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Month: October 2018

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.

How can we tell when descriptions of quantum mechanics are woo-woo and not scientific?

[This is my answer to a question on a website that others answered also.] Many of the answers to your question express the view that quantum physics is a mathematical theory that can be verified by experiment. Of course, this is true. However, some of the answers suggest that if the person discussing quantum physics is not writing math equations or reporting on experimental results, they’re talking woo. This is not true.

Many populizers of quantum physics, such as Brian Greene and Stephen Hawking, are themselves physicists. When they address lay audiences, they usually do not write math equations. They may or may not report on the details of experimental results. They are attempting to communicate the theory of quantum physics and important implications to a lay audience.

While a lot of the math of quantum physics is understood by physicists and a lot of experiments have been done, there is a great deal more to know about both. This is an evolving science; more is being learned every day. In such a situation, it is premature to say that we understand what can be said legitimately about quantum physics and what is woo.

Theoretical quantum physicists do math and attempt to interpret the math and the experimental results so that they can envision new experiments. This has been the role of important physicists like Einstein and John Bell. Theoretical interpretations allow quantum physics to progress just as experiments do.

Currently, there are many different theoretical interpretations of quantum physics. They mostly SOUND like woo. For example, the Many Worlds hypothesis proposes that innumerable universes are created be quantum events. The Bohmian approach proposes that every event is connected with every other event in the universe, thus explaining quantum entanglement.

Yet, serious physicists propose these kind of hypotheses and work on math regarding them. These are the easiest ones to describe—other interpretations are as weird when you get down into the details. And I’m including the Copenhagen (orthodox) interpretation as being weird because it depends on a very weird view of what reality is. That’s why Einstein challenged it for decades. “Weird” here does not mean woo and does not mean wrong. It simply means unfamiliar to us who live in the world of chairs and tables and don’t deal with electrons and photons on a daily basis.

It is easy to attack people who rely on quantum physics for some of their far-out philosophies. But when quantum physicists, themselves, propose Many Worlds or universal connectedness, one wonders who is calling the kettle black.

So, bottom line, I don’t think there is a way to distinguish between woo and serious physics without a heavy background in math and physics. Not yet. Quantum physics is currently an anarchy of interpretations, with each physicist either turning away from any interpretation at all or supporting their own preferred view. This is not a good time to make fun of someone else’s interpretation or of a philosophy based on an interpretation that one isn’t familiar with and, therefore, one supposes could not possibly have scientific support.

he answers to your question express the view that quantum physics is a mathematical theory that can be verified by experiment. Of course, this is true. However, some of the answers suggest that if the person discussing quantum physics is not writing math equations or reporting on experimental results, they’re talking woo. This is not true.

Many populizers of quantum physics, such as Brian Greene and Stephen Hawking, are themselves physicists. When they address lay audiences, they usually do not write math equations. They may or may not report on the details of experimental results. They are attempting to communicate the theory of quantum physics and important implications to a lay audience.

While a lot of the math of quantum physics is understood by physicists and a lot of experiments have been done, there is a great deal more to know about both. This is an evolving science; more is being learned every day. In such a situation, it is premature to say that we understand what can be said legitimately about quantum physics and what is woo.

Theoretical quantum physicists do math and attempt to interpret the math and the experimental results so that they can envision new experiments. This has been the role of important physicists like Einstein and John Bell. Theoretical interpretations allow quantum physics to progress just as experiments do.

Currently, there are many different theoretical interpretations of quantum physics. They mostly SOUND like woo. For example, the Many Worlds hypothesis proposes that innumerable universes are created be quantum events. The Bohmian approach proposes that every event is connected with every other event in the universe, thus explaining quantum entanglement.

Yet, serious physicists propose these kind of hypotheses and work on math regarding them. These are the easiest ones to describe—other interpretations are as weird when you get down into the details. And I’m including the Copenhagen (orthodox) interpretation as being weird because it depends on a very weird view of what reality is. That’s why Einstein challenged it for decades. “Weird” here does not mean woo and does not mean wrong. It simply means unfamiliar to us who live in the world of chairs and tables and don’t deal with electrons and photons on a daily basis.

It is easy to attack people who rely on quantum physics for some of their far-out philosophies. But when quantum physicists, themselves, propose Many Worlds or universal connectedness, one wonders who is calling the kettle black.

So, bottom line, I don’t think there is a way to distinguish between woo and serious physics without a heavy background in math and physics. Not yet. Quantum physics is currently an anarchy of interpretations, with each physicist either turning away from any interpretation at all or supporting their own preferred view. This is not a good time to make fun of someone else’s interpretation or of a philosophy based on an interpretation that one isn’t familiar with and, therefore, one supposes could not possibly have scientific support. It’s always good to keep in mind those who mocked and vilely threatened poor souls like Galileo who believed outlandish theories like the earth travels around the sun.

However, it’s an exciting time to try to understand as much as possible of the math, experimental results, and theoretical interpretations. With enough study, one can begin to draw one’s own conclusions about what is woo and what has theoretical, mathematical, and experimental support.


Does matter act simultaneously like a particle and a wave?



September 26, 2019


Actually, matter doesn’t simultaneously act like a particle and wave. It acts like a wave sometimes and a particle at other times, but not both at the same time. There isn’t a consensus among physicists on this particular description, but I’m going to give a current mainstream description of what’s… Read more


What would it be like to live in the quantum realm?



September 7, 2019


A number of interpretations of quantum mechanics postulate a “quantum realm.” These include the Transactional Interpretation.* One of its developers, Dr. Ruth Kastner, calls it “Quantumland.” Here’s a drawing of Quantumland that Dr. David Chalmers, the noted philosopher of physics, presented in a lecture : The quantum realm on the… Read more


How can I teach myself quantum mechanics, step-by-step?



July 17, 2019


I have taught myself quantum physics but without the math. In other words, I can calculate only the tiniest bit of anything quantum mechanical, but I have some understanding of quantum physics concepts. I also have a conceptual understanding of some of the equations. However, I believe that if I… Read more


What is the difference between quantum mechanics and quantum physics?



June 28, 2019


Quantum mechanics is the current name of the field of quantum physics. In quantum mechanics, physicists study how atoms, the components of atoms, and other tiny particles behave. These are called “quantum particles” and include electrons, protons, photons, and so on. They follow the laws of quantum mechanics. In the… Read more


What is wave function collapse? Is it a physical event?



June 1, 2019


In one view, a wave function is a piece of math, an equation. It’s not a physical thing. So, it can’t collapse in any physical sense. The collapse is metaphorical. This is one interpretation of quantum mechanics. It’s the interpretation taught in most university classes, the Copenhagen Interpretation. However, physicists… Read more


Are electrons waves or particles?



May 18, 2019


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,… Read more

How Can an Electron Be in More Than One Place at the Same Time?

A single electron CAN have a pretty specific location at a special moment in time, the moment that it interacts with another particle and creates a physical change in our universe (more later). However, an electron has no specific location when it acts as a wave in a quantum field. A wave can also be called an excitation or a disturbance of the field. In any event, the electron, when in the wave state, is spread out over a region of space.

However, upon interaction with something else in our universe, it will assume a pretty specific location. (This is, by convention, called the moment in which the electron is “measured.”) And the location in which the electron will be found upon interaction, is more probable where the amplitude of the electron wave is highest. Its PERMITTED locations are calculated using Schrodinger’s Wave Equation. Its PROBABLE locations are calculated as the square of the amplitudes of Schrodinger’s Wave Equation.

[Note: Schrodinger’s Equation is used to calculate the probable locations of a group of electrons. Using it to calculate the location of a single electron would be something like using the average height of women to predict the height of the next woman you see – not too sensible nor accurate. The average height of a women, however, would be helpful in predicting the heights that you’re likely to find in a random sample of all women. Same thing with Schrodinger’s Equation—it makes more sense to use it to predict the probable locations of a group of electrons.]

Returning to a single electron… by definition, the location of the electron won’t be found until it’s detected. It might be detected, for example, by a “cathode ray tube,” which includes a screen, the same kind of screen as on our TVs. The electron wave interacts with a spot on the detector screen; its energy is absorbed by the screen; and the screen gives off a tiny flash of light. A computer hooked up to the screen could record the location of the flash.

In this way, a single electron could be recorded at a specific location. However, the location is specific only in our macroscopic world. This would become clear if one looked at the flash with an overwhelmingly powerful microscope, stronger than we can currently even dream up the design of. If one used this microscope to look at the exact location of the electron as it hit the screen, the location wouldn’t be an exact point. Instead, at best, the electron would vibrate within a tiny space at the Planck-length scale. This is because of the Heisenberg Uncertainty Principle.

It’s important to note that when in the wave state, and prior to detection, it’s not just that we don’t know where the electron is. It actually has no specific location. Once it interacts with another particle, however, it randomly assumes a (fuzzy) specific location within the permitted locations specified by Schrodinger’s Equation. (Only when detecting a group of electrons can it be seen that the calculated probabilities have somehow guided the locations that the individuals in the group assumed.)

The electron has, at the moment of interaction, created information as to its whereabouts. Once that information is created, our universe has experienced physical change. Our universe works on the principle that that which has created a physical change (created information) goes into the past. And, now, that change will be a factor in creating the future.

 

What is the difference between an electron and a photon?

I had the same question. When I researched this at @Electricity is energy [the real title of the article is “Electricity Is NOT Energy”], I found out that electrons are tiny particles of matter. They are the bits of matter within an atom that vibrate around the nucleus of an atom. Electrons can also fly about freely or travel slowly and are not just found within atoms. In a copper wire, for example, they can be found loose, outside atoms, traveling slowly, a few inches per minute. 

Electrons have a negative charge, which means only that they move away from other negatively charged matter (other electrons) and are drawn to positively charged matter (protons, often ones in the nuclei of atoms). 

But photons are units (packets of energy) of an electromagnetic wave. They are not bits of matter. A type of photon that we experience very intimately all the time is the photons of visible light. These hit our retina and cause chemical changes or hit a photographic plate. In both cases, the photons create chemical changes which ultimately create images. 

Light is just one type of electromagnetic energy. Other types of electromagnetic energy are X-rays (a high-energy wave), waves that carry radio signals and TV signals, microwaves in a microwave oven, etc. All of the bits of energy that are associated with these waves are photons. 

Photons have neither negative nor positive charge. They are not matter and have no mass. They travel the speed of light when in a vacuum like in outer space (which is not a complete vacuum, really). But they can travel much slower when traveling through a medium like water or even air. 

Photons and electrons interact to create flows of electricity. Both are involved. Electricity is not merely a flow of electrons in a wire; it is also a flow of photons in an electromagnetic wave.