# Solid empty space?

1. Jul 24, 2014

### sambogrub

Learing about matter and what it is composed of, I'm spending a lot of time trying to wrap my brain around the idea that most of our mass is empty space, that the mass of our atoms is calculated and expressed in energy. How is this possible? Are the recreations moving so fast they almost create a solid field?

2. Jul 24, 2014

### e.bar.goum

I've got no idea what you mean by this. Could you perhaps expand on what you're thinking?

The fact that you can express the mass of atoms in terms of energy is nothing too exciting - it's just plain old E = mc2 - we know that mass and energy are equivalent. In fact, I can write my own mass in terms of energy - about 6x1018 Joules.

3. Jul 25, 2014

### sambogrub

Yeah but that is not what I'm asking. Our atoms are mostly empty space, if we add up the mass of the atomic and sub atomic particles that make up a person, the ones that gain mass from the Higgs field, it is only a fraction of our total mass. What we know of as mass on an atomic scale is energy correct? So how are we solid as a whole when most of what we are made of is empty space? I understand that it comes down to how the energy in atoms and molecules interact with each other, but still, an atom is 99.99999% empty space! why aren't we that way?

4. Jul 25, 2014

### Bandersnatch

But we are!

5. Jul 25, 2014

### sambogrub

So then why can't I see through my arm? Or pass through another object? Is it the energies interacting with each other?

6. Jul 25, 2014

### phinds

No, it is ALL of our mass. Where do you think the rest of our mass would be?

As bandersnatch stated, we are. Why would you think we are not?

7. Jul 25, 2014

### sambogrub

Thanks for putting up with my ignorance but I'm still confused.
I'm trying to define mass from the atomic level up. We are just empty space interacting with other empty spaces right? I mean the Higgs field doesn't even give us mass really, it just slows down the subatomic particles enough to interact with each other and to be perceived. Adding up the mass of all our protons, neutrons, and electrons is just a small fraction of our total mass correct? So why is all that empty space able to interact with anything at all?

8. Jul 25, 2014

### Bandersnatch

You're on the right track!
Imagine a pair of magnets with like poles facing each other. The space between them can be empty(in vacuum), yet they'll resist being brought together.
This space is only empty in so far as being devoid of massive particles, but it is filled with fields of various forces, with varying strengths.
From far away, the magnets are "unaware" of each other, but if you bring them close enough, the interaction will become noticeable, getting ever stronger as the distance decreases.

Atoms and molecules(bound collections of the former) act like a multitude of tiny magnets, whose fields collectivelly act to repel(or attract!) other atoms. In their case, it's not magnetic, but electric interaction(both being two facets of one fundamental, electromagnetic force).
When atoms attract, they tend to bind into more complex molecules, which is what chemistry is all about.

As for light, it only cares about the extent of the electromagnetically interacting "stuff" - i.e., the fuzzy cloud of electrons surrounding atoms/molecules. This is because light is the excitation of electromagnetic field.
It doesn't matter how much of the volume of an atom/molecule is occupied by mass, just what is the extent of the electron cloud.

As a rule of thumb, only light of wavelength comparable to the object it hits gets to interact, other wavelengths passing through.
Molecules are about the right size to interact with what we call the visible spectrum(360-720nm, iirc).

Particles that don't interact electromagnetically(like neutrinos) can pass through atoms as if they weren't there.

9. Jul 25, 2014

### phinds

No, it is ALL of our mass. Where do you think the rest of our mass would be? This is the second time I've asked you that question. Do you have an answer?

10. Jul 27, 2014

### e.bar.goum

So, just for the record, it's not *quite* true that the sum of the masses of the protons and neutrons and electrons in our body is equal to our mass - in the same way that the mass of oxygen is not the mass of 8 free protons and 8 free neutrons - you need to take binding energy into account.

It is also not true that the mass of protons and neutrons comes from the Higgs Mechanism alone. The Higgs Mechanism gives mass to the quarks, but the bulk (~99%, IIRC) of the mass of a proton or neutron comes from the gluon-mediated strong force, from E = mc2.

sambogrub, The reason we look "solid" rather than mostly empty space (or transparent) is actually rather easily answered! It's a matter of wavelength. We see the universe in the visible region of the EM spectrum, with wavelengths on the order of 500 nm. That's 5*10-7 m. An atom - the scale at which we're mostly empty space - has size of about 1 Angstrom - that's 10-10m. That is, an atom is one thousand times smaller than the wavelength of visible light!

A general rule of thumb is that to examine an object of some size, you need light of wavelength about the same or a bit smaller than the thing you want to examine. If you use a bigger wavelength, diffraction ruins whatever you're trying to examine. This is why to do something like nuclear physics or crystallography you use gamma radiation or x-rays which have smaller wavelengths.

So! Visible light is just way way way too big! But if you go to higher energy light, like gamma-rays from radioactive decay, which is on the order of MeV - about 10-12m, then you start to look pretty transparent!

11. Jul 27, 2014

### Drakkith

Staff Emeritus
It's true that you can't take the mass of each particle in a free state and multiply it by the number of each particle in your body to get an accurate number. You need to account for the loss of mass the system has due to a loss of energy (binding energy). However I don't know if I'd say that you can't add up the mass of every particle in your body to get the answer, as they don't really have the same mass as they did in their free, unbound states.

Of course, even defining the mass of a bound particle is touchy, as you have to measure the system as a whole or break it apart to measure each piece separately, which requires adding energy and thus mass to each piece.

All of which contributes to the "rest mass" of the proton or neutron as a whole.

12. Jul 27, 2014

### e.bar.goum

I think we're in exact agreement? Or rather, I'm not sure what part of my post you're disagreeing with?

13. Jul 28, 2014

### Drakkith

Staff Emeritus
Not really disagreeing, just wanted to expand a little on what you said for clarity's sake.

14. Jul 28, 2014

### e.bar.goum

Great! I was mildly concerned.

15. Jul 28, 2014

### Bandersnatch

I've heard something to this effect a few times already, but I must say I can't quite understand why it is so. Isn't binding energy of the strong force negative, like all attractive forces? Or does the positive contribution come from that region where the force becomes repulsive?

16. Jul 28, 2014

### Drakkith

Staff Emeritus
I see why you're confused. Binding energy typically refers to the energy needed to separate a system of bound particles and represents the amount of energy released when these particles enter their bound state. Thus a system of particle bound together are less massive than when they are not bound together due to this release of binding energy.

It may be more accurate to say that most of the mass of a proton or neutron is in the form of kinetic energy of the quarks themselves plus the energy of the gluons binding them together. I'm not certain why this is called binding energy, but then again I don't work in a QCD related field.

http://en.wikipedia.org/wiki/Proton#Quarks_and_the_mass_of_the_proton
http://terpconnect.umd.edu/~xji/mass.ppt

17. Jul 28, 2014

### e.bar.goum

I work in a QCD related field, and I would have said just what you did - it's called the binding energy just because that is the energy required to separate the nucleus or nucleon into constituent parts - to make it unbound.

18. Jul 28, 2014

### Drakkith

Staff Emeritus
Hmm... I may have to make a thread on this so we don't hijack this thread.

Last edited: Jul 28, 2014
19. Jul 28, 2014

### phinds

While I agree that all of the foregoing comments to the effect that our particle mass is not quite our total mass (because of binding energy) are correct statements, I think they are in this case an irrelevant side issue that detract from the fundamental incorrectness of Sambogrub's contention that they are "just a small fraction of our total mass". That statement is completely incorrect and I ask him again to say why he thinks it is correct.

20. Jul 29, 2014

### sambogrub

I guess I was exaggerating a bit when I said a small fraction, but the answers given actually help a ton. I was talking about exactly what was stated earlier, that you have to take into account the energies in our atoms and molecules to find our total mass. I guess I have the definition of mass wrong as I have it in my head as something that has substance or is solid, which quarks and other subatomic particles are, from the little I know. So its not really true that all of our mass is in these particles but a portion is also in the energies holding everything together.
And I really apprecite the answers. I sort of had the idea of it being light related that we cannot see through ourselves and also that we can interact with other objects because of the magnetic fields but this helps me know just a little more.