# I What is a realistic image of quarks?

#### Zeynel

In this CERN video quarks are represented as spheres. Is this how quarks look like. I thought they were fluctuations in the quantum field.

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#### BvU

Homework Helper
HI,

'Look like' is not a suitable expression in this context. Not even when the scale is still in the nm ranges: things that are smaller than the wavelength of visible light ( $\approx \; \mu$m) cannot be 'seen'. We can extend a bit with an electron microscope, but beyond that we have to use analogies, our imagination, or: math. In the video you see the outer electron orbitals represented as four little menhirs sticking out in four directions. If you know a bit of quantum mechanics you realize these are surfaces that are boundaries within which a certain percentage of the electron densities are to be found. Not even for simple orbitals, but for hybrid ones.

So by the time you are down another six orders of magnitude to the contents of the constituents of the nucleus it's really an appeal to your imagination by the maker of the video. But it looks reasonable and has more appeal than 'fluctuations in the field'.

And as long as folks don't consider results from experiments with three marbles in a paper bag as fundamental high-energy physics evidence, it's pretty harmless.

IMHO -- genuine theorists may well beg to differ.

#### DaveC426913

Gold Member
Subatomic particles manifest, not as an object, but as the effect of their properties. Our diagrams reflect this.

Think of a free electron. Its field strength is the same at a given distance (say, x) from the point centre. The points that define a strength of x will all be on the surface of a sphere. So, when an electron is diagrammed as if it's a sphere, it's simply a diagram of all points x.

Depending on what value for x you choose, it could be a point, a small sphere or a huge sphere. The choice is entirely arbitrary.

#### ZapperZ

Staff Emeritus
2018 Award
In this CERN video quarks are represented as spheres. Is this how quarks look like. I thought they were fluctuations in the quantum field.
Hint: videos such as this are made as visual eye-candy for the public. It is not meant for expert consumption. The public tend to be seduced by bells and whistles. Thus, one needs colorful depiction of the subject matter in ways that the public may be able to understand and latch on to.

Think of it as the "Bohemian Rhapsody" of quarks, i.e. a dramatization based on real-life characters and events.

Zz.

#### Zeynel

This is confusing. You say subatomic *particles* but then you say these are not *particles* (not objects).

Then you say "think of a free electron." So how do I think about it, visually? Because it is not a particle or an object. But it sounds as if you suppose that a free electron is a particle because then you ascribe to it a "field strength." Maybe you considered to be a "point particle". But to me this sounds like a particle in name only because a mathematical point is not a particle.

And then, if I understand correctly, you consider the spherical field to be the electron. What is this sphere made of? It's not made of particles. Just an imaginary sphere. Sorry, I'm not trying to prove anything I'm just confused.

#### BvU

Homework Helper
I'm just confused
In this case that's a good thing. It's all imagination and all we have is the observed behaviour of the 'things' that allows us to make sensible predictions.

And we get prety far with analogies like e.g. 'hard balls' for gas particles. But at some points the analogies each and all break down -- what remains is the mathematical abstraction.

#### ZapperZ

Staff Emeritus
2018 Award
This is confusing. You say subatomic *particles* but then you say these are not *particles* (not objects).

Then you say "think of a free electron." So how do I think about it, visually? Because it is not a particle or an object. But it sounds as if you suppose that a free electron is a particle because then you ascribe to it a "field strength." Maybe you considered to be a "point particle". But to me this sounds like a particle in name only because a mathematical point is not a particle.

And then, if I understand correctly, you consider the spherical field to be the electron. What is this sphere made of? It's not made of particles. Just an imaginary sphere. Sorry, I'm not trying to prove anything I'm just confused.
The problem here is that you attached too big of a significance to the English words that we used to describe things in physics. You should never do such a thing. There are many terminologies that we use that, taken literally, will get into into the wrong rabbit hole. The word "spin" is one clear example as used to designate the magnetic moment of electrons and other quantum "particles". The same thing with the use of the words "wave" and "particles" in QM.

It is why the first and foremost way to understand physics is to understand the mathematical description, because there is often no ambiguity in that description. Our interpretation of it may differ, but there is usually no similar ambiguity on what we are dealing with.

Zz.

#### Zeynel

the contents of the constituents of the nucleus
So we know that there is a nucleus and this nucleus has parts. But we have no idea what these parts inside the nucleus look like. Did I understand correctly?

#### ZapperZ

Staff Emeritus
2018 Award
So we know that there is a nucleus and this nucleus has parts. But we have no idea what these parts inside the nucleus look like. Did I understand correctly?
Once again, the problem is the words you use. What do you mean when you say "look like"? Do you mean visually?

We don't need to know what things visually look like. We know the content of a nucleus, and the content of nucleons. We don't need to know what they visually look like, because this isn't always necessary.

Zz.

#### DennisN

So we know that there is a nucleus and this nucleus has parts. But we have no idea what these parts inside the nucleus look like. Did I understand correctly?(my bolding)
At this scale, that is the atomic/subatomic scale, scientists detemine the properties and behaviors of particles from experiments (like scattering experiments) and the physical models describe and predict these behaviors. We can say a number of things about the particles and their behaviors, but we can not say what they look like, if we mean looking with our eyes, because we can not see these particles with our own eyes. We can say how they appear to behave with respect to the scientific instruments we use.

#### Zeynel

The problem here is that you attached too big of a significance to the English words that we used to describe things in physics. You should never do such a thing. There are many terminologies that we use that, taken literally, will get into into the wrong rabbit hole. The word "spin" is one clear example as used to designate the magnetic moment of electrons and other quantum "particles". The same thing with the use of the words "wave" and "particles" in QM.

It is why the first and foremost way to understand physics is to understand the mathematical description, because there is often no ambiguity in that description. Our interpretation of it may differ, but there is usually no similar ambiguity on what we are dealing with.

Zz.
Ok, thanks for this explanation. Let me write what I understood from it. Correct me if I’m wrong.

“...English words that we used to describe things in physics.”

So,

1. There are “things”. This is a given.
2. In order to refer to these things we need to name them.
3. As far as physics is concerned, these names are not important. We can call a thing “quark” or any word whatsoever and in terms of physics the name of the thing has no significance.

These names should not be taken literally. For instance, the word “spin” has a clear description outside of physics but in physics it has a different technical meaning. (But this meaning should be irrelevant too, no? Only its meaning in an equation should be relevant.)

Ok, so, physicists take well-known English words with clear and simple meanings and redefine them in physics. They make them physics jargon. I may criticize this practice of taking perfectly good English words and changing their meanings but this is standard practice in physics and must be accepted.

“...to understand physics is to understand the mathematical description, because there is often no ambiguity in that description.”

Is this really true that physics equations represent things without ambiguity? This is questionable.

First, only quantities can be represented in an equation. So words such as “particle” or “wave” cannot be represented in an equation because they are names not quantities. ("Strings" in Computer Science jargon.) Only some quantity associated with these “things” such as “mass” for the particle and “frequency” for the wave can be represented in the equation. But there is still ambiguity. Not only waves have frequencies. Every kind of oscillation will have frequency. So physics, reduced to equations and to “mathematical abstractions” cannot tell us the shape and form of the real object.

“Our interpretation of [mathematical description] may differ but there is usually no similar ambiguity on what we are dealing with.“

But if there are many interpretation of an equation then there must be ambiguity.

So the lesson for me, from your explanation, is this: If our goal is to understand the essence or the real shape and nature of things, physics will not help us because physics is the science of measuring quantities. Physics knows nothing about the general properties or forms of things. Physics knows only the quantity which is measured. This must be so because the fundamental unit of study of physics is the "physical quantity" which is defined as a number with a unit. Physics cares nothing about the name of a physical quantity or even if it exists at all. Investigations of the form or essence of this physical quantity falls outside of the realm of physics.

From these, I also conclude that, as you say, in physics the definitions of words are not important. In physics what is important is the definition of units. So the meaning of the word “mass” have no relevance in physics. Anything with the unit of mass is mass. But there is ambiguity here too because physicists defined two units of mass. One is the old definition of mass as kilogram the other is the definition of mass as multiples of electron volt (if I understand correctly). So in this case calling two different quantities with two different units with the same name is also confusing. Are these masses the same or are they different?

I still think that defining words used in physics uniquely and simply will be helpful in understanding these things.

Sorry, this turned out to be a long response but I think it is relevant to my original question. Thanks again.

#### A.T.

This must be so because the fundamental unit of study of physics is the "physical quantity" which is defined as a number with a unit. Physics cares nothing about the name of a physical quantity or even if it exists at all. Investigations of the form or essence of this physical quantity falls outside of the realm of physics.
To make this simpler: Physics is about making quantitative predictions about nature. Everything necessary for that goal is physics, the rest is not.

BvU

#### DennisN

Ok, so, physicists take well-known English words with clear and simple meanings and redefine them in physics. They make them physics jargon. I may criticize this practice of taking perfectly good English words and changing their meanings but this is standard practice in physics and must be accepted.
Well, quark is not what I would call a well-known English word with clear and simple meaning.
And the procedure of naming things, and the risk of choosing less wellsuited names, is not unique to physics nor to science in general. Human language is human, and naming is a human thing.
If our goal is to understand the essence or the real shape and nature of things, physics will not help us because physics is the science of measuring quantities.
You are using words here which tend to be very slippery when discussing science, like essence, real, nature of, but I'd like to say that I am of the opinion that the science branches, including physics, are the best branches of human knowledge for describing and understanding the physical world.
But there is ambiguity here too because physicists defined two units of mass. One is the old definition of mass as kilogram the other is the definition of mass as multiples of electron volt (if I understand correctly).
Kilogram is the standard SI unit for mass, while eV (electronvolt) is a unit of energy commonly used in atomic and particle physics. 1 eV is about 1.6 x 10−19 joules, where joule is the SI unit of energy. Switching between joules and eV is easy, it's just a unit conversion just like switching between kilometers and miles.

See
Edit: On a second thought, sometimes mass informally/sloppily is expressed using just eV. But to get from eV to kg you also have to divide by c2, where c is the speed of light.

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#### BvU

Homework Helper
Physics cares nothing about the name of a physical quantity
"Nothing" is an exaggeration. It has developed like that over the centuries: in the old days 'charge' and 'mass' were designated with names taken/derived from everyday experience. By the time mankind dug deeper such parallels were no longer available. I would venture the turning point was around the first signs of strange behaviour that led to the naming of the strange quark. After that, charm, top and bottom (they tried 'beauty' for a while) clearly distantiate from meaningful naming. Let alone the 'color' of quarks -- a term that seduces normal people to ask 'what do they look like ?' or even: 'what paint is used for that ?' (*)

It doesn't stop us from developing extremely succesful theories to explain how 'things behave', like QED, QCD, QFT and what have you. Sometimes they can even be validated or falsified with experiments.

(*) this one I made up

#### ZapperZ

Staff Emeritus
2018 Award
Ok, thanks for this explanation. Let me write what I understood from it. Correct me if I’m wrong.

“...English words that we used to describe things in physics.”

So,

1. There are “things”. This is a given.
2. In order to refer to these things we need to name them.
3. As far as physics is concerned, these names are not important. We can call a thing “quark” or any word whatsoever and in terms of physics the name of the thing has no significance.

These names should not be taken literally. For instance, the word “spin” has a clear description outside of physics but in physics it has a different technical meaning. (But this meaning should be irrelevant too, no? Only its meaning in an equation should be relevant.)

Ok, so, physicists take well-known English words with clear and simple meanings and redefine them in physics. They make them physics jargon. I may criticize this practice of taking perfectly good English words and changing their meanings but this is standard practice in physics and must be accepted.

“...to understand physics is to understand the mathematical description, because there is often no ambiguity in that description.”

Is this really true that physics equations represent things without ambiguity? This is questionable.

First, only quantities can be represented in an equation. So words such as “particle” or “wave” cannot be represented in an equation because they are names not quantities. ("Strings" in Computer Science jargon.) Only some quantity associated with these “things” such as “mass” for the particle and “frequency” for the wave can be represented in the equation. But there is still ambiguity. Not only waves have frequencies. Every kind of oscillation will have frequency. So physics, reduced to equations and to “mathematical abstractions” cannot tell us the shape and form of the real object.

“Our interpretation of [mathematical description] may differ but there is usually no similar ambiguity on what we are dealing with.“

But if there are many interpretation of an equation then there must be ambiguity.

So the lesson for me, from your explanation, is this: If our goal is to understand the essence or the real shape and nature of things, physics will not help us because physics is the science of measuring quantities. Physics knows nothing about the general properties or forms of things. Physics knows only the quantity which is measured. This must be so because the fundamental unit of study of physics is the "physical quantity" which is defined as a number with a unit. Physics cares nothing about the name of a physical quantity or even if it exists at all. Investigations of the form or essence of this physical quantity falls outside of the realm of physics.

From these, I also conclude that, as you say, in physics the definitions of words are not important. In physics what is important is the definition of units. So the meaning of the word “mass” have no relevance in physics. Anything with the unit of mass is mass. But there is ambiguity here too because physicists defined two units of mass. One is the old definition of mass as kilogram the other is the definition of mass as multiples of electron volt (if I understand correctly). So in this case calling two different quantities with two different units with the same name is also confusing. Are these masses the same or are they different?

I still think that defining words used in physics uniquely and simply will be helpful in understanding these things.

Sorry, this turned out to be a long response but I think it is relevant to my original question. Thanks again.
This still doesn't change the fact that you are hung up more on the names we give to these things than to understand the physics associated with those names.

BTW, these words ARE defined uniquely and "simply" (as simple as it can get). It is just that it appears that they are defined in ways you seem to unable to comprehend. This is not our fault. Mother Nature didn't make herself conform to be able to be understood by everyone, so blame HER.

And just in case this is overlooked, the repurposing of common words to mean something else in different context is not unique to just physics or to just science. I spent quite a bit of time earlier this year helping someone from another country starting a new career here in the US. And as much as he's highly proficient in English, he still struggled in trying to understand the meaning of "co-pay" and "deductibles", etc.. when he was evaluating his health benefits. So this isn't special. It occurs everywhere whenever the context change!

Zz.

#### DaveC426913

Gold Member
As far as physics is concerned, these names are not important. We can call a thing “quark” or any word whatsoever and in terms of physics the name of the thing has no significance.
This.

The formula is the thing; the math describes it.
Names can only ever be flawed descriptions.

#### Zeynel

Well, quark is not what I would call a well-known English word with clear and simple meaning.
You are right. I egree.

And the procedure of naming things, and the risk of choosing less wellsuited names, is not unique to physics nor to science in general.
This is true too.

Human language is human, and naming is a human thing.
I agree, you are right. Let me say that my real concern is about multiple definitions of symbols in physics. I agree with @ZapperZ that physics deals with quantities and not with “strings”, so all symbols must ultimately point to quantities.

Sorry, since this subject is not directly related to my original question maybe we can drop it and discuss it in another thread.

You are using words here which tend to be very slippery ... like essence
Ok, let’s say you are right about “essence” but the link you shared about units and dimensions used in physics made me realize that physics has an inherent limitation when “describing and understanding the physical world."

From the Physical Units page you linked:

Mechanics is the branch of physics in which the basic physical units are developed. The logical sequence is from the description of motion to the causes of motion (forces and torque) and then to the action of forces and torque. The basic mechanical units are those of Mass, Length and Time. All mechanical quantities can be expressed in terms of these three quantities.
This means that we cannot know the shape of the thing moving according to M, L and T. Because we only look at the motion and its weight (called mass), the distance it moves and the time interval it moves in. The moving thing can be a truck, a particle, a point, anything and everything, and the equation made up with M, L and T cannot tell the difference. If we are looking at colliding billiard balls, we would know that we are working with billiard balls. But if we do not know what we are working with, as in the case of quarks, then, we cannot use our equations to decide the shape of the signal. In terms of properties we can only know its mass and derived units like charge and energy, but not its shape. Our instruments measure behavior not shape or form. This is how I understand it.

This is confirmed, I think, because everyone here responded by saying that we do not know the shape or form of quarks. For instance, we don’t know if a quark is a spherical object.

The fact that we do not know the shape of quarks does not give CERN to picture quarks as spherical objects. I think it is more intellectually honest for CERN to admit that we do not know how quarks look like and put a disclaimer to that effect. Otherwise they are spreading misinformation and falsehoods to the public.

Kilogram is the standard SI unit for mass, while eV (electronvolt) is a unit of energy commonly used in atomic and particle physics.... See
Thanks again for the links, they were helpful, but can you explain how we get from 1/2 m vv to eV? The layers of units were too many and I couldn’t decipher them.

Thanks.

#### sophiecentaur

Gold Member
but can you explain how we get from 1/2 m vv to eV
It describes how the Kinetic Energy of a charged particle relates to the change in Electrical Potential through which it has 'fallen'. It's just like equating the gravitational potential of a mass on a high shelf to the KE it will have when it's fallen to the floor.

#### Zeynel

Ok. I understand. But I don't understand the steps to transform or substitutions to make to arrive to eV starting from 1/2 m vv.

"What is a realistic image of quarks?"

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