reality? This is not such a strange idea. Consider a human body—it can look quite solid and imposing, especially the body of Mr. Universe. But when considered at the reality level of atoms, 99.9999999% of that body is empty space.

Here’s another example: Imagine you were living the life of a character in a video game and, suddenly, you could see the computer coding that the video game was running on. Both the video game and the underlying coding are real. But they are different levels of reality. I called the quantum level underlying the regular physical universe level, “la-la land.” One of the developers of the Transactional Interpretation, Dr. Ruth Kastner, calls it “Quantumland.” Good name.

Quantum Field Theory--what is it?
The green film represents a photographic plate in physical reality and the red mesh wave represents Quantumland. [Image source: stills from Fermilab video by Dr. Don Lincoln, “Quantum Field Theory” (in the public domain) Jan. 14, 2016; https://www.youtube.com/watch?v=FBeALt3rxEA&feature=youtu.be]

The accompanying image shows the reality level of Quantumland as a red mesh and our physical reality as a green film. The position of the photon prior to measurement is possible anywhere in the red mesh hump. The hump represents a wave. This is the famous “wavefunction.” When the particle is measured, the wavefunction “collapses” to one particular position (orange dot) on the green film. The green film represents a photographic plate in physical reality.

The position we will be most likely to detect the particle is the highest point of the wave. As it happens, the photon was measured where it had the highest probability of being measured. Of course, other photons might be found in lower probability positions.

In short, measurement in quantum physics converts a multiplicity of possible states within a sublevel of reality (Quantumland) to a single definite state within physical reality.

These are just words trying to describe equations. 

What leads physicists to think that prior to measurement, the photon is in a superposition of many possible locations? This is what the math is saying. The results of experiments on quantum particles can be predicted using mathematical equations. Initially, Shrodinger’s Equation was a key quantum mechanics equation; later, Schrodinger’s was upgraded. Schrodinger’s and later equations which are used to correctly predict quantum behavior, generate only probabilities of possible positions of particles. So, if one runs an experiment repeatedly, the equations correctly predict the percentage of times that the particle will be detected in any particular position. Only when the particle is measured, can it be assigned a unique position. That information appears nowhere in the equations. So, many physicists consider that the equations describe a superposition of states.

I have just outlined the original interpretation of the meaning of Schrodinger’s Equation. This interpretation, which originated in the 1920’s and ’30’s, is called the Copenhagen Interpretation. Many later interpretations retain the view that the quantum particle exists as a superposition prior to measurement. Other interpretations, such as the Bohmian interpretation and Many Worlds Interpretation do not. This Wikipedia article lists the interpretations of quantum mechanics that hold that quantum particles exist in superpositions prior to measurement. These are labeled as involving “collapsing wavefunctions.”

Why take the equations so seriously? Why do our theories of what’s going on have to fit the math? Physicists have the goal of describing physical phenomena with mathematical equations. In fact, they do not honor a description of the world with the label “theory” unless it is a mathematical equation or a set of mathematical equations. It was not sufficient for Einstein to say that matter can convert to energy. That’s not a physics theory. When he wrote E= mc2  , he had written a physics theory. So, physics theories are mathematical statements. The verbiage describing these theories are called “interpretations.” In physics, the interpretations must fit the equations, not the other way around.