# Why do people say that mass is too fundamental to know exactly what it is?

## Main Question or Discussion Point

Isn't mass basically one or more atoms (in part or in whole)?

## Answers and Replies

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HallsofIvy
Homework Helper
Because diffferent isotopes of the same element differ in atomic mass, it is not sufficient to measure mass in terms of "number of atoms".

Because diffferent isotopes of the same element differ in atomic mass, it is not sufficient to measure mass in terms of "number of atoms".
So what they really mean is the "measurement" of mass, right? Mass itself is not a mystery.

Mass is very much a mystery.
We know it's effects but not it's cause(to my understanding)

Of course, that's also true with many other things.

Mass is very much a mystery.
We know it's effects but not it's cause(to my understanding)
The cause of atoms is protons+neutrons+electrons. Are you asking what these three components are made of?

The cause of atoms is protons+neutrons+electrons. Are you asking what these three components are made of?
No. Rather, what is MASS. Mass is not an electron, neutron or proton.
And an electron is a fundamental particle, having no further division known.
And all three have different charge characteristics.
Yet, all 3 have the attribute of "mass"
Very much a mystery.

Hopefully the LHC(Large Hadron Collider) will provide insight.

So you are essentially saying that we don't know what an electron (for example) is in and of itself?

So you are essentially saying that we don't know what an electron (for example) is in and of itself?
What I am saying is that in reference to "mass" we don't know.
However, I like to believe that we are getting closer to understanding.

For your question on the electron, you might want to peruse the quantum physics(or similar) section of PF, because I don't know. But I think I saw some good ideas there!

russ_watters
Mentor
Mass is very much a mystery.
We know it's effects but not it's cause(to my understanding)
A lot of people feel that way, but I don't like it. It implies a failing at something science doesn't even attempt to do. All science can do is define things by their properties. That's all we ask and all we expect. Ultimately, there is no "cause" of a fundamental property (unless it was decreed by God). It just is.

What do you guys even mean by "cause of mass"? All I was saying is that we do know what mass is. Mass is one or more fundamental particle(s). It is something that can be touched. You might say that we can't touch an electron. But we can. We just don't feel it. Mass is not a property of an object. Mass is the object itself. The reason I asked this in the first place was because it is a question that frequently comes up when people ask "what exactly is charge"? Charge is, indeed, difficult to know what it IS. We can only know what it does. As for mass, that is not true.

What do you guys even mean by "cause of mass"? All I was saying is that we do know what mass is. Mass is one or more fundamental particle(s). It is something that can be touched. You might say that we can't touch an electron. But we can. We just don't feel it. Mass is not a property of an object. Mass is the object itself. The reason I asked this in the first place was because it is a question that frequently comes up when people ask "what exactly is charge"? Charge is, indeed, difficult to know what it IS. We can only know what it does. As for mass, that is not true.
You "touch" photons all day long, photons have no mass. Are you going to claim that because photons have no mass they don't exist?

cepheid
Staff Emeritus
Gold Member
What do you guys even mean by "cause of mass"? All I was saying is that we do know what mass is. Mass is one or more fundamental particle(s). It is something that can be touched. You might say that we can't touch an electron. But we can. We just don't feel it. Mass is not a property of an object. Mass is the object itself.
That's where you're wrong. Mass is a property of a particle, just like charge and spin are properties of particles. The thing that we call inertial mass is the m in Newton's 2nd law, F = ma. It's the quantity that determines how much an object resists a change in its state of motion. An electron has a certain mass, which you could express in kg if you wanted. We can measure it, but we don't know* why it has the mass that it does, while other elementary particles have different masses (i.e. not the same mass as the electron). We don't know* why some particles have mass, and some don't. What, fundamentally, gives rise to mass?

*Caveat: a more accurate statement is that I don't know the answers to these questions. I know that there are theories, and I'm sure that people who knew a lot more about particle physics than I do would be able to explain them. I've heard that a particle called the Higg boson has been theorized to somehow give rise to mass in other particles, but I don't know how that works. I know that a lot of people are really hoping we detect a Higgs boson soon.

What do you guys even mean by "cause of mass"? All I was saying is that we do know what mass is. Mass is one or more fundamental particle(s). It is something that can be touched. You might say that we can't touch an electron. But we can. We just don't feel it. Mass is not a property of an object. Mass is the object itself. The reason I asked this in the first place was because it is a question that frequently comes up when people ask "what exactly is charge"? Charge is, indeed, difficult to know what it IS. We can only know what it does. As for mass, that is not true.
Just to elaborate a bit:

The charge of a particle is some quantity which determines how strongly it will respond to an electromagnetic field. It seems hard for us to understand what charge "is" because we don't have a sense of "charge" in every day life.

In an analogous way, the mass of a particle is some quantity which determines how strongly it will respond to a gravitational field. We may feel like we understand mass because we know the sensation of picking something up in every day life. But we are not feeling the objects mass, we are feeling the strength of the gravitational force between the object and the earth.

That's not really so different from holding two magnets and feeling the attraction between them. Most non-scientists think they don't understand magnetism. But if the earth were a big magnet (well, a stronger one), and our feet were magnetic, I think people would find magnetism to be a very "intuitive" concept. Just the way they feel about gravity now: objects fall down, what's difficult to understand about that?

Another thing to think about - when you reach out and touch the table, you may think you are feeling its mass. But actually, the gravitational attraction between your hand and the table is negligible, and the sensation you feel is due to electrostatic forces. So in a sense, it would be more accurate to say that you are feeling its charge.

That's where you're wrong. Mass is a property of a particle, just like charge and spin are properties of particles.
Or I could just say that mass is a measurement of the amount of matter. Ex. 2kg of matter. In that case, how would it be a property?

You need to think about individual particles here. An electron has a mass of 9.109x10^-31 kg. That's not the "amount of electron". Every single electron that exists has exactly that mass. Doesn't that sound like a property of the electron to you?

You need to think about individual particles here. An electron has a mass of 9.109x10^-31 kg. That's not the "amount of electron". Every single electron that exists has exactly that mass. Doesn't that sound like a property of the electron to you?
An electron has 9.109x10^-31 RELATIVE to the defined kilogram. Nature itself does not assign values to matter. We could have said that one electron is equal to 1kg. So I am still confused. Matter itself is the resistance to change in motion. Mass is just a measurement.

Drakkith
Staff Emeritus
An electron has 9.109x10^-31 RELATIVE to the defined kilogram. Nature itself does not assign values to matter. We could have said that one electron is equal to 1kg. So I am still confused. Matter itself is the resistance to change in motion. Mass is just a measurement.
Matter is NOT the resistance to change in motion. Matter is the description we give to the subatomic particles that make up all atoms, and to particles with mass. "Normal" matter is usually seen to be quarks that make up protons and neutrons, along with electrons. However one could easily consider the other types of quarks and bosons to be matter, and I believe they are.

Mass is a property of matter and energy that we have defined. The measurement of mass is a measure of this property, not some unkown "thing".

I've seen a lot of views like yours over the past week or two, in that they believe that we don't know "what" these properties actually are. All forces, properties, and everything can only be known by the effects they cause on other forces and particles and such. Attempting to say "what" these actually are is not science, as it cannot be observed. Do you understand what I am saying?

But there are are particles that don't have mass that we can observe? So how do we explain it, what is mass, why do some particles have it and some don't? Isn't this science?

Also the fact that mass can turn into energy, what exactly is the mass and what turns into energy, how does it do it etc. Aren't these all scientific questions?

Isn't it analogous to for example studying light absorption and reflection? Why some objects reflect light, some don't at all, some do it selectively, what is the difference, what is light and why does it behave like that etc. Aren't these the same questions which we have about mass except for these we have answers?

Drakkith
Staff Emeritus
But there are are particles that don't have mass that we can observe? So how do we explain it, what is mass, why do some particles have it and some don't? Isn't this science?
From wikipedia on Matter:

Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume. In practice however there is no single correct scientific meaning of "matter," as different fields use the term in different and sometimes incompatible ways.

Generally we say that all matter has mass. Particles that are massless, such as photons, are not usually considered matter.

Also the fact that mass can turn into energy, what exactly is the mass and what turns into energy, how does it do it etc. Aren't these all scientific questions?
Of course they are scientific questions. The mass is exactly that. Mass. It can be converted into energy via multiple ways. Annihilation with antimatter, nuclear reactions, and chemical reactions all convert mass to energy.

Isn't it analogous to for example studying light absorption and reflection? Why some objects reflect light, some don't at all, some do it selectively, what is the difference, what is light and why does it behave like that etc. Aren't these the same questions which we have about mass except for these we have answers?
Not exactly. A photon is a "thing". The energy that is transferred from one object to another object via EM radiation does so because the energy is contained in packets called photons. Now, these photons have properties which we define, such as momentum, frequency, etc. Trying to portray mass as "something" is like trying to portray frequency as "something". It is simply a property of the object.

I've seen a lot of views like yours over the past week or two, in that they believe that we don't know "what" these properties actually are. All forces, properties, and everything can only be known by the effects they cause on other forces and particles and such. Attempting to say "what" these actually are is not science, as it cannot be observed. Do you understand what I am saying?
Quanta of fields that interact with the Higgs field have a resistance to motion through space, exactly due to their interaction with the Higgs field. This property of resistance to motion is called mass.
People found that answer because they asked questions like "what exactly is the nature of mass" and the likes of it. Yes care must be taken to prevent unobservable and spooky stuff from being claimed science but likewise you should be very careful when labeling something unscientific just because you are having trouble understanding it. Even from the little literature I read, I know that the problems you guys are labeling unscientific are still being discussed in reviewed journals.

This property of resistance to motion is called mass.
I thought that was called inertia, which is a property of mass?

I doubt that there is any truely satisfying definition for mass. But that's one of the things that make the subject of mass and gravity such an interesting study for me. To quote H. A. Kramers, "In the world of human thought generally, and in physical science particularly, the most important and most fruitfull concepts are those to which it is impossible to attach a well established meaning".

Maybe "volume" is one other property besides inertia that, what we commonly refer to as mass, would be supposed to have but we now know that effective volume is just a manifestation of Pauli's exclusion principle. For example, in case of bosons for which Pauli's principle doesn't apply, two particles can be in the same place at once; I don't suppose one can define a volume for such particles.

nrqed
Homework Helper
Gold Member
Isn't mass basically one or more atoms (in part or in whole)?
There are two issues here.

Mass is a property of particles and as such it is not a mystery. In classical physics it is simply a measure of inertia and once we know the mass of an object (by first making some measurement) we can use that to do all sorts of predictions.

On the other hand, mass is a complete mystery in the following sense: we have no way to actually calculate the mass of anything! we must [it] measure [/it] the mass of fundamental particles, we cannot calculate them from fundamental principles. Why is the electron's mass $$9.11 \times 10^{-31} kg$$? Most physicists believe that if we had a truly fundamental theory of nature, we could actually calculate the mass of the electron from some fundamental principle. Or at least, we could calculate the masses of all particles in terms of only one of them (for example, we could predict the mass of the muon and tau once given the mass of the electron).

mass is a complete mystery in the following sense: we have no way to actually calculate the mass of anything!
On the most basic level, mass is not measured but instead defined. For example, we pick up a piece of metal. We give it the name "kilogram". Now, everything else we measure is a comparison to this metal. For example, a brick has three times the amount of matter that the kilogram has, however much that amount of matter may be (We don't know because matter is a loosely defined term). BUT, what actually matters is THE RELATIVE IMPORTANCE OF THINGS. The brick is attracted three times greater to the Earth than this thing we call "the kilogram". This helps us to have a better perception of things. It doesn't matter what we choose to call "kilogram" as long as we do so. From these observations that I have made, it seems to me that mass is just "a quantification of matter with respect to a defined standard object". Why is this number that is used to describe an objects relative amount of matter, aka mass, exactly copied digit for digit in describing the objects inertia (F=ma)? We are implying that inertia is also a measurement of the amount of matter. Why? This is what I don't get. Why are we literally equating [a measurement of mass] to the object's [resistance in motion]? They are proportional. But why equal?

Drakkith
Staff Emeritus